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 soud field inside the tube, to extract the wave that travels from the loudspeaker to the other end of the main tube -at which a sample could be mounted- and the wave that is reflected and travels back. From this information, it calculates:

• reflection coefficient
• absorption coefficient
• input impedance
• series impedance (requires a second measurement)

The input impedance is the impedance seen at the sample end of the main tube. It includes the impedance of the sample holder, if any. If the backside is open, it also includes the radiation impedance.

If the properties of only the sample itself need to be determined, a measurement of a sample can be combined with a measurement of an empty sample holder: the reference measurement. These have the same input impedance, except for the contribution of the sample. ACME can combine the measurements to extract the series impedance of the sample.