Acoustics is a complex science, requiring the study of math and physics, as well as knowledge of construction techniques and materials. Often the first stumbling block in discussing the topic is vocabulary. In discovering the vocabulary, we will also discuss a few common problems and provide a little direction to rectify some common acoustic issues. Throughout the article I will refer to specific audio clips that demonstrate the acoustic phenomenon I am touching on.

Challenges

Echoes are reflections that occur long enough after the original signal, in which they sound separate from the original signal. Echoes can be distracting and difficult for performers on stage and distracting for audience members. Clip 1, Echo Example

Echoes also decrease intelligibility. They are the polar opposites of “early reflections,” which are echoes that happen within a few milliseconds of the original sound Clip 2, Early Reflections Example

Where an echo sounds like a separate event, early reflections are echoes that tend to blend with the original sound and give the impression of thickening it. Early reflections can be favorable for the per-formers on stage because early reflections often are perceived as "support from the house", a very helpful thing for the performer. "Reverberation" is the collection of all the reflected sounds in an auditorium. Reverberation is often characterized as the "sound of the room". It can be both helpful and harmful to the program material being presented in the space, and certain program materials work better with some reverberation characteristics than with others. For example, optimum reverberation times can vary from less than a second in a recording studio space to about a second in a lecture hall or movie theatre, 1.5 to 2 seconds for a theatre or music performance space up to 2.2 seconds in a symphony hall. In addition to reverberation time, one must also consider the timbre or "color" of the reverberation. Different materials reflect and absorb sound waves at different frequencies. For instance, 14 oz. drape at 50 percent fullness absorbs six times as much sound at 1 kHz as it does at 125 Hz. The net effect can be a room that sounds dark
Clip 3, Dark Room

or light and bright
Clip 4, Bright Room

(Both Clips have 1.8-second reverb time.) "Standing Waves" are characterized by lack of vibration at certain points, between which areas of maximum vibration occur. Standing waves can be demonstrated by playing a tone (usually a low tone) through a speaker in a room. One then moves around the room, noticing how the level of the tone changes based on listening position. In most rooms, some of this effect can be attributed to standing waves. While certain amounts of echo, early reflection and reverberation are desirable, standing waves are almost never desirable and can be difficult to eliminate completely. Square rooms or rooms with parallel surfaces most often create standing waves. Echoes, reverberation and standing waves are all products of sound waves bouncing off sur-faces. Three things can be done to the surfaces to lessen or change the amount of bounce that occurs. Each of the treatments affects each of the challenges in unique ways.

Treatments

Absorption is best accomplished with fiberglass or other fibrous natural materials. Contrary to conventional wisdom, carpet is not as good at absorbing acoustic energy as these materials. Most carpeting does exhibit some absorptive properties at some frequencies, but also efficiently reflects sound energy at other frequencies, so it should be used with caution. Synthetic foam is also generally not as efficient as fiberglass. Fiberglass insulation is inexpensive, readily available and naturally fire resistant. For acoustic applications, semi-rigid fiberglass panels, such as Owens Corning 700 series of products, are well suited. These semi-rigid panels can be mounted in frames or mounted directly to walls using construction adhesive. A frame also allows acoustically transparent material to be easily attached to the panel and is used for both aesthetic reasons and the practical purpose of keeping the fiberglass contained. For a less permanent installation, a bag may be fashioned from acoustically transparent material into which a panel of fiberglass can be inserted. These panels can then be hung on a wall or suspended from the ceiling. Because the “bags” are finished on both sides, they can be hung in the middle of rooms and used as room dividers. They also happen to be quite light and easy to handle. It is important that the material be acoustically transparent, as the sound needs to be able to travel through the material and be absorbed by the fiberglass underneath. Acoustically transparent fabric comes in many colors and patterns. Fabric made for industrial use is generally fire rated and very resilient. (See Guilford of Maine, sold through www.acousticalsolutions.com or www.silentsource.com.) Some fabric manufactured for the audio industry is actually woven from vinyl-coated fiberglass. In addition to being acoustically transparent, it is impervious to staining and carries a top fire rating (www.acoustonegrillecloth.com).

Materials made for the diffusion of sound waves are interesting. There is a wide range of products available with different looks and varying properties. Most of the shapes are based on mathematically generated patterns for diffusing acoustic energy. Diffusion panels can be made of wood, extruded or vacuformed plastic, or constructed from thin panels comprised of several layers of materials, which in combination are capable of diffusing sound waves evenly. Before the advent of mathematically designed diffusion patterns, designers used ornate plasterwork, friezes and statuary to provide solid, yet uneven, surfaces off of which sound waves can reflect. This concept often can be an acceptable, although less precise, method of diffusing sound waves. Old movie houses constructed in the first half of the 20th century provide many great examples of this type of diffusion.

Diffusion is an important tool because it can be used to combat echoes without reducing the overall acoustic energy in the room, as would be typical of absorptive panels. When a sound wave impacts a diffusive surface, the sound waves are reflected in multiple directions, unlike a flat surface, which would reflect the wave at an angle opposite to the angle of incidence.

Creating reflective surfaces is fairly easy. A flat, rigid surface is an effective reflector. The lowest frequency that a panel can effectively reflect is determined by the size of the surface. Larger surfaces reflect lower frequencies more efficiently. For acoustic performances or for sound-reinforced theatre requiring a natural sound, the placement of reflective surfaces behind or above can help to direct sound off of the stage and into the audience. Reflective surfaces around the stage also help to create early reflections, which can help performers onstage to hear them-selves. Which tool works best? As with most things, there is no one right answer. Combinations of these tools often are applied to create a balance between absorbing, diffusing and reflecting sound waves. A couple of things to look for: To combat standing waves, try to eliminate parallel walls. Building walls out to create uneven relationships with other walls is a good place to start. Take care to fill any voids with fiberglass insulation. The void spaces are resonant cavities and can do more harm than good. Additionally, covering about 50 percent of a wall surface with absorptive material reduces reflections. Alternatively, placing sculpture or sculpted wall hangings on a flat wall can be an effective and attractive diffusive treatment. Too much absorption will make a room sound dead. Using diffusion instead can break up the echo without eliminating the acoustic energy completely. Acoustics is a very complicated science. Hiring a good acoustics consultant is highly recommended. Be sure to get references and, if you can, go and listen to spaces that were treated using the consultant’s recommendations. When it comes to hanging or placing materials on walls and ceilings, be sure to check with local building codes to make certain you adhere to local building and fire code.

Analyzing Acoustics Originally Published July 2007