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Hi All, Tough question from John, as it appears to have many dimensions – nonlinear distortion, noise, time-varying noise, effort and resources. I have not studied the problem comprehensively but the thoughts below occurred to me. Hope they’re
helpful. Cheers, Bill Woods 1.
Philip brought up a good point, but technically speaking the sweep does not have to be longer than the response, but the recording duration should be. For example,
in the ideal situation the impulse response of any room could be measured with one impulse (i.e., a click). Just make sure you record the whole thing. ;) 2.
Speaking of clicks, I imagine the shorter the sweep, the more click-like the stimulus. There are reasons sweeps are used instead of clicks (or MLS for that matter),
so might be useful to consider them. Perhaps they underlie the inaccuracies of the results with shorter sweeps? 3.
I think Dick was speaking to the difficulties of working with the large amount of data comprising recordings of very long signals, though that would be odd to hear
from someone at google. ;) Maybe I’m missing something, but clearly if there are several channels measured simultaneously at a high sampling rate and long duration then the data storage, transfer, and processing could become an issue if on a limited budget. 4.
Speaking of long signals, I imagine the longer the sweep the more you are stimulating the system with something like a sequence of long sinusoids. I can’t see any
particular disadvantage of that relative to averaging an equivalent number of responses obtained with moderate-length sweeps, and thus can’t explain why your results with moderate-length sweeps are better than with the longer. You mentioned the difficulties
in the comparison of results of the two lengths. Perhaps the averaging is not equivalent across your moderate-length and long methods? Have you tried making a long recording in the room without a stimulus and using that recording in a simulation of the two
methods? At least you’d be able to compare them under the same background noise conditions and check your implementation as well. Perhaps there are fluctuations in the background noise that are not averaged-out similarly under the two methods? From: AUDITORY - Research in Auditory Perception [mailto:AUDITORY@xxxxxxxxxxxxxxx]
On Behalf Of John Culling Dear all, Basic Q… Does anyone have insight into the optimum sweep duration using Farina's method for measuring room impulses responses? More detailed background… We are planning to make an extensive series of measurements, and in preparation have been testing the method using different sweep durations. One way to check the method is to correlate the impulses respones from repeated measurements or those generated
with different durations. To our surprise short sweeps (1-2 seconds) appear to give more
reliable results (repeated sweeps correlate, r>0.98) than longer ones. Comparing sweeps of different durations is a little trickier, because we find a temporal offset that reduces the correlation and can only be partially overcome by using cross-correlation. Nonetheless, it is apparent that durations from 1 second upwards correlate well, while going below one second leads to reliable IRs, but ones that are inaccurate when compared with those from longer sweep durations. Our surprising conclusion is that ~2s should be fine, but Farina refers to an ISO standard that
recommends very long sweeps (Farina has an example of 50s) to help overcome noise. This seems an unintuitive rationale to us, since longer sweeps should increase both the
signal energy captured and the noise energy, and the method does not involve averaging as far as I understand. Longer durations should help address brief interupting sounds, but
I am unsure if that it what was the idea. In the presence of continuous noise, we did not
notice any improvement in the IRs produced by longer sweeps. The nascent plan is to take >1 short sweep for each measurement and reject IRs that that don't correlate well with another. Any insights/advice appreciated, John.
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