Creating high level sound in a duct using an axial array of sources

Abstract

In this paper, both a frequency domain method for creating high level harmonic excitation and a time domain inverse method for creating large pulses in a duct are developed. To create controllable, high level sound an axial array of six JBL-2485 compression drivers was used. The pressure downstream is considered as input voltages to the sources filtered by the natural dynamics of the sources and the duct. It is shown that this dynamic behavior can be compensated for by filtering the inputs such that both time delays and phase changes are taken into account. The methods developed maximize the sound output while (1) keeping within the voltage constraints of the sources and (2) maintaining a suitable level of reproduction accuracy. For harmonic excitation, pressure levels of over 155 dB were created experimentally over a wide frequency range (1000–4000 Hz). For pulse excitation there is a tradeoff between accuracy of reproduction and sound level achieved. However, the accurate reproduction of a pulse with a maximum pressure level over 6500 Pa was achieved experimentally. It was also shown that the compliance of the throat connecting the driver to the duct makes it difficult to inject sound into any of the cut-on modes in the duct just below the cut-on frequency of each new acoustic mode (pre cut-on loading effect).