Spectrum of audio data

robert bristow-johnson wrote:
> i think that should be " 10*log10(abs^2) " where abs^2 = Re^2 + Im^2 .
> 

Yes, of course.

-- 
Jim Thomas            Principal Applications Engineer  Bittware, Inc
jthomas@bittware.com  http://www.bittware.com    (603) 226-0404 x536
Anything is possible if you don't know what you are talking about.

Hi Stereo,
    The procedure what i have followed is i have read the PCM samples
from the wave file passed throught the Hanning window and taken FFT of
it and then computed the spectrum so i think here there is no need to
take care of conversion to dB SPL.
  i will try some more audio files.

Rgds
-Prasad

<mailprassi@gmail.com> wrote in message
news:1147237208.666338.272260@j73g2000cwa.googlegroups.com...
> Dear All,
>
>    i want  to do a spectrum plot of audio data, for this i am doing
> like this
>    1. Taking the FFT of the data;
>    2. Finding the absolute(magnitude) of the fft output abs =
> (sqrt(Re^2 + Im^2)).
>    3. Then i am normalizing Norm = (2 * abs )/N) where N is FFT Length.
>    4. I am taking  20 * log10(Norm).
>
>   when i check the values i am getting spectrum values more -120 dB
> like -132dB.
>
> My query is
> 1. Can the spectrum of audio can be more than -120dB
> 2. Is the normalization what i am doing is right.
>
> Please help me out,
> Thanks a lot in advance,
> With warm regards,
> -Prasad B C
>
dB is a relative measure. On it's own it means nothing - you need something
to compare it with.

M.P





*** Posted via a free Usenet account from http://www.teranews.com ***

Mad Prof wrote:
>
> dB is a relative measure. On it's own it means nothing - you need something
> to compare it with.

that true, strictly speaking.  but they have defined reference voltages
and SPL levels.  you know, the "dBFS" and "dBv" and "dBu", the 1 mW
into 600 ohms thingie, and the threshold of hearing at 1 kHz.  i can
never keep them straight.

~~~~

robert bristow-johnson wrote:

> Mad Prof wrote:
> 
>>dB is a relative measure. On it's own it means nothing - you need something
>>to compare it with.
> 
> 
> that true, strictly speaking.  but they have defined reference voltages
> and SPL levels.  you know, the "dBFS" and "dBv" and "dBu", the 1 mW
> into 600 ohms thingie, and the threshold of hearing at 1 kHz.  i can
> never keep them straight.

SPL is standardized on acoustic pressure in air. Conversion to voltage
is a matter of calibrating the microphone system.

100 dB-SPL = 2 pascal = .01 watt/meter^2 in air.
http://www.sengpielaudio.com/calculator-soundlevel.htm

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

I wrote:

> 100 dB-SPL = 2 pascal = .01 watt/meter^2 in air.

That's wrong.
2 pascal = 100 dB-SPL (sound pressure level)
.01 watt/meter^2 = 100 dB-SIL (sound intensity level)

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Jerry Avins <jya@ieee.org> wrote in 
news:2f2dnXn2247bVfnZnZ2dnUVZ_sGdnZ2d@rcn.net:

> I wrote:
> 
>> 100 dB-SPL = 2 pascal = .01 watt/meter^2 in air.
> 
> That's wrong.
> 2 pascal = 100 dB-SPL (sound pressure level)
> .01 watt/meter^2 = 100 dB-SIL (sound intensity level)
> 
> Jerry

dB(SPL) is sometimes written dBre20uPa. You will also see dB(A) for sound 
pressure level. This is db(SPL) with A Weighting.

-- 
Al Clark
Danville Signal Processing, Inc.
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