Although
the acoustic treatment required to optimize the sound is different
for every room, every setup, and each unique application, there
are still some basic acoustic concepts that are applied universally
in a properly treated room. In this section of our web site we will
present 5 of the more fundamental acoustic topics addressed in a
properly treated control room or other mixing environment.
"Room" acoustics
takes on a whole new meaning with ASC's trademark ATTACK
Wall™ and Quick
Sound Field™ portable, modular acoustic sub-spaces.
With these systems, you get to define the acoustic signature of your
recording or mixing space.
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 people's 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 Tube Traps 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 the mix position or mic 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 or mic setup will simply shift the locations of reflections
and alter the problem frequencies, but does not remove the problem.ASC
Sound Panels, Sound Planks, and Fractional Tube Traps 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 engineer or
artist, 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 or recorded sound
sources located between parallel surfaces are constantly sending sonic
wavefronts into the repeating loops of these flutter paths.
Speaker Flutter in the Mix Environment
Flutter in the Tracking Room (top
view)
Placing ASC Tube Traps, Sound Planks, or Sound
Panels 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 involved.
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 cacophony.
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.
