Sound is acoustic energy and rooms store this energy. Resonance
is nature's most efficient way to store acoustic energy in a room.
Resonant energy easily lasts two times longer than sounds that are
not resonant, and this is how the coloration of sound occurs in
Originally written and published in dB Magazine, November/December
1991, and January/February 1992. Reprinted with permission of db
Magazine, Commack, NY.
An all-concrete reverberation chamber can store sound
for at least ten seconds, an empty gymnasium is good for five seconds,
and an empty room in a house has a decay time of two seconds. In
pro or semi-pro audio rooms, a decay time of no more than 1/2 second
is preferred. The typical furnished but untreated residential-type
room has decay times of 1 ¼ seconds. So, serious audio rooms
need serious acoustic treatment. Midrange and high frequency sound
is easily absorbed, but the lows are problematic. Sound absorbers
that handle the lower octaves are called bass traps.
Room Resonance Almost everyone can read about “room acoustics,”
which actually discusses the midrange and high frequency, the upper
three octaves of the keyboard. Now, the domain of low frequency
acoustics in small rooms is to be explored. This article will provide
an overview of the theory, history and practice of bass trapping
with an eye towards home and project studios.
Without proper decay times, mic work or listening
in an audio room is hampered by excessive reverberation. Resonances
color the acoustic signature because they are a group of specific
tones that overhang longer than the others. Excessively sustained
overtones cover over, blur and mask out the low level musical inner
detail. The control of decay times in the audio room means controlling
the resonances, and giving the room a neutral voice.
Resonant frequencies are not always the same; they
will vary depending on speaker position. With a walking, talking
person, the position of the sound source changes, stimulating different
resonances. The loudspeaker however, is fixed in position. It stimulates
the same group of resonances over and over again, The coloration
is fixed; it penetrates and stains all recorded and playback material.
Instead of capturing the “infinity” of musical variations
that create evanescent luster in audio recordings, resonance forces
a redundant tonal emphasis which renders music essentially boring–no
matter how much talent is applied.
Electronic upgrades in the studio should develop enhanced
performance. The need for any improvement springs from some dissatisfaction
with the present system. The room acoustic is the first and last
link in the audio chain. It is staggering to consider how many pieces
of electronic gear have been purchased out of frustration with a
system whose real problem was not electronic at all, but was driven
by the colorations due to room resonance.
There are only two ways to get residual low frequency
sound energy out of a room. The first and most common is leakage.
Unlike the downtown recording studio, deep bass leaks out of most
home and apartment construction. Leakage paths can be direct transmissions
through the walls, ceiling, floor, doors and windows. The heavier
the surface, the less leaky it becomes. Other leakage paths are
through openings such as under the door.
Absorption is the second method by which acoustic
energy is removed from a room. Downtown recording studios are heavy-walled
and sealed airtight to keep unwanted sound out. This is called isolation.
If sound is kept out, it is also kept in, and so studio builders
have developed a variety of low frequency sound absorbing techniques.
Hopefully, most of these will be reviewed in this article. The designer/contractor-built
studios usually have bass traps built in. The rapid expansion of
MIDI equipment has resulted in many serious home-based project studios
that are virtually without acoustic control.
The single most important result in a properly bass-trapped
room is that it has more bass, deeper punch and smoother extension.
This sounds contradictory–that bass trapping a room gives
more and not less bass. Actually, what you get is the bass you always
had; you just could not hear it because the resonant colorations
Once the basic concepts of room resonance and bass
traps are developed, the practical matter of setting up a room needs
to be discussed. This is broken into two sections. Trapping the
front or driven end of the room requires special considerations
because of its proximity to the loudspeakers. The back of the room
is more intuitively obvious and belongs to the world of deep bass
Virtually every downtown recording studio uses some
type of bass trap to control distortion and coloration of the frequency
response in the room due to low-end build-up. Bass traps in these
studios can be found hidden above the ceiling, inside the walls,
below the floor and sometimes even in adjacent rooms. The nagging
problem for home and project studios is that most engineers cannot
consider contractor renovations as an option for an acoustic upgrade
of their living rooms.
ROOM ACOUSTIC BASICS Before considering bass traps in detail, a review of acoustics
is in order. This will develop a sense of perspective and scale.
The behavior of sound waves and objects depends on the size of the
wavelength, in comparison to the size of the object. Simply put,
long wavelengths go around small things and small wavelengths get
reflected by big things.
wavelength of a sound is mathematically related to its frequency
or tone. The higher the frequency, the shorter the wavelength. Our
range of hearing officially spans ten octaves from 20 Hz to 20 kHz
and we can perceive or feel sound even below 20 Hz. (1 kHz = 1,000
Hz of cycles per second.) An octave is the doubling of frequency:
20 Hz, 40 Hz, 80 Hz, and so on. For audio playback in small rooms,
bass is considered to be the first four octaves (20 Hz to 320 Hz);
mids comprise the next two (640 Hz to 5.12 kHz); and the highs occupy
the last four octaves. Sounds of the piano keyboard are familiar
to most of us; middle-C is a frequency of 256 Hz. The bass range
on a piano occupies more than half of the piano keyboard, and about
forty percent of the full auditory spectrum.
Bass wavelengths are similar in size to the room in
which they exist. It’s easy to calculate the size of a wavelength
from the formula: wavelength n_\ = speed of sound (c) /frequency
(f). By comparing sound wavelengths to the size of a house, the
size of bass wavelengths are evident.
shortest “bass” note–A440–has a wavelength
of about 2.5 feet. The longest wavelength is 56 feet, and it belongs
to 20 Hz.
Full range speakers generally produce sound extending
down through most of the lower end of the piano keyboard. Subwoofers
produce sound specifically in the last octave of the piano’s
keyboard and the one just below it, the first audible octave.
Speakers possess frequency-dependent directional qualities. For
both mids and highs they produce adequate sound levels only in the
forward direction towards the listener. Lower frequencies from the
same speakers, however, radiate equally in all directions. This
directionality means that mids and highs are efficiently beamed
towards the listener, and little acoustic energy is wasted on illuminating
the rest of the room.
The lows easily require six or more times the acoustic/electric
power than the mids and highs to achieve the same sound level at
the listener’s position. Speaker efficiency is one reason
for power gulping; the other is directionality. Because bass waves
are bigger than the speaker, they travel with equal strength in
all directions. The speaker is an “omni” pattern sound
source. Often much of the bass wavefront has bounced off of the
walls, floor and ceiling of the room before it even reaches the
is an airborne ripple or wave whose speed (c) is about 1,128 ft/second.
Consider the piston of a loudspeaker that is vibrating to and fro
at 100 Hz. In the exact amount of time it takes for the speaker
cone to make one cycle, or complete a round trip (1/100 second),
the sound wavefront it generated will have moved away from the speaker
(1/100 x 1128) some 11.28 feet. For a continuous tone, this becomes
a repeating event. As you move away from the speaker, every 11.28
feet would be the same acoustic condition.
THE BREATHING MODE This review of small-room acoustics begins with the lowest
octave. Here, the wavelength is quite long as compared to the size
of the playback room. The room as a whole experiences internal pressure
changes. Acoustic activity in this region below the room’s
so-called “cut-off frequency” remains quite audible.
Here the speaker is acting on the room as if it were a pneumatic
plunger, alternating between pressurizing it and pulling a partial
vacuum on it. The walls, floor and ceiling react to what seems to
be a rapidly changing “barometric” pressure in the room.
Room surfaces billow out and then cave in with each cycle.
structural resonances are easily stimulated by breathing mode acoustics,
a common problem in playback for the larger power systems of today.
The surfaces of the room simply shudder in the bottom end as the
speakers stimulate, then overpower the mechanical stability of the
room. The result at high sound levels is a total loss of control
for low-frequency musical reproduction, as if sound in the room
“crumbles” when it is overloaded. This LF breakup of
the room itself is particularly evident in the concussive punch
bass beat attack transient.
ROOM MODES As the tone from the speaker is raised in pitch, out of
the deep bass octave and into the piano’s first bass octave
(40-80 Hz), a new class of room acoustics develops, called Room
Resonant Modes. The lowest frequency at which this can occur is
called the long dimension axial (1,0,0) mode.
fundamental room resonance is easily stimulated when the speaker
is located at one end of the room and the wavelength of the tone
played happens to be twice as long as the room. The wave from the
speaker travels down the room only to bounce off the rear wall and
return to the front of the room. During this time the speaker makes
one full cycle of motion itself. It generates a tone exactly in
step (or in phase) with its reflection. These two waves–the
old reflected wave and the new one–add together exactly, without
confusion. After a number of cycles the sound levels build, enveloping
the room in resonance.
For a non-resonant tone, sound builds up in the room
in highly disorganized manner. With resonance, however, the air
is stimulated into a “sloshing” mode of behavior, not
too unlike what can happen with a child in the bathtub if their
to and fro movement happens to keep time with the water’s
natural end-to-end slosh motion, called first harmonic.
RESONANCE It is interesting to explore acoustic resonance with a
SPL meter. Such a meter is very useful, can be found at stores like
Radio Shack, and cost as little as $30.00. You can also use a mic
patched into your board, keeping an eye on the VU meter. Sound meters
measure the strength of “sound pressure changes.” If
the SPL meter reads 90 dB, that means the air pressure at the microphone
is fluctuating strongly above and below ambient air pressure with
a strength of 90 dB. Compare this to a 60 dB reading and notice
that the fluctuations in pressure are much smaller and the sound
By the way, dB, A is not a flat response curve. It
is rolled off gradually below 1 k as our own hearing response does.
The dB, C scale is “flat” for most purposes. A mic,
patched through without equalization will be close to dB, C levels,
not dB, A levels. The dB, C or flat response weighting is best for
room acoustic measurements and the mic should be an omni mic.
the mic or SPL meter is moved from one end to the other end of a
room that is in the fundamental mode of resonance, data points can
be taken and plotted against position. High SPLs are detected at
both ends of the room, and a low SPL in the middle. These are known
in audio as “hot” and “cold” spots; the
“hot spot” is where pressure changes strongly occur
and the “cold spot” is a location where pressure only
Just because we don’t hear sound in the cold
spot doesn’t mean the acoustic energy is gone. The sound may
be “cancelled,” but the kinetic part of acoustic energy
is in full presence. Although we can’t hear acoustic kinetic
energy, a ribbon mic properly oriented can pick it up. Note that
the same ribbon mic in a pressure zone will not register any sound.
This is because ribbon mics pick up the air motion of sound while
condenser mics pick up the air pressure of sound. For a ribbon mic
to pick up the acoustic kinetic energy, it must be aligned per indicator
to the direction of air motion.
rotated ninety degrees so the plane of the ribbon is aligned with
the direction of the acoustic kinetic energy motion, the mic will
not give a reading.
The frequency of the lowest room resonance (1,0,0)
is easy to calculate from f100 = C/2L. Measure the length (L) of
your room and use the equation to calculate the room’s fundamental
resonant frequency. The graph of the equation is also useful to
TO RESONANCE The size, shape and internal details of a simple room will
affect its resonance frequencies. By using a f100 tone burst, lasting
about one second, as a test signal and feeding it to a speaker,
we can watch the SPL meter to illustrate overall frequency response
of the room. By listening first to the burst over headphones and
then again while using the room as an acoustic coupler, a very clear
audition of room acoustic resonance effects can be heard.
This kind of test, called a MTF (Modulation Transfer
Function) test, is the basis for checking the quality of any communications
channel. The Studio Reference Disk by Prosonus (list $69.95) has
this test on track 50. MTF testing is the more full bandwidth, musical
cousin to speech intelligibility tests that sound contractors are
wrestling with these days.
The “Hot” f100 location to illustrate
the presence of excessive reverberation is at the back wall of the
room. Here one hears the slow “turn on,” excessively
high sound levels, and a sluggish “turn off” response
characteristic. The sound of the tone burst sound is not sharp,
but “blooms” and “fades.” This can be characterized
as the difference between the test “boop” sound and
the “moo” sound delivered to the listening position.
The fact that a distinct, sharp signal is not really heard is clear
evidence that it is the room we are listening to and not, as we
usually presume, the speaker!
we hear, in fact, is the gradual build-up of energy in the room
as the speaker begins to move or slosh the air in the room. With
each cycle of continuous tone, the sound level continues to build,
but only until the power being pumped into the room by the speaker
exactly equals that being lost and dissipated by friction and leakage.
Only then can a steady-state sound level be reached.
When the speaker quits vibrating, the sound does not
just simply stop. There is built-up and stored acoustic energy in
the room which requires time to damp out. Acoustic friction reduces
the energy of sound in the room, as does the leaking of sound out
through windows, doors and the walls. It’s the leaking part
that neighbors will comment on.
THE COLD SPOT When sitting about the middle of the room at the “cold
spot” while the first resonance is set up, the very curious
effect of “sound cancelling” occurs. Here, the sound
from the speaker is exactly out of phase with that of the room resonance
at that location. Sound pressure may be cancelled, but nature does
not give up so easily; acoustic energy is not cancelled. If sound
(acoustic pressure) is “cancelled” in one part of the
room, it has only been replaced with acoustic kinetic. Conversely,
sound pressure will be found substantially louder elsewhere in the
room at locations that have been stripped of acoustic kinetic. Acoustic
energy is an interplay of acoustic pressure and acoustic kinetic.
Ocean waves have a similar action–the water wave has height
(pressure) and motion (kinetic) energy.
When we audition the one second tone burst here, we
first hear clearly the initial sound from the speaker. But it becomes
quieted as the buildup of the resonance in the room reaches full
strength and cancels the direct sound at the listening position.
When the speaker is turned off, suddenly we hear the sound of the
reverberant field as it decays. The response of the burst is not
the clean, crisp “boop” sound. It is more like a “bow-wow.”
In either case, and depending where one sits, the
in phase or out of phase room resonance/speaker coupling effects
dramatically rewrites musical dynamics and intonation. This illustrates
why the engineer can hear magic and the producer on the talent couch
still thinks it needs work–what you hear in the bottom end
depends on where you sit.
Farfield playback monitors strongly couple to the
room acoustic–that’s why they aren’t used very
much except in well-designed downtown studios. It costs a lot to
buy the monitors and a lot to fix the room to play them in. The
move has been towards nearfield monitors that give strong direct
signals and weak room resonance coupling.
The problem here is no bottom end–engineers
have to just punt into the mix below 60 Hz. The next move up is
to midfield monitors, a compromise, but still no bottom below 45
Hz. Another attempt is to add subwoofers into the system to get
the bottom end back up.
ACOUSTIC COLORATION So far, the distortion of amplitude modulation has been
shown to result from room resonance. The mic or listening position
has a tough time tracking the low frequency (LF) transients in musical
passages. The fast tracking of a room is one important aspect of
pro room acoustics. There remains another acoustic gremlin that
impacts musical accuracy: coloration. By playing a tone burst into
the room at a frequency just off a nearby resonant frequency, both
the attack and the sustain of the burst develop a “vibrato”
a beat frequency related to the difference between the applied tone
and the nearby resonant frequency.
For example, if a 45 Hz note is played into a room
with a resonance mode at 42 Hz, there would be a beating effect
in the attack and sustain of a vibrato at the difference frequency
of 3 Hz. A further coloration problem occurs when the speaker is
shut off; the sound decays at the nearby room resonance of 42 Hz,
and not with the sound of the musical note of 45 Hz. Essentially
the note sours in decay. This effect, like the other resonance-controlled
playback defects, remain clearly audible by means of an A/B headphone
In Part Two, Mr. Noxon explores
what has been done to make bass
a welcome guest in the studio.
There seems to be a popular misconception about the
role of bass traps. The uninitiated often say, “I want to
kill my resonances with some bass traps”. When absorption
is added to any resonant circuit, be it electronic or acoustic,
only the rate of energy drain from the system is increased. It must
be stressed, that from a practical basis, absorption can never eliminate
resonance: resonance exists because the room exists. Absorption
can only reduce the strength and sharpness of the resonance, (its
“Q”) but not eliminate it.
Sound will build in intensity until there is a balance
between the power delivered into the room and the power absorbed
or leaked out of it. Increased absorption means the room reaches
its peak sound level more quickly. Why? Because the equilibrium
sound level attained in the room is lower and not because the energy
rise rate is any more abrupt. Adding absorption, however, increases
the sound decay rate in the room.
benefits are noted at the cold spot. The resonant field strength
is weaker overall due to the added bass absorption. The reverb field’s
reverse phase cancelling effect of the direct wave from the speaker
is less strong. As a result, the cold spot “warms” up
and the pulses at turn-on and off are accordingly diminished.
As to the coloration effects, added absorption reduces
the “Q” of room resonance, the sharpness of its response.
Low “Q” rooms lose attack transient and sustain distortion.
The beating effects have disappeared and the tone in the decay is
the same as that of the driven frequency.
Absorptive damping of room resonances, as we have
seen, will improve the dynamic response characteristics of the room.
It is quite clear by now that it is the room that we listen to in
the lower registers. Accordingly, the better behaved the room, the
better the track and mix will sound.
A caution needs to be noted at this point. Nearly
all recording engineers have access to an RTA, typically 1/3 octave
bands. Their experience with electronic equalization, particularly
parametric, leads to the desire to see a flat room acoustic response
curve. Good luck! It is always a surprise to realize that dynamic
transient stability in the room can be developed to satisfaction,
and yet the 1/3 octave RTA shows less than 1 dB improvement. Just
as it is impossible to fix room acoustics with an equalizer, it
is likewise impossible to read room acoustics with an equalizer
meter, the 1/3 octave RTA. The narrow band Modulation Transfer Function
(MTF) type of test is how room acoustics must be evaluated in the
TRAPS Many ingenious designs have been developed to provide low-frequency
absorption. In the beginning, no doubt a bass trap probably was
little more than “great balls of fuzz,” fiberglass insulation
or batting stacked to the ceiling in the back of the room. Such
a system was so ugly that it was covered over with “scrim
cloth.” It did, however, provide absorption for frequencies
whose wavelength is up to four times the fill depth. A 3 foot deep
fuzz trap is effective to the 12 foot wavelength, about 94 Hz.
It is instructive to calculate how deep this trap
would need to be to dampen the fundamental room mode now that digital
tape can store such low frequencies. Calculate:
24 foot room would need a bass trap about 12 feet deep. Obviously,
converting half the room into a bass trap is not an option for most
alternative to filling the back of the room with fuzz is to remove
the closet doors at the back of the room and fill them with fiberglass.
The frequency response curve of the ¼ wavelength trap system
shows strong absorption on the first, third and fifth harmonics,
because the air friction occurs at the position of “sound
cancellation” or maximum air motion, typically ¼ wavelength
and ¾ wavelength from the trap’s wall.
BASS TRAPS The basic mechanism for sound absorption is the friction
of air as it moves across a surface. The more surface and the more
air motion, the better the absorption. But large scale bass traps
are physically unacceptable in the smaller home recording studio.
Another problem with giant absorption is that it makes for an uncomfortable
and distracting listening environment, because it is anechoic or
too dead sounding.
Consequently, wooden slats are added to most traps,
somewhat like a fence. The frequency response for such a system
is much more acceptable, since the mids and highs remain lively,
yet the bass becomes damped. Larger wavelengths pass easily through
the openings between the slats. But when the wavelength is less
than four times the slat width, the sound is back scattered.
MEMBRANE TRAPS The
need for low-frequency absorption, combined with the back scattering
of mids and highs, has been around for a long time. A different
solution was developed early on and became a standard in studio
design for forty years. “Membrane traps” utilize thin
sheets of plywood, 1/8 inch typically, that are bent into a sequence
of curved surfaces around the perimeter of the room. The airspace
between the membrane and wall ranges from inches to feet and is
packed with building insulation batt.
This technique provides low frequency absorption with the important
benefit of continuously curved surfaces creating lots of mid and
high frequency diffusion. Rooms with membrane traps are lively,
diffuse and well-damped. The efficiency of this technique is only
fifty percent at best. This means that twice as much surface area
is needed, but we end up with twice as much sound-scattering power.
All in all, it’s a reasonable tradeoff. These rooms are expensive,
but not too different than building a giant acoustic guitar. Their
concave curve sections produce local sound focus effects, a problem
for mic setups especially in a smaller studio.
PERIMETER TRAPS Another style of big room acoustics that has been used
in control rooms is to lay up row after row of lightweight building
insulation along the walls, but angled out from the walls. The hanging
batt curtains occupy the outer two-foot to three-foot perimeter
of the room. This technique is acoustically comfortable and stable.
As the entire room surface has been converted into a great ball
of fuzz, there will always be erosion of even the deepest bass energy.
The depth of these fuzzy walls can vary depending on the location
of the kinetic energy zones for certain problematic modes. The actual
volume of room is about twice that of the apparent room. It is somewhat
like a welter-weight anechoic chamber. This room can be successful
in a downtown designer/contractor studio, but is not an option in
the limited floor space of the home or project studio.
ZONE TRAPS Yet another version of deep bass absorption utilizes the
sound pressure-zone concept. The fiberglass batt used in a ¼
wavelength trap is compressed by ten to twenty times into a medium
density fiberglass board (commonly referred to as 703). This board
is then ‘furred out’ a number of inches from the wall
to produce a very effective sound trap. The major difficulty with
this technique is keeping the fiberglass from vibrating as air moves
in and out. When the fiat sheet of fiberglass moves, it shorts out
the bass trap. Its response curve is spotty, and some frequencies
are absorbed while others are not.
The trap design can also be outfitted with spaced
slats to back scatter the mids and highs, and if properly made can
develop high acoustic efficiency while staying close to the wall.
The most common mistake in slat/pressure zone traps is that the
slats are set flush against the fiberglass. This chokes off the
bass breathing ability of the trap. There needs to be at least a
½ inch air gap between slats and the face of the fiberglass.
pressure zone trap is a different type of sound trap than those
mentioned. It uses lumped parameter acoustics while typical fuzz
type absorption uses distributed parameter acoustics. Lumped parameter
devices are designed like an electronic circuit with discrete items
such as resistors, capacitors and inductors, and can be quite small;
The distributed acoustic devices use the wave-guide approach to
design and are sized directly to the wavelength of the note. For
example: the pan pipe (¼ wavelength) and a soda bottle (lumped
parameter) can both sound out the same note and equally loud, but
the pan pipe will be many times longer than the soda bottle.
IMPROVED QUARTER -WAVELENGTH TRAPS Rather than a loosely packed fiberglass batt, which always
settles, we can glue it to sheets of sound board which can be suspended
by wires inside the closet. Nothing much new here; the same response
curve as for the “ball-of fuzz” ¼ wavelength
fiberglass does not settle out and so the trap keeps working for
SYMPATHETIC RESONANCE TRAPS The sympathetic resonance or panel trap is a creative cousin
to the sound board and fiberglass trap. Often suspended in, supposedly,
random overhead positions, these panels are each tuned by trimming
to size and adding weights. Particular frequencies set these panels
into sympathetic vibration motion, and the incident acoustic energy
is converted to vibrating panel energy.
of the energy occurs with the air moving back and forth across the
face of the panel as it “twangs.” Its own internal friction
also dampens its motion. These panels have to be ¼ wavelength
in size, otherwise they would not be able to interact with the sound
wave. An 8-by-8-foot panel would function at 40 Hz, if it was correctly
tuned. Panel traps work best if aligned to meet the sound wave face
on (like a ribbon mic) to engage action. The flat of the panel needs
to face the wave front. Too often it is physically impossible to
set up a real room with these panels because of size constraints.
TRAP A classic never-to-be-forgotten sound trap is the Helmholtz
trap, which carries the name of a great, old-time German acoustical
scientist. Conceptually, the Helmholtz is little more than a jug,
tuned with loose batt stuffed inside. However, it usually looks
like a panel of ¼ in. pegboard behind which is a 1-3 in.
air space fluffed with light building insulation.
The absorption curve illustrates the strong frequency
selective property of this type of absorber. Two difficulties exist
with using such a trap:
1. It is a single-frequency type, and must be tuned
to a known room mode, and
2. The trap’s performance is strongly dependent
on the amount of batting placed in the cavity and the rigidity of
its wall, especially the perf panel. It is difficult to tune.
FUNCTIONAL TRAPS In the early 1950s, Dr. Harry Olsen, director of RCA Labs
and a prolific masterful contributor to audio practice and theory,
presented his “functional sound absorber.” It was especially
unique because of its unprecedented one hundred and sixty percent
efficient handling of low frequency sound. He envisioned its use
overhead in large rooms and halls. But elsewhere in his literature
he advises that low-frequency sound absorbers are best located in
the corners of smaller rooms.
“functional sound absorber” is a close cousin to the
flat pressure zone trap. The density of the fiberglass for this
type system is impedance matched to the radiation impedance of free
sound waves in air. Essentially, if the fiberglass is too dense,
sound bounces off; if it is too loose, sound goes right through.
The resistance of the surface combines with the volume of the airspace
inside to provide a very low frequency response curve for the trap,
similar to an electronic RC circuit. By adjusting the value of R
and C, the desired RC time constant can be picked for the trap’s
Sound absorption is always a function of two factors:
the surface of acoustic material exposed to the sound field and
the efficiency frequency response of the surface. Dr. Olsen’s
cylinder bass trap has just over three times the apparent frontal
surface area. Secondly, it is very efficient into the lower frequencies
because it is an acoustic circuit of RC time constant design, rather
than the more traditional ¼ wavelength “fuzz ball”
approach to acoustics.
with all traps, midrange and high frequency partial reflectivity
remains of value. Accordingly, today’s pro style functional-type
bass trap is usually outfitted with a membrane section to back scatter
mid-range frequencies (usually above 400 Hz). These traps are extremely
efficient, and particularly when located in the corners of a room.
To increase absorption in a selected frequency band or to extend
the low frequency response curve, the interior volume can be fitted
with a low Q Helmholtz resonator. It is particularly suited as a
corner-loaded bass trap in small audio rooms because it is small,
efficient, modular and easy to set up, more like studio equipment
than a remodel construction project.
RECTANGULAR ROOM DISEASE–HEAD END RINGING Home/project studios in rectangular rooms suffer from a
malady that most designer studios do not have–head-end ringing.
Speakers are usually located near the front of the room. From this
location they easily stimulate room resonances along the length
of the room. It takes about ten exchanges of sound between the front
and back of the room to build up the condition of resonance, typically
may be far from the back wall, but they are very close to the side
walls and floor/ceiling walls in the front of the room. Because
of these short lateral dimensions, side to side and vertical resonances
can build very quickly (within 1/20 second in the front end of the
room), long before the entire room Can be engulfed in the resonance.
This fleeting, quick resonance is called “head-end ringing”
and because of the time scale, dramatically affects imaging and
the color of attack transients.
Head end ringing is not a deep bass problem–it
is a mid bass coloration effect due to a lack of bass traps in the
front end of the room. Designer studios with the Reflection Free
Zone (RFZ) cup shaped front end don’t have this problem. The
raked walls and ceiling eliminate any opportunity for reflections
to stay and build up in the front of the room. But with home and
project studios set up in rectangular rooms, head end ringing is
a major problem that near-field or mid-field monitors cannot even
avoid. Typically, playback monitors are located about halfway between
floor and ceiling, and about one-third in from the side walls. The
classic head end ringing problem occurs at about 140 Hz. A substantial
distribution of mid-bass traps on the walls and in corners of the
front end of the audio room is the only way to control head end
EPILOGUE Over the years bass trapping has matured unique to the
recording industry. We don’t usually see them in press release
photos because they have always been built in behind the walls of
the designer/contractor studio. Nevertheless, bass traps are a tradition
that is integral to the definition of a recording studio or control
room. They are the primary acoustic consideration that separates
recording rooms from regular rooms. Although many versions have
evolved, one thing is for sure: bass traps have been, are now, and
will most probably continue to be the cornerstones for the pro room
But these are modern times and the availability of
personally affordable studio grade equipment is changing the face
of the recording industry. Home and project studios are being set
up at a ratio of ten to one compared to the traditional designer/contractor-built
studio. This new and rapidly developing division of the recording
industry may be wired like downtown studios, but their room acoustic
is all too often set up with no more than a couple of pieces of
foam tiles and particularly depleted of bass traps. Consistency
is always important, and the first rule in studio design is that
it must “look like a studio.” In this sense the topic
of bass traps in the designer/contractor-built studio and the home/project
studios do have one thing in common–no bass traps are visible.
is only one reason that studios have to look like studios–
to help establish client confidence. But this requirement for designer/contractor
studios does not apply in the home/project studio.
To a large degree, the owner of the home/project studio
is the client of the studio, The home/project studio may not have
to look like a designer/contractor studio in order to do its business,
but it certainly has to act like one. Since bass traps won’t
be built in behind the walls of any home/project studio, they will
have to be set up in front of the walls and corners of the room.
For the first time, engineers will simply have to look at bass traps.
Essentially, bass traps are “coming out of the
closet” in order to get back to work in the home/project studio.
After all, any chain, even the home/project studio audio chain,
is no stronger than its weakest link, and bass traps are critical
to the last link of the audio chain—the room acoustic.