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 ten years experience with the method, many practical problems of measuring accuracy were encountered. This paper will discuss the causes and suggested remedies for reducing measuring errors, particularly at low frequencies, due to drive unit nonlinearities, signal averaging in the presence of both random and synchronous background noise, and DC offset in the A/D converter. In addition, a novel method for accurately determining the free field response of a loudspeaker system down to very low frequencies will be presented. It will be shown that it is possible using this method to estimate a loudspeaker's frequency response to an accuracy of better than 1 dB down to 20 Hz using only 10mS of its impulse response measured in a non-anechoic environment.
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