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Introduction to Noise Control
Definitions and Background
What is Noise? Noise is disagreeable or unwanted sound. In many instances, noise is a relative definition of a sound since one person's music may be another person's noise. It is difficult to give a very clear definition of an irritating noise. Generally, noise is an unwanted
sound, regardless of its intensity or duration.
Noise Pollution has been recognized as a major threat to human well being. Much discussion and legislation has evolved in an attempt to recognize and combat the problem of noise pollution. It has been recognized that noise, of sufficient intensity, can damage hearing and be
classified as a hazard.
In combating the problem of noise pollution it is necessary to use a means of measuring noise levels and a system of classification. The decibel is a number which relates sound intensity or sound pressure. When most people use the term decibel or discuss noise levels in decibels they are referring to decibels as related to the "A-weighted" scale or, dBA. The A-weighted scale parallels the sensitivity of the human ear and uses the lowest audible sound that the human ear can detect as the reference point for determining the decibel level of a noise.
The reference intensities used above represent the threshold of audibility where sound is just loud enough to be heard. At 140 decibels or more acute pain is experienced. Some common noise values are as follows:
Ordinary conversation - 60 dBA
Heavy traffic - 80 dBA
Cocktail Party - 90 dBA
Moving subway train - 100 dBA
Riveting gun - 130 dBA
Hard rock band - 100 to 138 dBA
Jet plane heard at close range - 150 dBA
Any noise rating above 80 dBA produces physiological effects and any long term exposures at much or above 90 or 100 decibels will cause permanent damage to a person's hearing. An increase of 10 dBA is a doubling of loudness with respect to the human ear.
Noise generally consists of many tones with varying rates of vibration or frequency. The frequency expressed in cycles per sound or hertz (HZ) usually is in the range of 20 to 20,000 cycles per second. The ear is not very responsive to very low or very high tones as it is to the tones of medium frequency. The dBA scale matches the response of the ear, and is therefore well suited for evaluating noise as it relates to human beings.
Noise originates from a sound source and the ultimate recipient is the ear. Distinction is made between airborne sound and sound traveling through solid objects or structural-borne sound. In controlling noise, the distinction is very important as to the two types of sound and, particularly, the medium of propagation. Sound waves are one form of a general class of elastic waves that can occur in any medium having the properties of mass and elasticity. Mass, or inertia, allows displaced particles to transfer momentum to adjacent particles. The elasticity of the particles tends to restore a displaced particle to its original position like a spring.
The propagation of momentum from one particle to another particle is the manner in which sound waves travel. All materials have both mass and elasticity and can, therefore, carry sound waves or allow sound waves to be propagated within the material. Whenever an object moves back and forth, it disturbs the air next to it, and causes the air to move back and forth. In a much smaller scale, the individual particles making up the air vibrate and transmit vibrations to particles surrounding the vibrating particle.
These vibrations propagate and in this manner, waves propagate through the air. As the particles vibrate they cause variations in normal atmospheric pressure and as these pressure variations propagate through the air, and reach the listener, the sensation of sound is produced. The eardrum is sensitive to the pressure variations produced by the vibration of the individual particles.
Structure-borne sound is sound carried through solid objects such as steel of air and metal differ, the speed of sound waves traveling in these media will also differ. Solid objects vibrating also produce airborne sound. The best example of this would be a drum, which produces sounds as a consequence of setting the stretched skin or membrane of the drum into vibration. Any solid object, with significant area, which is set into mechanical vibration will produce airborne sound by vibrating the air mass surrounding it., concrete, wood, or any flexurally stiff material.
A typical example of structure-borne sound is the sound produced when a railroad train moves over rails and the rumbling can be detected several miles away by placing one's ear to the rail. The train sets mechanical vibration in the rail and the rail is set in flexural
vibration. The waves propagate just as waves in air propagate by vibrating the individual particles and having each particle excite its neighbor.
The noisy industrial machine can be viewed as a sound generator. The noise generated, generally, will be made up of sound waves which encompass the spectrum of 125 cycles per second to 8,000 cycles per second with a certain frequency band being dominant. The noise
emitted is either direct airborne sound or noise generated by mechanical vibrations setting up vibrations in sheet metal panels or large solid areas.
Sound waves decrease in length as the frequency increases or, more simply, the wavelength is inversely proportional to the frequency. Generally, it is easier to control noise in the higher frequency bands than the lower frequency bands since it is difficult to absorb sound which is made up of long wave lengths.
Approach to Analysis of Problems and Applications of Acoustical Materials
As pointed out earlier, the end recipient of noise is the human ear. An expedient solution to a noise problem comes through the use of earplugs or other ear protection. Legislation has taken the position that hearing protection of this nature will not be tolerated if effective means for controlling noise at the source exists. The first step in quieting a noisy environment is to clearly identify the problem.
An ideal starting point is to obtain the dBA levels emitted at various frequency bands. This should also be done at several positions around the machinery. This can be done with a simple sound level meter, however, with more sophisticated equipment, more detailed data can be compiled regarding the frequency characteristics and nature of the sound source.
In approaching the majority of industrial applications from a practical standpoint, the problem should be looked at keeping four basic solutions in mind:
Absorption of airborne sound
Blocking airborne sound
Damping
Vibration isolation
For most applications a solution will consist of one to all of these categories.
Absorption of airborne sound:
Most industrial noise generators such as air compressors, business machinery, textile machinery, heavy equipment, and the like have enclosures. The enclosure is basically made-up of sheet metal or plastic construction and its primary function is for cosmetic purposes or as a safety feature to protect the work force from a possible hazard.
Whatever the case may be, it is convenient when an enclosure, of sorts, is present and can be utilized. As we mentioned previously, some type of evaluation of the sound spectrum is desirable.
If analysis indicates the major problem lies in the medium frequency range, 500 hz to 4,000 hz, acoustical absorption can be a good starting point. By lining the interior walls of the enclosure with an absorber a significant amount of the acoustical energy incident on the walls can be absorbed and dissipated.
How Sound Absorbing Materials Reduces Noise Energy:
As sound waves, which are pressure waves traveling in air, pass through a porous open cell material the movement of the air molecules through the openings across the strands and membranes generates heat through friction which takes energy away from the sound wave.
In an enclosure or partial enclosure application, a sound absorber cannot reduce the sound energy below what is being produced. The ideal situation is to absorb enough energy so the reflected sound waves do not reinforce the sound energy being produced.
To illustrate this point, if a noise source produces 90 db of sound energy and a partial or full bare metal enclosure is put over the noise source, the reflected sound will reinforce the sound produced and the level within the enclosure may increase to 100db. By lining the enclosure with a properly selected absorption product the energy of the reflected sound waves do not reinforce the incident sound wave. The sound level within the enclosure can be reduced to 90 db, but not below this level since this is what is being produced.
Blocking Airborne Sound
In areas where low frequency airborne sound is predominant, an absorber is limited in its efficiency. At frequencies from 250 hz to 750 hz, a barrier can be extremely effective if a complete enclosure exists. The major stumbling block associated with barrier materials is that they are best utilized with complete enclosures.
A small amount of open area, even 1% opening will significantly reduce the barrier’s performance. A barrier can also be utilized effectively on the exterior of an enclosure where it may not be feasible to treat the interior. This also applies to pipe applications, where a “pipe wrap” is required. The barrier essentially acts as a second skin, which due to its mass, reflects incident sound waves back towards the center of the pipe.
Vibration Damping
Most acoustical absorbers have some damping characteristics, particularly if used with a pressure sensitive adhesive. If it is determined that vibration of sheet metal is causing the noise problem, rather than airborne sound from the machinery itself, vibration damping is needed to dissipate the energy within the sheet metal.
Damping materials can be applied to the sheet metal to help stop noise generated by solid panels vibrating. Damping can work effectively
when there is at least a 1 : 1 ratio between the thickness of the damping pad and the panel to be damped.
Panels thicker than .090 inches to .125 inches cannot effectively be damped by extensional damping pads. Vibration damping of thick panels or thin metal sections can be effectively achieved by forming a constrained layer damping system. This is achieved by applying a layer of viscoelastic material and a rigid outer panel to constrain the viscoelastic.
This produces a composite which allows a thin viscoelastic layer to be put into shear deformation.
Floor Mat
Another common application for a barrier material is in the areas where partial blocking of incident noise is desirable. A typical example of this would be the interior or exterior of the firewall of a vehicle. The barrier reflects the noise towards the engine compartment keeping the cab or operator area from receiving it. An acoustical floor mat, which is a barrier with a functional facing material, also reflects sound waves back through the floor to deep the cab area quiet.
