Ways to measure audio noise level?

Discussion in 'General Electronics Chat' started by MikeA, Jan 28, 2015.

Jan 20, 2013
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I'd like to sample ambient noise once every 15 seconds. What about this idea: electret mic -> op-amp -> capacitor with a high value resistor to ground. Then an ADC will sample the voltage on the cap every 15 seconds. Is this a reasonable and straight forward way to accomplish this?

2. #12 Expert

Nov 30, 2010
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You need a precision rectifier in there to give a unipolar peak level.

3. MrChips Moderator

Oct 2, 2009
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No. If you sample every 15 seconds, the highest frequency you can measure is 0.03Hz according to the sampling theorem.

What kind of noise do you want to measure? White, pink, brown, blue, violet, grey?

Colors of noise

4. #12 Expert

Nov 30, 2010
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It took me most of a minute to figure out what you said, MrChips.
I assumed MikeA wanted to find the general loudness level of the background noise. Kind of like taking a photo of a db meter every 15 seconds.

I guess we're about to find out.

planeguy67 likes this.
5. wayneh Expert

Sep 9, 2010
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That was my assumption as well.

6. MrChips Moderator

Oct 2, 2009
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Yes, I understood your idea even before you posted (amazing, isn't it?).

What you are proposing is to rectify the AC signal and measure the sound envelope. That will work also.
I wanted to put some theory up front on the table for the OP to understand.

7. #12 Expert

Nov 30, 2010
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I know you think differently from me, and that is a major part of why this forum is good. It's just that sometimes I have to back up a step to get on your track.

Jan 20, 2013
135
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For now I'd like to measure just an average noise level. Bringing in a peak measurement might come in the future.

9. #12 Expert

Nov 30, 2010
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Average compared to peak is just a 2 resistor voltage divider, (and you're going to do that at the averaging capacitor anyway). You can add that circuit before or after the precision rectifier, but you can't easily build an, "average" rectifier that works properly across a wide range of amplitude.

It's just: Mic---precision rectifier---resistor--capacitor---resistor.

or you can do: Mic---resistor---resistor----precision rectifier---resistor---capacitor---resistor.

The second method rectifies the average instead of the peak, but it needs more parts to get to the same place.

Last edited: Jan 28, 2015

Jan 20, 2013
135
19
Is there a sample circuit diagram that I can look at to wrap my brain around this?

11. #12 Expert

Nov 30, 2010
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Here's a start. One of them shows the secret internals of an electret mic and the other is a peak detector with gain.

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12. joeyd999 AAC Fanatic!

Jun 6, 2011
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You have just stoked my curiosity, #12. Why do you suggest a differential amp for an electret? In what way is that better than single-ended?

13. #12 Expert

Nov 30, 2010
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I did not mean to suggest a differential amplifier. That's just the drawing I had available in less than 3 minutes. It shows that an electret mic needs some power supply and can be connected to the input of an op-amp.

That one also has a switchable filter on it for some reason I can't remember.
That's what the OP needed that day...whatever day that was.

Last edited: Jan 28, 2015
14. joeyd999 AAC Fanatic!

Jun 6, 2011
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Boy, you are just one big enigma today.

Ok...well, for the sake of the OP, a cursory (not rigorous!) analysis of the differential circuit tells me it might not work so well. Buyer beware....

#12 likes this.
15. #12 Expert

Nov 30, 2010
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It was right for some other person on some other day. That drawing just needs a LOT of circuitry chopped off of it for today's job.

Like this:

• Average noise detector.png
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16. ronv AAC Fanatic!

Nov 12, 2008
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Range may be a problem. What is quiet? Like the bedroom at night or like normal conversation. And loud?

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17. #12 Expert

Nov 30, 2010
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Maybe I should make those 2 resistors on the + input of the op-amp a volume control?

18. MrChips Moderator

Oct 2, 2009
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OP/TS needs to say what he wants to do with this information.
He may want to consider using a log amplifier.

#12 likes this.
19. #12 Expert

Nov 30, 2010
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You're right. There is a HUGE range of loudness levels and a log amplifier could make a much more useful output range.

20. studiot AAC Fanatic!

Nov 9, 2007
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It's nice to know that there are at least two members of this forum that think.