Controlling Noise Within a Space

When controlling noise within a space, there are usually two main   problems to remedy: a noisy space due to reverberation or a noisy   space due to equipment noise.

  General rules of thumb for controlling noise within a space:

• You have to at least double the absorption in a space before there is a noticeable difference. Every time you double the absorption, the reverberant noise field is reduced by 3 dB, which is classified as “just perceptible.”
• Adding absorption to a space can provide a clearly noticeable improvement if the space is fairly reverberant to begin with. The practical limit for noise reduction from absorption is 10 dB, which sounds half as loud.
• The improvement will not be as noticeable as you get closer to the noise source.
• Carpet is not a cure-all. In fact, it is typically only 15-20% absorptive. It would take four times as much carpet to have the same impact as a typical acoustic material, which is about 80% absorptive.
  

Case Study 1

Location: Retirement Village

Area of concern: Multi-purpose clubhouse

Additional information: The original thought was that the sound system needed to be upgraded or fixed because it wasn’t “working” properly. Further review showed that it was the lack of absorption in the room, not the sound system that was causing the problems.

Questions asked of client:

• Please describe the problem.
• What are the dimensions of the space?
• What activities take place in this room?
• Is there a noise issue? A sound system issue? A reverberation (“echo”) problem?
• When is it the loudest?
• Is it difficult to hear someone speaking when there is no loud noise?
• Do presenters on stage complain about reflections?
• Please describe the ceiling. Is it domed? Peaked? Flat?
• What materials are used in this room? Drywall? Wood? Carpet? Tile?

Client feedback:

• The room is too loud whenever there is a group in it, especially during dinners.
• It’s difficult to hear presenters and understand announcements. Small group conversations are hindered by excessive surrounding noise.
• The space is 65’L x 54’W x 18’H.
• The room is used for large dinners, performances, presentations, and other group activities.
• The original assumption was that the problem was the sound system, but we don’t have problems hearing announcements when the room is quiet. It must be a noise issue within the room itself.
• It’s the loudest during dinner when everyone is talking at once.
• It is not difficult to hear a presenter when there is no other noise.
• Presenters on stage do complain about reflections.
• The ceiling is flat drywall.
• Drywall and carpet are used throughout the room. Draperies and curtains are used on the stage.
  

Evaluation: After speaking with the client and visiting the site, it was obvious that a lack of absorption was causing the excessive noise in the room. Frequently, in a situation such as this, a reflective ceiling, which is a large area that will project noise back down to the floor, causes a majority of problems.

Addressing the ceiling alone would improve the noise level, but would not protect performers from the problematic reflections called slap-back*. There are a variety of products available for such applications. The products you choose are dependant upon the look and feel of the room and your budget. In this case, acoustics improved as a result of adding material to the ceiling (to control the overall noise) and acoustic wall paneling to the back wall (to control slap-back and the overall reverberation time).

*Slap-back = A reflective back wall will reflect, or “slap,” the noise back to the source causing a delay.

Case Study 2

Location: Headquarters for a large credit card company

Area of concern: Credit card processing center

Additional information: The first step in solving a problem related to equipment noise is to call the manufacturer. Sometimes there is a problem in the installation or in the equipment operation. Certain pieces of equipment have a retrofit noise reduction kit that can be purchased to reduce problems.

Questions to ask client:

• Please describe the problem.
• What are the dimensions of the space?
• What activities take place in this room?
• Is there a noise issue? A sound system issue? A reverberation (“echo”) problem?
• When is it the loudest?
• Is it difficult to hear someone speaking when there is no loud noise?
• Please describe the ceiling. Is it domed? Peaked? Flat?
• What materials are used in this room? Drywall? Wood? Carpet? Tile?

Client feedback:

• The processing center houses equipment that generates noise at 85-90 dB.
• Workers are annoyed by this noise and the company is on the borderline of an OSHA violation.
• The space in question is 260’L x 90’W x 20’H.
• This room facilitates automated printing and folding of statements and stuffing envelopes.
• Equipment noise is the primary problem.
• It is the loudest when all of the equipment is operating, which is during business hours.
• There are no communication issues when the equipment is not running.

Evaluation: It is always best to control noise at the source, which, in this case, is the equipment itself. The level of improvement is related to the reverberance of the space. The more reverberant a space is, the more dramatic the possible improvement. For this project, the space was not too reverberant, so the improvement would not be remarkable, but it would be noticeable.

Hanging vinyl-covered acoustic baffles from the ceiling, particularly the areas directly above the equipment, controlled the noise from emanating within the space, but did not reduce the noise level for the equipment operator (though it did help the other operators).

If adding absorption does not provide enough noise control, it might be necessary to isolate the noisy areas from the quieter areas. Doing so would result in the implementation of a hearing protection program for those employees working in the unavoidably louder areas. In this case, enclosing the equipment with an acoustic shield (of plexi-glass) reduced the noise level for the operator by about 10 dB. The combination of the absorptive material and the acoustic shield reduced the overall noise by about 4 dB for all employees in the area, which met the client’s needs and brought them into OSHA compliance.