Selection guidelines

Outline

An outline of the impedance ranges for each method is shown in the table below. It is not possible to assign an impedance range to each method, because it is frequency-dependent. It also depends on the hole diameter of the sample holder disk.

Selection procedure

The selection method consists of the following steps:

1. Pick a starting configuration
2. Perform series impedance measurement
3. Evaluate choice
4. If required, update the choice and redo the measurement with the new configuration

Starting configuration

The sample diameter gives a first indication what method to use. Only count the diameter through which sound can pass, i.e. exclude the outer edge if there is an adhesive ring.

Perform series impedance measurement

Next, perform a series impedance measurement, as described in Series impedance measurements.

Evaluate & update choice

The first part of the evaluation is done by ACME.

• If using the 5 mic (0.5 cc) configuration and ACME complains that the tip microphone gets no signal, try the 0.1 cc cavity.
  • If that does not help, double check that the signal generator is set to the highest level that does not result in clipping.
  • If that does not help, the impedance is too high to measure. Please contact us to discuss the possibilities.

The second part of the evaluation is done manually.

• If using the 4 mic configuration, check that the impedance is not too high. To do so, go to the Analyze tab, µZ tool and plot the Input impedance, normalized to the Plane wave impedance of air. This value should not exceed 10$\times$ (20 dB). If it does, switch to the 5 mic (0.5 cc) configuration.
• Go to the Analyze tab, µZ tool and plot the Series impedance. The graph should be as expected.
  • Overall: both the magnitude and phase response should be smooth. Raggedness or 'grass' can indicate that the signal strength is close to the noise floor or that the sample impedance is towards the lower end of the measurement range.

Examples

The following examples are measured with all three configurations, of which two are incorrect each time. They help to see what effects can be observed when the configuration is not suitable for the selected sample.

Low series impedance: constant 3 M Rayl/m²

This impedance is best measured with the 4 mic configuration. Beyond 2 kHz, the 4 mic configuration fails, because the sample is stuck to a sample holder disk with a 2 mm hole diameter. This adds a significant acoustic mass to the sample and therefore a signficant series impedance at high frequencies. Even though the µZ system later subtracts the effect of the acoustic mass, that is only done after the measurements have been performed and have been subject to noise.

• The 5 mic (0.1 cc) configuration drifts away at both the lower and upper end of the frequency range. Furthermore, it is not completely smooth at low frequencies.
• The 5 mic (0.5 cc) configuration does the same, but to a lesser degree.

Medium series impedance: constant 80 M Rayl/m²

This impedance is best measured with the 5 mic (0.5 cc) configuration.

• The 4 mic configuration gives a ragged, incorrect result. The sample impedance is too high for it to measure.
• The 5 mic (0.1 cc) configuration gives a reasonable result, but it drifts at both low and high frequencies. Note that, at high frequencies, this drift is not accompanied by a phase shift, which indicates that it is not a physical phenomenon.

High series impedance: constant 920 M Rayl/m²

This impedance is best measured with the 5 mic (0.1 cc) configuration.

• The 4 mic configuration gives a ragged, almost random result. The sample impedance is too high for it to measure.
• The 5 mic (0.5 cc) configuration gives a reasonable result. At high frequencies, the line gets noisier. This occurs because the signal level at the tip microphone approaches the noise floor. The volume velocity through the sample is not large enough to build op a sufficient sound pressure level within the larger back cavity.