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| Are here some acoustic experts? Chamber concentrating sound to microphone. |
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| Miyuki:
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 dBA So I need some horn or other way to concentrate it to microphone with noise floor about 10dB I want to measure only small objects (classic power brick size) Distortion is not an issue here, only absolute levels. Have you some ideas? |
| RJSV:
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. |
| Miyuki:
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 200Hz So I need to have some gain at 100-200Hz and reaching maximal gain somewhere around 500Hz. This gives reasonable dimensions. |
| coppercone2:
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 |
| 2N3055:
--- Quote from: Miyuki on March 31, 2021, 07:01:15 pm ---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 dBA So I need some horn or other way to concentrate it to microphone with noise floor about 10dB I want to measure only small objects (classic power brick size) Distortion is not an issue here, only absolute levels. Have you some ideas? --- End quote --- 0 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. |
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