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Overall Noise Levels: Acoustical Considerations
by: Art Barkman
Bounce
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.
Absorption
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 - moves. 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 he 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% 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 Architectural Acoustics
Modifying sound levels to make them compatible with the intended use of the space. One size does not fit all. Most of the spaces that the facility manager, architect or engineer deals with should be quieted. There are also spaces where it does not much matter one way or the other.
The following are examples of space 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 work stations and other adjacent space. An expansive discussion of these types of spaces can be found at OPEN PLAN OFFICE ACOUSTICS. 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 space, etc. also tend to be a problem. They are just too loud and hence, unpleasant. This may or may not interfere with the use of the space - at least some of the time. The noise from these spaces can spill over to adjacent areas where quietness is important.
Again, 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 created 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. There are also corollary advantages (looks good, makes speaker phone conversations more intelligible).
Other Nuisances:
Copy rooms, with or without doors (which are never closed anyway), broadcast sound out into office space or corridors leading to office space. Acoustical wall treatments reduce the noise level in the copy room and should be installed on the on the corridor walls outside these rooms which act as conduits for sound.
Mechanical rooms, equipment rooms, etc., even when doors are tightly sealed and other violations of their envelope are blocked, often disturb adjacent space. Absorbing, thus reducing, noise levels in these rooms with absorptive industrial grade (cheaper) finishes can reduce the noise level by as much as 50% (10dBa). This is a lot easier than adding to the mass of the walls and ceilings of such space.
Reverberation
When sound bounces around it not only gets louder, it also becomes distorted. Announcements in the airport, an arena, or other similar space are good examples of what kind of problems excessive reverberation can cause. The "old railroad station effect" if you will.
This is caused by sound bouncing repeatedly off of 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 are also hearing that which preceded it. They overlap - echo. Sometimes many times before they become inaudible. You hear a lot, but understand little. In large space the effect is obvious, in 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 the time, 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 are not perceivable to the occupant of the room do cause problems with recorded or tele-transmitted sound. The section on VIDEO CONFERENCING - TELE CONFERENCING addresses this in more detail.
The introduction of absorptive materials will also provide for an acceptable level of RT for any space - large or small. Again, the function of the space dictates what the RT should be.
In some instances the use of acoustically reflective surfaces is essential. Auditoriums, for example, are designed to reflect and project the sound from the stage or podium to the audience. The section on AUDITORIUMS, MEETING AND TRAINING ROOMS addresses these issues.
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 would be apparent to the casual observer. Qualified suppliers or consultants can provide acoustical design support -use them.