Frequency response equalization is often performed to improve audio reproduction. Variations from the target system response due to playback equipment or room acoustics can result in perceptible timbre distortion. In the first part of this paper we describe experiments conducted to determine the audibility of artificially introduced spectral dips. In particular, we measured notch depth detection threshold (independent variable) with respect to notch center frequency and Q-factor (independent variables). Listening tests were administered to 10 listeners in a small listening room and a screening room (small cinema with approximately 100 seats). Pink noise was used as the stimulus as it is perceptually flat (with roughly 3 dB/octave spectral tilt with frequency) and is known to be a reliable and discriminating signal for performing timbre judgments. The listeners gave consistent notch depth results with low variability around the mean value. The notch audibility data was then used to develop multiple candidate algorithms that generate equalization curves designed to perceptually match a desired target response, while minimizing the equalization gain applied. Informal subjective results validated the performance of the final algorithm.
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