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Be an "Ace of Bass" - Part 2

Wed 15 December 2010 | Technical articles | Back to article list

Last week we discovered how waves have amplitude (size) a period (length of a cycle in time), frequency (number of cycles per second) and wavelength (physical size of a cycle in meters). This week we’ll talk about what happens when more than one wave arrives at our ears at once and, more specifically, what happens when walls reflect bass waves in the listening room.
 

A quick thought about the “listening room”…

To describe how reflected waves effect the sound in a room we’re going to assume a “perfect” room that reflects sound with 100% efficiency. By this we mean that the walls absorb none of the sounds energy. In practice this just never happens but to understand the principles of what happens this is what we’ll assume for now.

And now for those reflections…

When a bass speaker puts sound into a room it will propagate away in all directions. This is a little different from higher frequencies that tend to only propagate in one direction. Because of this bass sound will hit the boundaries of the room including the walls, ceiling and floor as well as going toward the listener. Thinking about a subwoofer placed on a floor the listener will now hear both the direct sound from the sub woofer and a reflected sound, which has hit the floor on its way to the listener.

Now it would simple to assume that these two sounds add up to make the sub woofer appear louder than it actually is. In fact that’s true but there’s one more factor we need to consider that makes this heppen for bass – phase.

Phase is the relationship between two different waves, it tells us how close or far apart they are in terms of their position. If the two peaks and troughs happen at the same time they’re said to be “in phase”. If not they are “out of phase”. We measure phase in degrees, a full wavelength being 360 degrees, just like the angles in a full circle.

Usefully, waves that are more or less “in phase” add up to make a single wave of higher amplitude (conversely two waves that are out of phase subtract and get smaller). Since bass waves have a long wavelength the reflected sound appears to be “in phase” with the direct sound (the difference in the distance they travel is small compared to wavelength) and they add up. So, we get extra volume for free! This doesn't stop with the floor either, subwoofers placed close to side walls add further "in phase" reflections and thus aparant volume.

That’s it for this week. We hope you enjoyed this article. If you did, why not let our webmaster know? Next week we’ll take a look at how to set crossover frequency.



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