A theory question

Shrum · 8th July 2011

Hy guys!
I have recently become interested in acoustics and I have read several sources on the theme flush mounting of monitors. Among others I have come upon something interesting on genelec's site.

"In a flush mounted system where the drive units are in the plane of the wall, the standing waves in the room may be excited more than with other speaker placements. To effectively excite a standing wave one needs to have a pressure source at a standing wave pressure maximum. A standing wave has a pressure maximum for all resonance frequencies at the wall surface. An enclosed dynamic speaker behaves like a pressure source, and placed flush to the wall will excite all longitudinal standing wave modes."

I have read in several other sources, among others the book Masterclass of Acoustics that the nodes-minimum spl level of a standing wave are located on the wall surfaces, and NOT antinodes! This would mean that genelec has the whole concept wrong ( I doubt that they dont know what they are talking about but wtf?!)
Having the node (minimum spl) of a standing wave on the plane of the wall means that if a monitor is flush mounted the standing wave is being emitted from a place where it has from the start the least amount od pressure level and thus it seems logically to me that it would NOT awake room resonances even more, but it would make the standing waves effect smaller, quieter...
I am aware that flush mouting a speaker into a wall makes its bass response stronger but the upper issue is now a bit unclear to me, since I dont know why genelec would have it wrong.
Please help me become a bettter acoustics enthusiast and fill me in on what I might be missing
Thanks!

DanDan · 8th July 2011

Gotta be careful with the words. Modes are one thing. Nodes are another.
Node, afaik, comes from No Displacement of air particles.
Antinodes are Not No Displacement

At the wall the particles have to momentarily or statistically stop moving, No Displacement. Energy is never lost, just changes form. In this case it changes from movement of particles to pressure. High pressure at the boundary, high Sound Pressure Level.
Calculate the first length mode of your room. Lets say it's 34 Hz in a 5M room.
Play a 34Hz with the speaker at the wall boundary. Huge level at the two end walls, scarily silent null in the middle of the room. Move the speaker to that null. What happens now?
DD

Shrum · 9th July 2011

Dear Dan!
thank you for making something so obvious clear to me

I feel a bit stupid now, but hey, I learned something new

Anyway, thank you very much for the time taken to explain it to me :D
Cheers man!!

8th July 2011	#1
Shrum Gear nut Joined: Feb 2010 Posts: 115 Thread Starter	A theory question Hy guys! I have recently become interested in acoustics and I have read several sources on the theme flush mounting of monitors. Among others I have come upon something interesting on genelec's site. "In a flush mounted system where the drive units are in the plane of the wall, the standing waves in the room may be excited more than with other speaker placements. To effectively excite a standing wave one needs to have a pressure source at a standing wave pressure maximum. A standing wave has a pressure maximum for all resonance frequencies at the wall surface. An enclosed dynamic speaker behaves like a pressure source, and placed flush to the wall will excite all longitudinal standing wave modes." I have read in several other sources, among others the book Masterclass of Acoustics that the nodes-minimum spl level of a standing wave are located on the wall surfaces, and NOT antinodes! This would mean that genelec has the whole concept wrong ( I doubt that they dont know what they are talking about but wtf?!) Having the node (minimum spl) of a standing wave on the plane of the wall means that if a monitor is flush mounted the standing wave is being emitted from a place where it has from the start the least amount od pressure level and thus it seems logically to me that it would NOT awake room resonances even more, but it would make the standing waves effect smaller, quieter... I am aware that flush mouting a speaker into a wall makes its bass response stronger but the upper issue is now a bit unclear to me, since I dont know why genelec would have it wrong. Please help me become a bettter acoustics enthusiast and fill me in on what I might be missing Thanks!

8th July 2011	#2
DanDan Lives for gear Joined: Aug 2003 Location: Cork Ireland Posts: 6,825 1 Review written	Mixed Gotta be careful with the words. Modes are one thing. Nodes are another. Node, afaik, comes from No Displacement of air particles. Antinodes are Not No Displacement At the wall the particles have to momentarily or statistically stop moving, No Displacement. Energy is never lost, just changes form. In this case it changes from movement of particles to pressure. High pressure at the boundary, high Sound Pressure Level. Calculate the first length mode of your room. Lets say it's 34 Hz in a 5M room. Play a 34Hz with the speaker at the wall boundary. Huge level at the two end walls, scarily silent null in the middle of the room. Move the speaker to that null. What happens now? DD __________________ www.irishacoustics.com www.soundsound.ie

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9th July 2011	#3
Shrum Gear nut Joined: Feb 2010 Posts: 115 Thread Starter	Dear Dan! thank you for making something so obvious clear to me I feel a bit stupid now, but hey, I learned something new Anyway, thank you very much for the time taken to explain it to me :D Cheers man!!