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.
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. It has both a magnitude and a phase.
Absorption coefficient
The absorption coefficient is the amount by which the reflected wave is attenuated. It has a value in the range of 0...1 and no phase. The system cannot dinstinguish wheter 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 impedance can be normalized in three different ways:
| Normalization | Units | Description |
|---|---|---|
| Absolute impedance | MKS Rayl / m² | No normalization |
| 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 |
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 |
| 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.
note
Some manufacturers specify a value for 'Air Permeability ( @ 70mbar)'. This can be converted to (MKS Rayl = and is the specific impedance for static airflow. If the material is resistive, it describes the specific impedance. If the material is capacitive, membrane-like, it does not describe the its primary acoustic behavior. It only describes the leakage.