Introduction to ACME

Welcome to the ACME user guide. ACME (ACoustic Measurement Environment) is a multi-purpose acoustic measurement and analysis tool. It provides tools for measuring, a signal generator, power spectra and sound level analysis.

Additionally, ACME is the required software for performing μZ impedance tube (product page) measurements and analyses. More information on the measurement and analysis procedures for this system can be found at the µZ impedance (userguide).

For more detailed description, please continue reading this user guide.

Reporting bugs

If you find any software bugs, inconsistencies, errors or improvement hints, please let us know on support@ascee.nl and we will fix it as soon as possible.

Privacy notice

We value your privacy, so we do not store unnessesary (personal) information. No (tracking) cookies are stored. We do a small bit of analytics for this site, just to see how many people are watching our documentation and have a rough idea of where they are from¹. Basically the only thing happening is a counter increase of the number of unique visitors we get.

Notes on this manual

The user interface of ACME picks up the style from its operating system. Therefore, colors, styles and places of buttons can vary a bit depending on your used operating system.

We did our best to provide a proper manual, free of errors. However, if you do find an error or incomplete part, please let us know by sending an email to support@ascee.nl.

Enjoy!😄

1. This is done by masking the last byte of the IP address ↩

System requirements

Operating system

ACME is compatible with the following operating systems:

• Microsoft® Windows 7¹
• Microsoft® Windows 10
• Microsoft® Windows 11
• Linux Mint 22.1
• Ubuntu 24.04 Noble Numbat

Note that ACME is currently only compatible with machines that have an x86-64 instruction set.

Hardware requirements

Minimum

• RAM: 4 GiB
• CPU clock speed: 2 GHz
• Processor: Dual core
• At least one free USB 2.0 A connector port
• Screen resolution: 1920 ⨉ 1080 pixels
• Free disk space: 1 GiB

ACME requires 400 MiB of disk space. However, ACME stores the raw measurement time data for each measurement. The size of a measurement file may vary. It depends on the number of channels, sample rate and the record duration. As a working metric, measurement files are approximately 10 MiB. Make sure there is sufficient free disk space to store your measurements.

Recommended

• RAM: 8 GiB
• CPU clock speed: 3 GHz
• Processor: Quad core
• At least one free USB 2.0 A connector port
• Screen resolution: 2560 ⨉ 1440 pixels
• Free disk space: 20 GiB

1. Not recommended, as it is no longer supported by Microsoft®. ↩

Installation guide

• Before continuing, please first check whether the system requirements are fulfilled.
• Download the latest version of ACME from the ACME page, specific for your operating system (OS).
• Double-click the installer, and follow OS-specific instructions below:
  • Windows instructions
  • Linux instructions

Windows installation instructions

Hardware drivers

Compatible general ASIO driver (optional)

In order to use any other ASIO device on Windows, an ASIO driver is required. ACME is compatible with ASIO4ALL 2.16. Please follow the instructions below to download ASIO4ALL 2.16:

• Go to ASIO4ALL.org and download version 2.16.
• Double-click on the installer. Use all default settings.

Audient EVO 16 (comes with μZ systems)

To use all channels on the Audient EVO 16 (as is required for the µZ case) on a Windows system, the EVO ASIO driver needs to be installed.

• Go to the Audient EVO 16 web page, and download the Windows driver (v4.4.0). Use all default settings when installing.

Once installed, the DAQ configuration in ACME will show the ASIO driver on API PortAudio Windows ASIO, as shown below:

Optional: USB Driver installation for DT9837A

For interacting with the DT9837A USB data acquisition, the driver needs to be installed. A general LibUSB driver needs to be coupled to the device, which is done using Zadig.

• Go to the website, or download Zadig using this link.
• Connect your DT9837A to the PC
• Run zadig-2.9.exe
• For the Zadig update policy, press No
• When the program opens, you see the following window:
  
  • Select the DT9837A, if not already selected, in the drop down list
  • Select the WinUSB driver, if not already selected, in the box at the right of the green arrow.
  • Press Install Driver
• The driver will then be installed:
• When finished you will see:
  
• Done! After installation, and when the device is connected, it will be visible in ACME's DAQ Configuration

ACME Installation

• Double-click acme_v<X>_installer.exe, where <X> is the version specifier:
  
  • Click Yes
• You will see:
  
  • Click Next
• Wait till ACME is being installed:
  
• When the installation is finished, the following page is shown:
  
  • Choose to launch ACME directly, by clicking Finish, or choose to quit by deselecting the Launch ACME box, and the clicking Finish.

Linux installation instructions

Under construction. If you need help, send us a message and we will arrange a video conference.

Overview of ACME

When starting up for the first time, you have to Accept the license agreement. After that, you will see the interface with the Measure tab activated.

ACME consists of two main tabs: Measure and Analyze. The Measure tab contains functionality for recording measurements, including a signal generator and signal monitors. ACME measurements are stored in separate files and are grouped together in a working folder. The measurements can be processed and plotted on the Analyze tab.

Measure tab

The figure above shows an overview of the Measure tab.

• The DAQ configuration allows editing and configuring the DAQ devices. This works using presets. Presets can be editing by right-clicking with the mouse on the current configuration. The example above shows a configuration called My DAQ Config
• You can switch to the Analyze panel by clicking the Analyze tab. This can be done at any moment, except when performing a measurement.
• The Signal Generator box is used to control output test signals.
• The Measure box is used to perform measurements.
• The PPM shows Peak Programme Meter results, these are instantaneous and peak levels measured on the activated input channels.
• The Real time Viewer has two tabs and allows for real time investigation of incoming signals.
• The System status indicator is part of the status bar and shows current CPU and memory usage. If these are too high, values will turn red. This might also lead to buffer under or overruns during recording.

Analyze tab

The figure above shows an overview of the Analyze tab.

• The measurement list shows an overview of the available measurement files in the current folder. The current folder is also used in the Measure tab, as the place where measurement files are stored. Each measurement file in ACME is self-contained.
• The compute box shows settings for performing post-processing computations. Post-processing is any tasks that extracts valuable information from the raw data stored in a measurement. For example, this can be the computation of sound levels, power spectra and insertion loss.
• The figure shows computed results in a plot. Multiple results can be added in the same plot if they are compatible.
• Figures can be annotated / marked up using the layout options. Lines can be hidden, edited and legend labels can be changed using the Plot operations... button. The dialog that opens with this button also allows for exporting the result data.

DAQ device support

As data acquisition, ACME supports sound cards through the OS provided APIs:

• PulseAudio / ALSA (Linux),
• WASAPI / ASIO / DirectSound (Windows),

as well as the DT9837A USB DAQ. The backend of ACME is LASP, our in-house developed acoustic signal processing library. Through ACME's backend-independency, we strive to support multiple DAQ devices around. If you would like ACME to support a device we currently do not yet support, please contact us and we will discuss the possibilities.

µZ Impedance tube

ACME is the required software for performing measurements with the μZ impedance tube. See the product page and section in the user guide for more details.

Menu bar

The menu bar is located at the top of the screen.

It is organized as follows:

• File

  • Open measurement folder
  • Recent folders
  • Reload measurement folder
  • Exit

• Actions

  • Toggle output stream
  • Toggle input stream
  • Start measurement
  • Rescan DAQ devices
  • Calibrate microphone
  • Calibrate µZ system¹

• Settings

  • Post processing
    • Show general power spectra settings

• Help

  • Open user guide in browser
  • Show license agreement
  • License key info
  • Restore to default
  • About

Detailed information

The items which might require more explanation are are discussed below.

Rescan DAQ devices

If a DAQ device is connected to the computer while ACME is running, it does not recognize the device. Press this button to scan and recognize the new device.

Calibrate microphone

This opens a tool to determine the sensitivity of the microphone, which is required for measuring absolute sound pressure levels.

1. The Calibrate µZ system action is only available if the µZ module has been purchased. ↩

Toolbar

The toolbar is located near the top left of the screen and contains commonly used buttons.

Buttons

1. Open measurement folder: Change the working folder in which ACME stores the measurements.
2. Enable/disable the input stream: If in the DAQ configuration is set to duplex mode, this enables the duplex (input and output) stream.
3. Enable/disable the output stream: No function when in duplex mode.
4. Start measurement: Start a measurement with the settings defined in the Measure tab.
5. FFT settings: Show the power spectra settings. For more information, see the FFT Settings page.

FFT settings

The Fast Fourier Transform (FFT) settings can be selected by clicking the purple Erlenmeyer flask in the toolbar, near the top of the screen.

This menu controls two sets of settings, depending on whether it is click in the Measure or Analyze tab.

• Measure tab: Real Time Spectrum Viewer
• Analyze tab: For plotting the measurements

The button pops up the following screen:

The settings are as follows:

The resulting frequency resolution is calculated with:

$\Delta f=\frac{f_s}{N_{\mathrm{FFT}}}$

In which $f_s$ is the sample rate and $N_{\mathrm{FFT}}$ the FFT length. A sample rate of 48 kHz and an FFT lenght of 48 k result in a frequency resolution of 1 Hz.

Available windows are:

1. Hann
2. Hamming
3. Bartlett
4. Blackmann
5. Rectangular

The difference between the Window types is subtle and is beyond the scope of this userguide. An exception is the Rectangular window, which turns the windowing function off and is unsuited for most analyses. The Hann window is a good all-round choice.

Measure tab

Access the Measure tab by clicking its tab:

The various functions are explained in the subchapters.

Configure Data AcQuisition (DAQ)

A DAQ configuration defines the device used for generating signals and reading measurement data. ACME provides an API abstraction layer around a DAQ, and is able to work with different API's to read/write signal data. The easiest way to read signal data in ACME is by using an audio interface.

Managing DAQ configurations

On the Measure tab, create a new device configuration by right clicking on the dropdown box in DAQ configuration:

Here you can choose to:

• Edit the current configuration (if any)
• Create a new configuration
• Create a new configuration by copying the currently selected configuration
• Delete a configuration

DAQ configuration presets are stored on your computer right after creation and will be visible every time you restart ACME.

Editing a configuration

Global settings

When you edit an existing configuration, or create a new one, a dialog will open:

The DAQ configuration has two tabs. The first is the global configuration. The settings apply to both input and output settings. You can control input and output independently, but the configurations stores settings for both input and output simultaneously. ACME allows the usage of only one input and one output device at the same time.

• The Configuration name is a free text field where you can enter the name of the configuration. Duplicate names are not allowed.
• The Input API / Output API show the currently selected API for the input and output.
• When a device supports duplex mode, the input and output can be coupled to the same device. In that case, the Outpput device configuration is greyed out, and the Input device configuration settings apply to both input and output.
• For the Input device configuration the sample rate can be set, this is the amount of samples that are taken per second (for each value). Lower values allow for smaller measurement file size, but also limit the highest maximum frequency that can be analyzed.
• The Frames per block setting is advanced, and determines the real time latency in the real time viewer. Too low values might result in buffer X-runs. Advised is to leave this value to the default of 8192.
• Data type is the format for which samples obtained from the device. The allowed settings here are dependent on the device and the device driver. Floating point values are the most precise, but sometimes they are already converted from fixed point. This depends on the underlying device. It is advised to use either 32 bits fixed point of floating point.
• The Run in duplex mode checkbox switches the device to duplex mode.
• For some devices, a hardware loopback exists. In that case this button is enabled and an artificial input channel appears, that copies over the output signal generated with the Signal generator.

Channel settings

The second tab contains settings for each channel. Depending on the selected API / device combination, some settings might be grayed out.

Depending on the selected input / output device, the available input and output channels is listed. From the given channels of a device, a subset can be enabled for input. These have to be checked next to the name.

• The checkbox next to the Name field toggles enabling this channel. Any channel not enabled is not visible in the PPM, Real time viewer nor taken into account in a measurement.

• The Name field allows for a unique name for each channel. Duplicate names are not allowed.

• The Sensitivity field allows for setting the sensitivity. See background info below.

• Physical Quantity is the kind of signal that is measured ultimately. For a microphone, this is acoustic pressure. Changing this setting is useful e.g. for measuring sound pressure levels.

• A Digital highpass applies a first order digital highpass filter to the incoming data. This can be used to remove unwanted DC offset from the measured signal. When the value is <=0, the digital highpass is turned off. Otherwise, it determines the cut-on frequency of the used digital highpass filter.

• Hardware AC coupling is another option to remove unwanted DC offset from the measured signals. If the device supports it, checking this box will enable a hardware AC coupling. This can be done, for example, by placing a capacitor in series before the ADC.

• IEPE is a sensor power supply. This can be turned on for some sensors, for example measurement microphones.

Sensitivity settings

The sensitivity $S$ has a unit of $1/U$, where $U$ is the unit of the physical quantity measured. For example when measuring acoustic pressure, the sensitivity has unit Pa${}^{-1}$. The following equation is applied to scale the data:

$$u_i={\mathrm{data}}_i/S_i,$$

where $u_i$ is the physical measured quantity for channel $i$ and $S_i$ is the sensitivity for this channel. When data is stored as floating point, this equation is directly applied in this form.

Fixed point data caveats

Fixed point data is interpreted as fraction of full scale. So for fixed point data, the value is computed as

$$u_i=\frac{{\mathrm{data}}_i}{{\mathrm{data}}_{\max }}\times \frac{1}{S_i},$$

where ${\mathrm{data}}_{\max }$ is the maximum positive value that can be stored for the integer data. For example, for 16-bit integers this value is $2^{15}-1$, for 32-bits integers this is $2^{31}-1$.

When fixed point data is shown, it is always scaled to this maximum value, representing a number between -1 and 1.

Signal generator

The signal generator supplies the signal for a measurement. It consists of three parts:

1. Type: Adjust the signal.
2. Level: Slider for adjusting the output level. Press Start / Stop to enable of disable the generator.
3. Equalizer: Modify the spectral content.

Type

The generator supports three types:

1. Sine
2. Noise
3. Sweep

Sine

Generate a continuous tone with a fixed frequency.

Noise

Generate white or pink noise. Pink noise has a Color 0-dB point parameter. The noise is only pink from this frequency up. A higher Color 0-dB point results in a stronger power spectral density, within the pink range. The parameter Interruption period makes the generator alternate between noise and silence.

Sweep

Generate a sweep. It has the following parameters:

1. Sweep type: Sets whether the frequency increases (decreases) linearly between to start and stop frequency or exponentially. An exponential sweep puts equal power in each octave and is sometimes called a logarithmic sweep. If the results are to be viewed on a logarithmic frequency axis, use the exponential sweep.
2. Sweep repetition: Sets whether the sweep increases in frequency, decreases in frequency or has an increase followed by a decrease.
3. Start frequency: Sets the lower bound of the sweep.
4. Stop frequency: Sets the upper bound of the sweep.
5. Sweep time: Sets how fast the signal sweeps through the frequency range.
6. Quiescent time: Adds a silent phase after each sweep.

Level

The slider does not directly control the signal level, it is only a gain knob. The peak output level depends on the signal type and equalizer settings. Consider the following chain:

[Type] ➔ [Level] ➔ [Equalizer]

The raw output of Type is set to peak at 0 dBFS, for the Sine and Sweep output types. The gains of the Level slider and Equalizer are then added. For the Noise output type, the peak value is not defined.

For Sine and Sweep output types, the peak level matches that of the Level slider, if the equalizer is disabled. A value of 0 dB then results in a full scale signal.

Equalizer

Right-click the preset box to create a new equalizer:

After entering a name, the settings window will show:

Adjust the sliders as desired. Scroll to the right to view the higher frequency bands. Connect all levels will make all sliders move simultaneously. Reset all levels sets each band to 0 dB.

To disable the equalizer, press the Disable EQ button.

Measurement settings

The Measurement box contains the settings for the measurement.

Settings

The optional counter adds an underscore and a two digit number to the file name. It uses the lowest two digit number which has not already been taken. A third digit is added when required. This is handy when performing multiple measurements, which do not need individual annotations.

The measurement ends after Measurement time, unless Measure indefinitely is checked. Then it must be stopped manually.

The signal generator can automatically start together with a measurement, by checking Control signal generator. Because of input and output buffers, the measurement misses the first part of the generated signal. The measurement can wait for it by using a value for Measurement delay of at least two times the buffer size. In case of 8192 samples and a sample rate of 48 kHz, this equals 0.34 s.

For the Measurement type, the following options are available:

1. Generic
2. Insertion loss
3. µZ¹

The Generic option does not need additional explanation. The other options are explained in their own parts of the userguide.

Start

After clicking Start measurement, a window is shown with the measurement progress:

Notes can be added as a Measurement comment while it is running. The measurement automatically stops after Measurement time has elapsed, unless Measure indefinitely was checked. If desired, the measurement can be stopped manually in two ways. If the result should be kept, click Stop and save. If the result should be discarded, abort by clicking the X in the top right corner.

Once the measurement is ready, it is added to the Measurement list on the Analyze tab.

1. The µZ option is only available if the µZ module has been purchased. ↩

Signal monitor

The input signals are monitored on the signal monitor. It consists of three parts:

1. PPM: Peak programme meter
2. Real time signal viewer: Time domain view of one input channel
3. Real time spectrum viewer: Frequency domain view of one input channel

Peak Programme Meter

The Peak Programme Meter (PPM) shows the peak level for each input channel. In one eyesight it shows the signal levels.

As the name suggests, the meter responds to the peak signal values. It can be used to help set the signal generator level or gain knobs in the system. Ideally the signal is as strong as possible, without overloading the input. A typical target level is -10 dBFS.

Features

For each channel there is a vertical bar. The bar is updated each time the DAQ sends a block of data and is green under normal circumstances. If clipping is detected, the bar turns red. To make it easier to read the peak levels, a horizontal line holds the peak value.

Options

Right-click inside the PPM to open an options menu. It has two settings:

1. Minimum visible level: Sets the vertical range.
2. Decay rate: Sets how quickly the bars drop, after the peak has passed. It does not affect the horizontal line that holds the peak value.

Minimum visible level

Decay rate

Real time signal viewer

This viewer shows the time domain data of one input channel.

It has two settings:

1. Channel selector: Select the input channel with the drop down menu near the top. Only one channel can be viewed simultaneously.
2. Time history: Adjust the slider to set the time scale.

Real time spectrum viewer

This viewer shows the spectral content of one input channel.

By default, the viewer is frozen to lessen the computational load. Click Unfreeze to enable it:

A blue line will be drawn:

Settings

• Freeze: Hold the current graph for inspection. It also stops the computational load.
• Reset: Clear the time history, related to Time weighting.
• FFT settings: See FFT settings.
• Settings menu: See below.

Settings menu

Select the input channel to be viewed and what property should be calculated. The settings shown below work for most situations. More information about the Power Spectra Settings can be found on the page Power Spectra.

Analyze tab

Access the Analyze tab by clicking its tab:

Overview

The analyze tab is divided into three sections:

1. Measurement list: red
2. Compute: green
3. Plot area: blue

The vertical divider (purple line in the image below) between the computation settings (left) and plot area (right) is draggable. If it is dragged to far to the left some of the options may become (partially) obscured.

Workflow

To plot one or more measurements, select them from the measurement list, pick the desired computation settings and click Compute. A plot is created in the plot area. Only compatible measurements can be plotted simultaneously. In practice, this means that they must share the same DAQ Configuration.

The various sections are explained in the corresponding subchapters.

Measurement list

The measurement is located in the top left of the Analyze tab and lists the measurements of the current measurement folder. See Toolbar how to change this folder.

1. Folder: shows current measurement folder
2. Select All / None: select all or no measurements
3. Select new: select recently added measurements
4. Filter: search measurements
5. Name: click to sort by name
6. Timestamp: click to sort by date and time when the measurement was performed
7. Comment: click the field next to a measurement to add a comment

Details are shown when the mouse hovers over a measurement:

Right-clicking on a measurement opens a menu:

Right-click --> Export selected measurement(s) will export it as a wave audio file. It is exported to the measurement folder and gets the same name as that of the measurement.

Supported output type are:

• 32 bit float
• 16 bit integer
• 32 bit integer

Normalize maximizes the amplitude without clipping. If Overwrite existing files is unchecked, the export is cancelled if an there is already a wave file with the same name in the measurement folder.

Right-click --> Export measurement metadata... opens a screen with detailed information and allows to change the channel configuration.

Compute

The computation settings are located at the bottom left of the Analyze tab.

They contain the following items:

1. Start position: select from which point the data should be processed - will be set to the start of the signal by default.
2. Stop position select up to which point the data should be processed - will be set to the end of the signal by default
3. Compute: choose what to calculate
4. Compute-specific settings: the contents of this sub-menu depend on what has been chosen at Compute
5. Output figure: select in which figure the results should be plotted.
6. Compute button: press to calculate and plot

Start and stop position

The start and stop position select what part of the measurement is to be processed. Usually, they can be left at their default values, which uses the whole measurement. This option is disabled if multiple measurements with different lengths are selected.

Compute

This setting determines what is calculated. It offers the following options:

1. Sound level meter - statistics
2. Sound level meter - time dependent
3. Power spectra
4. Insertion loss
5. µZ tool¹

These options are further described in their respective sections.

Output figure

The option Output figure sets where the calculated result should be plotted. A result can be plotted in a new figure, or in any compatible existing figures. A figure is compatible if it has the same axes (with or without phase) and contains similar data, of the same quantity. For example, it is not possible to plot sound levels and a waveform in one figure.

1. The µZ tool is only available if the µZ module has been purchased. ↩

Sound level meter - statistics

The sound level meter - statistics computes the sound levels within octave or third octave bands. It operates on one channel and takes the statistics across the whole measurement time. Three statistics are available:

1. Peak sound level (Lpk)
2. Maximum sound level (Lmax)
3. Equivalent sound level (Leq)

1. Channel: channel to take the statistics of
2. Statistics type: Lpk, Lmax or Leq
3. Time weighting for Lmax: smooth out peaks; only applies to Lmax
   0.1 ms; Fast (0.125 s); slow (1 s); 10 s
4. Frequency weighting: A, C or Z
5. Frequency bands: octave or third octave
6. First band: limiting the number of bands helps to keep the figure easy to read
7. Last band:

Peak sound level (Lpk)

This is the strongest sound anywhere in the measurement, without time weighting to smooth out short peaks. Even though there is no time weighting, the bandpass filters for each band still mean that there is no instantaneous reaction to a short peak, especially at the lower frequency bands.

A sine wave has an Lpk of 3 dB above Lmax or Leq.

Maximum sound level (Lmax)

This is the strongest sound anywhere in the measurement, with time weighting to smooth out short peaks.

Equivalent sound level (Leq)

This is the average power. It can be viewed as the level that a sine wave would have, to have the same average power as the measuement.

Example

An example plot with all three statistics is shown below.

Power spectra

Power spectra compute the frequency domain data of the measurement.

1. Auto power channel
2. Frequency weighting
3. Output type
4. TF input
5. TF output
6. Smoothing

Start by selecting the Output type. Depending on the choice, some items are greyed out.

Output type

The following output types are available:

• Auto power
• Bode diagram
• Signal coherence
• Transfer magnitude
• Transfer phase

They are further explained below.

Auto power

Auto power shows the average spectral content of channel Auto power channel. It is the sum of the positive and negative frequencies, so the plot shows the total signal power at this frequency bin. This corresponds to what a sound level meter would do. The power is plotted on a logarithmic (dB) scale, with a reference depending on the type of signal.

• For acoustic pressure, the reference is 20 µPa.
• For voltages, the reference is 1 V.
• For unscaled numbers, the reference is the rms level of a sine wave with unit amplitude, which is $1/\sqrt{2}$.

here are some examples / properties of these definitions:

• Assuming no spectra leakage and a rectangular window, an unscaled sine wave with an amplitude of 1.0 will have an auto power of 0 dBFS ("decibel full scale").
• An acoustic wave with an amplitude of 1 Pa rms, will show up with an overall level of 94 dB SPL.
• An acoustic sine wave with an amplitude of \sqrt{2} Pa, will show up with an overall level of 94 dB SPL.
• For white (Gaussian) noise, represented as unscaled numbers with variance of 1.0 and zero mean, up to the Nyquist frequency has an expected overall signal power of 1.0, and 0.0 above the Nyquist frequency., Therefore the expected auto power in each frequency bin $\approx 2/N_{\mathrm{FFT}}$. Therefore computed level in each bin is: $10{\log }_{10}\left(\frac{2}{N_{\mathrm{FFT}}\times {\left(1/\sqrt{2}\right)}^2}\right)$ = $10{\log }_{10}\left(\frac{4}{N_{\mathrm{FFT}}}\right)$ dBFS.

Note that due to this, for broadband signals the height of the auto powers in the spectrum depend on the frequency resolution: the higher the frequency resolution, the lower the levels. On the other hand, for narrow band signals, the height of the peaks are more / less independent on the frequency resolution.

To compute the signal auto power, select the right measurement(s) in the Measurement List, select the Auto power channel, frequency weighting and possible smoothing:

Ten press Compute.

Bode diagram

This type plots both the estimated magnitude and phase of a transfer function between two channels.

The transfer function is taken from TF input to TF output. For best performance, tick the box Low noise next to the channel with the lowest noise. This is usually the TF input channel.

Signal coherence

The coherence between two signals is useful to check the signal to noise ratio and linearity. A value of 1 represents perfect coherence (the signals are perfectly related) and a value of 0 represents completely uncorrelated signals. The signal coherence is defined as:

$${\gamma }_{ij}=\frac{C_{ij}{C}_{ji}}{C_{ii}{C}_{jj}},$$

where $C_{ij}$ is the cross-power spectrum estimate from channel $i$ to channel $j$.

Transfer magnitude

This is the magnitude part of the Bode diagram. Additionally, Smoothing is available.

Transfer phase

This is the phase part of the Bode diagram.

Frequency weighting

Human hearing is less sensitive at low and high frequencies. Moreover, the frequency sensitivity depends on the level of the sound source as well. Frequency weighting can be applied as a coarse representation of that. The weightings are according to the IEC 61672:2003 standard.

• A: A-weighting
• C: C-weighting
• Z: no frequency weighting

Smoothing

Smoothing makes the graph easier to read, by removing details. It is only available on output types Auto power and Transfer magnitude. As power spectra are about the frequency domain, the data are always plotted on a logarithmic x axis. Fractional octave smoothing has been chosen to keep the window width visually constant.

Plot

The plot area is located on the right side of the Analyze tab.

1. Tabs for each figure
2. Reset axes
3. Undo
4. Redo
5. Move
6. Zoom
7. Save figure (image)
8. Cursor position
9. Change layout
10. Plot operations
11. Preset for x axis
12. Fit x axis to data
13. Fit y axis to data

Change layout

In this menu, the formatting of the labels and the settings of the axes and can adjusted.

The yellow arrows on the right side are 'undo' buttons.

Plot operations

The plot operations menu contains the list of plots. This menu offers the options to change the line style, apply mathematical operations, reorder the plots or export the data. Depending on the number of plots selected, some buttons can be disabled.

1. Plot list
2. Select & remove
3. Operations
4. Arrange

Plot list

This area lists all plots within the figure.

1. Checkbox: a plot is selected when this checkbox is ticked; the selection is used at the buttons at Select & remove, Operations and Arrange
2. Name: for your own reference
3. Channel: lists Auto power channel or (TF input, TF output) for power spectra
4. Line color: click the color to change
5. Line width: light (1) to heavy (8)
6. Line style: solid, dashed, dash-dot, dotted
7. Marker: diamond, triangle etc. - not always available
8. Offset: vertical offset applied to plot; to make overlapping plots visible or to move widely spaced plots together
9. Visible: show / hide 10: Custom legend: tick the checkbox to modify the legend label

Select & remove

These buttons can quickly select or hide all plots, reset the line styles or remove one or more plots.

• Select All/None: select all or no plots
• Show/hide all: hide or show all plots
• Reset line styles: reset the line color, line width, line style and marker to the default cycler
• Remove: delete selected plots

Operations

These operations apply to the selected plots.

• Average: calculate the power average and plot the result in the same figure
• Difference: calculate the difference - y values are simply subtracted, without any conversion to power
• Sum: calculate the sum, with the option to weigh each differently and a choice between summing power or decibels
• Export: export as Numpy array (.npz), Spreadsheet (.xlsx) or Comma Separated Values (.csv)

Arrange

Move the selected plots up or down in the list. Multiple plots can be moved simultaneously.

µZ impedance tube

Introduction

µZ (pronounced: /'mju:zi:/) is a state-of-the-art impedance tube designed for precise acoustic testing of small material samples. The device gets it name from the symbols for micro (µ) and impedance (Z). It can be used to measure the acoustic properties of small material samples, like meshes, membranes, or vents, typically used in hearables. µZ operates effectively over a wide frequency range, depending on the specific type of device.

Use-cases

Although it is designed to measure acoustic properties of thin samples, µZ can also be used for regular impedance tube measurements. Combined with the ACME measurement and analysis software, the system can be used for:

• Acoustic material characterization: Characterization of the series impedance of thin materials such as meshes and membranes.
• Sound absorption measurements: Evaluation of the normal incidence sound absorption and reflection coefficient of an acoustic sample.
• Quality control: Monitoring acoustic products to ensure consistent quality.

General measurement procedure outline

Usage of the µZ system consists of the following steps:

1. Measurement configuration: Decide whether to use the 4 or 5 microphone method and (in case of the 5 microphone method) pick a cavity size.
2. Setup: Connect the hardware and set a DAQ configuration in the ACME software.
3. Calibration: Determine system and environmental parameters to ensure accurate measurement results.
4. Reference measurement (optional): Measure the impedance of an empty sample holder, so it can be (automatically) corrected for in the analysis phase.
5. Sample measurement: Measure the impedance of a sample, including its sample holder.
6. Analysis: Plot various acoustic properties of the sample, as a function of frequency.

Package contents

The µZ system is delivered in a sturdy case and includes the following components:

1. Main tube assembly
   • Contains an enclosed speaker and 4 MEMS microphones.
2. Supportive base
3. Calibration/measurement tube (2x¹)
   • Can also be used to mount samples during regular impedance tube measurements.
4. Hard end cap without tip microphone (2x¹)
   • Used for measurements in 4 microphone mode.
5. Hard end cap with tip microphone
   • Used for measurements in 5 microphone mode.
6. Sample holder with 0.1 cc back cavity
   • Used for 5 microphone measurements on samples with relatively high series impedance.
7. Sample holder with 0.5 cc back cavity
   • Used for 5 microphone measurements on samples with relatively low series impedance.
8. Sample holder plate
   • Clamps the sample to the back cavity.
9. Sample holder plate mounting tool
   • Used to lock the sample holder plate in place.
10. Audio interface
    • Audient EVO 16
11. Required cabling²
    • XLR cable (5x)
    • 1/4" TRS Jack cable, short
    • 1/4" TRS Jack cable, long
    • USB-A to USB-C cable
    • Power cable
    • 5 V power supply
12. Spare O-rings²
    • 22.5 x 1.5 mm (5x)
    •   7.0 x 1.5 mm (5x)
    • 10.5 x 1.5 mm (5x)

1. Only one is shown on the photograph. ↩ ↩2

2. Not shown on the photograph. ↩ ↩2

Measurement configurations

µZ works in two different configurations:

• 4 microphone mode

• 5 microphone mode

The difference is the presence of a tip microphone, which measures the sound that passes through the sample. Depending on the sample characteristics, one or the other method is preferred. How the configurations work is outlined below, including guidelines how to select the best configuration.

4 microphone mode

Inner workings

This configuration only uses the 4 MEMS microphones that come pre-assembled along the length of the main tube. The system uses plane wave decomposition on the soud field inside the tube, to extract the wave that travels from the loudspeaker to the other end of the main tube -at which a sample could be mounted- and the wave that is reflected and travels back. From this information, it calculates:

• reflection coefficient
• absorption coefficient
• input impedance

The input impedance is the impedance seen at the sample end of the main tube. It includes the sample holder and radiation impedance.

If the properties of only the sample itself need to be determined, a measurement of a sample can be combined with a measurement of an empty sample holder: the reference measurement. These have the same input impedance, except for the contribution of the sample. ACME can combine the measurements to extract the series impedance of the sample.

5 microphone mode

The characteristic impedance of the tube is given by¹:

$$Z_0=\frac{z_0}{S_{\mathrm{tube}}}\approx \frac{412\mathrm{Rayl}}{\frac{\pi }{4}{D}_{\mathrm{tube}}^2}$$

where $z_0$ is the characteristic specific acoustic impedance of air (MKS Rayl), $S$ the inner cross-section of the tube (m${}^2$) and $D$ the inner diameter of the tube (m). The units of $Z_0$ are Pa$\cdot$s/m${}^3$.

For samples with a higher impedance than 20$\times Z_0$, the plane wave decomposition method by itself is not sufficient achieve good results over the entire frequency range. The acoustic volume velocity through the sample is so small than it cannot be determined with a good relative accuracy. It must be measured in a more direct way. This is done using an additional microphone in a cavity behind the sample. The microphone capturing the signal is called the tip microphone. The pressure measured in the back cavity is used to determine the volume velocity through the sample, which in turn is used to calculate the sample's acoustic properties.

Back cavity size

The pressure levels measured at the tip microphone are determined by the size of the back cavity. Therefore, the ideal back cavity size depends on the specific sample that's being tested. µZ-20 comes with cavities in two sizes:

• 0.5 cc: This is the default cavity.
• 0.1 cc: This cavity is suitable for samples with the highest impedances. With the extra small cavity size, only a tiny volume flow is required to get a usable signal at the tip microphone.

Choosing the most suitable measurement configuration

An overview or use cases for both measurement configurations are shown in the table below. The diameter is only a rough guideline.

The provided guidelines are all qualitative as the combination of a specific acoustic impedance, with a certain cross sectional area combined result in a series impedance. If no information is available about the sample under test, the following procedure can be followed:

1. Measure the series impedance in the 5 mic (0.5 cc) configuration.
2. If ACME warns that the signal on the tip microphone becomes too low, re-measure in the 5 mic (0.1 cc) configuration.
3. If ACME showed no warning, plot the Input impedance, normalized to Plane wave impedance. The value should be larger than 10$\times$ (20 dB). If not, re-measure in the 4 mic configuration.

The 4 microphone mode is suitable for low-impedance samples with respect to the characteristic impedance of the tube.

1. when we specify Rayl , we always mean MKS Rayl. 1 Rayl is 1 Pa$\cdot$s/m ↩

Setup

The following sections describe the steps required to make µZ ready for use. The setup is split into two parts:

• Hardware: Setting up the impedance tube and connecting all the in- and outputs.
• Software: Creating a DAQ configuration in the ACME software that is compatible with µZ applications.

Hardware

This section covers the hardware setup of µZ, including cabling and setting up the Audient EVO 16.

Tube

Place the main tube in the base and mount the end cap, as shown above. Remember to mount the appropriate end cap for the desired measurement configuration:

• without tip microphone for 4 microphone mode
• with tip microphone for 5 microphone mode

All part are connected using a twist an latch system. An example of the procedure for mounting a part B onto another part A is as follows:

1. Position part B such that its two grooves line up with those on part A. On sample holder parts only look at the grooves that extend towards the part it's being connected to.
2. Slide part B onto part A.
3. Twist part in a clockwise direction until only one of the grooves on parts A and B line up.
4. Tighten the connection by closing the latches

The video belows shows the procedure. Here part B is the end cap and part A is the main tube. For all connecting parts the procedure is the same.

To disconnect the parts, the procedure should be reversed.

The main tube, calibration tube and sample holders contain O-rings to ensure airtight seals between the parts when the system is assembled. It is possible that repeatedly removing and mounting parts can cause the O-rings to fall out of their grooves. When doing measurements, periodically check whether the O-rings are still in place, especially when to results are not as expected. Spare O-rings are included with the system to replace ones that might get lost during use. If additional parts are needed, please contact ASCEE.

Cabling

Audient EVO 16

µZ comes with an Audient EVO 16 audio interface. Connect the interface to power using the power cable and connect it to the computer running ACME via the USB-C cable. Depending on your operating system, additional drivers might be required to fully use the EVO 16's capabilities. For more information, check the Audio driver requirements section.

Loopback

µZ measurements require a loopback of the speaker signal. To create this loopback, connect line output 2 to mic/line input 8 using the short TRS jack cable. Both the output and input are located on the back panel of the EVO 16. See figure below:

Microphones

Connect the microphones to mic/line inputs 3 to 7 on the back panel of the EVO 16, using the XLR to XLR cables. The suggested connections are listed in the table below. The microphone indices are marked on the µZ main tube and end cap with microphone.

The microphones will receive power from the phantom power on the EVO 16. Below a picture of the channel connection. The colors indicate the channel / microphone connectons.

Speaker

Connect the speaker amplifier to power using the 5V AC adapter. When using an alternative adapter, make sure to use one with the following output specifications:

Connect the signal input socket of the speaker to the to line output 1 on the EVO 16 using the long TRS jack cable.

EVO settings

Set the volume of the line outputs on the EVO 16 to 100 by pressing the Speaker button on the front panel and turning the Control Wheel.

Turn on the phantom power on all the mic/line input channels that are connected to the microphones. The phantom power needs to be enabled on each of the channels separately, which can be done as follows:

1. Select the channels by pressing the associated number button on the front panel
2. Turn on the phantom power by pressing the 48V button. The button will light up and the screen will indicated that phantom power has been enabled.

The green LEDs in the microphones will light up if the phantom power is enabled properly, as shown in the picture on the first page of this section.

DAQ configuration

To work with µZ, first a compatible DAQ configuration needs to be created in ACME. To do so, start ACME and open the Measure tab. Right-click the drop-down menu labeled DAQ Configuration and click Create new configuration.... This will open a dialog.

In the Global configuration tab set the settings as shown below, taking note of the following:

• The configuration name can be set to whatever you prefer, but it is recommended to make it something clearly linked to µZ.
• The Output settings are not used, since the EVO 16 will be used in duplex mode. Therefore, their values do not matter.
• The Input API is operating system dependent. It should be set to the PortAudio version for your operating system.
• The exact device name for the Audient EVO 16 may differ on your system, but it will be clearly recognizable as the Audient EVO 16.

In the Channel configuration tab set the settings as shown below, taking note of the following:

• The indices of the in- and output channels in the DAQ configuration do not match the channel indices on the Audient EVO 16, since the former starts counting at 0, while the latter starts at 1. Therefore, DAQ configuration channel 0 corresponds to channel 1 on the EVO 16.
• The input channel names should correspond to the order in which the physical hardware is connected. The configuration shown below corresponds to the connections that were suggested in the previous section.
• The input channel names must be exactly as shown below, because they will be used by the µZ toolbox to determine the system configuration.

• The tip microphone channel should be enabled or disabled, depending on the desired measurement configuration. If it is enabled, the software will assume the system is in 5 microphone mode. Otherwise it will assume the system is in 4 microphone mode.
• In the Output channel configuration channels 0 and 1 must be enabled. Their names are optional, but are added for clarity.

Calibration

To achieve accurate measurement results, µZ needs to know certain parameters about itself and the environment it's being used in. To determine these parameters, the response of the system needs to be measured in two configurations:

• Short: End cap (with or without microphone, depending on the measurement configuration) mounted directly onto the main tube.
• Long: Calibration tube mounted between the main tube and end cap.

Since µZ is sensitive to changes in climate conditions, the calibration procedure needs to be performed at the start of every measurement session. If the temperature changes by more than 2 °C (3.6 °F) over the session, a new calibration is required. ACME considers a calibration valid if it was performed in the last 12 hours but cannot warn for temperature changes.

The calibration procedure

To calibrate your µZ system, open the Actions menu in ACME's toolbar and click Calibrate µZ system:

This will open the µZ calibration wizard, which will guide you through the entire calibration process. Some additional information about the calibration process is listed below. This information can also be found in the calibration wizard.

Setting speaker output level

To get the best results, the speaker output should be as high as possible, without overloading the microphones. You will get an opportunity to check you levels prior to starting the short calibration measurement. This check is only necessary the first time you calibrate your µZ system.

During the signals check the PPM bars should peak around -15 dB. A speaker output level of -45 dBFS usually works well. If you do find any difficulties, please contact directly.

Two-stage measurements

The calibration measurements are performed using a sweep signal. To maximize the signal quality for each measurement, an amplitude envelope is applied. For this reason, two measurements are performed for each configuration. The initial measurement is used only to calculate the amplitude envelope.

Signal checks

The µZ toolbox performs a number of checks on each measurement to determine whether the system is operating as expected. The checks include:

• Signal levels on the input channels: Are the power supplies of the speaker and all microphones enabled?
• Channel names: Is the data as expected based on the channel name?
• System identification: Is the system that's being calibrated a µZ-20 or µZ-30?
• System configuration check: Is the data compatible with a short or long calibration measurement?

In case any of these checks fail, the calibration wizard will show an error message and give instructions for how the issue can be resolved.

Saving measurements

Whenever the calibration wizard is closed, either after a successful calibration or after something has gone wrong, you will have the option to save the measurements that were performed so far. In some cases the definitive measurements can be useful for analysis. The initial measurements are generally only useful for debugging purposes.

Performing a measurement

The following sections describe the procedure for performing measurements with a µZ system. Two types of measurements are considered:

• Generic impedance tube measurements: General impedance tube measurements, e.g. to determine the sound absorption coefficient of acoustic samples.
• Series impedance measurements: Measurements to characterize thin acoustic material samples.

Both types of measurements require a valid calibration, which must be performed before the actual measurements and must not be older than 12 hours. Additionally, measurements using 5 microphone mode can only be performed if the system was calibrated using all 5 microphones, thus including the tip microphone.

Make sure to use the same DAQ configuration for the measurements as the one that was used during the calibration.

Setting up the software

The following section contains information about the signal generator and measurement settings for all µZ measurements. Other measurement settings can be specific for the type of measurement. Information about these settings can be found in the relevant sections of this user guide.

Measurement tab

Head to the tab Measure. Everything related to performing measurements can be found here.

Start the data streams

To start a measurement, first start the input and output streams in ACME by clicking the microphone button. This will start both streams, since the Audient EVO 16 is set to run in duplex mode.

Signal generator

µZ measurements can be performed using either a sweep or a noise signal.

Sweep

The recommended signal generator settings for a sweep signal are shown below.

1. Sweep type: Set to Exponential.
2. Sweep repetition: Set to Continous.
3. Start frequency: Set to one octave lower than the lowest frequency to be captured in the measurement. Typical values are in the range of 10 - 200 Hz.
4. Stop frequency: Sets the upper bound of the frequency range for the measurement. Its maximum allowable value is limited based on the specific µZ system being used.
5. Sweep time: Determines how fast the speaker signal sweeps through the frequency range and as a result how long the measurement will take. This affects the quality of the measurement data. It is recommended to set this setting to 20,000 ms (20 s).
6. Quiescent time: Not to be used for μZ applications: set to 0 ms.

Noise

The recommended signal generator settings for a noise signal are shown below.

1. Noise color: Set to Pink.
2. Color 0-dB point: Set to a value in the range of 20 - 200 Hz. A lower value will improve the signal to noise ratio at the lower end of the measurement range, while a higher value results in a stronger and cleaner signal above the Color 0-dB point. If a bandpass filter is used, there is no advantage to picking a high value and it can be left at 20 Hz.

When outputting a raw noise signal, it will be broadband, containing all frequencies from 0 Hz to the Nyquist frequency. The specified measurement range for µZ systems is much narrower. Additionally, some samples may exhibit non-linear behavior in certain frequency ranges and should ideally only be excited in a narrow frequency band.

For this reason it is recommended to band-pass the speaker signal to either the measurement range of your µZ system, or the sample under test, whichever is narrower. In ACME, this can be achieved by applying an Equalizer to the output signal. To create an equalizer, right-click the drop-down menu labeled Select Preset and click Create new equalizer.... This will open a dialog.

In the dialog, drag the sliders corresponding to the third-octave bands that are outside the desired measurement range fully down. Leave the other sliders unchanged. The EQ shown below is set up to use the entire measurement range of the µZ-20.

Save the specified equalizer by clicking OK and enable it by clicking the Enable EQ button.

Signal level

The optimum signal level takes the following factors into account:

1. Maximize the signal to noise ratio
2. Avoid overloading the microphones
3. Avoid overloading the sample
4. Avoid creating turbulence through the empty sample disk¹

A quick method is to re-use the value entered in the calibration wizard. This only works when using a sweep and the 5 microphone method.

The full procedure to find the right level is as follows:

1. Enable the signal generator by clicking the Start button. See image below.
2. Set the level slider such that the microphone channel with the highest level peaks at around -15 dB².
3. Listen for harmonic distortion (4 microphone method) or perform a measurement and plot the waveform or the tip microphone signal (5 microphone method) and visually check for harmonic distortion.

Two-stage measurement

µZ measurements using a sweep signal are performed in two stages, like during the calibration. For this reason, an additional dialog will be opened which tracks the progress of the measurement.

A sub-set of the checks that are performed during the calibration will also be performed here. Unlike the calibration, these measurements do not need to be saved manually afterwards. The definitive measurement is saved automatically. The initial measurement that is used to calculate the amplitude envelope for the sweep signal is automatically discarded.

1. This seldom is a problem. It can be occur when measuring a sample disk with a small hole, using the 4 microphone method, but that method often is not the right one for those disks. ↩

2. Capacitative / compliant materials, like membranes, may resonate in the measurement range. If this is case, it is possible that the previously suggested signal levels may result in distorted results. If this occurs, try redoing the measurement with a lower signal level. ↩

Generic impedance tube measurements

Generic measurements are all measurements other than series impedance. The measurements generally involve placing acoustic samples of non-negligible thickness inside the measurement tube. An example is measuring the absorption coefficient of a fibrous absorber.

Hardware

The following hardware is required, in addition to the main tube assembly:

1. End cap, without tip microphone
2. Measurements tube: This tube is identical to the calibration tube and can be used interchangeably. If two tubes are available, we recommend to dedicate one to calibration, to guarantee it remains clean on the inside.
3. Sample

Place the acoustic sample inside the measurement tube. If its thickness is smaller than the tube length, it should touch the end cap and leave empty space on the other end. The sample should cover the whole cross-section of the tube, without leaving a gap to the tube wall.

Mount the end cap to the measurement tube and mount the assembly onto the main tube.

Software

These settings are specific to generic impedance tube measurements. Other settings, like those for the signal generator can be found here.

Signal generator

Set the signal generator to sweep, according to Performing a measurement.

Measurement settings

The recommended measurement settings for generic measurements on a µZ system are shown below.

The following settings are specific to generic impedance measurements and are of interest to the user. All other settings should generally be set as shown above and are explained in Measurement settings.

1. µZ measurement type: Indicates how the measurement should be processed during analysis. Set to Generic.
2. Reference plane offset: Indicates how far the sample is located behind the reference plane, which is the end of the main tube (see image below). To calculate this, subtract the thickness of the acoustic sample from the length of the measurement tube, which is 200 mm¹.

Performing the sample measurement

1. Start the measurement.

1. For example, the reference plane offset for a 130 mm thick acoustic sample would be 70 mm. ↩

Series impedance measurements

A series impedance measurement measures the series impedance of a sample, as a function of frequency.

It differs from generic impedance measurements, in the sense that it includes a reference measurement. This reference measurement is used to cancel the acoustic effect of a sample holder, so the properties of the sample itself can be extracted. Both measurements share the same procedure and settings. The only differences are:

1. Which sample disk (empty or with sample) is mounted in the sample holder.
2. Whether the checkbox Reference measurement is ticked.

The reference measurement must always be performed before the measurement with sample. A reference measurement remains valid for 12 hours. The reference measurement will automatically be tied to the sample measurement.

Hardware

To perform a series impedance measurement, the following hardware is required in addition to the main tube assembly:

1. Sample holder / back cavity:
   • Two options: 0.1 cc or 0.5 cc. In 4 microphone mode, always use the 0.5 cc cavity. In 5 microphone mode, see Measurement configurations for selection guidelines.
2. Sample holder disk (empty):
   • This disk will be used for the reference measurement. Make sure that this disk has the same hole shape and size as the one with the sample on it.
3. Sample holder disk (with sample):
   • The hole diameter and shape in the disk should match the active area of the sample under test.
4. Sample holder plate:
   • This plate will hold the sample holder disks in place during the measurements and is required for an airtight seal.
5. Sample holder plate mounting tool:
   • Is used to lock the sample holder plate in place.
6. Hard end cap with tip microphone (optional, not shown):
   • This end cap should only be used for measurements in 5 microphone mode. When measurements in 4 microphone mode, no end cap should be used.

Place the sample holder disk in the sample holder and lock it in place with the sample holder plate and mounting tool. Push the plate down before rotating. The plate rotates only a few millimeters at the edge. If the plate does not fall out when held upside down, it is mounted correctly.

Mount the sample holder directly onto the main tube. Remember to remove the calibration tube if you have just finished calibrating.

For measurements in 5 microphone mode, mount the end cap with microphone onto the sample holder. For 4 microphone mode measurements, leave the back of the sample holder open.

Software

These settings are specific to series impedance measurements. Other settings, like those for the signal generator can be found here.

Signal generator

Set the signal generator to sweep, according to Performing a measurement.

Measurement settings

The recommended measurement settings, for series impedance measurements on a µZ system, are shown below.

The following settings are specific to series impedance measurements and are of interest to the user. All other settings should generally be set as shown above and are explained in Measurement settings.

1. µZ measurement type: Indicates how the measurement should be processed during analysis. Set to Series impedance (4 microphones) or Series impedance (5 microphones) depending on the hardware configuration being used.
2. Reference measurement: Check this box when performing the reference measurement. See below for more information.
3. Cavity size (5 microphone mode only): Indicates what size back cavity was used during the measurement (0.1 cc or 0.5 cc). This is only relevant for measurements performed using 5 microphone mode.
4. Sample size: Indicates the active diameter or area of the sample under test. This is used to calculate the specific impedance of the sample. It must be the same for the sample measurement itself and the accompanying reference measurement.

Performing the reference measurement

1. Mount the empty sample disk in the sample holder and attach it to the tube.
2. Check the box Reference measurement.
3. Start the measurement.

Performing the sample measurement

1. Replace the empty sample disk by one with a sample.
2. Ensure the checkbox Reference measurement is unchecked. Leave all other settings as they are.
3. Start the measurement.

Analysis

After the measurements have been performed, it is time to analyze them. Move to the Analyze tab in ACME.

FFT settings

Open the FFT settings menu and use the following settings:

1. FFT length: 48000
2. Window: Hann
3. Overlap: 90%

If the overlap cannot be set, the most likely cause is that the Measue tab is active instead of the Analyze tab,

Compute settings

µZ tool

Select the µZ tool.

The other tools like Sound Level Meter - time dependent and Power Spectra work on the measurements as well, but access the raw microphone data instead. They can be useful for trouble shooting and are normally not required when using the impedance tube.

Output quantity

Further settings depend on the output quantity and are described on page Output quantity.

Message box

The message box near the bottom shows information about the calibration tied to the selected measurement. It also warns if a measurements cannot be plotted.

Output quantity

The software can calculate four quantities:

• Reflection coefficient
• Absorption coefficient
• Input impedance
• Series impedance

Series impedance only works if a reference measurement is associated to the measurement.

Reflection coefficient

When sound from the loudspeaker hits the sample, some of it is reflected back towards the speaker, some is transmitted and some is absorbed. The reflection coefficient is the ratio between the reflected wave and incident wave from the loudspeaker. It has both a magnitude and a phase.

Absorption coefficient

The absorption coefficient is the amount by which the reflected wave is attenuated. It has a value in the range of 0...1 and no phase. The system cannot dinstinguish wheter the remainder is absorbed or transmitted and assumes there is zero transmission.

Input impedance

The input impedance is the ratio between the acoustic pressure in front of the sample and the volume velocity flowing through it. It has both a magnitude and a phase. The impedance can be normalized in three different ways:

The plane wave impedance is useful to check whether the 4 microphone method can be trusted. See Measurement configurations for details.

Series impedance

The series impedance is the ratio between the acoustic pressure drop across the sample and the volume velocity flowing through it. The series impedance can be normalized in two different ways: