Chapter 6: Impedance measurements

Acoustic impedance measurements
dB(V)
-60 pressure response
silent
-70
PSD free field
R=1
-80
dB(V)
-50
-60
-70
-90
-100
-110
-120
-130
10 100 1k 10k
Frequency [Hz]
-80
-90
-100
-110
-120
velocity response
silent
Free field
R=1
100 1k 10k
Frequency [Hz]
Fig. 6.27 (left): Pressure signals and (right): velocity signals (PU match). Red lines: silent
response, blue lines: pink noise on the loudspeaker and close to fully reflective plate,
black lines: free field response to pink noise.
Measurement of a plane with R=1
The measurement of a fully reflective plane is the most difficult scenario
for this method. This is because the particle velocity level is low close to a
fully reflective plane (see also paragraph below: ‘Signal to
(background)noise level’). Apart from that, the measurement is very
sensitive for phase errors. The only thing that is simple about a fully
reflective plane is that the spherical reflection coefficient equals the planar
reflection coefficient. The model that is used here is therefore valid.
In the measurement (at 4.5mm distance) of the impedance of the
(80cmx80cm) reflective plane a ripple occurs. This ripple is caused by
standing waves between the reflective plane and the loudspeaker. The
ripples can be removed by the moving average algorithm. It is our believe
that a larger source-sample distance can reduce this effect. This is a point
of further R&D.
reflection coefficient
2.0
1.5
1.0
0.5
reflection coefficient
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2mm
5mm
10mm
20mm
40mm
-0.2
0.0
100 1k 10k
Frequency [Hz]
-0.4
100 1k 10k
frequency [Hz]
Fig. 6.28 (left): A measurement result of a 80cmx80cm fully reflective plate
(measurement distance h: 4.5mm 18cm source-sample). Black line without moving
average smoothing red line: with moving averaging smoothing. Right: An 80cmx80cm
fully reflective plate measured at several probe-sample distances (source-sample
distance: 18cm).
6-32