Room Acoustics 101
Room dimensions with non-equal or divisible dimensions are best. Vaulted ceilings, non-parallel walls and irregular surfaces help reduce slap echoes, but have little effect on low frequency standing waves. Room construction affects bass reinforcement, the noise floor, and adjacent room noise. The average drywall wall resonates around 70Hz. Doors rattle, windows sing, air vents whoosh. Just grab your test CD or tone generator, play a sweep tone and listen. The difference in sound you will hear during the sweep is almost entirely due to room coloration.
Let's begin by discussing how the length of a wave relates to its frequency. This understanding will allow you to take a methodical approach to understanding room response problems. Sound nominally travels at about 1130' per second. The human ear can typically detect frequencies from 20 vibrations per second (Hertz)to roughly 20,000 vibrations per second. We can calculate the wavelength ("l") of any frequency by simply dividing 1130 ("v" or velocity) by the frequency ("f"), using the formula l = v/f.
Frequency (f) Hz. 20 50 100 150 200 250 500 750 1,000 5,000 10,000 15,000 20,000 Wavelength (l) Ft./In. Rounded 56' 5" 22' 6" 11' 3" 7' 5" 5' 7" 4' 5" 2' 3" 1' 5" 1' 1" 2" 1" .08" .06"
From the previous table we can see, for example, that trying to dampen a 100Hz bass wave that is 11'3" long with a pillow 12" x 12" x 1.5" stuck into a corner is futile.
We can also use the formula to determine the fundamental standing wave frequency for a given room dimension by dividing the round trip of that dimension by 1130 (v). That is, if our room is 20' long, the round trip distance is 40'. Divide 1130 (v) by 40'(f) for a fundamental standing frequency of 28.3Hz (l). Let's go one step further with our formula and newfound wavelength knowledge and see how we can apply it to understanding the problems of room acoustics.
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