In order to control noise, designers must first determine where the noise source or sources are how much each is contributing to the overall level, and their frequency signatures. This can be done with a combination of instruments: a Type I 1/3-octave band sound-level meter, a sound intensity analyzer, and if there may be significant structural-borne noise, by model analysis of various components.
Once the sources have been identified and quantified, they can be ranked by how each contributes to the overall noise level. This is most important because if lower-level noise contributors are silenced first, this will not reduce the overall level. For example, when the exhaust is louder than the air intake on a gas or diesel engine, reducing the air intake noise gains little noise reduction until a properly sized muffler is installed.
When trying to control noise in any type machinery, designers generally end up employing some, if not all, four elements of noise control. These are absorption, barrier, dampers, and gasketing materials.
Absorption material: A good candidate will "soak up" airborne sound-energy waves by changing the wave energy into heat as it passes through the absorption medium. Absorption materials are generally either fibrous or cellular. Common fibrous material are fiberglass, mineral wool, and ceramic. These are applied as blanket or semirigid sheets which can be cut to shape. Generally, they are film faced or bagged to prevent the fibers from being dislodged and causing problems in air-handling systems or rotating machines when bearings or other components could prematurely wear out if they became contaminated.
Newly developed melamine and polyimide cellular materials offer significant advantages over traditional urethanes in many respects. The melamine required only 40% the weight of a comparable urethane, costs 30% more, and has no smoke or toxic by-product of combustion. The polyimide foam has similar characteristics to the melamine but costs 10 to 12 times as much, and is not as hydrolytically stable.
Film facing will reduce overall absorption at higher frequencies. This may be a problem if most of the noise energy generated is also at these frequencies.
Barriers: There are two types of barriers -- those that already exist (walls, cabinets, enclosures, etc.) and supplemental barriers. Supplemental barriers are those which you add if the existing enclosure wall is not thick enough. In this case, mass is the key to controlling noise. The mass law for homogeneous materials gives a rough approximation of the amount of noise that can be reduced given a specific materials mass.
However, the same reduction cannot be achieved at all frequencies. There is less attenuation (noise reduction) in the lower frequencies than the highs for a given mass, in this case, 9.6 oz/ft″, or lead. The mass law predicts only 6-dB additional attenuation if the weight is doubled. But separating the two sheets of lead with a …-in. decoupling layer of open-cell urethane foam produces a significantly greater increase particularly above 500 Hz. Bonding a single layer of 10 oz/ft″ to a 20-gage steel panel would result in even greater noise reduction.
These figures all assume "perfect" walls in which there are no openings. This is not very practical in most everyday applications. When openings are created for pipes, wires, air, and products to enter and exit, noise is let out. The amount of noise reduction expected from a perfect wall or enclosure diminishes drastically in the real world. As the opening size increases as a percentage of the total enclosure area, the actual noise reduction decreases.
For example, with a transmission loss potential of 20 dB or greater and 10% opening, designers can never get more than 10-dB noise reduction. However, this can be drastically increased by sealing around wires, pipes, and air ducts. In addition, noise reduction is increased by providing absorption-lined tunnels for materials that have to be fed into and out of the enclosure. These "supplemented" barriers are easy to fabricate and install either mechanically or with preapplied pressure-sensitive adhesive.
Damping: All materials have a natural frequency. When they are excited by some source at this natural frequency, they will vibrate. This causes the air surrounding the material to vibrate and produce noise. Sometimes referred to as "oil can" or "drum head" phenomenon, this type of noise can be controlled by damping. Properly applied damping materials will only work if the metal or plastic to which they are applied is vibrating at or near their resonant frequency. If a mechanically driven plate is vibrating, damping will not stop it. And it should be noted that all damping materials are temperature sensitive, so they must be selected both for their temperature range and the operating temperature of the material to which they are applied.
Damping materials work to reduce the vibration in the material to which they are applied by dissipating the vibrating energy as heat, rather than radiating it as acoustic energy or noise. Damping materials are termed "viscoelastic," having both elastic and viscous properties. Essentially, the material is stretched when it is bonded to a vibrating surface. There are two types of damping material, homogeneous or free-layer damping and constrained layer. Homogeneous/free-layer materials are generally vinyls which have platelet-type fillers in them. As the material to which they are applied vibrates, the platelets slide against one another, and this friction between platelets converts the vibration energy into heat.
Free-layer damping material, made from stable vinyl and other polymers, work over an extremely wide frequency (50 to 5,000 Hz) and are very stable over a long period of time (10 to 20 years or more).
The other type of damping is constrained layer. Here, the viscoelastic polymer is homogeneous (not filled) and is sandwiched between two plates. These are bonded together, usually with a structural epoxy adhesive. The ratio of the base thickness to the constraining plate thickness is between 1:1 and 4:1. Better damping is achieved at the 1:1 ratio.
Polymer thickness is determined by the frequencies to be attenuated. Generally, the thicker the polymer, the lower the frequency and conversely the thinner the polymer, the higher the frequency.
Polymer selection is a function of the operating temperature of the material to which it will be applied. Again, each formulation has a finite temperature range over which it will be effective. This temperature range is somewhere in the neighborhood of 80°F, for any given polymer, so that one that works are -20 to 60°F will not be suitable for the 80 to 160°F range. All manufacturers of damping materials give the specific temperature range, frequency, and thickness of the constraining layer.
Damping treatment can achieve considerable noise reduction. As much as 14 dB have been achieved by this method alone, but it must be properly designed.
Gasketing: Although this is a subject which generally receives little attention, it is essential to achieving the full potential of a cabinet enclosure. Gasketing materials are generally soft, pliable foamed vinyls, urethanes, or neoprenes, although other materials are used.
The most important characteristic they exhibit for noise control is sealability or conformability to the irregular surfaces between which they are placed. Closed-cell materials make better gaskets than open cell, but their design is more complicated because they are harder to compress due to the entrapped gas in each cell. This is overcome by extruding them with cross sections having profiles such as "H, X, and Z." Costs for gaskets are minimal compared to the noise reduction they can achieve. A good gasket material properly designed for the application will easily reduce noise by 6 dB or more, and cost about $0.25/linear ft.