The far field is the space outside the near field, meaning that the far field begins at a point at least one wavelength distance from the noise source. Standard sound level meters (i.e., type I and type II) are reliable in this field, but the measurements are influenced by whether the noise is simply originating from a source (free field) or being reflected back from surrounding surfaces (reverberant field).
A free field is a region in which there are no reflected sound waves. In a free field, sound radiates into space from a source uniformly in all directions. The sound pressure produced by the source is the same in every direction at equal distances from the point source. As a principle of physics, the sound pressure level decreases 6 dB, on a Z-weighted (i.e., unweighted) scale, each time the distance from the point source is doubled. This is a common way of expressing the inverse-square law in acoustics and is shown in Figure 4.
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Free field conditions are necessary for certain tests, where outdoor measurements are often impractical. Some tests need to be performed in special rooms called free field or anechoic (echo-free) chambers, which have sound-absorbing walls, floors, and ceilings that reflect practically no sound.
The net result is a change in the intensity of the sound. The sound pressure does not decrease as rapidly as it would in a free field. In other words, it decreases by less than 6 dB each time the distance from the sound source doubles.
As sound power radiates from a point source in free space, it is distributed over a spherical surface so that at any given point, there exists a certain sound power per unit area. This is designated as intensity, I, and is expressed in units of watts per square meter.
The hierarchy of controls for noise can be summarized as: 1) eliminate or minimize noise exposure by installing equipment that produces less noise (e.g., buy-quiet programs), 2) prevent or contain the escape of noise at its source (engineering controls), 3) control exposure by changing work schedules to reduce the amount of time any one worker spends in the high noise area (administrative controls) or by changing practices such as distancing from noise-producing equipment (work practice controls), and 4) control the exposure with hearing protection. This hierarchy highlights the principle that the best prevention strategy is to eliminate exposure to hazards that can lead to hearing loss. Corporations that have started buy-quiet programs are moving toward workplaces where no harmful noise will exist. Many companies are automating equipment or setting up procedures that can be managed by workers from a quiet control room free from harmful noise. When it is not possible to eliminate the noise hazard or relocate the worker to a safe area, the worker must be protected with PPE.
Just because a surface area vibrates, it is not correct to assume it is radiating significant noise. In fact, probably less than 5% of all vibrating panels produce sufficient airborne noise to be of concern in an occupational setting. However, vibration damping materials can be an effective retrofit for controlling resonant tones radiated by vibrating metal panels or surface areas. In addition, this application can minimize the transfer of high-frequency sound energy through a panel. The two basic damping applications are free-layer and constrained-layer damping. Free-layer damping, also known as extensional damping, consists of attaching an energy-dissipating material on one or both sides of a relatively thin metal panel. As a guide, free-layer damping works best on panels less than -inch thick. For thicker machine casings or structures, the best application is constrained-layer damping, which consists of damping material bonded to the metal surface covered by an outer metal constraining layer, forming a laminated construction. Each application can provide up to 30 dB of noise reduction.
It is important to note that the noise reduction capabilities of the damping application are essentially equal, regardless of which side it is applied to on a panel or structure. Also, for practical purposes, it is not necessary to cover 100% of a panel to achieve a significant noise reduction. For example, 50% coverage of a surface area can provide a noise reduction that is roughly 3 dB less than 100% coverage. In other words, assuming that 100% coverage results in 26 dB of attenuation, 50% coverage could provide approximately 23 dB of reduction, 25% coverage could produce a 20-dB decrease, etc. For free-layer damping treatments, it is recommended that the application material be at least as thick as the panel or base layer to which it is applied. For constrained-layer damping, the damping material again should be the same thickness as the panel; however, the outer metal constraining layer may be half the thickness of the base layer.
Simple free-layer damping materials consist of rubbery "viscoelastic" materials that can be painted, sprayed, troweled, or adhered (i.e., with adhesive or magnetism) onto the noisy surface. Typically, on sheet metal, a layer of damping material half the thickness of the metal (or 10% by weight) will eliminate the "ringing" from impact. A much thicker layer of damping material, two to three times the thickness of the metal, will increase the sound-absorption coefficient of the metal to approximately 0.3 to 0.6 (see section 2.i. below for more information on the sound absorption coefficient).
Keep in mind that the machine, the product being manufactured, and the process itself can all create and radiate noise. Consider the illustration in Figure 31 (conveying rocks into a hopper). In the example on the left side, the rocks impacting the metal-paneled walls of the hopper cause it to ring like a bell. As shown on the right side, reducing the free-fall height (by backing up the conveyor) such that there is only a short drop significantly reduces the potential energy, which reduces the resultant noise. Additionally, a durable rubber-like material is added to damp the hopper and minimize the ability of the metal panel to flex and vibrate, which eliminates this noise at the source. Damping material can be added to either side of the metal surface (Driscoll, Principles of Noise Control).
A common way to reduce reverberation in a room is to install sound-absorbing materials, such as acoustic tiles, in strategic places on the walls and ceiling surrounding the noise source. Reverberation can be greater when the room surfaces are hard (e.g., concrete, cinder block, corrugated metal); in these environments, sound-absorbing materials can be beneficial. This is a common treatment in theaters, broadcast studios, and sound-recording booths. Figure 37 shows a large, open room in which sound-absorbing baffles and acoustic tiles are hanging from the ceiling. This engineering control will do nothing to reduce the noise level from the noise source but will reduce the reflection of noise back into the room. As was mentioned previously, this type of control works best in a small room (less than 10,000 square feet) with low ceilings (less than 15 feet). In a room with high ceilings, the main source of noise to which workers are exposed is most likely direct noise from the source. Sound-absorbing materials should never be painted, as this would cover the pores in the material, thereby preventing noise from being absorbed.
If a sound is generated at a point source in a free field, meaning there are no walls or other obstructions, the sound pressure level, Lp, will be reduced by 6 dB each time the distance from the noise source is doubled. Alternatively, Lp will increase by 6 dB in a free field each time the distance to the noise source is halved. Consider the following example:
Calculating the sound pressure level at a specific distance from a noise source is often useful. The following equation allows one to calculate the sound pressure level at any distance from a noise source in a free field:
Free-Standing Overhanging Objects. (Shows a horizontal panel mounted on two posts). When the horizontal distance between detectable posts or detectable objects is greater than 12 inches (302 mm), the maximum height to the bottom of the free-standing object is 27 inches (685 mm).
Slopes of curb ramps shall comply with 4.8.2. The slope shall be measured as shown in Fig. 11. Transitions from ramps to walks, gutters, or streets shall be flush and free of abrupt changes. Maximum slopes of adjoining gutters, road surface immediately adjacent to the curb ramp, or accessible route shall not exceed 1:20.
Spout Height and Knee Clearance. The front edge of the fountain must extend 17 to 19 inches (430 - 485 mm) from the wall. The 27 inches (685 mm) high minimum clear knee space must be free of equipment or obstructions for a minimum of 8 inches (205 mm) extending from the front edge of the fountain back toward the wall. In addition, a minimum 9 inches (230 mm) high toe clearance space must be provided extending back toward the wall to a distance no more than 6 inches (150 mm) from the back wall. The toe clearance space must be free of equipment or obstructions.
Lavatory Clearances. The minimum knee clearance must be free of equipment or obstructions for a minimum of 8 inches (205 mm) extending from the front edge of the lavatory back toward the wall. This knee clearance must be 29 inches (735 mm) high at the front of the lavatory and no less than 27 inches (685 mm) high at a point 8 inches (205 mm) back. In addition, a minimum 9 inches (230 mm) high toe clearance must be provided extending back toward the wall to a distance no more than 6 inches (150 mm) from the back wall. The toe clearance space must be free of equipment or obstructions.
(7) Kitchens, Kitchenettes, or Wet Bars. When provided as accessory to a sleeping room or suite, kitchens, kitchenettes, wet bars, or similar amenities shall be accessible. Clear floor space for a front or parallel approach to cabinets, counters, sinks, and appliances shall be provided to comply with 4.2.4. Countertops and sinks shall be mounted at a maximum height of 34 in (865 mm) above the floor. At least fifty percent of shelf space in cabinets or refrigerator/freezers shall be within the reach ranges of 4.2.5 or 4.2.6 and space shall be designed to allow for the operation of cabinet and/or appliance doors so that all cabinets and appliances are accessible and usable. Controls and operating mechanisms shall comply with 4.27. 2ff7e9595c
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