Is this how I should be thinking about modal bandwidth?

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Is this how I should be thinking about modal bandwidth?

Postby obieJon » Thu Apr 27, 2017 8:33 pm

Hi Everybody!

Trying to wrap my mind around the concept of modal bandwidth. Lets say I have an 1st order axial room mode with a frequency of 120Hz. When I excite the room with a sine sweep, there is a bump at 120Hz in the frequency response and it rings out longer. Lets say 122Hz also rings out longer than expected.

The way I've got it visualized in my head is that the wavelengths of 120HZ and 122Hz are close enough that the nodes and anti-nodes of 122Hz will basically line up with a half wavelength between the walls during the first reflection, then they will gradually phase and by say the 8th reflection the anti-nodes are not superimposed at all. Is this correct? And when the absorption coefficient of those two surfaces is high, the 120 and 122Hz will both decay swiftly enough that the difference in the amount of reflections when the anti-nodes are superimposed will be a moot point?

Second question:

If there is more than 20Hz between adjacent modes (why is this the number that gets thrown around? Is there a study behind it?), will a frequency caught in the gap between the modes not reverberate at all or will it simply reverberate the same amount as the diffuse field RT60?
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Re: Is this how I should be thinking about modal bandwidth?

Postby bert stoltenborg » Mon May 15, 2017 10:14 am

What happens is measured and then analyzed using a FFT. The bigger the FFT is, the higher the resolution in the frequency domain, the question is: what are the relevant psycho acoustics behind it? In the low frequencies research seems to indicate a 1/3 octave resolution is sufficient to explain what you hear; a higher resolution is nice to see what is actually happening.
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bert stoltenborg
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