## 5pAA11. An equivalent phase error on the acoustical impedance measurements using a single microphone and pseudorandom sequence.

### Session: Friday Afternoon, December 6

### Time: 4:50

**Author: Jing-Fang Li**

**Location: Dept. of Mech. Eng., Univ. of British Columbia, Vancouver, BC V6T 1Z4, Canada**

**Author: Murray Hodgson**

**Location: Univ. of British Columbia, 2206 East Mall, Vancouver, BC V6T 1Z3, Canada**

**Abstract:**

An experimental system has been developed for in situ measurement of the
acoustical impedance of surfaces using a single microphone, pseudorandom
sequences, and transfer-function techniques. It is well known that the impedance
determined from two transfer functions is very sensitive to phase errors. The
biased impedance has been expressed as a function of equivalent errors [J.-F. Li
and J.-C. Pascal, J. Acoust. Soc. Am. 99, 969--978 (1996)]. In the system used
here only one channel is used; thus the error does not include the electronic
phase mismatch. However, since no experimental system is perfectly linear and
repeatable, an equivalent phase error occurs. This error can be represented as
the difference in phases between two average transfer functions measured
sequentially at two locations. The objective of this work is to show how this
error affects the impedance. The error causes a large bias at low frequencies.
The results suggest that to reduce errors the distance between the two
microphone locations should be large enough at low frequencies, and the number
of transfer functions averaged should be as large as possible.

ASA 132nd meeting - Hawaii, December 1996