Output quantity
The software can calculate four quantities:
- Reflection coefficient
- Absorption coefficient
- Input impedance
- Series impedance
Series impedance only works if a reference measurement is associated to the measurement.
All quanties are calculated at the reference plane.
Reflection coefficient
When sound from the loudspeaker hits the sample, some of it is reflected back towards the speaker, some is transmitted and some is absorbed. The reflection coefficient () is the ratio between the reflected wave and incident wave from the loudspeaker, at the sample:
where is the incident wave amplitude in frequency domain, and the reflected wave amplitude in frequency domain.It has both a magnitude and a phase.
Absorption coefficient
The normal incidence sound absorption coefficient () is the amount by which the reflected wave is attenuated in power. It has a value in the range of 0...1 and no phase:
where denotes the complex modulus of . The system cannot dinstinguish whether the remainder is absorbed or transmitted and assumes there is zero transmission.
Input impedance
The input impedance is the ratio between the acoustic pressure in front of the sample and the volume velocity flowing through it. It has both a magnitude and a phase. The input impedance can be normalized in three different ways:
| Normalization | Units | Description |
|---|---|---|
| Absolute impedance | MKS Rayl / m² | No normalization, also known as acoustic ohms |
| Tube cross-section | MKS Rayl | Multiply by the cross-section of the tube; for samples that cover the whole tube cross-section, it results in the specific impedance |
| Plane wave impedance | - | Relative to the plane wave impedance of air, which is ~415 MKS Rayl, but the exact value depends on the calibration. |
The plane wave impedance is useful to check whether the 4 microphone method can be trusted. See Measurement configurations for details.
Series impedance
The series impedance is the ratio between the acoustic pressure drop across the sample and the volume velocity flowing through it. The series impedance can be normalized in two different ways:
| Normalization | Units | Description |
|---|---|---|
| Absolute impedance | MKS Rayl / m² | No normalization, also known as acoustic ohms |
| Specific impedance | MKS Rayl | Multiply by the cross-section of the sample, to get the properties of the sample material, irrespective of its size |
tip
This quantity is useful for characterizing vent materials.
note
For resistive materials (i.e. series impedance irrespective of frequency), using the specific impedance option makes sense: the absolute impedance scales linearly with the sample cross-section. The calculated specific impedance describes the sample material. However, for capacitive, membrane-like materials, this normalization does not result in anything useful, as there is a non-linear relationship between the absolute impedance and cross-section.