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 small rooms.
Originally written and published in dB Magazine,
November/December 1991, and January/February 1992. Reprinted with
permission of db Magazine, Commack, NY.
Part
One
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 covered it.
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
traps.
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.
The
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.
The
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.
SPEAKER
DIRECTIVITY
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 listener.
Sound
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.
Major
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.
The
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.
MEASURING
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
is quieter.
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.
If
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
slightly changes.
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.
If
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 use.
LISTENING
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!
What
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.
SOUND
“CANCELLING,”
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
test.
BOOM
BUSTERS
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.
Other
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
low end.
BASS
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:
A
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
people!
An
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.
SLAT
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.
PRESSURE
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.
The
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
trap. The
fiberglass does not settle out and so the trap keeps working for
years.
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.
Dissipation 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.
HELMHOLTZ
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.
The
“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
roll-off characteristic.
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.
As
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
¼ second.
Speakers
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 ringing.
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
acoustic.
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.
There
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.