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 as used in the 4 microphone configuration is not sufficient to 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.

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