Experimental study of the influence of low frequency flow modulation on the whistling behavior of a corrugated pipe
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.
A probe-corrected theory is presented for computing the acoustic far fields of transducers and scatterers from measurements of near fields on a cylindrical surface. The near-field data is truncated at the top, bottom, and angular edges of the scan cylinder. These truncation edges can cause inaccuracies in the computed far fields. Correction techniques are developed for the top and bottom truncation edges. The cylindrical wave expansions automatically apply an angular taper to the near-field data that reduces the effect of the angular truncation edges. The taper function depends on the probe and the angular sample spacing. The theory is validated through numerical examples involving a point source and a baffled piston transducer probe.