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Thread Subject:
Standard Deviation of sound in dB

Subject: Standard Deviation of sound in dB

From: Sheida

Date: 20 Oct, 2010 18:21:04

Message: 1 of 2

Hi, I know someone already asked something similar to this, but the answer they got just confused me more.

I have raw data of sound intensity vs depth. I need to find the mean and standard deviation at each depth and plot them in dB. Originally I converted everything to dB and found the mean and standard deviation that way. However, I learned that I cannot find the standard deviation this way and must somehow find it while it is in intensity and then convert it to dB. Clearly I am very confused on how this needs to be done. I tried to find it this way: 10*log10(standard deviation of intensity at each depth) but the answer I got made no sense.

Does anyone know how to do this in matlab and can explain it to me clearly?

Thanks very much!

Subject: Standard Deviation of sound in dB

From: someone

Date: 20 Oct, 2010 18:52:04

Message: 2 of 2

"Sheida " <s.d.not.this@gmail.com> wrote in message <i9nbug$3qs$1@fred.mathworks.com>...
> Hi, I know someone already asked something similar to this, but the answer they got just confused me more.
>
> I have raw data of sound intensity vs depth. I need to find the mean and standard deviation at each depth and plot them in dB. Originally I converted everything to dB and found the mean and standard deviation that way. However, I learned that I cannot find the standard deviation this way and must somehow find it while it is in intensity and then convert it to dB. Clearly I am very confused on how this needs to be done. I tried to find it this way: 10*log10(standard deviation of intensity at each depth) but the answer I got made no sense.
>
> Does anyone know how to do this in matlab and can explain it to me clearly?
>
> Thanks very much!

As you've discovered you can't simply compute the standard deviation of the intensity and take 10*log10 of that. You must treat the problem as a transform function of the random variable. If the "dBs" were gaussian distributed, then you would have what is known as a log-normal distribution. Google that term or see:

http://en.wikipedia.org/wiki/Log-normal_distribution

to get you started.

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