A device to generate high frequency noise from commercially available low frequency acoustic drivers

Abstract

A device has been developed to generate high frequency noise with low frequency acoustic drivers. The principle behind this mechanism is the use of the non-linear propagation effects of high intensity sound waves inside hard-walled ducts, which alter the shape of the sound wave. This modification makes the sound wavefront steeper and, thereby, extends the frequency range of the wave signature. Two means are used to achieve this objective. In the first case, a converging duct is connected to an acoustic driver, where use is made of the fact that for a fixed acoustic power output from the driver, the intensity of the sound wave increases as the duct cross-sectional area decreases. In turn, a strong non-linear effect is produced. Thus, the larger the contraction ratio, the stronger the non-linear effect and the higher will be the frequency range. In the second case, a long, hard-walled duct of constant cross-sectional area is connected to the acoustic driver. In this second case, a reasonably strong sound wave emanating from the driver propagates through the long duct and, thereby, experiences non-linear propagation effects for a longer time. Therefore, the longer the duct, the greater the non-linear effects and the higher will be the frequency range. This paper presents the results obtained by using these two methods to generate high frequency noise from a low frequency driver. Higher frequencies can also be generated by combining both the means of creating non-linear effects: i.e., using a long and converging hard-walled duct. Examples of such devices developed for specific applications are illustrated in this paper.