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Hello John,
The sweep method at a single frequency is an approximation to a steady-state measurement with a pure tone. Longer sweeps give higher signal to noise ratio per sweep because it spends more time per frequency. Short repeated sweeps (but not shorter than the length of the impulse response) are good, if time-varying or sudden noise that doesn't average out is likely to contaminate the measurement. Sweep duration (and "rate" in general) also matters if the system (room in your case) is nonlinear, time-variant, or both, but that's another discussion. Something is wrong with your implementation if the temporal offset of the impulse responses you measure depends on the sweep duration. You should be able to check this by connecting the output of your sound card directly to its input. Also note that wrongly measured or wrongly computed impulse responses may be very reproducible in terms of correlation. Best, Anders PhD student in acoustics Aalborg University, Denmark From: AUDITORY - Research in Auditory Perception [AUDITORY@xxxxxxxxxxxxxxx] on behalf of John Culling [CullingJ@xxxxxxxxxxxxx]
Sent: Friday, July 24, 2015 5:25 PM To: AUDITORY@xxxxxxxxxxxxxxx Subject: Optimal sweep duration for BRIR measurements 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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