Volume 103, Issue 3, March 1998
Index of content:
- ACOUSTICAL MEASUREMENTS AND INSTRUMENTATION 
On the multiple microphone method for measuring in-duct acoustic properties in the presence of mean flow103(1998); http://dx.doi.org/10.1121/1.421289View Description Hide Description
Nowadays, the two-microphone method is accepted as the standard as specified in ASTM E1050-90 for measuring in-duct acoustic properties. However, research results on using the least square method with multiple measurement points and broadband excitation have been reported for enhancing the frequency response of the two-microphone method. In this paper, the effects of varying the relative measurement positions on errors in the estimation of the acoustic quantities is studied for the multiple microphone method. Both the theoretical and experimental results show that, among possible sensor positioning configurations, the equidistant positioning of sensors yields the smallest error within the effective measurement frequency range. In addition, it is noted that the measurement accuracy can be increased and the effective frequency range can be widened by increasing the number of equidistant sensors.Measurement examples are shown and the results support the findings.
Suppression of distortion product otoacoustic emissions (DPOAE) near removes DP-gram fine structure—Evidence for a secondary generator103(1998); http://dx.doi.org/10.1121/1.421290View Description Hide Description
Since the discovery of distortion product otoacoustic emissions (DPOAE) there has been a controversial discussion about their cochlear generation sites. Suppression experiments suggest that the place near is the main generation site. On the other hand, the fact that DPOAE can be perceived subjectively indicates that there is also a cochlear excitation at the place of resulting in a stimulus frequency otoacoustic emission (SFOAE). The contribution of this SFOAE to the overall emission is still unknown. Different studies showed contradictory results. We demonstrate a secondary generator by successive suppression of the SFOAE with a sine wave close to the frequency Suppression growth functions (SGF) showed a three-step behavior. For low suppressor levels, the emission either decreased or increased when increasing the suppressor. For intermediate suppressor levels, DP amplitude was constant and independant of suppressor level. For high suppressor levels, the emission always decreased with further increase of the suppressor. The behavior of the SGF in the first section depends on the fine structure of the DP-gram, which shows minima and maxima. Emissions at a maximum decreased while emissions at a minimum increased in the first section of the SGF. We conclude that the fine structure of the DP-gram is produced by alternate constructive and destructive interference of the two generators. By adding a third tone near the SFOAE and thus the interference are suppressed. The fine structure of the DP-gram vanishes and the resulting DP-gram should be more closely related to the cochlear status near
103(1998); http://dx.doi.org/10.1121/1.421265View Description Hide Description
In this work experimental techniques developed for acoustic temperature oscillation measurements in gases using a cold wire anemometer are reported. The techniques also proved accurate for measuring of the relative phase between temperature and pressure oscillations. Specifically, the structure of the oscillatory thermal boundary layer with isothermal conditions in the solid boundary of an acoustic standing wave in air with 130-Hz frequency and pressure oscillation amplitude up to 200 Pa is determined. Experimental results are satisfactorily compared to theoretical results of a linear theory.