Room Acoustics

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 environment.

 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. 

Room Size:        

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 smaller speakers.

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![1] Fortunately, we need only one-quarter of this dimension to achieve adequate bass response.

 Rigidity And Mass:

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 today’s 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. [2] 

Reflectivity:

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[3]. 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 diffusive materials.

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 Traps[4]. These 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 problems. 

Available Materials: 

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 Alignment Kit.

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 GREEN’S ROOM TUNES: Offers a variety of free-standing, wall mounted and custom installation products are available. Price: moderate.

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.

[1]  To 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.

[2]  These modifications are relatively simple to execute, however a detailed description is not within the scope of this paper.

[3] So named after Helmut Haas, a German Acoustician.

[4] 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.