Modes and diffraction

[DanMilo]

Hi,
this is just a more physical question...
When there´s a room mode in the height and its frequencies are typically 70/140/210 for example .... what happens when i insert an obstacle (a typical desk for example). Does the mode build up the same way but because of the longer way (to travel around the desk) its freqencies shift up? Or is the mode damped/diffused by the obstacle and a new mode develops between the desk and the ceiling? I think a typical desk has about half the wavelenght of 140hz...so i´m interested what happens in reality....i know the difraction formulas...but i´m not sure if this works the same with a resonance.

[Soundman2020]

what happens when i insert an obstacle (a typical desk for example).

Unless it is a very large desk, it won't have much effect. At 70 Hz, the wavelength is 16 feet. Unless your desk is close to 16 feet wide / long / high, it won't have a very large effect. There might be some diffraction going on around it that would tend to "smear" the mode slightly, or perhaps shift the frequency slightly, but I doubt that it would have a substantial effect on the mode.

Also, since the mode is an effect across the entire surface area of the ceiling and floor, and the desk only represents a small fraction of that surface area, the overall total effect on the entire room will not be huge.

Desks usually cause far more serious problems with reflections, mid-range frequency response, and comb filtering, with regard the to the path from the speakers to the listening position. I'd be more concerned about those,than about the effect your desk might have on modes. There will be some effect, yes, but not large enough to be useful.

- Stuart -

[DanMilo]

yes, i see. Perhaps its to small to influence a mode. I thought it could have some effects on dampening or moving a mode.