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“Room resonances at low frequencies behave as “minimum phase” phenomena, and so, if the amplitude vs. frequency characteristic is corrected, so also will the phase vs. frequency characteristic. If both amplitude and phase responses are fixed, then it must be true that the transient response must be fixed – i.e. the ringing, or overhang, must be eliminated” (Toole, The Acoustical Design Of Home Theaters, 1999)There has been debate about whether this theory applies in small rooms and also whether small rooms have any areas that are minimum phase. On the latter point, John Mulcahy, creator of the acoustic measurement package Room EQ Wizard has written a useful article that explains how a measurement called excess group delay can be used to understand which areas of a room's response are minimum phase. On the former point we present the measurements in this article as proof that the ringing can indeed by reduced or eliminated by correctly deployed equalization. The following measurements were taken in a dedicated 9 seat home theater designed by my friend and acoustics accomplice Jeff Hedback of HdAcoustics. On this project we split the workload - I did the calibration and Jeff did the acoustic design. Practical limits of space and cost limited the amount of low frequency absorption that could be deployed, so it was known from the outset that equalization would be appropriate in helping to improve performance. The design was completed in such a way that the seat to seat variation in frequency response was low, allowing equalization to be applied effectively. It was confirmed during the verification measurements that a couple of parametric EQ filters would be beneficial in improving the sound quality in the room. Two equalization filters were applied, at 25Hz and 50Hz. Since room modes have a very narrow bandwidth - typically around 5Hz - you need narrow bandwidth or high Q filters to properly address them. The filters used had a bandwidth of 0.35 octaves. The attenuation at 25Hz was 13dB and at 50Hz was 6dB. Before the equalization was applied there was a slight subjective loss of articulation of piano, bass guitar and drums due to the resonance at 50Hz. Furthermore there was also a significant 'energizing' of the room at 25Hz such that low frequency effects from movies were slightly blown of out proportion and were even making the screen noticeably shake! After the equalization there was a very clear improvement in articulation and that energizing of the room at 25Hz was almost completely nullified, creating a very articulate and dynamic bass presentation. Certainly one of the better home theaters I have had the pleasure of calibrating, and a great testament to the combined skills of a home theater installer, a specialist calibrator and a specialist acoustic designer all playing their part in achieving the results. Three different measurements are presented: the frequency response, the time/energy/frequency response and the impulse response. The frequency response measurement clearly shows how the equalization filters have flattened out the response. The time/energy/frequency measurement, presented both as a waterfall and a spectrogram, clearly shows the reduction in decay times. The impulse response measurement most clearly shows the reduction in ringing.
Nyal Mellor, Founder, Acoustic Frontiers