Sound is conveyed through waves
in the air. Waves that exist between a pair of surfaces can
create standing wave resonances whenever the distance between
the surfaces is any even multiple of one-half of the wavelength.
At resonant frequencies (tones), the sound is louder and decays
much more slowly than at non-resonant frequencies, causing
uneven tonal quality and interference with clarity. Resonant
frequencies occur mainly in the bass range, due to the relationship
between the wavelengths of low-frequency sounds and the typical
sizes of listening rooms.
This wave is in a standing
wave resonance since it's wavelength equals the
distance between the pair of surfaces.
Every room has its associated resonant frequencies.
Rooms built using preferred dimensions ratios have potentially
more even distributions of these resonant frequencies. Room
built with angles walls or ceilings have more complicated
resonant modes than typical rectangular rooms and the resonances
can be potentially less severe. But, no matter what the size
or shape of the room, resonant frequencies can be controlled
through the use of bass traps.
"Bass" frequencies occupy all the
notes on the left half of the keyboard (Everything below middle
C). Since this is such a large portion of the musical spectrum,
and because every room has potential resonant frequency problems
in this bass range, it is imperative that the low frequencies
be the first issue to address in improving any room's acoustics.
Of course, each specific room's geometry, setup, and application
dictate how to best optimize the bass performance. However,
there are some general enhancements that can be made using
ASC TubeTraps that are sure to offer improvement
in any room.
Sound and music propagate
through waves and, therefore, must abide by the laws of wave
physics. This means that when 2 waves "collide",
they do not bounce off one another as is the case with physical
objects. Instead, at that location in space and moment in
time, they either add their combined amplitudes to some degree
or cancel their combined amplitudes to some degree.
Waves exactly in phase add to make a
wave with twice the amplitude.
Waves exactly out of phase add to make
a wave of zero amplitude.
Waves out of phase to a small degree
add to make a wave with
slightly higher amplitude than either wave individually.
The wavelength of the 2 sound waves and the
difference in the distances they have traveled determine whether
they add to or subtract from the combined resulting amplitude.
This means that there are a series of adds and cancels at
various frequencies of sound for any given room setup.
There are many potential reflection points
that can cause a sound launched from a source to return to
that source and interfere with itself. There are also many
potential ways for sounds to travel from one source to another
and cause interference. Likewise, there are many ways for
sounds launched from single or multiple sources to arrive
at a central listening position at different times and interfere
with one another there. All of these interfering waves cause
the resulting amplitude of the sound to either increase or
decrease to some degree depending upon the frequency (tone)
of the wave. The resulting adjustment to the amplitudes at
each frequency is called a comb filter.
Comb filtering effects are reduced by placing
acoustically absorptive materials at the reflection points
responsible for the interfering waves. The materials must
be of a size and type to properly address the frequencies
of each specific problem. Rearranging the speaker setup will
simply shift the locations of reflections and alter the problem
frequencies, but does not remove the problem.
ASC SoundPanels, SoundPlanks, and
Fractional TubeTraps are often used to control comb
filter reflections, with the appropriate device chosen based
upon the frequency of the problem. Although locating the precise
positions of problem reflections can be a complex task, there
are a few locations where controlling the reflected wave is
sure to make an improvement to the sound.
There are certain paths for sound
that produce a repeating loop. Every time the wavefront passes
the listener it is heard as the sound is intended, but with
a twist. Just as when you "click" the individual
prongs on a comb in quick succession, the quickly repeating
sound of the wavefront continuously passing the listener produces
a distinctive "zinging" tone. This is known as flutter
echo and is due to our brain's desire to interpret air pressure
fluctuations at some frequency as a particular tone. For this
is exactly what is occurring as the wavefront continuously
passes your ear at some rapid rate.
The flutter paths are most commonly located
along lines between parallel surfaces. Speakers located between
parallel surfaces are constantly sending sonic wavefronts
into the repeating loops of these flutter paths.
Placing ASC TubeTraps, SoundPlanks,
or SoundPanels at the reflection points for these
flutter paths breaks up the flutter. This removes the tonal
discoloration caused by the "zinging" sounds our
brain interprets from the repeating wavefronts it encounters.
As seen in sections 2 and 3,
controlling room reflections is fundamental to creating accurate
sound reproduction in any room. In addition to utilizing precisely
selected panels addressing comb filter and flutter problems,
it is also generally desired to include the proper combination
of absorption and diffusion to control sounds reflected throughout
the room. The desired balance of absorption and diffusion
is obtained through selection of appropriate absorptive material
and proper placement to create diffractive diffusion and/or
multiple time-delayed specular diffusion.
Edge-effect diffractive diffusion
Multiple time-delayed specular diffusion
The proper placement and selection
of panels to attain the desired reflection control is determined
on a case-by-case basis due to the large number of variables
Sound produced within any enclosed
space will continue to exist in that space for some amount
of time after it is created, decaying away until it is inaudible.
If this decay time, known as the room's reverberation time,
is too long, sounds will linger within the space and begin
to overlap with new sounds being made, creating an unintelligible
Long reverberation time = Poor Intelligibility
Short reverberation time = Good Intelligibility
A sufficient amount of acoustic absorption
is required at all audible frequencies of sound in order to
keep the reverberation time in a room short enough to have
good intelligibility. The measurement of the reverberation
time in a room is often referred to as RT60. The desired RT60
at an frequency varies from room to room. All ASC acoustic
treatments alter the RT60 of a room to some degree. Acoustic
treatment is developed with desired RT60 levels in mind.