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.