The ABC’s of Acoustics

Acoustic Education
Controlling noise and creating a comfortable acoustic environment relies on three basic strategies known as the ABC’s of acoustics. “A” is for absorption, “B” is for blocking, and “C” is for Covering. The method chosen depends on the goal of the project. It is worth noting that the most effective noise control methods will incorporate both sound blocking and sound absorption, and depending on the application may include sound covering as well.

If the goal is to control sounds within a space from reverberating, then absorption should be added to the room. Reverberation occurs when sound is reflected off of the hard surfaces in a room. The sound waves bounce around until their energy is diminished making speech intelligibility difficult. If a large area has a lot of hard parallel surfaces, this can take a long time. Sound absorption products are fabricated from light porous materials such as fiberglass or open cell foams. By adding absorption to the room the sound waves are attenuated within the fibers or cells of the absorption material and converted into heat energy thus preventing the continuous ricochet effect.

If the goal is to keep sound from leaving or entering a space, then sound blocking should be used. A material’s ability to block sound is a direct relation to its mass, so sound blocking products are fabricated from dense heavy materials. The first step in blocking sound between adjacent spaces is to eliminate any short circuits such as common HVAC ductwork and registers, pipe and conduit passing between walls and ceilings, walls that stop above a dropped ceiling but do not extend to the above plenum, and doors and windows. Once the common sound paths have been remedied, the next step is to add mass to common partitions, floors, or ceilings. This can be accomplished by adding additional sheetrock over a layer of mass loaded vinyl barrier.

Additional sound blocking can be achieved through sound isolation by decoupling a wall, ceiling, or floor with resilient isolation clips. Sound isolation works by disrupting the sound path between two solid objects thus eliminate structure borne noise. The same principals also apply to acoustic pipe and duct hangers and vibration isolators.

Sound can also be covered in order to provide speech privacy. This is accomplished with the installation of a sound masking system. Sound masking systems are typically installed as individual speakers directly above the dropped ceiling to create a uniform field of sound. Sound masking systems work by introducing a scientifically engineered contour of white and pink noise adjusted just above the threshold of human speech. This provides just enough background noise to make nearby conversations unintelligible, thereby eliminating distractions.