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April 26, 2010

How Can We Have Better Sound?

Many public facilities and even some homes have problems with sound. These problems may be classed as "too much sound", "too little sound", "poor-quality sound", or "uneven sound". Often the owners/occupants of these facilities do not know what to do about the sound problems. This can lead to wasting money on the wrong "solutions", attempted fixes by self-styled experts, or more commonly, just putting up with the frustration and sub-optimum performance of the room or building.

Under the heading of "too much sound" are external and internal noise sources. External sources can include highway noise, "boom cars", noisy neighbors (individuals or businesses), or various kinds of miscellaneous machinery noise. Short of moving, one may find quieting these noise sources difficult or unfeasible. Well-constructed new buildings can incorporate walls having a high sound transmission coefficient (STC) that will help block external noise. Some retrofit measures may be applicable for existing buiuldings, including special windows and doors. Noise sources such as air conditioners and ventilating fans can be quieted; this is much less troublesome if the air conditioner is owned by the person wishing to reduce the noise, as otherwise conflicts of interest may hamper efforts to mitigate the noise problem.

Internal noise sources include heating/ventilating/air-conditioning systems, or occupants of adjoining rooms (especially rooms beside or above the affected room). As with external noise, proper initial design goes a long way toward avoiding excess internal noise. Some retrofit measures can be applied, primarily through the use of special doors or door seals and carpeted floors.

Problems with too little sound are usually found in rooms especially used for listening, such as classrooms, auditoriums, churches, theaters, etc. Often these rooms are served by electronic sound systems, and all too frequently these systems are improperly designed, installed, or operated. but too little sound can also be the result of uninformed attempts to rectify other sound problems by using the wrong kind, quantity, or location of acoustical treatment. The solution to sound system problems may well not involve purchase of new equipment, but may involve adjustment, training, and/or acoustical work.

Poor-quality sound almost always is associated with electronic sound-system problems as described in the previous paragraph. Uneven sound -- a defect in which some people in a room hear good sound but others don't -- can result from either sound-system problems or problems in the architectural acoustics of the room.

In any of these cases, the building owner/occupant is faced with the choice of whether to involve a professional, and if so, what kind of professional. It is not necessarily true that someone who works for the telephone company, a radio or TV station, or a pro-sound or musical instrument store is qualified to assist in correcting sound problems. Nor is a person necessarily qualified by virtue of being responsible for operating the sound system at a church or theater, or for a musical group. The combination of public ignorance about the technical aspects of sound, along with the sizeable egos of many professionals, leads to a proliferation of poor advice, wasted money, and frustrated listeners. An experienced consultant who has an appropriate education can provide trustworthy guidance. Consultants may specialize in noise control, architecturel acoustics, and/or sound system design. It is always best to choose a consultant with all the specialties that may be needed to solve the specific problem. Usually a consultant will give a certain amount of free advice before "starting the clock" on billable time. Many times, this free advice can at least point the owner in the right direction for solving the sound problem. If not, then the consultant will provide a specific proposal for further investigation, with costs included so that the owner can make an informed decision of how best to proceed.

Comment thread:

Our church has a longstanding problem with reflections from the front balcony wall interfering with the sound coming directly from the front speakers. This makes it hard for some people to understand what's said, even when the sound system is loud enough. Two different acoustical consultants suggested a product to cover the balcony face. The church bought the product, but it was not installed the way the consultants recommended.

Does it really make a difference if the acoustical board is mounted directly on the wall instead of having space behind it filled with fiberglass?

Janet Miller

Thanks for your question, Janet . [Rant coming up!] Unfortunately, this type of problem (consultant recommendations only being partially followed) is not rare. I have a book written by an eminent Russian acoustician, in which he describes various projects on which he consulted during the Communist years. In other words, the government paid him for his recommendations, owned the building, and paid the building managers. Yet in many if not most cases, acoustical problems remained due to his instructions not being followed. [Rant finished.]

More to the point of your question: If you need acoustical absorption to be effective at bass frequencies to avoid echoes and excessive reverberation, that absorption must be very thick ( 4" or more), or must be mounted over an airspace or building fiberglass to achieve a 4" or greater effective thickness. The fiberglass-in-the-airspace approach is cheaper and almost as effective in many cases. But if the fiberglass and airspace are omitted, the bass echoes and boominess will remain.

--Richard Honeycutt, Ph.D.