Two effects can be considered: 1. The room response to a sustained tone rises in level towards a maximum (due to absorption), with time constant equal to the reverberation time. A higher level gives better signal to noise at the microphone, and hence better signal to noise of the calculated IR. 2. Once the level at the microphone has saturated the calculated IR noise at this frequency falls as 1/sqrt(t) for ‘time at each frequency’ t - this can be seen by viewing the deconvolution in terms of fourier transforms: Integration over time has an averaging effect on a random variable causing the familiar reduction in noise level. In practice the signals don't stabilse fully as the frequency is changing, but the same principle applies. Phase 1. gives rapid gains initially compared with 2., which is why its a good idea that the sweep length is greater than the reverberation time, although not necessary. Actually it should be several times this because the ‘time at each frequency’ is a fraction of the sweep time. -- Dylan Menzies Senior Research Fellow Institute of Sound and Vibration Research University of Southampton, UK From: Trevor Agus <t.agus@xxxxxxxxx<mailto:t.agus@xxxxxxxxx>> Reply-To: Trevor Agus <t.agus@xxxxxxxxx<mailto:t.agus@xxxxxxxxx>> Date: Tuesday, 28 July 2015 09:53 To: "AUDITORY@xxxxxxxxxxxxxxx<mailto:AUDITORY@xxxxxxxxxxxxxxx>" <AUDITORY@xxxxxxxxxxxxxxx<mailto:AUDITORY@xxxxxxxxxxxxxxx>> Subject: Re: Optimal sweep duration for BRIR measurements Dear list, There's a conundrum in this discussion that: (1) increasing the duration of the sine-wave sweep increases the signal-to-noise ratio (which seems intuitively true) versus (2) increasing the duration of the sine-wave sweep does not affect the signal-to-noise ratio (as John noted; which is literally true if the "signal" is the sweep, whose level is unaffected by duration). It's a fun paradox, and I don't want to take away from anyone's pleasure by my stab at a resolution... (Potential spoiler alert.) Is it that the SNR of interest is the impulse-response-to-noise, only over its relatively short time period? As such, a large amount of the energy of the noise (in a long sweep) would be outside of the IR's expected time period (after the convolution stage that 'extracts' the IR), but all the energy of the sine sweep (and its reverberation) would be kept within this time period. So the SNR is constant (if you consider the full duration) yet increased (if you focus on the time period of the IR). Or is there more to it? All the best, Trevor Brian FG Katz wrote: 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 Rue John von Neumann Campus Universitaire d'Orsay, Bât 508 91405 Orsay cedex France Phone. + 33 (0)1 69 85 80 67 - Fax. + 33 (0)1 69 85 80 88 http://www.limsi.fr<http://www.limsi.fr/><http://www.limsi.fr/> web_group: http://www.limsi.fr/Scientifique/aa/ web_theme: http://www.limsi.fr/Scientifique/aa/thmsonesp/ *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<mailto: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<mailto:AUDITORY@xxxxxxxxxxxxxxx>] on behalf of John Culling [CullingJ@xxxxxxxxxxxxx<mailto:CullingJ@xxxxxxxxxxxxx>] *Sent:* Friday, July 24, 2015 5:25 PM *To:* AUDITORY@xxxxxxxxxxxxxxx<mailto:AUDITORY@xxxxxxxxxxxxxxx> <mailto:AUDITORY@xxxxxxxxxxxxxxx><mailto: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 [cid:part1.06010402.07030506@qub.ac.uk] Brian FG Katz<mailto:brian.katz@xxxxxxxx> 27 July 2015 17:46 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 [cid:part2.04070804.09090909@qub.ac.uk]LIMSI - CNRS Rue John von Neumann Campus Universitaire d'Orsay, Bât 508 91405 Orsay cedex France Phone. + 33 (0)1 69 85 80 67 - Fax. + 33 (0)1 69 85 80 88 http://www.limsi.fr<http://www.limsi.fr/> web_group: http://www.limsi.fr/Scientifique/aa/ web_theme: http://www.limsi.fr/Scientifique/aa/thmsonesp/ 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<mailto:AUDITORY@xxxxxxxxxxxxxxx> Objet : Re: Optimal sweep duration for BRIR measurements
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