Roland Kruse

Measuring the free field acoustic impedance and absorption coefficient of sound absorbing materials with a combined particle velocity-pressure sensor

Acoustic surface impedance of sound absorbing materials can be measured by several techniques such as the impedance tube for normal impedance or the Tamura method for normal and oblique surface impedance. In situ, the acoustic impedance is mostly measured by use of impulse methods or by applying two-microphone techniques. All these techniques are based on the determination of the sound pressure at specific locations. In this paper, the authors use a method which is based on the combined measurement of the instantaneous sound pressure and sound particle velocity. A brief description of the measurement technique and a detailed analysis of the influence of the calibration, the source type, the source height, the sound incidence angle, and the sample size are included.

Manfred Schroeder and Acoustical Impedance

Manfred Schroeder was a man with many interests. Between the years 1966 and 1967, he invented devices for measuring the surface impedance of materials as well as the vocal tract impedance. At the time, he would only consider normal incidence; however, he already anticipated that the behavior of materials at grazing incidence would have relevance to room acoustics, a field of research he is well known for.

Measurement of impedance at grazing incidence with a standing‐wave tube.

As guest editor of Ando’s book “Concert Hall Acoustics” Schroeder wrote 1985 in his foreword “This lack of low frequencies in the first overhead reflection revealed another low‐frequency deficiency that had hitherto gone unnoticed: a progressive attenuation of low frequencies in the direct sound as it grazes across the rows of seats. (This “seat effect” must exist in many other halls, but it is usually masked by the presence of low‐frequency components in the early overhead reflections).” Schroeder addressed the acoustics in a famous concert hall. Early digital computing helped to suggest improvements. Even today it remains a problem to exactly determine properties of acoustic material. Determination of the effective impedance of acoustic materials at normal incidence is a standardized procedure (ISO 10534). However, at grazing incidence, the situation is more complicated. Using a modified impedance tube, in which the material under investigation is placed parallel to the direction of wave propagation, an...

Optimized geometry for ground impedance measurement

The two‐microphone method is a convenient and well‐known procedure for the in situ determination of the surface impedance both in room acoustics and for outdoor use. In light of its possible use in a future revision of ANSI S1.18‐1999, its sensitivity towards measurement errors has been investigated. Unfortunately, both measurements and simulations show that the method is very sensitive to errors, especially in the transfer function, at low frequencies, and for high surface impedances when using one of the three suggested geometries from the standard. Therefore, an improved geometry is proposed to reduce the effect of errors. However, even with this optimization, the use of the two‐microphone method cannot be recommended for surfaces with a high impedance at low frequencies as the necessary precision is unacceptably high.

In situ measurement of absorption of acoustic material with a parametric source in air

Measurement of properties of acoustic material in situ is quite desirable in numerous applications. But pulse‐echo methods often lack from spurious reflections in a closed‐space environment. It is therefore necessary to irradiate the sample material with a highly focussed acoustic beam of defined width. The principle of parametric transducers, which is well known in underwater acoustics, can be used in air as well, if the sound pressure of the ultrasound carrier is high enough. This was recently demonstrated by Hibral and Zakharia (Proc. of the Joint Congr. of CFA/DAGA 04, Strasbourg 2004, p. 541). The new measurement set‐up is in principle best suited for in‐situ measurements due to the narrow beam‐width. But ultrasound intensity, efficiency of generation of audio sound etc. give rise to several limitations. Applications of in‐door measurements, in particular at grazing‐angle, are presented.

Psychophysical analysis of sound and vibration in the cabin of passenger aircrafts

The vibroacoustics within the fuselage of several types of aircrafts is recorded with microphones, ear‐related devices, and accelerometers at different locations of passengers seats, and the workplace of the cabin and cockpit crew. The signals are analyzed according to standard psychoacoustic and vibration parameters. The requirements for the reproduction of the signals in a ground‐based test‐bed (e.g., mock‐up) are identified. Results are reported on how well test facilities at ground meet real‐flight conditions. [Work supported by the European Community (www.heace.org).]