It is established, using the reciprocity principle, that if a voltage-driven loudspeaker has a flat axial frequency-response, then such a response will also be obtained when it is used as a microphone, provided the associated amplifier has zero input impedance and a V out/I in response rising with frequency at 20dB/decade throughout the audio spectrum. The design of a suitable amplifier raises problems with regard to signal-to-noise ratio and the ability to handle the very large low-frequency input levels that are liable to be produced. Two different approaches are described, both leading to practical designs. In conclusion, consideration is given to the use of the technique for subjective loudspeaker quality assessment, and for the measurement of frequency response and absolute sensitivity.
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