Chapter 6: Impedance measurements

Acoustic impedance measurements
Output [V]
1
0,1
R=1
0,01
0,001
0,0001
0,00001
R=0
100 1k 10k
Frequency [Hz]
Fig. 6.71: Velocity output of the three steps. Upper (Black R=1) line: output close to a fully
reflecting plate, middle (grey) line: output close to the acoustic material, lower (black R=0) line:
anechoic output.
Step 3: measurement of acoustic sample
If the setup is placed close (and r 1 =r 2 ) to a sound absorbing plate the
particle velocity that is measured equals:
Q ikr + 1
u R e
2 ikr2
R
= (6.47)
2
4π
r2
The results of the three measurements are shown in Fig. 6.71.
The ratio between the anechoic reference response (step 1) and the
response on fully reflecting plate (calibration measurement) is called the
quality. This ratio gives the maximal dynamic measurement range of the
method. A high ratio means that the zero adjustment of the particle velocity
sensor versus point source is done well. The quality is shown in Fig. 6.72.
Quality [dB]
40
absorption [-]
1,0
20
0,5
0,0
0
100 1k 10k
Frequency [Hz]
100 1k 10k
Frequency [Hz]
Fig. 6.72: Quality of the zero adjustment in the mirror source technique. Fig. 6.73:
Absorption coefficient of a sample obtained with the mirror source technique.
The reflection coefficient of the measurement method is derived from the
ratio of the measurement on the acoustic material and the calibration
measurement (the measurement on the fully reflecting plate). Note that the
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