The technique of coupling two loudspeakers to obtain a signal proportional to the velocity of the cone is known. The sound at a distant point from a loudspeaker is related to the diaphragm displacement. Since the induced e.m.f is proportional to the voice coil velocity,measurement of induced e.m.f enables us to obtain the displacement of the cone and hence we can plot the SPL curve for a loudspeaker. This method has some limitations in that it is particularly applicable only when one is interested in using the output of the second speaker used as a sensor in some further applications.
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