The evaluation of a loudspeaker's transfer function or frequency response (amplitude and phase) via digital processing of its measured impulse response is now an established technique. During 10 years' experience with the method, many practical problems of measuring accuracy were encountered. The causes and suggested remedies for reducing measuring errors are discussed, in particular at low frequencies, due to drive unit nonlinearities, signal avceraging in the presence of both random and synchronous background noise, and dc offset in the analog-to-digital converter. In addition, a novel method for accurately determining the free-field response of a loudspeaker system down to very low frequencies is presented. It is shown that by using this method it is possible to estimate a loudspeaker's frequency response to an accuracy of better than 1 dB down to 20 Hz using only 10 ms of its impulse response measured in a nonanechoic environment.
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