Microphones in parallel to reduce noise
in [Design]
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From: George Herold on 18 Dec 2009 09:34 On Dec 17, 6:14 pm, "Phil Allison" <phi...(a)tpg.com.au> wrote: > "George Herold" > whit3rd > > "> I'm not happy paralleling the (FET) outputs; seems like the DC > > > levels would interact poorly." > > Hmm, can't you just AC couple them into the opamp? > > ** Yep - as I have already said, there will be an improved s/n ratio as a > result. > > When several similar mic capsules are paralled - the random noise voltage > drops, compared to one. However, the signal output will remain essentially > the same (as from one) long as the capsules are sited together in a > directional array. > > This simple fact has got some here bamboozled. > > ..... Phil "> ** Yep - as I have already said, there will be an improved s/n ratio as a > result." I saw that, sorry I didn't mention it. (Guess I was a bit cranky cause you wouldn't give me the name of a good/cheap dynamic mic... google found nothing for less than about $100. But as is usual I was probably searching for the wrong thing... half the battle is knowing what something is called.) "> This simple fact has got some here bamboozled." Correlation effects in noise are not always obvious. (At least it took me a while to figure it out when I first encountered it.) George H.
From: GregS on 18 Dec 2009 09:51 In article <e0185ff4-a7f2-4b4f-9927-1924e7b93988(a)k17g2000yqh.googlegroups.com>, osr(a)uakron.edu wrote: >My boss laid a unit on my desk today. Its got ten electret mics fed >into one low noise opamp. Any ideas how they did this without a ton >of noise. All I can see from the board layout is it is NOT the classic >op amp summer. > >Any suggestions for papers on low noise signal combining? > >Steve For your failure to explain or show a picture or diagram of how the mics are placed, I can not determine what the purpose was. It would be a big problem to do this just to reduce noise. There has to be some kind of directionality involved. greg
From: Adrian Tuddenham on 18 Dec 2009 13:52 <osr(a)uakron.edu> wrote: > My boss laid a unit on my desk today. Its got ten electret mics fed > into one low noise opamp. Any ideas how they did this without a ton > of noise. All I can see from the board layout is it is NOT the classic > op amp summer. > > Any suggestions for papers on low noise signal combining? Parallel arrays of identical small capsules can be used as a substitute for a large-diaphragm capsule to reduce the electrical and Brownian movement noise. If identical in-phase signals from two capsules are combined, the resulting signal increase will be 6dB. If the random noise from two identical capsules is combined, the noise increase will only be 3dB because the two noise sources, being random, will partially cancel each other. The result is an improvement of 3dB in the S/N ratio. This can be done with many more capsules, to give a significant improvement in S/N ratio (I have used a stack of 32 capsules for one specialised low-noise array). The result will not be quite as good as a large diaphragm, because the capsules (being round in shape) will have to be spread out over a larger area for a given size of diaphragm surface, so the phase differences between capsules for off-axis signals will be greater. However, the capsules can be arranged in a vertical stack, which reduces phase problems in the horizontal plane at the expense of worse phase effects in the vertical plane. That may not matter as long as the performers are all in the same horizontal plane (which they often are). -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
From: osr on 18 Dec 2009 15:42 They are in a arc, 3 each side with uneven spacing and 4 touching each other in the middle about the vertex. Steve
From: GregS on 18 Dec 2009 16:08
In article <5c8b1b84-ad7a-4fa9-b7f3-37d94bc74ec8(a)n35g2000yqm.googlegroups.com>, osr(a)uakron.edu wrote: > > > They are in a arc, 3 each side with uneven spacing and 4 touching >each other in the middle about the vertex. > When you talk uneven spacing it sounds like your not trying to get high phase addition or cancellation. Its still going to be frequency dependant. greg |