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.

note

The µZ system uses normal sound incidence. This is different from the ISO 354:2003 standard 'Acoustics — Measurement of sound absorption in a reverberation room', which uses random incidence.

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

Photo showing the measurement tube, the end cap without microphone and a polyester wool 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.

Recommended measurement settings for µZ system

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.

Photo showing the measurement tube, the end cap without microphone and a polyester wool sample, explaining what the reference plane offset is.

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. ↩