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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

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