Sound Control 101
Insulation materials frequently are used to control noise problems. Noise is defined as an unwanted sound (for some of us, this is the basic definition of several types of so-called music). Sound (noise) is generated by, among other things, combustion, reciprocating engines/compressors, impact or water hammer, high velocity flow through pipes or ducts, gears, and bearings.
Sound is a vibration that travels along a path to a receiver (such as your ear). Sound consists of the vibration of various frequencies. It travels through air at about 1,100 feet per second, much faster through water or steel.
Sound is measured in decibels. The decibel is a logarithmic ratio that expresses the intensity of a sounds loudness. Table 1 provides the decibel levels of various familiar sounds.
The sound levels in Table 1 are taken close to the source of the noise. The more distance there is between the source and the receiver, the lower the sound level.
The Occupational Safety and Health Administration (OSHA) says that constant noise over 90 decibels (dB) in the workplace is not good for people. Options include wearing ear plugs or reducing the noise level at the source. This is where the insulation contractor can pick up some business-by reducing noise.
Methods and Materials
Sound transmission is reduced by the following methods and materials:
- Absorption-fibrous materials, open cell foams
- Mass-concrete, lead
Path barriers-flexible couplings, seal air holes
Fiberglass has been a leading sound-absorbing material for the past 40-50 years. Fiberglass duct liner, ceiling tiles, batts, and blankets are mainstays of the acoustical industry. Keep in mind that if the noise you are trying to control is a low-frequency rumble, the thickness of the fiberglass absorber must be greater than if the noise is a high frequency whine.
If you build a concrete wall (which provides plenty of mass) between the noise source and the receiver, but then allow the electrical contractor to install back-to-back outlets, the wall will be acoustically compromised. If air can flow from the sourceto the receiver, noise will alsotravel to the receiver. The caulk-type fire sealants that evolved from the nuclear power industry are ideal for acoustical sealing of holes in walls or floors.
If you build a wall of two-by-fours and drywall, the wall acts like a drum head and the sound passes through the wall with minor dB losses. Take the same wall, fill the cavity with fiberglass, then put 1/2 of drywall on one side of the studs and two layers of drywall on the other side of the studs. The added mass of the extra layer of drywall-but more importantly, the imbalance-causes the two sides of the wall to vibrate at different frequencies, and the wall no longer acts like a drum.
Lead is a terrific acoustical barrier because it is limp and does not vibrate (much) and hence effectively blocks the sound as long as it is sealed airtight. As with holes in vapor barriers, air holes in noise control barriers cause failures in the acoustical system. Making the enclosure airtight requires caulks, sealants, and adhesives-materials that insulation contractors use.
When you combine fiberglass, lead, and adhesives you have an optimum system for many industrial noise problems. Some manufacturers combine lead with PVC plastic to reduce costs. Sprayed-on fireproofing, such as rock wool or even sprayed-on cellulose, can also be an effective sound absorber.
Frequently, plastic pipes are insulated to control noise that was seldom a problem when cast iron pipes were used for similar drains. If the pipes are noise generators, it could be more effective to insulate them with calcium silicate instead of fiberglass due to the greater mass (weight) of calcium silicate. For the same reason, stainless steel jacketing (with the joints sealed) is a better sound barrier than aluminum jacketing.
Filled asphalt or other thick mastics are used on sheet metal as a “damper” or deadener in some applications. Sheet metal ducts in air handling systems pose two problems that need to be addressed for noise control: reducing fan noise and preventing “cross talk.” These problems can be addressed by isolating the fan with flexible connectors and installing duct lining in the supply and return ducts.
In summary, insulation contractors have all of the materials at their fingertips to control noise: fibrous insulations, mastics, tapes, and jacketing.
|Ramjet at takeoff
|Turbojet at takeoff
|WW II fighter plane
|Vane Axial fan