4 microphone mode

This configuration only uses the 4 MEMS microphones that come pre-assembled along the length of the main tube. The system uses plane wave decomposition on the sound field inside the tube, to extract the incident and reflected waves (see diagram below). From this information, it can calculate the acoustic properties of the sample under test.

4 microphone mode is applicable for series impedance measurement on samples with relatively low impedance compared to the characteristic impedance of the tube. 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 an impedance magnitude higher than 20$\times Z_0$, the plane wave decomposition method cannot accurately determine the volume velocity through the sample. As a result, the sample's acoustic properties cannot be accurately measured with the 4 microphone mode. In this case, the 5 microphone mode should be used.

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