Dear John,
As others have pointed out, increasing the length of the sweep increases your signal-to-noise ratio. For large room acoustics, we typically use sweeps of 20 to 40 seconds. This of course depends on the size of your room, its reverberation time, and the power of your source. The weaker the source, the longer the sweep. You should basically do a test and see what SNR you get. If you need more, and your measurement chain is at its limit, the only option is longer sweep. Longer sweeps will not help much with impulsive interruptions, while averaging will.With a single sweep, basically that freq-range during the noise is lost. If you are just measuring from the RIR, this may not be an issue, as measurement parameters are often in octave-bands, but this is not true for auralization usage; corrupt data is corrupt data, though I haven’t gone through a thorough study of this actual case.
The sweep should definitely be longer than the RT, for room measurements. Then, don’t forget that you need to record the sweep length PLUS the RT or more if your SNR is better than 60dB!
We have also compared averaging repeated sweeps vs. longer sweeps. Avoiding the recent developments in overlapping sweep processing, this basic repetition approach ‘requires’ the decay of the first sweep to finish before you launch the second sweep. As such, 3x20 second sweeps take longer than 1 x 60 second sweep, due to the additional pauses. Of course, without repetitions, you have no backup in case something goes wrong, like a door slam or something. So, I tend to use repeated sweeps and take the best 1.
We do all our processing in MatLab, and have never had an issue (within the last 20 years) of processing long sweeps in real halls.
As you are considering BRIR, and not anechoic HRTFs, you are subject to the same conditions. If you want to convolve the BRIR directly, you will need to ensure that the SNR is sufficiently high that the noise-floor is not audible as a late reverb part of the BRIR. Some noise extension methods exist from older studies on basic auralization and scale model RIR auralizations. I cannot imagine a 2s sweep for BRIR unless you are measuring an office of other room with <1 sec RT. I think there is a fault in your correlation-based analysis for reliability, and there are too many factors to consider in comparing BRIR of different lengths. First, examine your SNR.
Regarding distortion of the source, this should be an issue for the processing element (different from burning out you speaker). This is because one of the strengths of the sweep method (when done correctly) is that any harmonic distortion components of a higher frequency that the excitation signal at the time or folded back to BEFORE the direct sound after deconvolution by the excitation signal (as that frequency has yet to be generated). We presented a through work on this feature, extending Farina’s earlier works, to general conditions:
M. Rébillat, R. Hennequin, E. Corteel, and B. Katz, “Identification of cascade of Hammerstein models for the description of nonlinearities in vibrating devices,” J. Sound and Vibration, vol. 330, pp. 1018–1038, 2011, (doi:10.1016/j.jsv.2010.09.012).
This method lets you actually extract and analyse the different harmonic distortions (THD etc.) as well as allowing for the modelling of non-linear responses.
Cheers,
Brian
--
Brian FG Katz, Ph.D, HDR
Resp. Groupe Audio & Acoustique
LIMSI - CNRS
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De : AUDITORY - Research in Auditory Perception [mailto:AUDITORY@xxxxxxxxxxxxxxx] De la part de Anders Tornvig Christensen
Envoyé : lundi 27 juillet 2015 09:07
À : AUDITORY@xxxxxxxxxxxxxxx
Objet : Re: Optimal sweep duration for BRIR measurements
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.
Prof. John Culling School of Psychology, Cardiff University Tel: +44 (0)29 2087 4556 | Yr Athro John Culling Yr Ysgol Seicoleg, Prifysgol Caerdydd Ffôn : +44 (0)29 2087 4556 |
