Chapter 6: Impedance measurements

Acoustic impedance measurements
6 Acoustic Impedance Measurements
6.1. Summay............................................................................ 3
6.2. Introduction ....................................................................... 3
6.3. Techniques to determine the reflection coefficient ................... 4
Free field methods under (semi) anechoic conditions 5
Reverberant field method 6
Impedance tube methods 7
Two microphone standing wave tube methods 7
One microphone impedance tube methods 8
Drawbacks of impedance tube methods 8
6.4. The Microflown in the Kundt’s tube........................................ 9
PU based Kundt’s tube, Ratio of intensity and energy 13
Comparison PU based tube measurement 2p and pu method 14
PU based Kundt’s tube method, an example measurement 14
Relation between impedance and reflection coefficient 17
6.5. Surface impedance technique ..............................................18
Image source model with plane wave reflection coefficient 19
Moving average filtering 23
Surface impedance technique comparison with Tamura method 24
PU Kundt’s tube vs. surface impedance technique 25
Measurements at DAF Trucks / Paccar 28
Surface impedance technique, F-term correction 29
Surface impedance technique, low frequencies 31
Surface impedance technique, signal to (background)noise level 31
Measurement of a plane with R=1 32
Surface impedance technique, influence of measurement distance33
Surface impedance technique, influence of environment 34
Surface impedance technique, influence of sample size 36
Surface impedance technique, influence of horizontal obstacles 36
Surface impedance technique, influence of vertical obstacles 37
Stability over the time 37
Reproducibility 38
Effect of different operator 38
Effect of background noises and reverberant surrounding 39
Effect of ambient temperature change 39
Measurement with wind / moving surface [34], [35] 40
Measurements under an angle 40
Measurement errors 41
Measurements in a car 42
6.6. Measurement of inhomogeneous materials............................45
Single quarter lambda resonator 46
Movement over the surface (x-axis) 46
Movement in the resonator (z-axis) 49
Kundt’s tube reference 51
Three quarter lambda samples 51
Kundt’s tube comparison 54
6.7. Measurements of a slotted panel absorber ............................54
6.8. Measurement of the vocal tract impedance ...........................58
6.9. The mirror source method...................................................59
The mirror source method, influence of sample size 62
The mirror source method, influence of environment 63
6.10. References ......................................................................63
Fig. 6.1 (previous page): First test of the mirror source set up and as expected is does
not function as expected (2006).
6-2