Special Report: Sound Masking Part I
Mission Shhh …Combating Noise

by Art Barkman

Let’s face it doing business is noisy. In an office there are phones ringing, co-workers conversing, printers processing and a variety of other miscellaneous sounds to distract and annoy employees. In spaces like manufacturing facilities there’s a whole other set of problems caused by things like loud machinery. Then there are large public spaces like airports and convention centers where sound distortion is a leading concern. But regardless of what type of facility you’re responsible for, dealing with noise is a three-pronged process. The first step is becoming familiar with the scientific characteristics of noise. Next is to take those principals and assess your facility. The last part is to arrive at a solution that suits the needs of your space based on the first two steps.

The physics of sound

Aside from structure born sound, sound travels by three paths; 1) direct, 2) reflected, and 3) diffracted. The reflected paths that sound travels by are the bounces that sound waves take when they impact hard surfaces. If that surface is soft, it responds to that impact and absorbs the sound or a portion of it. Throw a ball (a tennis ball, not a bowling ball) against a brick wall. Now throw the ball against a pillow. Get the idea?

When sound reflects from the interior surfaces of a space, two things happen:

            1) The space gets louder, and
            2) The sound becomes distorted.

Both of these simultaneous occurrences are usually not desirable.

Sound is a form of energy and, you can’t get rid of energy. Energy is either dissipated because it spreads out or is converted into another form of energy or matter. Acoustically absorptive materials actually convert sound energy into mechanical energy—wiggles. When sound waves (fluctuations in air pressure) impact a material that is acoustically absorbent, that material responds by moving. The ability of a material to absorb sound in this manner is measured in a test that provides a NRC (Noise Reduction Coefficient). This is the simple mathematical average of absorption at four frequencies: 250, 500, 1000 and, 2000 Hertz (Hz) also called cycles per second. These four frequencies are the ones we hear the best and are generally those most dominant in the sounds created in the real world.

So, a material with a NRC of .80 absorbs an average of 80 percent of the sound that impacts it. Whether this is good or bad depends on the acoustical need. Certain surfaces should reflect sound. A band shell or the angled ceilings and forward sidewalls of an auditorium are supposed to reflect and project sound away from the stage or podium. The rear wall, on the other hand, should absorb sound.

Dealing with reflected noise

The following are examples of where excessive noise may be a problem and how to deal with it:

Open plan office space—Because there are no full height barriers, there are many reflected paths that sound can take to travel between workstations and other adjacent areas. Perimeter walls and storage (files, shelving, etc.) that provide reflective surfaces should be treated with acoustical wall panels.

Large open areas—Spaces such as cafeterias, atriums, multi-purpose spaces, etc. also tend to be a problem. They are just too loud and hence, unpleasant. This may interfere with the use of the space—at least some of the time. The noise from these spaces can also spill over to adjacent areas where quietness is important.

The ceiling presents the best opportunity to reduce noise levels. Where a suspended ceiling is not practical or in keeping with the design of the space, suspended banners or baffles may be employed. Acoustical panels may be surface mounted to hard ceilings. Perimeter walls should also be treated with absorptive panels.

Closed office space—Sound (noise to everyone else but the people that create it) that enters or is created within an office is made louder if it can bounce around. Acoustical wall treatments can serve to correct this and may provide a cost-efficient alternative to creating a tighter envelope of walls and ceilings.    

Other nuisances

Copy rooms with or without doors (which are never closed anyway), broadcast sound into the office or corridors leading to office space. Acoustical wall treatments reduce the noise level in the copy room and should be installed on the corridor walls outside these rooms which act as conduits for sound.

Mechanical and equipment rooms often disturb adjacent space even when doors are tightly sealed. Absorbing, thus reducing, noise levels in these rooms with absorptive industrial grade (cheaper) finishes can reduce the noise level by as much as 50 percent (10dBa). This is a lot easier than adding to the mass of the walls and ceilings.

Understanding reverberation

When sound bounces around, it not only gets louder, it also becomes distorted. Announcements in airports, arenas, or other similar spaces are good examples of the problems excessive reverberation can cause, the “old railroad station effect” if you will.

This is caused by sound bouncing repeatedly off ceilings, walls, floors and whatever else there is that doesn’t absorb it. While you are hearing the first syllable of a word, a specific note of music or other sound, you also are hearing that which preceded it. They overlap and echo sometimes many times before they become inaudible. You hear a lot, but understand little. In a large space the effect is obvious, in a smaller space, less so.

The measurement of reverberation in a space is called reverberation time (RT). It is the amount of time it takes for a sound to diminish (decay) 60 decibels. The longer it takes the more distortion. In certain environments, such as concert halls and recital rooms, a certain degree of reverberation is desirable.

In many instances, degrees of reverberation that cannot be perceived by the occupant of the room can unknowingly cause problems with recorded or tele-transmitted sound. This may pose difficulties if the room is intended for videoconferencing or teleconferencing.

The introduction of absorptive materials like carpeting, fabrics and wall and ceiling panels will help achieve an acceptable level of RT for any space, large or small. Again, the function of the space dictates what the RT should be.

When absorptive materials are introduced into a space that space gets quieter and the “echoiness” diminishes. Calculating the correct amount of treatment and its placement can be a little more complicated than it seems. It’s important to seek advice from qualified suppliers or consultants that can provide acoustical design support.

Art Barkman is president of Sound Management Group, Inc., Hillsborough, New Jersey.
www.smg-corp.com