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. ↩