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.
Continue >>>
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