Volume 118, Issue 1, July 2005
Index of content:
- TRANSDUCTION 
118(2005); http://dx.doi.org/10.1121/1.1928787View Description Hide Description
This paper deals with the problem of reproducing two signals at two points in space by using two acoustic sources. While much is now known about the techniques available for the design of matrices of inverse filters that enable this objective to be achieved in practice, it is still the basic physics of the sound field produced that controls the effectiveness of such systems and which ultimately dictates their design. The basic physical processes involved in producing the cross-talk cancellation that enables the reproduction of the desired signals is revisited here by using a simple two source/two field point free field model. The singular value decomposition is used to identify those frequencies where the inversion problem becomes ill-conditioned and to explain physically the origin of the ill-conditioning. As observed previously, it is found that cross-talk cancellation becomes problematic when the path length difference between the two sources and one of the field points becomes equal to one half the acoustic wavelength. The ill-conditioned frequencies are also found to be associated with a limited spatial region of cross-talk cancellation and with large source outputs manifested in the time domain by responses of long duration.
118(2005); http://dx.doi.org/10.1121/1.1931087View Description Hide Description
The noise in a telephone handset produced by the bursts of airflow that accompany plosive sounds such as a “p” or “t” has been investigated. Measurements have been made on a series of modified handsets, using various arrangements of ports and tubes between microphone and the exterior of the telephone mouthpiece. A narrow stream of air was used to probe the mouthpiece and the signal picked up by the microphone recorded. The use of one, two, and multiple exterior holes was considered. Two-hole approaches alleviate the static pressure buildup that occurs when a single microphone hole is used and in many cases they result in reduced noise generation; for some directions of incident airflow, though, noise can be markedly increased. This was found to be due to interaction between intersecting streams of air. By using a large number of small (less than ) exterior holes, the airflow noise can be reduced by over for all incident directions of airflow. It is also beneficial to ensure that none of the exterior holes is in direct line with the sound tube leading to the microphone.