Chapter 6: Impedance measurements

Acoustic impedance measurements
The 2mm distance is varied in the software from -3mm to +7mm. The
result is shown in Fig. 6.29, left. As can be seen: the influence is most when
the reflection coefficient is high.
Same shows for the same material measured at 10mm. If the absorption
is calculated for a distance varied from 4mm to 15mm, the results are
influenced when the reflection coefficient is high.
A large sample of Foamex 1 is measured at several measurement
distances. These distances are also used for the processing. The results are
shown in Fig. 6.30. As can be seen, the measurements in a range from
2mm up to 20mm are consistent and when the distance is increased
further, the results deviate.
The measurement results have a reflection coefficient higher than 1
which is obviously wrong. Why this error occurs is not clear.
reflection coefficient
1.5
1.0
2mm
5mm
10mm
20mm
40mm
80mm
0.5
0.0
1000 10000
frequency [Hz]
Fig. 6.30 : Foamex 1 measured at several distances.
Surface impedance technique, influence of environment
For this test 25mm Foamex is used and two source-sample distances are
used: 17cm and 32cm. To check the influence of the acoustic environment
several measurements where done:
a) a large sample of 60cmx60cm was measured as reference
b) a 30cmx30cm sample that was cut free
c) a smaller (15cmx15cm) sample that was cut free
d) a very small (4.5cmx4.5cm) sample that was cut free
e) a very small (4.5cmx4.5cm) sample put in the end of a kundt’s tube
(see Fig. 6.22)
f) 60cmx60cm sample two reflective vertical planes (see, Fig. 6.31, left)
g) 60cmx60cm two reflective horizontal planes (see, Fig. 6.31, right)
In Fig. 6.32 the influence of several environments is shown.
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