Combining noises
Combining noises
(OP)
An old textbook of mine says to use the power, not pressure, formula when combining noises but later that ten identical sound sources have an SPL 10 dB louder than just one source. Shouldn't it read PWL (power level of the noise), not SPL (sound pressure level)?





RE: Combining noises
Here is a Matlab script that can be used to demonstrate the calculation:
disp(' ');
disp(' add_dB.m ver 1.0 February 9, 2010');
disp(' by Tom Irvine ');
%
disp(' ');
disp(' This script assumes that the dB values represent ');
disp(' a sound pressure level or a power spectral density ');
%
clear a;
clear sum;
clear length;
clear C;
%
disp(' ');
disp(' Enter vector name of dB values ');
a=input(' ');
n=length(a);
%
C=10;
sum=0;
for(i=1:n)
sum=sum+10^(a(i)/C);
end
sum=C*log10(sum);
%
out1=sprintf('\n overall level = %8.4g dB \n',sum);
disp(out1);
Tom Irvine
www.vibrationdata.com
RE: Combining noises
- Steve
RE: Combining noises
10 identical sources could produce an SPL anywhere between 20 dB greater and -infinty dB less, depending on the type of noise and where you are in relation to the sources!
M
--
Dr Michael F Platten
RE: Combining noises
- Steve
RE: Combining noises
RE: Combining noises
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Combining noises
RE: Combining noises
For random noise or for sources with different frequencies, the 3dB rule of thumb applies.
- Steve
RE: Combining noises
Instantaneously x1+j*y1+x2+j*y2 = xresult+j*yresult always applies. If people are trying to confuse you about that then walk away. You can derive all results for a linear system from that, tho broadband results need a bit of work.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Combining noises
If I have 2 uncorrelated random signals with the same rms amplitude (say 1 to make things nice and easy) and I add the two signals together, what is the rms amplitude of the combined signals?
The answer is sqrt(2). Some of the random bits of one signal happen to add together and some happen to cancel out, but on average the increase is a factor of 1.414.
Now what happens if I add 2 identical signals (say add signal 1 to itself)? Obviously the signal is doubled so the rms is doubled
If those signals were sound pressures, then adding the 2 uncorrelated signals would mean an increase of 3dB in sound pressure level (add 20 log 1.414), but adding the 2 identical signals would mean an increase of 6dB (add 20 log 2).
M
--
Dr Michael F Platten
RE: Combining noises
RE: Combining noises
http://w
Ted
RE: Combining noises
- Steve
RE: Combining noises
Your are correct that coherent sounds are handled differently when combining levels.
Ted
RE: Combining noises
RE: Combining noises
For example geared electric motors running at the same speed? Or maybe a number of turbines sitting next to each other in a power station? Or perhaps a pair of loudspeakers connected to a hifi system?
M
--
Dr Michael F Platten
RE: Combining noises
3+3 = 6dB
3+3+3+3 = 9dB etc
i.e. it assumes 10 incoherent sources all with the same LW and directivity.
If you double the pressure on a single output then add 6dB.
If you double the sound power on a single output then add 3dB.
If you add sources together IN A FREE-FIELD, consider their realtive phases. Coherent - include phase, incoherent - don't worry.
Guess what, you need a better textbook ;) consider Beranek or Fahy.
RE: Combining noises
Walt
RE: Combining noises
"With two equal tones the pressure is 2x or 6 dB higher."
No, not necessarily. Or even, not very often.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Combining noises
To be more specific: two tones of same frequency and amplitude that are in phase will have a peak amplitude 2x or 6 dB higher than one tone alone.
Walt
RE: Combining noises
This has nothing to do with pure tones. It is to do with whether the sources are correlated or not.
If the sources are uncorrelated then sum the rms sound pressures. If they are correlated then you need to sum the instantaneous sound pressures, then take the rms.
Take 10 random noise generators and connect each one to its own amp and speaker.
Now take 1 random noise generator and feed the signal to 10 amps and speakers.
Assume everything is in a free field, the speakers are small, omnidirectional and far enough apart that no speaker is in any other's nearfield. i.e. each speaker produces an independent pressure field.
For the 10 noise generators the time averaged sound pressure level will be 10 dB more than for just one noise generator.
For the single noise generator connected to 10 speakers this is not true and there will be anything between a 20 dB increase and nothing at all depending on the degree of correlation of the sources (in this case, 100% correlated) and the location of the measurement point with respect to the sources.
The confusion has arisen in this thread because of the original post which says "identical sources". As the above example shows, it very much depends on what you mean by "identical sources"
M
--
Dr Michael F Platten
RE: Combining noises
- Steve
RE: Combining noises