ANY EFFECTIVE effort to lessen noise pollution dictates a familiarity with its most basic constituent - sound. How sound is generated in industrial machinery and how sound can be inhibited are the subjects of this article.
The sound a person hears can be either pleasant or annoying depending upon the volume and frequency of that sound. Annoying sounds most commonly are referred to as noise. Accurate measurement of sound is very important in the analysis and abatement of noise.
Sensitivity of the human ear varies with the frequency and pressure level of the sound. Therefore, in measuring and analyzing noise, a weighting system must be employed. Originally, three weighting scales were established such that for low volume sound scale "A" was used. Scale "B " was used on medium volume sound and scale "C" on high volume sound. The "A" scale., however, has emerged as most closely approximating the ear and is the most commonly used weighting scale.
Measuring instruments consist of a microphone, amplifiers, weighting circuits and an output meter. To get an accurate reading to determine the noise level of a given source, background. noise must be considered. Anechoic (sound absorbing) or more often semi-anechoic rooms are used to eliminate background and reflected noise.
SOURCES OF MACHINERY NOISE
Sound is generated whenever there is sufficient disturbance in a medium to cause a detectable pressure fluctuation. In the operation of industrial machinery where moving parts and fluid flow are involved, there are numerous disturbances which may be considered noise sources.
Gear noise is related directly to the design, manufacturing technique and operation of the gears. Friction associated with the rolling and sliding of metal to metal contact causes vibration and noise. Accuracy of tooth form and spacing is necessary to reduce the small changes in acceleration and impacts that cause the vibration and excitation resulting in excess noise.
Although bearing noise generally is small compared to other noise sources, it still deserves consideration. In journal bearings, noise is caused by relative motion between the shaft and the bearing surface. Inadequate oil supply and irregularities in shaft or bearing surfaces tend to increase noise. With ball bearings, the ball movement between the inner and outer races generates noise. Again, imperfections in the surfaces and inadequate oil supply can cause excess noise. Tilt pad type bearings have mechanical movement of the pads that can add to the noise generated.
Noise generated by couplings is due mostly to windage and can be greatly reduced by proper design of the coupling guard.
Noise caused by a mechanical imbalance generally is very small. However, noise generated from the mechanical or hydraulic reaction associated with this imbalance can be quite noticeable. Mechanical or hydraulic forces caused by imbalance are proportional to the square of the speed. Therefore, the effect of an imbalance becomes of greater concern at higher speeds.
Sound produced by a machine normally is related to input horsepower. Actual increase in noise associated with a given increase in power can be determined only by test data. However, a general rule of thumb for centrifugal pumps and compressors is that the sound pressure level will slightly more than double with a doubling of input horsepower (approximately 3 to 4 dbA).
In general, high speed machines seem noisier than low speed machines. However, there is no simple formula which relates rotational speed to noise. The increase in noise generated is a function of speed, the ratio of rotating mass to the base, type of machine, mounting configuration, machine alignment, excitation frequencies and structural resonance. Test data is the best way to establish the effect of rotational speed on a given machine design.
In addition to mechanical noise, pipe flow noise presents problems as well.
The major cause of noise in piping systems is related directly to turbulent flow. In most every industrial piping system, turbulent flow is present. Turbulent flow is caused by various items such as high velocity, changes in pipe diameters, bends, restrictions and obstructions in the flow path.
This piping noise may well exceed the noise radiated from the machinery installed in the system. Therefore, it is important to consider reducing the pipe noise as well as the machinery noise.
Aerodynamic noise in compressors. Noise associated with the flow of gas through a system is referred to as aerodynamic noise. In centrifugal compressors this aerodynamic noise is generated mostly by turbulence in the flow path. Centrifugal compressors work on the basis of increasing gas velocity then diffusing the velocity head to pressure head. This, of course, requires high velocities and, therefore, considerable turbulence. This turbulence also is increased by Von Karman vortices which follow the trailing edge of the impeller blades.
If flow into the eye of the impeller is turbulent, this will tend to increase the noise as well. Therefore, it is important to have smooth, well finished flow passages in the case and diffuser. Obstructions and abrupt t changes in passage diameters also should be avoided.
When a compressor operates in a surge condition, a hunting situation is established and flow reversal occurs. This, of course, increases turbulence and flow noise. Since there is also a considerable amount of vibration associated with compressor surge, this will increase the mechanical noise as well.
The terms choke or stonewall describe the maximum flow condition in the compressor. In this situation, the relative condition of sonic flow occurs in the compressor diffuser. This generally results in an increase in the noise due to the increase in turbulent mixing downstream of the sonic flow point and also due to the shock mechanism. Both surge and choke situations should be avoided not only for noise reduction but also to prevent equipment damage and maintain system performance.
Hydraulic noise. Noise generated by turbulent flow of liquid in a pump is similar to aerodynamic noise in compressors. High speeds associated with the centrifugal impeller and vortices created by the impeller blades result in added turbulence. It again is necessary to maintain smooth, unobstructed flow passages to keep turbulence to a minimum.
In both pumps and compressors, increased turbulence is associated with a decrease in unit efficiency. Operation at or near the best efficiency point will tend to reduce hydraulic noise.
Pump cavitation is a situation where, due to a local pressure drop, cavities filled with vapors are formed and then collapse at higher pressures than the liquid vapor pressure. In this situation, the cavity collapse locally exerts forces on the pump. This not only in- creases turbulence and flow noise but increases mechanical noise and can cause damage to the equipment.
Since viscosity of a liquid is greater than that of a gas, hydraulic turbulence associated with a given fluid velocity will be less. Hydraulic noise, therefore, is less prevalent than its counterpart, aerodynamic noise.
Electric motor noise. Electric motor noise is complex and a combination of the following:
· Windage noise due to cooling air flow turbulence .
· A siren or whistling noise produced by the fan blades passing close to stationary members
· Rotor-slot noise caused by rotating open slots
· Noise generated by combination of rotor bars and stator slots
· Noise due to high flux density
· Bearing and other mechanical noise such as misalignment and dynamic unbalance
· Unbalanced line currents in three-phase power supply can increase noise produced by the motor.
INHIBITING SOUND
Now that the many sources of machinery noise have been delineated, what methods of sound control can be employed?
The best means of controlling sound is to prevent noise generation or at least reduce its level. In some cases this is possible, but generally, machinery is designed with performance in mind, and to change internal components to reduce noise also can reduce efficiency. Therefore, it is necessary to find other ways to reduce the noise levels that reach the listener.
The basic methods of reducing noise are:
Sound absorption. Machinery installed in enclosed rooms has the added problem of noise that reverberates off the walls, thus increasing the noise level the operator will hear. This reverberant noise can be reduced substantially by lining the. walls of the room with a soft porous material that will absorb the sound and reduce sound reflected back into the room.
Sound isolation. A common way of reducing machinery noise is to place a sound barrier between the sources and the listener. Effectiveness of this barrier is described by its transmission coefficient, which is defined as the fraction of the incident sound transmitted through the barrier.
Sound is transmitted through a solid barrier by forced vibration of the wall caused by sound wave striking the surface. The heavier, more rigid, and more airtight the barrier is, the more resistant it is to sound transmission. Internal dampening and bending stiffness effect sound transmission as well.
Vibration isolation. Airborne noise generated by a vibrating part generally can be reduced by isolating this part from the rest of the structure or machine. The simplest isolating device is a flexible support that reduces the magnitude of the: force that would be transmitted to the structure or machine. By the same token, an isolator may reduce the amplitude transmitted from a vibrating support to a part of the machine generating excess noise.
Vibration dampening. Vibrating parts have certain resonant frequencies. When an exciting force has the same frequency as the resonant frequency of that part, the amplitude will be limited only by the amount of dampening in the system. Noise generated by this resonant part can be reduced by increasing the dampening in the system.
In the absence of all dampening, the amplitude of a vibrating part will go to infinity. Of course, there is always some dampening in all systems. Adjustment of this dampening is one of the most important factors in vibration and noise control.
Mufflers. Silencers or mufflers generally are divided into two categories: absorptive and reactive. The distinction between these two is somewhat arbitrary since nearly all mufflers use a combination of both to accomplish noise reduction.
Absorptive mufflers rely on the dissipation or absorption of the sound into sound-absorbing material. Internally-lined ducts, plenum chambers and baffle mufflers fall in this category. They often are used with fans, jet engines, gas turbines, and air ejectors.
Reactive mufflers utilize the reflective properties of sound and do not rely on sound absorption. Conical connectors, expansion chambers, resonators and tail pipes are used in these mufflers to accomplish noise reduction. The most common of these is the automobile muffler.
Acoustic enclosures. When a large amount of noise reduction is desired, an acoustic enclosure is often the most direct solution. The enclosure serves as a noise barrier that completely encloses the machine. It is de- signed such that its resonant frequency and the noise frequency of concern do not coincide.
Enclosures with hard, reflective walls may have an internal sound pressure level higher than if no enclosure was present. This can be avoided by lining the enclosure with a sound absorptive material.
The number and size of the openings in these enclosures are of concern as they can greatly reduce its effectiveness. Of course, each individual plant situation will differ in types of noise and possible control methods. As a practical matter, it may not be possible to reduce noise as much as we would like. Nevertheless, by being aware of the sources of sound pollution and possible alternatives it is possible, in most cases, to reduce greatly the problem of industrial noise pollution and make the work place safer and more productive for worker and manager alike.
