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Old 23rd April 2003, 03:00 PM   #1
Kevinbd is offline Kevinbd  United Kingdom
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Default Measuring very low levels of acoustic distortion

Hello,

Just wondering how to go about measuring very low levels of acoustic distortion. In the 0.001% region or with that sort of resolution. 0.01% would be OK too, but not sure what sort of equipment i need.

I do have an old Bruel & Kjaer SPL meter, this is an old unit but high quality. I want to get a digital one - this would be much easier for making graphs.

Can microphones go down to 0.001/0.01% distortion level from 20hz - 20khz? I suspect these could cost many thousands though.

It amuses me, when people go on about unnecassery oversampling in sound cards etc. They negate factors like a decent mic ( a few $K) and room acoustics to boot. Plus pc's through out emi, noise like you wouldn't beleive - they miss the point. Makes a mockery of pc soundcards though.

Perhaps knocking up a rig to feed this back into an op amp with some limiting might be an idea.

Obviously the room acoustics are important or lack of them more to the point. There was a recording i seen in a hi fi mag that had a anechoic (spelling?) chamber recording with no reflections. Never did get that CD.

Would be interesting to see what effect reflections does have on sound and the importance of dealing with it. ie. room damping/sound proofing.

I would think that, if normal sounds in a reflectionless
chamber was not discernible, then conversely, room acoustics is a _massive_ area for optimisation.

Any views/knowledge welcome

Kevin
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Old 23rd April 2003, 04:35 PM   #2
SY is offline SY  United States
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That's going to be tough to do, but it CAN be done with a high-precision mike and preamp. You'll need to budget four or five thousand bucks. Then you'll need a suitable environment for measuring, where the noise is unbelievably low and reflections are extremely well-damped. We're talking about 5 figures of renovation. At very low frequencies, you can get away with near-field measurements, which gets you back to the $4-5 grand range. Ultra-low distortion measurments are rarely useful, unless you've got some special need...?

There's a lot of literature on the interaction of rooms, speakers, and the ear/brain system. You can start with Floyd Toole's introductory papers (they're on Harman International's website in pdf format), then if you want to get a better handle on these issues, start burrowing through JAES.
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