BOUNDARY CANCELLATION
How to eliminate low frequency noise cancellation with
proper woofer placement
By Marty McCann
Copyright 2001
Many local bands and sound companies are unknowingly victims
of a type of low frequency cancellation known as boundary cancellation.
This phenomenon occurs when a loudspeaker is 1/4 of a wavelength
away from a boundary. Assuming that the boundary (like most
walls or floors) absorbs little or no energy, the reflection
off of the boundary plays against the energy still coming from
the speaker source.
Here is a thought experiment that is a good analogy. Imagine
a subwoofer that is placed 1/2 of a wavelength, at certain
frequency, away from another sub, and the second sub has had
its polarity reversed. It is easy to imagine that the loudspeakers
will be opposing each other because one will be moving inward
while the other moves outward, causing mutual cancellation.
A similar thing occurs when a loudspeaker is placed 1/4 of
a wavelength away from a boundary. It is as if the boundary
is not there, but there is a phantom speaker 1/2 of a wavelength
away that has had its polarity inverted.
Now lets investigate where and why many people unknowingly
have a problem at a very critical frequency. The kick drum
frequency can be a fundamental problem for many people as they
have a misconception as to where this frequency actually is.
I will not say that the fundamental frequency is exactly at
80 Hz, but usually the 80 Hz, 1/3 octave equalizer control,
will bring more of the kicks thump out than say 50 Hz, 63 Hz
or 100 Hz. Usually, this fundamental is somewhere between about
72 and 80 Hz.
It is not always easy to get a great sound from a kick drum;
however, most people reproduce the characteristic kick drum
sound established in an LA studio by New York drummer, Steve
Gadd. You can hear similar kick drum tunings in most genres
of music today, and the reason is very simple. It fits, and
it does not step on the vocalist or rhythm instruments. The
sound consists of the fundamental frequency and the second
harmonic that is one octave above of course, but subsequent
harmonics are suppressed. This is what the proper use of the
pillow in the kick drum results in. (If you would recall the
early sound of John Bonham´s kick drum sound in Led Zeppelin.
It rang quite a bit, and just wasn't tight by today's standards.)
We used to have a product called the Electric Pillow, which
was a kick drum conditioning filter, that employed a special
filter attenuating the region where most of the ring came from.
This filter can be simulated via an equalizer. If you would
like to dry up and tighten a kick drum in a mix, insert an
equalizer into the kick channel, roll it off below 40 Hz; boost
it +4 to +6 dB at 80 Hz; and leave 100, 125, and 160 Hz flat
(0 dB). Set 200 Hz to -3 dB, 250 Hz to 6 dB. Cut 315 Hz through
500 Hz completely (-12 dB). Set 800 Hz to 6 dB, and leave 1
kHz through 5 kHz flat. Then cut 6.3 kHz to 4 dB, 8 kHz to
8 dB, and cut 10 kHz, 12.5 kHz and 16 kHz all the way (-12
dB). This equalization curve can help when wanting more punch
out of your kick drum in a mix, but other problems may arise
when you choose where to place the loudspeaker.
Here is an example of conditions where the kick drum can be
diminished due to boundary cancellation. Lets assume that a
typical stage is 3 1/2 ft. high (42 in. or 1.07 m), and the
subs are placed on the corners of the stage. If we divide the
speed of sound by a given frequency, we get the wavelength
or physical size of that wave form in air. In most warm rooms,
the speed of sound is 1130 ft. per second (344.5 m).
80 / 1130 = 14.125 ft. (4.3 m)
One-fourth of this dimension is 3.53 ft or 42.4 in. (1.08 m)
Guess what? This height stage is canceling half the energy
of the kick drum frequency. This is very common yet unrecognized
by most. If you move the sub closer to the floor or boundary,
you raise the frequency of cancellation. And of course if you
move it further away from the boundary, you lower the frequency
of cancellation. No one is really the wiser if the note of
cancellation is in some key in the bass players domain because
he doesn't play the same note over and over. However, in the
case of the kick drum you decide.
A Worst Case Scenario of Boundary Cancellation
In the early 70s, in a club in McKeesport, Pennsylvania, I
was mixing for a band that played this room for the first time.
The stage was 3 1/2 feet from the corner, and it was also about
3 1/2 feet above the floor, and yep, only about 3 1/2 feet
deep. We lost a woofer on the left-hand side, halfway through
the first set. Trying to compensate on the EQ, I only succeeded
in trashing the woofer on the right side before the set was
finished.
We decided to put two woofers together in the center of the
dance floor and plug them in. Back then I had heard about center
floor coupled woofers before but never tried them myself.
When the band began the next set, I had an incredible mount
of kick drum with the current EQ setting. In fact, I had to
turn it DOWN and change the EQ. Below is the math.
Boundary cancellation was occurring in five places, so here
are the losses:
Left Woofer
-3 dB off of floor
-3 dB off of side wall
-3 dB off of rear wall |
Right Woofer
-3 dB off of floor
-3 dB off of rear wall |
Total loss due to boundary cancellation = -15 dB
By placing the two subs on the floor, we gain +3 dB. Because
they were placed together and mutually coupled, and were thus
coincident, we gain an additional +6 dB of performance for
a total of +9 dB. The difference between -15 dB and + 9 dB
is a 24 dB margin. Now you can see why the kick drum was so
pronounced when we made the change.
For some reason, this woofer placement became a default arrangement
by most bands that played this room. No one knew why it worked
so well at the time, but the thump on the dance floor was incredible
when we did it this way. (Alas though, one night the club mysteriously
caught fire in several places at once and burned to the ground).
Several years later when I attended a Don Davis Syn-Aud-Com
seminar, Don explained the math involved with boundary cancellation.
Many common stages unfortunately present this case of boundary
cancellation at the kick drum frequency. Should the woofers
therefore go on the ground? It depends on the crossover frequency.
If crossed over low enough (<125 Hz) the woofers can be centered
on the floor or ground in front of the stage.
In a permanent installation, the sub can be placed under the
stage, however, when placed under the stage, care should be
taken to insure that any gap is sealed. It is also a good idea
to put absorption material on the top, sides and back of the
enclosure. All of this is necessary to prevent the subwoofer
from driving the stage itself. If this is not addressed, the
cavity under the stage becomes a massive Helmholtz resonator
(a type of acoustical resonator with an enclosed volume of
air connected to the acoustical environment by a much smaller
opening). With front-loaded, direct radiating woofers, the
loudspeakers can be serviced or replaced without removing the
enclosure.
Many three-way systems that are crossed over between 200 and
300 Hz will have a problem with an octave or more of information
being muddied due to some blockage by the audience if the subs
are placed on the floor. Using four-way systems with separate
center stage subs running 100 Hz and below, so that they can
be placed on the ground, works better than the three-way systems.
The best crossover frequency for a separated woofer is below
125 Hz. The audience can NOT block the low frequencies below
125 Hz. The wavelengths are simply too large.
I hope this article will help you understand the acoustic principles
involved and perhaps when you apply this information, you will
get an even better sound from your performances. |
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