Why do room modes become insignificant above bass frequencies?

[Kerr Forrest]

Hi,

I have been researching the physics behind room acoustics as part of a university project to design a speaker, and I keep noticing that in both room mode simulations and waterfall graphs only the 20Hz - 200Hz range is considered, suggesting that the influence on SPL of room modes becomes irrelevant above 200Hz.

Can anyone explain to me why this is? Considering that constructive/destructive interference should be possible when a room dimension is any integer of a half or quarter wavelength (respectively) of a certain frequency, I would assume that room mode effects would be noticed across the full audible spectrum.

My only guess would be that as frequency increases, the waves become more directional, making self-interference more difficult and hence room modes less of a problem.

If anyone is aware of why room mode effects are only looked at in the low frequency I would really appreciate it if you could explain it to me.

Cheers,
Kerr.

 

[DanDan]

Modal density is very low at LF. This increases dramatically as frequency rises. Quite soon there are so many modes and reflections that there are no gaps between them. Almost diffuse. Reverb or Room Tone.

 

[Kerr Forrest]

Modal density is very low at LF. This increases dramatically as frequency rises. Quite soon there are so many modes and reflections that there are no gaps between them. Almost diffuse. Reverb or Room Tone.

Thanks very much!

 

[DanDan]

You are welcome. https://amcoustics.com/tools/amroc?l=700&w=400&h=300&r60=0.6

 

[TheMaddScientist]

Do be aware of room resonances, though. Most rooms I have measured have resonance peaks in the single digit kilohertz range, strongly dependent upon room geometry and where you are in the room. I found out the hard way when trying to record my violin. Cheap violins have resonance peaks in that range, as well, while good ones have a smooth frequency response. So a bad room will make a good violin sound like a whiny mess. Fortunately, it is fairly easy to dampen those frequencies.

 

[JLM1948]

Modal density is very low at LF. This increases dramatically as frequency rises. Quite soon there are so many modes and reflections that there are no gaps between them. Almost diffuse. Reverb or Room Tone.

Not only that. At higher frequencies, boundaries are generally more absorbent, thus reducing the effect of wave combination.

 

[JLM1948]

Do be aware of room resonances, though. Most rooms I have measured have resonance peaks in the single digit kilohertz range, strongly dependent upon room geometry and where you are in the room.

That would b ein a room that has not been treated acoustically. Indeed, windows and mirrors are pretty good at reflecting higher frequencies, particularly if their dimenions produce diffraction in the same range.

 

[w_sizer]

There’s a good discussion of this topic in The Acoustics and Psychoacoustics of Loudspeakers and Rooms, 3rd ed., by Dr. Floyd Toole. This is an excellent reference and a must for everyone’s A/V library.