Acoustic Treatment of Small Rooms on a Budget:The Rationale for AIAB

A White Paper by Thomas P. Bergman, Ph.D., M.B.A., C.P.A.

Sponsored by

The Problem

Rooms where people gather and converse, present to a small audience, and/or communicate with others at a distance via Audio-Video linking equipment, commonly suffer from poor acoustics. This deficiency generally presents as an excessive acoustic reverberation time, or in extreme cases, as a discernable echo. Reverberation refers to the repeated reflection of sound by surfaces in the room, that is, a sound that bounces back and forth between parallel, opposite walls, is said to reverberate. The longer the sound continues to bounce back and forth, the longer is the reverberation time of the room.

Such rooms possess this negative acoustic behavior pattern as a direct result of modern construction methods that employ hard surface walls, floors, and ceilings. Large windows that are highly reflective of sound further aggravate the problem. Carpeted floors and acoustic tile ceilings tend to mitigate the problem, but often these changes are not possible for esthetic or budgetary reasons, and both floor and ceiling treatments differentially affect acoustic performance at frequencies across the audible frequency range. Generally, floor and ceiling acoustic treatments can help to reduce reverberation at higher frequencies, but usually do little to help reduce reverberation at lower frequencies resulting in “boomy” rooms that still reverberate in the lower frequencies.

Large rooms such as auditoriums and sanctuaries are often designed with acoustic performance in mind, however, when acoustic performance has not been integrated into the fundamental design, such rooms nearly always require post-construction acoustic redesign services that can easily run to 6 figures for even a small auditorium. Such complex and expensive corrections are simply not an option for most owners/managers of small rooms that suffer from the same class of acoustic problems.

Breaking Down the Problem

Many terms are used to describe poor acoustics: a live vs dead room, too much echo, a noisy room, unintelligible room, a cold vs warm room, and many more. The reality is that all these descriptions result from the same condition: excessively long duration sound reverberation. That analysis suggests the cure is to shorten the reverberation time. You could theoretically completely “kill” the room, meaning eliminate any bouncing of sound from any surface in the room, however, that is rarely practical and can be quite expensive if it is even possible.

As a practical solution though, you can shorten the reverberation time of the room, meaning you reduce the length of time the sound will reverberate within the room once it is initially generated.

While desirable for studio recording, a completely dead room with zero reverberation is usually not especially comfortable for most other applications. When a room is too dead, it feels stuffy, unnatural, and an uncomfortable place to have a conversation. By contrast, when a room is too lively (the most common problem,) conversations are difficult because each sound produced in the room continues to bounce around the room until it dies out, all the while, interfering with new sounds being produced. Because each person speaking is having trouble understanding what was said to them due to this interference from previously spoken sounds, the tendency is to speak louder, leading to more sound energy being injected into the room, thereby increasing the time it takes for each new sound to die out.

The obvious solution is to reduce the reverberation time of the room to a level that is appropriate, given the use of the room. A room that reverberates badly at the frequencies most common in human speech will be highly undesirable as places to hold conversations, for example, though they may be desirable as presentation rooms. When a single speaker addresses an audience, as in a TED talk, a generous amount of reverberation is desirable as it imbues the speaker with power and control. Such a room would not work as well for a musical presentation by a band or choral group because the mix of sounds would be damaged by the addition of reverberating sounds. In the latter case, very little reverberation is desirable.

The reverberation time at different frequencies is also an important consideration. A room that has been treated in a way that reduces reverberation at higher frequencies, such as by the addition of acoustic tile ceilings and draperies may still sound "boomy" due to excessive reverberation at low frequencies.

Practical Solutions for the Small Room

A wide range of acoustic foam products exist for installation in small rooms, each designed for specific, targeted, frequency range improvements. These acoustic foam sheets, egg crate blocks, wave blocks, and bass trap blocks are made specifically for recording studio applications, and they tend to be sensitive to placement, unattractive, and expensive.

Perdue Acoustics pioneer, Jay Perdue, has developed the first do it yourself acoustics management system that is suitable for treating a wide range of rooms. Called appropriately, “Acoustics in a Box”, the patented system uses simple 24” x 48” acoustic panels, attached to a patented wall or ceiling mount that requires nothing more than adhesive or drywall screws to attach. The panels can be mounted vertically or horizontally on walls and/or ceilings anywhere there is room available. Each panel added to any given room will incrementally reduce the reverberation time across the entire audible spectrum. Still too much reverberation? Add more panels.

The secret to the system lies in the custom made stone wool panels which are made similarly to rock wool sheets, but with a stronger adhesive, more densely packed stone wool, and a special membrane on the back of each panel that greatly increases the sound absorption characteristics of the panels. The membrane enhances the sound absorption performance of the panels by causing the entire panel to move as a unit, and thereby utilizing the entire mass of the panel, rather than just the local area as is the case with acoustic foam products.

The mounts tilt the panels at the correct angle to act as a bass frequency trap that augments the higher frequency absorption of the 24” x 48” x 1” thick panels. Placed at the recommended intervals of 3-6’ apart around the perimeter of a room, the AIAB system can tame the acoustics of most rooms that are likely to be addressed in a do-it-yourself manner by the owner/manager of the room. The company that produces the AIAB product line makes no claims that this system can completely kill the acoustical reverberation of any room, just that each panel added will incrementally reduce the reverberation time of the room in the audible frequency range.

Because professional acoustic treatment runs as much as $100 per seat, professional acoustic treatment is simply not a realistic alternative for most organizations. So the rationale of the approach taken by AIAB is that "getting 80% improvement in the acoustical environment in a room, for 10% of the cost of professional acoustic treatment, is a lot better than doing nothing!"

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