Are here some acoustic experts? Chamber concentrating sound to microphone. - Page 1
Forum section: electronics / general

author: Miyuki
author_rank: Frequent Contributor
timestamp: March 31, 2021, 07:01:15 pm
content: Hi folks.I need some help with acoustic. I want to build a chamber to measure the acoustic noise of power supplies.But I need some way of concentrating sound on the microphone to detect even weak noise. If possible down to 0 dBASo I need some horn or other way to concentrate it to microphone with noise floor about 10dBI want to measure only small objects (classic power brick size)Distortion is not an issue here, only absolute levels.Have you some ideas?

author: RJSV
author_rank: Super Contributor
timestamp: March 31, 2021, 08:07:31 pm
content: YES I agree a (closed off) horn shape should help in capturing or 'funneling' the mid and higher acoustic frequencies. But that is, also, assuming that your concern is not so much, for lower frequencies, such as a 60 or 120 hz sound wave.I believe you could still pick up the lower frequencies, it's just that basic 'exponential flared' horn enclosures are obviously tailored to match the acoustic impedance of the driver, to the open air impedance, for a sound being emitted.I would also pay attention to noise sources close by (acoustic noises). That is a small pressure (wave), and that sort of noise seems to penetrate any barriers by way of tiny cracks/ holes. That gets frustrating when building with thick, supposedly impenetrable walls, around your device under test.So, in building an enclosure, every small opening matters a lot, and should be closed off, methodically.Plus, you're going to want to isolate whatever box you build, usually by way of a soft blanket or other material for setting on. Don't forget the microphone cable: that also should have soft material supporting it, and maybe a few loose 'S' shapes in the cable (not a tight run from microphone.Once you get that far, try it: when GAIN is turned up, be wary of feedback if using a speaker.Mostly though, be aware that tiny openings can let in more extraneous sound noise than one would think, ordinarily.

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 01, 2021, 12:17:39 pm
content: Low frequencies are not that much a concern as hearing vs microphone sensitivity. A microphone is linear, while ears are low sensitive to frequencies under 200HzSo I need to have some gain at 100-200Hz and reaching maximal gain somewhere around 500Hz. This gives reasonable dimensions.

author: coppercone2
author_rank: Super Contributor
timestamp: April 01, 2021, 11:13:42 pm
content: i think a omni-direcitonal microphone at a fixed distance is the best way to measure noise, its about putting it in a big open environment with little reflections

author: 2N3055
author_rank: Super Contributor
timestamp: April 02, 2021, 07:51:10 pm
content: Quote from: Miyuki on March 31, 2021, 07:01:15 pm0 dBA ??25 dBA is a sound of a human breathing at 1 meter...There is no way to isolate from environment to achieve 0dBA background noise..Average anechoic chamber will be in 10–20 dBA range. Going lower than that you would need millions.

author: coppercone2
author_rank: Super Contributor
timestamp: April 02, 2021, 09:03:05 pm
content: the best you can do is get as close as possible to find noisy components IMO for a engineering precompliancetrying to do it at any sort of distance is cost and room prohibitive. you would get it tested later at a chamber, its like RF, you use near field, even though sound does not have a near field, you pretend it does

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 03, 2021, 10:30:33 am
content: I was thinking about something like this sketchI have an available underground room with low noise levels. It's in rock outside of the city. Where I can place a test setup covered with some sound absorbent.

author: coppercone2
author_rank: Super Contributor
timestamp: April 03, 2021, 03:49:09 pm
content: I am not sure that will help you determine how loud it actually sounds when you put the product on a real thing like a table

author: Nominal Animal
author_rank: Super Contributor
timestamp: April 03, 2021, 04:34:31 pm
content: Standard anechoic chambers cover the walls with uneven foam structures, to disperse sound waves. The idea is that the soft foam absorbs most of the pressure waves, and the rest that is reflected, is reflected in different directions. If I've understood correctly, it can be difficult to place the foam structures without causing "resonances", where some frequencies are not dampened as much as others, because of how they happen to bounce between the foam structures in opposing walls; that it is a bit of a hand-crafted art to make anechoic chambers. Place foam, measure, replace, measure, etc.I could have understood wrong, though; I've never made any anechoic chambers myself.Personally, I would make the chamber ellipse-shaped (as viewed from top), with the equipment to be measured placed in one focal point, and a cylinder of fuzz at the other focal point, and cover the walls with standard sound-absorbing pyramid foam.In an ellipse, all sound generated at one focus will reflect to the other focus. (There are buildings that use this as an acoustic effect; a whisper in one focus can be heard in the other focus, even if the distance is much greater than one would think a whisper could carry.)The cylinder of fuzz is basically an omnidirectional sound absorber. Whatever sound it reflects, will be reflected back to the device, so the cylinder of fuzz would need to be as absorbent as possible. I'm thinking of something like a 4" diameter foam cylinder covered with a thick shag carpet.The microphone would be placed at exactly 1m away from the equipment, as per standard. Actually, I'd use several microphones, so that the directionality of the noise could also be analyzed. If the equipment is typically used on a table, it would be on a table in the measurement chamber too. If handheld, then on a thin pedestal at roughly 1m height.Essentially, instead of trying to make the walls uneven, I'd be assuming the opposite, and focusing the sound reflections to a dedicated dampener. Whatever covers the walls will hopefully absorbmostof the acoustic energy, but some of it will always reflect. Having those reflections meet at a dedicated dampener, from which they're reflected back to the walls, should – I think and hope! – yield maximum damping, without relying on the small-scale structures in the walls dispersing the sound. In a way, such an anechoic chamber would have only one "really quiet spot", but that should be fine for measuring noise generated by equipment.

author: penfold
author_rank: Frequent Contributor
timestamp: April 03, 2021, 07:34:38 pm
content: The first thing that jumped to mind in measuring sounds with poor SNR: is the noise of interest correlated to any other measurable signal (line frequency, switching frequency etc..) and is it likely to be constant or varying? Or is it related to something which can be varied (such as load current)? There are potentially some ways you could use any correlated events to your advantage. But of course, if you're just interested in total random spurious noise, then it's not useful.