Absorption

The first choice for reflection control
The sound produced by your speakers, as well as its reflections from your room's walls, ceiling, floor and furnishings, is actually sound energy, or acoustical energy. These sound waves cause air particles to vibrate, and when they vibrate against our eardrums, we hear sound.

A basic law of physics states that energy can neither be created nor destroyed, but can be converted into another form. If it's impossible to simply destroy all these unwanted sound reflections, how can we control them? This is where the concept of sound absorption enters the picture.

If you've ever been inside a recording studio, radio or TV station, concert hall, or music practice room at a school or music store, you've probably seen some type of sound-absorbing material, even if you didn't know what it was for.

For nearly 60 years, applying absorptive material to walls and other reflective surfaces has been the primary method for taming unwanted reflections. Dense, porous materials like polyurethane foam and fiberglass have been the most popular choices. These materials absorb sound by converting the acoustical energy (the sound) into heat.

This happens when the air particles are driven into motion by the sound waves, then attempt to pass through the dense sound-absorbing material, resulting in heat-generating friction. (Don't worry, this energy conversion process generates tiny amounts of heat.)

Whether we're talking about common room materials (see table) or professionally designed room treatment products, a material's ability to absorb sound varies according to the frequency of the sound. As the table shows, soft, fibrous materials like carpet and drapes will absorb most reflected sound above 500 Hz, yet have little or no effect on reflections below 125 Hz.

The illustration above left shows that a 1" thick fiberglass panel provides excellent absorption of sounds above 500 Hz, but that controlling lower-frequency reflections requires the use of thicker panels. As an alternative to thicker fiberglass, the illustration above right shows how creating an air space between the panel and wall surface increases low-frequency absorption.

This makes sense when you remember the huge differences in the wavelengths of high- and low-frequency sounds. Fibrous materials, which are so effective at absorbing 1000 Hz sound waves a little over a foot long, can do very little when it comes to 125 Hz wavelengths that are 9 feet long. These long-wavelength reflections simply pass right through these soft materials with almost no resistance.

The table on the previous page shows that drywall and window glass provide significant absorption in the 125 Hz range. This conversion of acoustic energy is accomplished in a different way than that of the soft, fibrous materials described previously. When a low-frequency sound wave strikes drywall or a window, those surfaces convert some of the sound energy to motion; they actually flex a tiny amount, thus absorbing some of the acoustic energy.

Notice the increase in the absorption of reflected sounds — especially for sounds at or above 1000 Hz (1kHz) — when the fabric is folded into drapes.

Tips on absorptive treatments
Although absorptive treatments are very effective at taming flutter echo and mid- and high-frequency reflections in general, they won't cure all room acoustics problems. In fact, using too much absorptive material can itself cause problems.

If your system was in a room with thick carpeting on the floor, acoustic tile on the ceiling, and heavy drapes covering much of the wall surfaces, you would have nearly all of the high-frequency reflections being absorbed and nearly all of the bass sounds being reflected. The sound in this room would be unpleasant: thick and boomy in the bass with little or no sense of spaciousness. An over-absorptive room can make spoken dialogue sound unnaturally dry.

At the other extreme, a room with painted drywall on the walls, drywall or plaster on the ceiling, linoleum over concrete on the floor, and no sound absorbing drapes or rugs, would sound extremely bright, thin and echoey. And as we discussed, too many echoes can negatively affect movie dialogue, making it more difficult to understand.

Your goal should be to balance the amount and frequency of the absorption in your room to achieve some bass and high-frequency absorption. Typically, bass absorption is the more difficult to achieve.

Meanwhile, here are a few tips and ideas to keep in mind concerning sound absorption:

• Before turning to professional room treatment products for absorption, try to get the most out of ordinary room materials (see table).
• Large expanses of glass such as picture windows or French doors should be covered with drapes.
• You don't have to treat every surface in your room. There are a few key spots which, if treated, will give you maximum sound improvement for your investment.
• The pad beneath a carpet contributes to its sound-absorbing ability. While your first considerations should be durability and comfort, it's worth knowing that an "open-cell" pad such as foam rubber will absorb more sound than a "closed-cell" pad.

Absorption is an important ingredient of room treatment, and is especially effective at treating side wall reflections. But absorption is not the only answer, and in many situations, it's not the best choice. In a small listening room, overuse of absorptive material for reflection control can result in a room that is too acoustically "dead."

Some music lovers think of professional recording studios which have been heavily treated with absorptive materials as an acoustic model, but keep in mind that studios are able to add artificial reverberation through electronic signal processing. Music lacking the richness contributed by the room effect is less involving.

Fortunately, there are other ways to control room reflections, and increasingly, audiophiles and musicians are turning to diffusion.