World's Most Reverberant Room

Bennett Prescott

Bennett Prescott

Just This Guy, You Know?
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Jan 10, 2011
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I posted this on Facebook and had a good geek out about it, but I thought you guys might like it too.

Now we've heard it all: Acoustic scientists shatter the world record for longest ever echo - Science - News - The Independent

Deep inside a complex of secret tunnels in the Highlands stirs a sound which will reverberate through the ages. The world record for the longest echo ever discovered has been shattered by a hidden network of oil storage tanks in Rossshire. Acoustic scientists emerged from the Inchindown oil storage tanks, an underground fuel depot constructed during World War II, with proof that a gun-shot fired inside the tunnel resonates for a full 112 seconds.
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Smaart thinks 109.5 seconds at 125Hz, but who's counting?

pg-24-echo-rcahms.jpg
 
Re: World's Most Reverberant Room

This does not hold a candle to the length of the reverberation of a spousal reprimand for dumb-assery...
 
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I think we can manage 150 seconds in the Lærdal tunnel :)~:-)~:smile:


But the record was set by Jimmy McGregor and Gregor McJimmy ages ago, "It's offa dark in here Jimmy, strike a match, that's a good lad!"
The boom reverberated for two weeks :D~:-D~:grin:
 
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Professor Trevor Cox's blog is really interesting too, you'll enjoy this one about the sound of a gunshot in an anechoic chamber:

Listening to the B of the Bang

I recorded myself walking into B&C's larger chamber (good to about 90Hz) the last time I was in Italy, it's not quite as interesting as actually doing it yourself - just sounds like I walked into a recording studio. In person the effect is really strange because your brain is missing all the cues for room size and volume of your voice. The majority of the sound you hear comes through bone conduction while in the chamber, and it feels like you're tiny and that your head is being squeezed. Uncomfortable for a little while.

[video=youtube;nFf-29F1_iI]https://www.youtube.com/watch?v=nFf-29F1_iI[/video]

Another amazing thing from Professor Cox's blog is that the chamber he is doing these tests in has a noise floor of -12.4dBA. Not NC30, or NC20 - pretty much the standard for high end mastering rooms. Negative! I have never experienced a room that quiet.
 
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Frank Koenig said:
Wow! How do you even measure that? Maybe they extrapolate from the measured sound transmission into the chamber from an artificial (very loud) source. Interesting stuff. --Frank
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As long as your measurement microphone and preamp have a low enough noise level, you can measure negative sound levels. Remember that 0dBA is not zero sound, but the lowest level of sound the human ear can detect (with a lot of individual variation)
 
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Per Søvik said:
As long as your measurement microphone and preamp have a low enough noise level
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Do you have some part numbers? That's a pretty low self-noise for direct measurement. I'll stick with the extrapolation story, although I do question the .1dB resolution in this context.

Signal averaging, although a very useful class of techniques for many low-noise measurements, doesn't help you here as the statistical properties of the signal are not sufficiently distinct from those of the noise.

(Oh, and I do know how dB SPL is defined, the negative value wasn't the point. This is an AUDIO forum.) --Frank
 
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Frank Koenig said:
Do you have some part numbers? That's a pretty low self-noise for direct measurement. I'll stick with the extrapolation story, although I do question the .1dB resolution in this context.
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Brüel & Kjær (4179) have stuff that is just about able to measure at that level.

Signal averaging, although a very useful class of techniques for many low-noise measurements, doesn't help you here as the statistical properties of the signal are not sufficiently distinct from those of the noise.
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Brüel & Kjær use narrow band analysis to take the noise floor down further, but just comparative analysis can fairly easily get you useful results 10 dB below the noise floor if the measurement system is stable and consistent enough to establish a baseline noise floor reference.
(Oh, and I do know how dB SPL is defined, the negative value wasn't the point. This is an AUDIO forum.) --Frank
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Of course you know, I was just stating the obvious because that fact was pivotal to my argument. ("Remember")
 
Re: World's Most Reverberant Room

Yes, B&K's best will get you close but not quite there. Perhaps there are some completely different microphone technologies that have very low noise. After all, unlike most audio applications, we don't need a lot of dynamic range for these measurements, just a very low noise floor. Superconductors and the tunneling effect (as used in the scanning tunneling microscope) come to mind, but I'm just making this up.

I think the trouble with narrow-band measurements is that, in general, the noise we're trying to measure is broad-band, so its power is reduced by the same ratio as the measurement system's noise and the signal-to-noise ratio remains constant. Were the signal we're trying to measure narrow-band, then band-limiting works. This is essentially what synchronous detection does.

Back in the late '70s I worked on displacement transducers with solid 100 A (10^-8 m) resolution that used linear variable differential transformers and synchronous detection, but they only had a bandwidth of a few Hz -- neither fast nor likely sensitive enough for this.

Another thing I wonder about is what is the thermal acoustic noise floor of air at room temperature (in a given bandwidth)? It must have one. Someone handier than I with that kind of physics probably can figure it out pretty easily.

Still, if I were charged with estimating the noise floor in the chamber I think I'd put a speaker out in the street (actually at more than one location) and produce a narrow band test signal that could be synchronously detected (or otherwise signal-averaged) in the chamber. This would give an estimate of transmission loss from the street to the chamber. Then do a regular noise measurement outside, subtract band-by-band, apply the weighting, and I'm done.

The big assumption here is that all the noise that matters is coming from "the street". Internally generated noise or noise from other sources, say transmitted through the earth, would be missed. I'm also assuming that the transmission medium is linear, but that's a pretty safe bet.

I believe some of you folks here do this sort of thing for a living. How would you do it?

--Frank
 
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Frank Koenig said:
I believe some of you folks here do this sort of thing for a living. How would you do it?
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Frank, if I told you how the "secret sauce" worked, then it wouldn't be "secret sauce" :twisted:
and no, I don't do "this sort of thing for a living". I gave up building shit long ago. these days, I just use (or abuse) the products that the smarter guys build for us......
 
Re: World's Most Reverberant Room

Frank Koenig said:
I think the trouble with narrow-band measurements is that, in general, the noise we're trying to measure is broad-band, so its power is reduced by the same ratio as the measurement system's noise and the signal-to-noise ratio remains constant. Were the signal we're trying to measure narrow-band, then band-limiting works. This is essentially what synchronous detection does.
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Yeah, you're probably right, but the result referred seems to be inside of the limit of what can be measured with the B&K equipment and a statistical approach.

I guess you can't just cool the equipment down with liquid nitrogen either, as that would render the microphone useless even if acoustic effects of a massive temperature differential could be ignored.
 
Re: World's Most Reverberant Room

I was once told that the National Acoustic Lab's anechoic chamber in Sydney held the record for the quietest place on earth for a number of years. As the story goes, a mic was hung, the doors were closed and the whole chamber was left for a week to allow any internal air movement to settle.

It's a great facility with many other test chambers and tunnels, but the government (in its infinite wisdom) is demolishing the place and selling the prime real estate to developers for apartments.
 
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drew gandy said:
Some anechoic chambers have no or very limited ventilation systems which leads to other "uncomfortable" things as well.
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I did some work in a chamber once, building a cable pass through panel. I found it uncomfortable as well. My worst fear, was if the door closed behind me and I got locked in somehow. I'm pretty sure no one would have heard me screaming.
 
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Aaron McQueen said:
I did some work in a chamber once, building a cable pass through panel. I found it uncomfortable as well. My worst fear, was if the door closed behind me and I got locked in somehow. I'm pretty sure no one would have heard me screaming.
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"In anechoic chamber no one can hear you scream", a famous quote from the horror movie "Digital"
 
Re: World's Most Reverberant Room

The worlds most reverberant room is a sound forum room where someone has suggested moving faders or recallable gains are not critical features on a digital mixer. Immediately someone will respond with the negative and the signal will then return over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and.
 
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