Noise and noise reduction are major problems in many work areas. Most of us are aware of many of the issues surrounding noise in the factory. It can cause damage to hearing, it induces stress in workers, reduces worker efficiency and is generally undesirable.
Beyond the effect on workers, however, there is a cost to the machinery that produces noise. Noise is caused by vibration, shock or some other action that not only causes sound to be transmitted through the air, but also causes a weakening of structures through fatigue, wear and, in the case of shock loads, catastrophic damage.
One of the increasingly important factors in product design is avoidance and abatement noise. As with other important factors, characteristics that cause noise and its control must be considered in the initial design. Not considering these factors can result in more costly, less adequate "retrofit" solutions that should be avoided.
E-A-R Specialty Composites is one company that has made a business out of noise reduction. Their products are based principally on proprietary urethane materials, vinyl foams and several solid materials that control noise, vibration and shock. They market their products worldwide, enabling OEMs to design in feature to control unwanted material machinery, precision equipment, health and medical products, and vehicles of all types.
What to consider first
In any situation where noise is generated, all vibrational sources that work in the product must be considered. Along with this, the paths through the product that will transmit the noise must be located.
If a product is a prototype, it is possible to make sound level measurements that provide references and guidance in "fine tuning" the design. This information will also allow accurate determination of engineering controls that are used for noise control. Instruments are available on the market today that mimic the human ear in response to sound, making accurate, quantifiable results possible.
Materials are a key ingredient in the sound transmittal and noise equation. Often, desires and requirements for noise control are at direct odds with other characteristics of the design. For example, cabinets and housings that are made of lightweight, rigid materials may be effective at reducing cost and improving efficiency, but they may offer little if any contribution to noise reduction. They may actually increase noise through vibration of their own.
Ventilation grates, while necessary for cooling parts of the machine, do little to contain noise inside the cabinet. Both heat and noise are allowed to escape.
It may be desirable to mount motors and drive systems tightly to casings and other internal components to save space and weight, but this also increases the danger of shock damage if the unit is dropped or struck with another object. These tightly mounted items also provide a superb source for noise transmittal throughout the unit.
Once the sources and methods of noise transmittal are established, methods of noise control can be addressed.
How to control it
Over recent years, quite a bit has happened in the area of energy control materials. Most of this work in the area of sound energy has focused on the use of vinyl and urethane materials. These materials can be formulated and engineered to meet specific requirements for weight, space, temperature and other environmental requirements. Where the need is there, custom formulations can be developed to perform in specific applications.
These materials are manufactured into products that take the form of sheets, foams, tiles and combinations of these. In use, they have the ability of reducing vibration and other forms of mechanical energy by dissipating it as low grade heat.
Vinyl damping sheets are made in a wide range of thicknesses and ranges of thermal stability. From these, shapes can be died out to fit particular locations and needs. Many come with peel-and-stick pressure sensitive adhesives for ease of installation.
Where needed, products have been formulated to meet specifications for flame resistance, as well as smoke and toxic gas emission. Metal foil facings layers can also be added to enhance flame and heat resistance performance.
Products are also manufactured as multi-layer composites. Ultra-thin constrained layer materials are also manufactured. The multi-layer materials are made using foams with sheet damping materials. This combination can be used to produce products that perform over a broader range of temperature than sheet products alone. The ultra-thin materials are made by constraining viscoelastic pressure sensitive adhesives between metal foils. These are able to provide damping with minimum weight and thickness.
Structural vibration isolation requires use of highly damped elastomers. These effectively address both vibration and shock energy. These products also require minimum sway space and rapidly restore equilibrium to the system. These structural materials can be provided in sheet form or can be custom molded to specific configurations. Use of metal or other rigid inserts in the product adds physical strength and widens the range of possible installations of the part.
Airborne sound control is provided by combining a variety of materials. Open cell foams and fiberglass blankets are excellent at removing sound energy from an air space while dense sheets will provide a sound barrier. Often in enclosed equipment, the normal walls provide an effective sound barrier and noise reduction is easily accomplished by adding absorptive foam or blankets.
What about foams and barriers?
Polyurethane acoustical foams achieve optimum sound absorption by controlling the air permeability of the foam cell structure. Effectiveness of a given product is judged by the overall predictability and consistency of the air flow resistance levels in the foam.
Design flexibility is achieved by combining the urethane foams with a variety of facing materials. These add durability, resistance to contamination and acoustical performance. In addition, various surface treatments can be added.
Barrier materials act as sound reflectors by virtue of their mass. As such, they restrict or reduce the amount of sound transmitted from one space to another. Barriers are usually highly loaded vinyl or urethane based sheets. They are formulated to not only provide a sound barrier, but also retain their flexibility and mechanical properties over a wide range of temperatures and environments. In addition, these can be faced with materials, including metal foils, film products, cloth or other materials.
Acoustical properties of barriers are best at middle and high frequencies. By using these in conjunction with acoustical foams, performance can be altered to achieve specific reductions across a discrete frequency range.
Noise control is becoming more important in all areas of our lives. Studies have proven time and again that exposure to elevated levels of sound, whether in the factory or at a rock concert, will damage our overall hearing ability.
As we look at our work areas and as we design new products, concern with noise reduction should not be overlooked. By using these types of materials, significant improvements can be achieved. As with E-A-R Specialty Composites, there are companies that specialize in meeting requirements for noise reduction and control of other types of mechanical energy that can hurt workers of the machinery itself. Achieving noise reduction and energy absorption is both important and something that requires a degree of understanding of how the energy is generated and transmitted. In addition, methods of avoiding it and constraining or converting it to a less harmful form of energy often requires more than a casual look.
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|Title Annotation:||control of mechanical noise; damping materials|
|Date:||Oct 1, 1991|
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