This article focuses on pneumatic tools that are among the most damaging sources of noise pollution. The National Institute for Occupational Safety and Health estimates that more than 30 million workers in the United States are exposed to hazardous noise, costing the economy about $1 billion every year. There is no magic solution to all noise control problems, and this remains true even when we are dealing just with pneumatic tools. Each case must be considered separately, and the solution to be used must satisfy the often conflicting demands of at least five criteria simultaneously. The sound power level produced by a pneumatic tool is a product of many interrelated parameters: operating pressure, pressure drop, expansion volume, exhaust air velocity, speed of device, exhaust air, airflow, length of exhaust path, and tool power. Good standards are available for evaluating the noise of many components, and suppliers provide data on their products that can be used for comparison.
While pneumatic tools are vital to many areas of modern life, they are also among the most damaging sources of noise pollution. Most of us have at some time held our hands over our ears to escape the cacophony of jackhammers, but those who need to work daily with these tools have a more serious problem. The National Institute for Occupational Safety and Health estimates that more than 30 million workers in the United States are exposed to hazardous noise, costing the economy about $1 billion every year.
There is no magic solution to all noise control problems, and this remains true even when we are dealing just with pneumatic tools. Each case must be considered separately, and the solution to be used must satisfy the often conflicting demands of at least five criteria simultaneously: the acoustical criterion, which specifies the sound level required; the aerodynamic criterion, which specifies the maximum acceptable average pressure drop; the geometrical criterion, which specifies the maximum allowable volume and restrictions on shape; the mechanical criterion, which specifies the permissible materials (which must be durable and require little maintenance); and the economic criterion, which is vital in the marketplace: The techniques used must be as inexpensive as possible, not only in the initial purchase price but also in operating costs.
These five criteria do not make the problem of silencing impossible, but they do restrict the use of muffling techniques commonly used in other fields.
To meet the first criterion, one must quantify the sound level to be reach ed. It is not sufficient to say "make it quieter"; a target must be set. The exposure level for an operator of pneumatic tools was set at 80 dBA for an eight-hour-duration exposure by the Occupational Safety and Health Administration in 1993. How does this relate to the noise level produced by the tool? Those who have been involved in noise measurement know it is possible to come up with practically any number for an answer, depending on the method of measurement.
If we set as our objective an 80-dBA sound pressure level, we must consider the muffling techniques that are available to meet the other criteria of power, size, weight, materials, and economics. At present, all practical muffling systems used on portable pneumatic tools rely basically on one of four techniques, or a combination of the four, for their silencing success. First, one may pipe the exhaust away from the operator. Second, materials can be selected to make maximum use of sound-absorption qualities. The third method is the use of an acoustical filter, such as an expansion chamber or a resonating chamber; one or more of them along the flow route of the exhaust cancel frequencies to which they are tuned.
A fourth method of muffling noise is diffusion, generally applied at the exhaust exits. The diffusion method breaks up the sound wave front and also takes advantage of any cancellation effects of the induced exhaust turbulence.
Pneumatic tools may be classified in three groups: rotary tools, whose driving source is a vane-type air motor; percussive tools, whose driving source is a free-moving piston; and impact wrenches, where the vane-type air motor turns a hammer that strikes an anvil.
Rotary tools, such as grinders, nut runners, drills, and screwdrivers, are divided into governed and ungoverned tools. For governed tools, the maximum sound pressure level is usually at peak horsepower. For ungoverned tools, it is usually at the free speed condition. For percussive tools and impact wrenches, the sound pressure level is produced by the exhaust air and the "steel noise" produced by metal striking on metal.
The sound power level produced by a pneumatic tool is a product of many interrelated parameters: operating pressure, pressure drop, expansion volume, exhaust air velocity, speed of device, exhaust air, airflow, length of exhaust path, and tool power. In addition, there is pure tone noise caused by the repetitive passage of the vanes over the exhaust slots in the air motor liner or, in the case of percussive tools, the piston passing the exhaust holes in the cylinder of the tool. Whenever pure tones are present, one encounters resonance from the geometry of the tool that is triggered by the fundamental frequency of the pure tone.
A designer should learn which facto r is the major cause of noise and deal with it first. The basic requirements for this part of the job are a sound level meter, an octave band analyzer, a narrow band analyzer, a graphic level recorder, a calibrator, and a work area where the background noise is at least 10 c1BA lower than the desired sound level. Also, appropriate loading devices must be available so an operator can test the tool at peak power and free speed.
Several techniques can reduce pure tone noise. One is to use gradual exhaust to reduce the amplitude of the exhaust pressure pulse. Another method is to place partitions or baffies to prevent standing waves. A third is to design a reactive muffler that is tuned to suppress the pure tone frequencies.
Suppression of broadband noise requires different techniques, depending on which noise-producing parameters are at issue. When a steady airflow passes through a duct or muffler, there is a steady pressure drop that is related to the magnitude of the flow and the geometry of the air passages. If there is an alternating (acoustic) flow superimposed on the steady flow, it is the instantaneous velocity, the sum of steady and alternating flow, that deternlines the pressure drop. This back pressure is important, because it influences the mechanical performance of the system to which the muffier is attached.
Any muffler technique used requires consideration of the pressure drop it will cause. This drop is easily detected in the performance of the tool, usually showing up as a change in speed. For example, one of the more successful muffling techniques is to use porous materials, such as screens of various meshes, sintered metals, felts, or open-cell plastic foam. These materials obviously will offer more restriction than an open port and therefore will create a pressure drop. An analysis of the system will determine whether a pressure drop elsewhere in the system" can be reduced to compensate for the pressure drop created by the insertion of a porous material. The total pressure drop found on the exhaust side of an air motor is in the range of 4 to 8 psi, depending on the airflow and the geometry of the tool. The main disadvantage to the use of porous materials is field maintenance. They will clog if the air lines are not kept clean, and so will require periodic attention.
Another technique that is closely related to the pressure drop is the use of expansion volume. The more expansion volume one can use, the more one can reduce noise without noticeable power loss.
An important parameter is the exhaust air velocity, which is the average velocity of the air as it leaves the tool. This usually occurs at the exhaust deflector, which will have either slots or holes. The lower the air velocity, the better. The contribution to sound power level from velocity varies for the type of aero-acoustic source. For a monopole source, it is to the fourth power; for a dipole source, it is to the sixth power; and for a quadrupole so urce, it is to the eighth power of velocity.
As for the length of the exhaust path, the easiest solution is to have a long, tortuous path. The worst case from a noise standpoint would be if an observer could look through the exhaust holes in the tool housing and see the primary exhaust slots in the air motor liner. This, of course, would be the shortest path. The main consideration in the use of the tortuous path technique is that you do not end up with a length that can produce resonance.
As for operating pressure, the rule of thumb is to use the lowest pressure that will give you proper performance from the tool. This, of course, is up to the user. For example, the tool's performance, as rated by the manufacturer, is usually specified as 90 psi at air inlet.
In the case of abrasive tools, one is faced with an additional problem—icing. Most air motors that are operated near the peak power p oint for long periods will build up ice, which is produced when moisture in the air lines encounters the drop in the temperature as the air expands within the motor. Icing usually prohibits the use of dissipative materials for muffling, and may also restrict the engineer in using the optimum ratio of exhaust hole diameter to wall thickness.
Piping away exhaust can be a very successful noise-reducing technique. In a situation where free maneuverability of the tool is not important-as in the case of multiple nut runners, stationary motors, and suspended tools-this approach should be seriously considered.
One of the best noise controls available to a machine designer is to use quieted components. Many component manufacturers- such as those producing air motors, electric motors, fans, and hydraulic pumpsstarted their noise control efforts decades ago.
Good standards are available for evaluating the noise of many components, and suppliers provide data on their products that can be used for comparison. In many cases, acoustic data must be specifically requested. One must also ask how the noise data was obtained-distance, free speed, loaded, and measurement standard used. Still, the application of noise control techniques will remain a continual search for comprormises that will optimize the balance of mechanical performance, size, cost, and noise reduction.