Abstract: The Air-Modulated Speaker (AMS) is one of the most popular high-power acoustic sources. High intensity sound waves are generated by the modulation of high speed air stream. Although there should be a close relationship between unsteady flow and acoustic source formation from the aero-acoustic point of view, few studies available concerning the internal flow evolvement inside the source. The velocity fields of 2D AMS steady model were obtained utilizing Particle Image Velocimetry (PIV). Static pressure measurements of unsteady internal flow were recorded by the flow induced sound monitoring system. Dominant steady flow characteristics were observed including pressure recovery, flow separation on the out wall and alternate appearance of low and high pressure area on the internal wall. Experimental results in the sound energy conversion process show that pressure distribution variation is related to the modulate frequency. The frequency response of the source is affected by many factors such as chamber pressure, input signal strength and some geometry sizes. Flow near the nozzle exit has certain influence on the voice coil vibration. Peak value of the frequency response is between 0.5 and 1 kHz. When the drive current is lower than 10 A, SPL output of AMS increases with the chamber pressure and amplitude of the drive signal. Width of nozzle exit and vocal tract inlet also have evident effects on the performance of AMS. In order to attain a higher SPL value, the acoustic source has to work in the full modulation condition by the proper adjustment of nozzle parameters, and small inlet width for vocal tract is also advantagious. Harmonic components in the distorted pressure are obvious in the source generation zone after the monochromatic modulation. The spectral composition of the distorted signal in the internal flow is coincident with the sound signal in the far field under the same working condition.
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Received: 01 July 2009
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