Modulation transfer function is in audio applications describes well the clarity of sound, but conventional definitions and measurement methods are not easily applicable to transducer measurements, low-frequency acoustics, or capturing effects of narrow-band phenomena. A revised definition of modulation transfer function, taking into account the magnitude and phase of modulation transfer for each carrier and modulator frequency combination is presented. This function is derived from the complex frequency response by analyzing the response at the carrier frequency and at the modulation sidebands. Also the distortion of modulation envelope arising from the asymmetry especially in the phase transfer properties is discussed. Examples of the use of the complex modulation transfer function are presented for simple filters, anechoic response measurements of loudspeakers, and for loudspeakers in rooms.
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