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Old 11-24-2008, 04:20 PM   #18
Human being with feelings
Join Date: Aug 2006
Posts: 2,012

Causes and effects part II

To bring this back to sound, the problem with low frequencies is that the waveforms are much longer in terms of their cycle through positive and negative pressure. The low E on a bass guitar takes something like 33 feet to resolve in open air. If you have room dimensions less than 33 feet, especially if they are less than half of 33 feet, then that E note is not simply going to diffuse and dissolve into random room reverberation, but is instead going to settle into a fixed but haphazard path through the room, creating standing positive and negative pressure spots that are going to cause comb-filtering effects through the entire frequency spectrum. And it's not just one "snake" going through the room, because the bass amp is emitting sound pressure in all directions, and is playing other notes and harmonics of those notes that each finding their own patterns to settle into.

The net result is that you have have all sorts of effectively randomized acoustical peaks and valleys all over the room, and frequency response is different in every square foot, maybe even every inch. You're hearing a sharp ringing at 2kHz and overly-heavy bass while the guy right next to you is hearing scooped mids and papery highs. And the double cruelty is that the remarkable design of human hearing is such that your real perception is compensating for this, making the effect very hard for you to detect unless you are a practiced listener, but the mic is picking up all this ugliness just fine. So it perpetually sounds like your recordings just suck, even though it sounds groovy to you in the room.

Clap your hands and listen carefully to the decay. If you are in a typical, untreated, residential space, chances are the decay will not be the even, full-spectrum reverb we expect from a burst of white noise, but rather a ringing or boinging sound. Maybe even one that changes in pitch or "flutters." Walk a few feet away and try it in a different part of the room and it will be a different sound or pitch. What you are hearing is standing waves. Instead of sounding like the decay of a gunshot, it sounds like some weird ethnic percussion instrument.

The effect might sound subtle in the room, but it will not be subtle at all when you record it and play it back. Your ears compensate for the acoustical space that you are in, but they do not compensate for the playback of recorded material very well. We walk around and never "hear" reverb unless we are in a parking garage or a stairwell or some other unusual space, but it is present everywhere. If someone were to lead you blindfolded through your house, you'd be able to tell what room you were in just from the quality of the silence and the sound of your breathing, even though you might never be aware of how the different rooms sound in everyday life. However, if you were to take a tape recorder and walk around your house talking to yourself, the differences in sound quality would jump right out of the speakers on playback.

It is very important to understand that these are not problems that can realistically be corrected with EQ. The frequency problems can be extremely narrow, and are time-, frequency-, and phase-dependent. These problems can only be properly addressed in the room.

The problem is a big and complicated one, but fortunately, the solutions can be quite simple. Read on.
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