Volume 130, Issue 4, October 2011
Index of content:
- AEROACOUSTICS, ATMOSPHERIC SOUND 
130(2011); http://dx.doi.org/10.1121/1.3624817View Description Hide Description
A randomly varying ground impedance is introduced into the solution for the sound field produced by a point source in a homogeneous atmosphere above a flat ground. The results show that in general the ground with a random impedance cannot be represented by an effective, non-random impedance. The behavior of the solution is studied with a relaxation model for the impedance in which porosity and the static flow resistivity are random variables. Mean values and standard deviations are adopted from measurements of two types of ground surfaces. For both surfaces, the mean intensity of the sound field above a random-impedance ground deviates only slightly from the intensity above a non-random impedance. The normalized standard deviation of intensity fluctuations can, however, be greater than one, thus indicating that for a particular realization of the random impedance, the sound intensity might significantly deviate from the intensity for a non-random impedance.
Experimental study of the influence of low frequency flow modulation on the whistling behavior of a corrugated pipe130(2011); http://dx.doi.org/10.1121/1.3628665View Description Hide Description
It is well known that airflow in a corrugated pipe can excite whistling at the frequencies of the pipe’s longitudinal acoustic modes. This short contribution reports on the results of experiments where a low frequency, oscillating flow with velocity magnitudes of the same order as the airflow has been added. Depending on the oscillation strength, it has been found that this flow may silence the pipe or move the whistling to higher harmonics. It is also shown that the low frequency oscillation itself may excite higher frequency whistling sounds in the pipe.