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Old 05-01-2008, 03:04 PM   #6
welwynnick welwynnick is offline
Senior Member
Sep 2007

Magnificent effort to explain in plain English one of the most common stumbling blocks to understanding audio.

For completeness I'd like to comment on this though.
In general, at most frequencies, the decay of sound waves is rapid, but when a sound’s wavelength is precisely twice the size of a room dimension (e.g., length), the waves from both directions reinforce each other at the wall boundaries and cancel each other in the midpoint of these two boundaries, creating a resonant condition. Like most other resonant conditions, standing waves produce a fundamental tone (the lowest-frequency resonance the space will support) and a series of harmonics. If the fundamental frequency is 25Hz, there will be other, progressively weaker ones at 50Hz, 75Hz, 100Hz, 125Hz and so on. Each of these harmonics causes a high energy peak points in the room, with a null (low energy) midway between each adjacent pair of peak points.
As you say, sound waves are longitudinal travelling waves, where the wave perturbation is a small displacement in the direction of travel (unlike water waves or light waves for example).

Standing waves are caused by reflections from a discontinuity in the medium. A wall is a discontinuity, simply because it’s solid and stationary, and doesn’t allow the air at the boundary to move in the direction it wants to go - in a perpendicular direction to the wall.
There are other types of discontinuity, but this one is a node because there can be no amplitude at the boundary (because the wall doesn’t move, so neither can the air right next to it). Simple, huh?

By contrast, the middle of the room may be an anti-node, because there is nothing stopping the standing wave from having its maximum displacement there. Equally, for higher order standing waves it MAY still be a node, but that just depends on the geometry of the standing waves – it can be either or anything in between.

However, the wall is always a boundary that forces the standing wave to a node. In the first resonant mode, the room will be a half-wavelength across, with nodes at the walls and an anti-node in the centre, where the amplitude is highest. In this situation, all the air in the room is rushing up and down the room altogether – all in phase – all the air molecules start and stop together and move in the same direction. The mass of the air is simply bouncing against it’s own compliance. Of course, there will usually be other resonant modes in other directions as well, but that’s basically what happens.

BR, Nick
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