Controlling noise in foundries.
The nature of products and processes in foundries makes it difficult to achieve a quiet work environment. Castings are "hard" objects. When these hard objects collide or vibrate against themselves or other hard surfaces--such as conveyors, chutes and bins--loud sounds result.
The short-duration impacts cause high-intensity, high-frequency noise that falls within the most sensitive frequency range of human hearing.
Many foundry operations involve the impact of hard bodies. Vibrating conveyors feed material to furnaces, and at each directional change in the material flow, the feed material may drop a foot or more. Conveyors also move finished castings from point to point.
The shakeout of molding sand and cores is another source of noise. At the cutoff station for sprues and runners, waste pieces are tossed into a chute and castings dropped into a bin. Other equipment, such as pumps and fans, also contributes to the noisy environment.
Fortunately, there are methods for controlling noise generated by such equipment. Noise problems can be attacked at the source with vibration damping and with traditional methods of acoustic absorbers and barriers. With careful analysis and design, significant noise and vibration reductions can be achieved. Vibration Damping
Damping is defined as a measure of the capacity of a material or structure to dissipate energy and is an inherent property of a material or structure. The primary physical effects of damping are that it limits the steady state motion of structures at their natural frequency and increases the rate of decay of vibration. This increased rate of decay of vibration reduces the "ringing" sound associated with repetitive impacts.
Damping also increases the fatigue life of structures and the decay of vibration with distance--which may be more important in some instances than reducing noise.
However, it is the reduced response to repetitive impacts that will control noise in the processes described earlier. Thus, the higher the damping in a structure, the less noise will radiate from it when it is struck by an object.
Since the inherent damping in structural metals is very low, it is necessary to increase the damping of structures made from these materials by the use of appropriately designed damping systems. These damping systems use some form of highly dissipative (or "lossy") material, usually a rubber or plastic compound. Known as viscoelastic materials, these materials "absorb" the energy of vibration by converting it into heat. Categorized by the arrangement of the damping material within the system, there are four types of damping systems.
Freelayer Damping System--In the freelayer damping system, the damping material is placed on the structure to be damped. As the structure is excited by impacts or other forces, it vibrates with the damping material, alternately stretching and compressing the damping material as it goes through a cycle of vibration. Figure 1 shows the cross section of a panel structure with a freelayer damping system. The stretching and compressing work the molecules in the viscoelastic material and dissipate the vibration.
To dissipate appreciable energy, the damping material must be stiff. Effective freelayer damping materials are made from various plastic, rubber and asphaltic-based compounds. Because the thickness of the damping material must be 1/2-2 times the thickness of the underlying structure, freelayer damping systems tend to be heavy, precluding their use in very weight-critical applications such as aircraft and spacecraft. However, the availability of low-cost, asphaltic-based compounds makes them more common in high-volume applications, like automobiles.
Constrained Layer Damping System--in this system, damping material is placed against the structure and covered with a "constraining layer." Thus, the damping material is sandwiched between the structure to be damped and the constraining layer. The constraining layer is usually a metal sheet in industrial applications. Constraining the damping material causes it to undergo a shearing action when the structure vibrates (rather than extension and compression as in the freelayer) to dissipate the energy of vibration.
The thickness of the damping layer in a constrained layer damping system can be 1/20-1/4 the thickness of the underlying structure. The constraining layer thickness can be from 1/10 as thick as the underlying structure. Because the damping and constraining layers can be relatively thin, the constrained layer damping system can be thinner and lighter than a freelayer damping system with equivalent damping, despite using a metal layer.
The thickness of the constraining layer has a great effect on the amount of damping attained. Thicker constraining layers give higher damping, assuming that the damping material thickness and stiffness are appropriate for the design. High levels of damping can be achieved in these systems.
Tuned Damper--The tuned damper is a spring-mass system that is tuned to vibrate at the same frequency as the structural vibration to be damped. It is similar to tuned absorbers or dynamic absorbers discussed in elementary books on mechanical vibrations. The difference is that the spring in the tuned damper is made from a viscoelastic material instead of being a completely elastic (metal) spring. The mass on the tuned damper vibrates out of phase with respect to the structure on which it is mounted, thereby applying a force in opposition to the motion of the structure. Also, when the tuned damper vibrates, the viscoelastic spring is worked and dissipates energy. Figure 3 shows a schematic of a vibrating structure with a tuned damper used to control a resonance. The tuned damper only works in a narrow frequency range around the design frequency. Because most noise problems involve a broad range of frequencies that need to be controlled, the tuned damper has limitations unless the noise has a dominant tone. The tuned damper is often used in vibration problems involving equipment such as fans or pumps (or the structures attached to or supporting them) that run at constant speed.
Direct Load Path Damper--A direct load path damper works by having a portion of the load go through, or be supported by, the damper. It differs from the freelayer and constrained layer damping systems in that the damping material is directly-stretched or sheared, rather than the motion being induced by bending action. Also, whereas freelayer and constrained layer damping use area coverage, the direct load path damper is located at a "point" or discrete location. Like the tuned damper, it is difficult to control a noise problem of a chute, bin or conveyor because of the point location of the damper, except in special circumstances. However, the direct load path damper is useful in vibration problems where the damper can be placed between the ground or the foundation and a vibrating part (in essence, becoming a brace that incorporates damping material). Applications include equipment such as fans, pumps and piping, and structural framing.
In freelayer and constrained layer damping systems, the actual movement and the heat generated are very small. In tuned dampers and direct load path dampers, the movement of the damping material can be large, and the possibility of heat buildup must be considered.
Damping systems shouldn't be designed by trial and error. Unfortunately, the deceptively simple nature of freelayer and constrained layer damping systems--combined with the availability of off-the-shelf materials--leads many people to "slap something on" a structure without understanding the underlying vibration characteristics of the structure or design parameters.
When common materials are used for the damping material and/or inappropriate thicknesses of the damping material and constraining layer are selected, the result is much effort with marginal performance. In some cases, there may not be improvement.
Constrained layer systems are difficult to design because damping is a function of many variables, including: damping material stiffness and thickness; constraining layer stiffness and thickness; the flexural rigidity of the base structure and the vibration modes involved; and the temperature of the structure and frequency of the vibration.
The properties of the damping material also must be known as a function of temperature and frequency to perform an effective design analysis. With so many variables at play, many combinations of designs must be evaluated. Computerized evaluation of the appropriate equations are imperative.
Daily experiences teach the basic concepts of acoustic absorption and sound insulation. Everyone notices how "live" an empty room is (lack of acoustic absorbing materials) or the effect of closing a door to a noisy room (blocking the airborne transmission of sound).
When a sound wave hits the surface of a wall or partition, three things happen to the energy: some is reflected, some is absorbed and some is transmitted through the wall.
Acoustic Absorption Materials--Acoustic absorption materials are usually highly porous materials such as mineral wool, fiberglass, foams and other materials. When sound waves enter the interconnecting pores, bubbles or interstices, the air molecules rub against the fibers or particles forming the small spaces. The acoustic energy is successively damped out and converted into heat. Effective absorbers are usually 1/2-3 in. in thickness. The thicker the absorber, the more effective it is at low frequencies, although the increased thickness has little effect on the high frequencies. Because the amount of absorption depends on frequency, the absorption is usually given for the octave bands between 125 Hz to 4000 Hz. (An octave is a doubling of frequency; there are six octaves between 125-4000 Hz, with center frequencies of 125, 250, 500, 1000, 2000 and 4000 Hz.)
The noise reduction coefficient (NRC) is the average of the four absorption coefficients at 250, 500, 1000 and 2000 Hz. This gives a single number for comparing different materials. However, material selection shouldn't be based on the NRC alone. One material could have a higher NRC than another but be less suitable for a particular application because the absorption coefficient is lower, for instance, at 125 or 4000 Hz (which may be especially important for a particular problem).
Absorbing materials are normally used in enclosures around equipment, but they also are used to absorb acoustic energy from nearby sources. Figure 4 shows a panel, lined with absorptive material, hung from a ceiling to reduce the noise from a roof ventilator. The panel's size is related to the fan's diameter and the distance of the panel from the fan. Such an installation can provide a 12 dB(A) noise reduction.
Acoustic absorption materials can also increase the overall absorption in a room. In a highly reverberant (acoustically reflective) room, even a small noise source can cause a high noise level because of repeated reflections. By increasing the room's absorption, reflections and, therefore, noise levels, are reduced. Thus, hanging absorptive panels from the ceiling or on the walls can help quiet noisy environments.
Acoustic Barrier Materials--Barrier materials block the airborne transmission of acoustic energy. In contrast to the lightweight, porous materials that comprise most absorbing materials, effective barriers are typically heavy, dense materials. When the sound wave strikes the barrier, the pressure sets the material in motion. The lighter the material, the more easily it can be set into motion.
Also, the tower the frequency of the sound wave, the more easily the barrier is vibrated. For example, people can feel the pounding bass notes of loud music against their chest or feel them move their clothes. The same can't be said for high-frequency sounds.
If the harrier vibrates, it retransmits the impinging acoustic energy from the other side. These facts are the embodiment of the empirically determined mass law. The mass law states: if for a given wall the frequency is doubled, the transmission loss is doubled (increased by 6 dB); or, if for a given frequency the mass of the wall is doubled, the transmission loss is doubled. Thus, it is more difficult to block low-frequency sounds, and the heavier the barrier, the more effective it is. Therefore, a concrete block wall is more effective than a plywood wall.
Combination Absorbers and Barriers--Products are available that combine acoustic absorption and transmission loss. These products are typically foams with a heavy, limp barrier layer bonded to one side or quilted blankets using fiberglass and a barrier material. The barrier material is usually a vinyl, though sheet lead also is used. If the product is bonded to a rigid surface, a decoupling layer of foam separates the barrier from the surface. This prevents vibration of the barrier from being directly transmitted to the rigid surface.
Helmholtz Resonator--The Helmholtz resonator (or cavity resonator), consists of a cavity that is connected to the noisy space through a short neck. The compressibility of the air in the cavity acts as a spring; the air in the neck tends to move as a unit and acts as a mass. When the plug of air vibrates in the neck, the viscous losses against the sides of the neck dissipate acoustic energy. The dimensions are chosen to give a specific frequency of vibration. Therefore, the Helmholtz resonator is similar to the tuned damper discussed earlier--it is tuned to work at a particular frequency and has a limited frequency range over which it is effective.
Simple Changes Improve Damping, Control Noise
One foundry significantly improved its work environment its through simple changes.
The schematic below shows a vibrating conveyor used in a foundry feed fine shavings to 200 lb pigs to a furnace. During feeding the average noise level, as measured by the LEQ (equivalent level) was 102.5 LEQ dB(A). When heavy parts dropped from one conveyor to the next at rights-angle turns, the level reached 125 dB(A).
A constrained layer damping system was designed to reduce the noise levels caused by the vibration of the feed material against the sides and bottom of the conveyor. The side walls and bottom of the conveyors were 3/8-in. steel plate. Although adequate levels of damping could be achieved with 1/8-in. constraining layers (1/3 the thickness of the base structure), a 3/8-in. constraining layer was used on the floor of the conveyor pans for durability, and gave high levels of damping.
Because the sides experience less wear, 1/4-in. constraining layers were used, which still gave high damping, although not as high as on the floor. A 1/8-in. damping material was chosen for the combination of 3/8-in. base structure and 1/4 in. to 3/8 in. constraining layer.
This system resulted in a 15 dB(A) reduction in the sound pressure level that occurs when parts drop from one conveyor to another. A greater difference would have been possible, but a smaller conveyor nearby wasn't damped at the time the noise measurements were taken. It was "short-circuiting" the noise reductions. Noise Control
In addition to the noise created by scrap material vibrating against the sides of the conveyor, a portion of the noise was created by part-to-part noise of the material vibrating against itself. The part-to-part noise couldn't be controlled by the constrained layer damping system.
To control this noise, a combination acoustic absorber/barrier material was incorporated into a cover over the conveyor. The absorbing layer of foam absorbs the acoustic energy, and the barrier layer prevents any remaining noise from passing through the sheet metal cover.
The conveyor system also included removable covers over the transition from one conveyor to another. These, too, were lined with the absorber/barrier material. Barrier strips sealed the opening in the transition cover to the conveyor.
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|Title Annotation:||includes related article|
|Author:||Kluesener, Matthew F.|
|Date:||Feb 1, 1994|
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