The single most important and influential link in the
audio reproduction chain is also the least understood and
most neglected - the listening room itself. Unfortunately,
this is also the most difficult or costly "component"
to change. What follows will be a brief overview of the
immensely complex and multi-faceted topic of room acoustics
and listening room design. Additionally, I hope to
pass along a few helpful tips that will allow you to realize
maximum benefit from your present (or future) listening
There are many factors that influence the "sonic
signature" of a given space. To try and illuminate
them all would require and in-depth course on acoustics.
The more conservative goal of this treatise is to explore
a few of the topics most germane to the Audiophiles' listening
room environment. Three that stand out as important considerations
are: room size, rigidity and mass, and reflectivity. Let
us examine each of these in more detail.
In our discussions, room size will be broken down into
two subclassifications: dimensions (height, width
and length) and cubic volume. From a practical standpoint,
room volume will be an important criteria in choosing loudspeakers
and the amplifier necessary to drive them to the desired
sound pressure (loudness) level. Assuming that the listener
wants to "fill the room" with sound, a large environment
will require both a larger loudspeaker and a more powerful
amplifier to do the job. Smaller spaces usually dictate
The dimensions of the room (and their ratios) do much
to influence the sound in a listening room. The height,
length and width will determine the resonant frequencies
of the space and, to a great degree, where the speakers
and listener should be located (see our separate article
on speaker placement). The longest room dimension, the diagonal,
will determine the ability of the room to support low frequencies.
Ideally we would like to have a diagonal dimension equal
to or greater than the wavelength of the lowest frequency
we expect to generate within the room. This ideal quickly
becomes impractical for most of us when we realize the gargantuan
nature of low frequency sound waves in air. A 20 Hz wavelength
is 56.6 feet in length!
Fortunately, we need only one-quarter of this dimension
to achieve adequate bass response.
Rigidity and mass both play significant roles in determining
how a given space will react to sound within. They have
a strong relation to the low frequencies - both qualitatively
and quantitatively speaking. Low frequencies can be tremendously
powerful, capable of flexing walls, ceilings and occasionally,
floors. Flexure of this type is termed diaphragmatic
action. To illustrate this concept, think of the room
as a small box. If the box is made of cardboard, the
walls vibrate easily. The same box made of concrete would
exhibit little movement. Diaphragmatic action dissipates
low frequencies, robbing the bass of both impact and extension.
Therefore, the more rigid/massive the walls, floor and ceilings
in our listening rooms, the less diaphragmatic action and
the tighter, more defined and powerful the bass.
An ideal room would have absolute rigidity and infinite
mass. While such a "perfect" room is theoretically
impossible, the closer we can approximate the ideal, the
better. The closest I have come to reproducing the "ideal"
room, was in the design of a West Texas recording studio
done a number of years ago. The availability of native materials
allowed the walls of this structure to be constructed from
solid rock to a thickness of 16". The bass reproduction
in the control room was absolutely the cleanest, tightest
and most powerful I have yet experienced. Although
the studio monitors and electronics in use were inferior
to many of todayís better hi-end audio systems, listening
to this system with master tapes was truly a religious experience.
Our goal then, is to reduce the amount of diaphragmatic
action in the listening room. We can accomplish this
task by increasing the mass and rigidity of all surfaces
within the listening environment. This can dramatically
improve low frequency detail, solidity and overall accuracy.
In existing rooms using drywall construction, we can simply
add an additional layer of sheet-rock, making sure to tightly
couple the new layer with screws and adhesive. In new construction,
we can look at using not only two layers of sheet-rock,
but double-wall techniques, more robust framing materials
and thicker drywall material. At this stage these changes
are quite inexpensive. 
In simple terms, reflectivity is the apparent "liveness"
of a room. Professionals prefer the term reverb
time or Rt-60. Rt-60 defined, is the amount of
time (in seconds) it takes for a pulsed tone to decay to
a level 6OdB below the original intensity. A live room has
a great deal of reflectivity, and hence a long Rt-60.
A dead room has little reflectivity and a short Rt-60.
Rt-60 measurements are most useful in determining the
acoustic properties of larger spaces such as churches, auditoria,
etc. In smaller environs the Rt-60 measurements become so
short as to be useless. In these confined spaces, individual
reflections from nearby surfaces dominate the sonic picture
and are the primary focus for the audiophile.
Reflections can be both desirable and detrimental.
This depends on their frequency, level and the amount of
time it takes the reflections to reach our ears following
the direct sounds produced by the speakers. Our brain blends
together all of the sounds reaching our ears within 5-30
ms of the original. Reflections arriving approximately 30-50
ms or more after the original will be perceived as separate
sounds. This phenomenon is known as the Haas effect.
It is these initial reflections that are most important
to the brain in determining the apparent size of the listening
room. By manipulating the ratio of direct vs. reflected
sound, we can fool the brain into thinking we are listening
in a larger room than actually exists. The idea is to reinforce
the direct output from the speaker with reflections of the
proper level, frequency and arrival time, while eliminating
the detrimental ones. This can be accomplished by proper
positioning of the speaker and listener, and through implementation
of various acoustic correction products such as those made
by Acoustic Sciences Corporation, RPG and others.
Comb filtering is another form of unwanted reflection.
This condition is created when a speaker is placed near
a reflective surface (wall, floor, furniture etc.). The
result is image smear and/or frequency response anomalies.
The comb filter effect occurs when the direct sound and
the reflected sounds arrive at the listeners' ears out of
phase, thus canceling each other. This problem can be avoided
by placing your speakers well away from reflective surfaces,
or by treating nearby problem areas with absorptive and/or
A simple test can help us to identify problematic reflections
in our listening rooms. The hand clap test is so
named for obvious reasons. Simply sit in your normal listening
position and clap your hands once, listening carefully to
how the sound is affected. Do you hear a slow, even decay,
a single hard reflection or a multiple of closely spaced
repeats. These faster echoes are known as flutter echoes
and are created when sound bounces back and forth between
two reflective surfaces. Flutter echoes and strong distinct
echoes that must be eliminated if optimum sound quality
is to be expected. Again, judicious use of acoustic correction
materials can be of great help.
Our hand clap test described above will not, unfortunately,
expose another common acoustical anomaly - that of standing
waves (Acousticians sometimes use the term room modes
to define this effect). Here we are describing a type
of low frequency reflection, caused by dimensional relationships
within the room. Low frequency standing waves can be predicted
mathematically when the dimensions of the room are known.
Standing waves build up in the listening environment and
conspire to sabotage the low-end performance of our stereo
systems. A low frequency standing wave is likely to "bloat"
the character of the bass, causing severe peaks at points
throughout the range. The only cost-effective method available
for the treatment of standing waves is the use of ASC Tube
units, placed in the comers (The point of maximum pressure)
can dramatically improve the quality of low frequency sound
in a space plagued by standing waves.
In our zeal to control every last reflection, a potential
problem should be understood and avoided. The over-damping
of the midrange and high frequencies is a common problem
resulting from the overuse of highly absorptive materials
(Sonex, Fiberglas insulation). Too much absorption here
will skew the proper tonal of music, causing the all important
midrange/high frequency region to be attenuated, and low
frequencies to become too prominent. Many acoustic treatment
products exist on the market today. Each has merit and is
designed for treatment of specific problems. It is unwise
however, to purchase any of them without a prior understanding
of the particular room deficiencies you are experiencing.
Careful selection of the acoustic correction methods employed
is important if optimum results are expected.
Each of the currently available acoustic control materials
represent an effective means of subduing or eliminating
a variety of room problems. However, choosing the
right "tool" for the job is tantamount to success.
Let's look at some of the more common problems encountered
by the audiophile, and chart a course for corrective action.
If your room is overly live (caused by midrange/high frequency
reflections), it can exhibit a bright character. If you
don't have a real problem with hard, distinct echoes, you
should try adding a few ASC Tube Traps, some Sonex or Owens
Coming 500 series compressed Fiberglas board.
Another way of taming reflections is through the use of
diffusion. Diffusers disperse hard reflections in a random
manner, eliminating reflections just as effectively as absorption.
Proponents of diffusion argue that the method is preferred
as it returns energy back into the room in the form of ambience.
The RPG Diffusers are the most prolific form of this kind
of device. Additionally, ASC Tube Traps offer a balanced
combination of absorption and diffusion.
If you have determined that your room suffers from flutter
echoes, try damping one or both of the reflective surfaces
creating the problem. Remember that flutter echoes occur
between two parallel surfaces (like two walls), and that
damping of at least one of these reflective surfaces should
control or eliminate the problem. Small rooms are often
rife with flutter echoes and are the major cause of imaging
problems within these rooms.
The science of acoustics is still in its infancy.
Each year we learn more about the interactions between transducers,
sound and the environment. Because of this growing
knowledge, the range of products available to control acoustic
problems is also increasing. Now more than ever we
are able to find cost effective solutions to our acoustic
ASC TUBE TRAPS: A wide range of products made unique by
their ability to work over a broad range frequencies. They
are easy to use, relatively unobtrusive and extremely versatile.
They can be used in the control of low frequency standing
waves, general dampening and specific reflection control.
Covered in a fire retardant, open weave material similar
to burlap, Tube Traps are available in a variety of colors
to match most any decor. Price: moderate to expensive.
ASC OPTICAL ALIGNMENT KIT: The Optical Alignment
Kit (OAK) offers a simple and unique way of helping one
to place ASC Tube Traps (or any other acoustical correction
materials) within the listening room. A bit of digression
is necessary to understand the idea at work here.
Let me explain.
Sound waves emanate from a loudspeaker in much the same
way as a beam of light radiates from its source. And, like
the beam of light, will be reflected off nearby surfaces.
These reflections can be beneficial or deleterious depending
upon their intensity and time of arrival, in relation to
the direct sound from the speaker. Reflections arriving
too soon (early reflections) after the direct sound, confuse
the ear/brain, creating chaos in the image, defocusing and
confusing the three dimensional recreation we strive for.
Absorbing these harmful early reflections significantly
enhances the performance of an audio reproduction system.
ASC Tube Traps are an ideal way of combating these reflections,
however their effectiveness depends upon accurate placement.
This can be done through experimentation (trial and error),
using expensive test equipment or now, with the Optical
A wide strip of reflective Mylar "tape"
is temporarily attached to the wall surrounding the speakers,
at ear level. Seated in your listening position, you
will be surprised to see multiple reflections of each speaker,
all around the room. These images represent the points along
the walls at which harmful reflections will occur.
Mark these points, remove the Mylar, pop a Tube Trap in
each location and viola, instant gratification. You can
also have some slide a mirror along the wall while you sit
in the sit I the listening position. But I have found that
being able to see all the reflection points at once is a
benefit. Price: inexpensive
MICHAEL GREENíS ROOM TUNES: Offers a variety of free-standing,
wall mounted and custom installation products are available.
OWENS-CORNING FIBERGLAS: This Company manufactures an entire
range of acoustic control products. The most common
are the 500 series of compressed Fiberglas panels.
These can be covered with an open-weave cloth and used either
as spot treatment for reflection control or general room
deadening. Price: inexpensive.
RPG DIFFUSERS: Looking rather unconventional, the RPG Diffusers
are constructed using a number of slats or thin metal strips
enclosed in a frame. As sound waves contact the strips they
are reflected in a random manner, resulting in diffusion
rather than absorption. Price: expensive.
find the wavelength for a given frequency, simply divide
1130 (sound travels at the speed of 1130 feet persecond
in air) by the frequency. As an example, a SOHZ wavelength
is 22.6 feet, 10OHz would be 11.3 feet.
modifications are relatively simple to execute, however
a detailed description is not within the scope of
 So named
after Helmut Haas, a German Acoustician.
 The ASC
Tube Traps are cylindrical devices designed to be placed
in room comers. They are effective at absorbing low frequencies,
thus controlling standing waves. In addition, they offer
both an absorptive (to high frequencies) and reflective
side. Thus they can be rotated to provide an absorptive
surface (to high frequencies) or a diffusive surface.