A novel technique is described for monitoring and adjusting the acoustic power-output/frequency response of a direct-radiator type loudspeaker system at low frequencies. The method proposed for monitoring the acoustic power output of a moving loudspeaker diaphragm involves the measurement of the sound-pressure distribution just above the diaphragm surface, together with the velocity distribution on the diaphragm surface. Suitable processing of these measurements can provide an electrical signal which is proportional to the product of the magnitudes of the pressure and velocity distributions multiplied by the cosine of the phase angle between them. This electrical signal is proportional to the acoustic power radiated by the diaphragm as a function of frequency. Because the acoustic power radiated is also a function of both the position of the loudspeaker enclosure relative to the room boundaries and the resonance modes of the room, it is proposed that the electrical signal be used to control the electrical input power fed to the loudspeaker in such a way that the acoustic power-output/frequency response is maintained uniform regardless of the environment in which the loudspeaker system is placed.
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