Abstract
We describe a series of measurements to assess the practicality of a length limited parametric array in air. This study shows that the length limited effect is a measurable phenomenon that can be produced using pairs of commercial off the shelf parametric array speakers. We generated the effect using parametric arrays mounted so that two directional audio beams were simultaneously co-propagating through the open air. Parametric arrays work such that after the ultrasound frequencies have attenuated, the remaining audio range acoustic frequency is linear. We used this principle to propagate 2 kHz signals from two parametric array speakers, adjusting the relative phase of the resulting audio-range signals to produce varying amounts of constructive or destructive interference in the resulting linear sound beams. We demonstrated that increasing the overlap of the audible sound beams increased the effectiveness of the length limited phenomenon. We also found that changing the magnitude of the sound projected through one of the speakers did not have significant impact on the length limited effect.
Highlights
Parametric acoustic array speakers create highly directional beams of audible sound by simultaneously transmitting two ultrasound frequencies [1,2,3,4,5,6,7,8,9]
Our results show that the length limited effect is controllable and can be generated through modulation of the difference frequency
Parametric arrays work such that after the ultrasound frequencies have attenuated, the remaining audio range acoustic frequency is linear and so the length limited effect is generated through interference between the difference-frequency sound beams propagating simultaneously
Summary
Parametric acoustic array speakers create highly directional beams of audible sound by simultaneously transmitting two ultrasound frequencies [1,2,3,4,5,6,7,8,9]. The nonlinearity of air creates both a sum and difference frequency as the overlapping ultrasound beams propagate [10,11,12,13]. These two colormap plots were generated with simulations of the parametric array using the Texas KZK code In these colormap plots the nonlinearity parameter on and off, respectively. These plots show the sound pressure levels in dB of the 5 Hz difference frequency. The plot on the left shows a simulation where the nonlinearity parameter is set to that of air In this plot the beam pattern of the 5 kHz signal can be seen in the lighter blue. The right image has no 5 kHz beam pattern because
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