Home Toys Article
- December 2004 -
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Introduction to Acoustic Treatment.
I love reading HomeToys.com to keep up with all the great new gear that's available. Loudspeakers, preamps, SACD players-they all promise to increase my listening pleasure with sound that's clearer and fuller than last year's models. Unfortunately, what you won't usually read about, even though it's arguably the single most important factor in achieving high fidelity, is acoustic treatment and especially bass traps. For reasons that elude me, room acoustics is often overlooked by even the most dedicated audiophiles. And that's a shame, because a rack full of the greatest gear in the world is utterly useless if the room you listen in skews the frequency response, and adds echoes and ambience that obscure musical detail.
In this article I will explain the basics of acoustic treatment as it applies to small rooms. That is, rooms the size you'll find in most homes. Acoustic treatment addresses three important problems that occur in all small rooms:
Effective acoustic treatment can transform a muddy sounding listening room, having poor midrange definition and erratic bass response, into one that sounds clear and tight, and is a pleasure to hear music and watch movies in. Even if you spent many thousands of dollars on the most accurate loudspeakers and other equipment available, the frequency response you actually realize in an untreated room is likely to vary by 30 dB or even more.
Please understand that acoustic treatment as described here is meant to control the sound quality within a room. It is not intended to reduce the amount of sound that travels between rooms. Sound transmission is improved by using heavy rigid walls, and by decoupling the walls, floor, and ceiling with shock mounts. This is very different from acoustic treatment which improves the sound quality inside the room.
ACOUSTIC TREATMENT BASICS
There are two basic types of acoustic treatment-absorbers and diffusors. There are also two types of absorbers. One controls midrange and high frequency reflections and the other, the bass trap, handles mainly low frequencies. All three types of treatment are useful in rooms intended for serious listening, but absorption is usually more important than diffusion and so will be the focus here.
Many audiophiles install thin acoustic foam or carpet over much of their walls, in the mistaken belief that avoiding all ambience is desirable. If you clap your hands in a room treated that way you won't hear any reverb or echoes. But such a room is unpleasant to be in, and makes the music sound flat and lifeless. Further, thin materials do nothing to control low frequency reflections and reverb, and hand claps won't reveal that. Basement home theaters and listening rooms having walls made of brick or concrete are especially prone to these problems-the more rigid the walls, the more reflective they are at low frequencies. Indeed, all acoustic problems are caused by reflections off the walls, floor, and ceiling.
I consider acoustic treatment as addressing two distinct ranges, with the dividing point at about 300 Hz. Below that frequency the main problems are a badly skewed frequency response and modal ringing, where the room sustains some bass notes longer than others. Figure 1 shows the low frequency response I measured in a room about 16 by 11-1/2 feet with a standard 8 foot ceiling. As you can see, the response varies by a staggering 29 dB between 108 Hz and 139 Hz, with numerous peaks and deep nulls at other frequencies too. Nobody would tolerate a response this terrible in their speakers or other gear, yet this variation, which is caused entirely by the room, is absolutely typical.
[Figure 1] - The horrendous low frequency response shown here is not only common, but typical of most home listening rooms.
Modal ringing occurs when a room's natural resonances are excited by musical notes played by bass instruments. (Mode is short for Mode of Vibration, which simply means a natural resonant frequency of a room.) Any bass note whose frequency corresponds to a room's resonance will sound louder than other notes, and will continue to sound after the note stops thus conflicting with subsequent notes. The result is a jumbled mess that makes it difficult to distinguish what notes are being played. You can tell that something is going on down there, but it's very difficult to pick out the individual pitches.
Above 300 Hz the dominant problems are excess ambience, flutter echo, and compromised imaging due to early reflections. Flutter echo is the "boing" sound you hear when you clap your hands in an empty room, and it's caused by sound waves bouncing repeatedly between opposing parallel surfaces. This type of echo is more damaging to music than normal ambience and reverb because the wall-wall or floor-ceiling spacing imparts a specific pitch. If that pitch coincides with musical notes in the music, those notes will sound louder and sustain longer than other notes. And if the pitch is near, but not exactly the same as, musical notes present in the music, the echo's tone can make the music sound out of tune as both pitches sound simultaneously.
PROBLEMS AND SOLUTIONS
Okay, now that you're aware of the acoustic problems that exist in all small to medium sized rooms, let's look at some solutions. Avoiding echoes and ambience at mid and high frequencies is not difficult because those frequencies are readily absorbed using thin, inexpensive materials. At the minimum, all rooms need absorption at the first reflection points above and to the sides of the listening position. Additional absorber panels can be placed on the walls and ceiling to tame any remaining ambience if needed. This article <www.realtraps.com/rfz.htm> explains in more depth the importance of avoiding early reflections, and shows how to use a mirror to determine the correct placement of absorber panels.
What separates the men from the boys in acoustic treatment is absorption at the lowest frequencies. Bass traps are by necessity large, since low frequency wavelengths are long, and several different types of traps are commonly used. I favor a broadband approach, which means that all low frequencies are absorbed rather than only the modal frequencies related to the room's dimensions. And this leads to an interesting point. I am often asked about the best way to measure a room's frequency response, in order to determine what type of treatment and bass traps are needed. In truth, all rooms have peaks and nulls at all low frequencies. Although some are related to the room dimensions, peaks and nulls also occur at frequencies related to the distance between the listener and the walls, floor, and ceiling. So the short answer is No, you don't necessarily need to measure your room's response because trapping all low frequencies improves the response regardless of the cause. Although you can definitely make a room too dead sounding at mid and high frequencies, it's probably not possible to have too much absorption below a few hundred Hz.
[Figure 2a and 2b] - These ETF graphs show the improvement in low frequency modal ringing when bass traps are added to a room. The upper graph shows the decay time versus frequency with the room empty, and the lower one shows the greatly reduced decay time after bass traps were added.
Figures 2a and 2b were created by the ETF program <www.acoustisoft.com> and show how the low frequency decay changes when bass traps (see Figure 3) are added to a room. Here, the rear portion of the graphs show the same response as Figure 1, but the decay is also displayed over time, progressing toward the front of the graph. In Figure 2a the various low frequency peaks decay over a period of about 0.3 to 0.4 seconds. After adding bass traps all of the decay times are reduced considerably, most to less than 0.2 seconds.
Another important benefit of bass traps is that they broaden the peak widths. Not only has the low frequency response been made much flatter in Figure 2b compared to Figure 2a, but the peak frequencies are much wider too. So a range of frequencies is emphasized now rather than specific notes, with the result that fewer individual bass notes stick out, and those that are boosted become boosted less.
It may be difficult for many people to accept that the skewed response shown in Figure 2b could be considered "good," but it's a huge improvement over how the room sounded before treatment was added. Please understand that it's impossible to make any room perfectly flat at low frequencies. But reducing a 30 dB variation to only 10 or 15 dB makes all the difference in the world, especially when the muddying low frequency reverb is also reduced substantially. When fellow musician and engineer friends visit my home theater they are always very surprised at the high quality of sound. Especially when I point out-boast, really-how little I paid for my receiver and other gear. Indeed, nearly any system based on mid-level components playing in a well-treated room will sound far better than even the most expensive gear in a typical untreated room.
Ethan Winer has been a professional musician, producer, and audio engineer since the 1960s. These days Ethan heads up RealTraps in New Milford, CT where he designs high-performance acoustic treatment products. You can read more of Ethan's articles at www.realtraps.com.