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A textbook definition of sound is "a rapid variation of atmospheric pressure caused by some disturbance of the air." Sound propagates as a wave of positive pressure disturbances (compressions) and negative pressure disturbances (rarefactions), as shown in Figure 1. Sound can travel through any elastic medium (e.g., air, water, wood, metal).

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When air molecules are set to vibrate, the ear perceives the variations in pressure as sound (OTM/Driscoll). The vibrations are converted into mechanical energy by the middle ear, subsequently moving tiny membranes across microscopic cilia (hair cells) in the inner ear, which in turn convert the sound waves into nerve impulses. If the vibrations are too intense, over time the cilia can be damaged, causing hearing loss. In the workplace, sound that is intense enough to damage hearing is a hazard that must be addressed by employers.

Several key terms describe the qualities of sound. These qualities influence how it affects hearing and health, how it is measured, and how it can be controlled. Effective occupational noise investigations require the investigator to understand these basic terms.

The wavelength (λ) is the distance traveled by a sound wave during one sound pressure cycle, as shown in Figure 2. The wavelength of sound is usually measured in meters or feet. Wavelength is important for designing engineering controls. For example, a sound-absorbing material will perform most effectively if its thickness is at least one-quarter the wavelength.

Sound frequency is perceived as pitch (i.e., how high or low a tone is). The frequency range sensed by the ear varies considerably among individuals. A young person with normal hearing can hear frequencies between approximately 20 Hz and 20,000 Hz. As a person ages, the highest frequency that they can detect tends to decrease.

Human speech frequencies are in the range of 500 Hz to 4,000 Hz. This is significant because hearing loss in this range will interfere with conversational speech. The portions of the ear that detect frequencies between 3,000 Hz and 4,000 Hz are the earliest to be affected by exposure to noise. Audiograms often display a 4,000-Hz "Notch" in patients who are developing the beginning stages of sensorineural hearing loss (hearing loss due to damage to the inner ear cilia or the auditory nerve).

The speed at which sound travels, c, is determined primarily by the density and the compressibility of the medium through which it is traveling. The speed of sound is typically measured in meters per second or feet per second.

The vibrations associated with sound are detected as slight variations in pressure. The range of sound pressures perceived as sound is extremely large, beginning with a very weak pressure causing faint sounds and increasing to noise so loud that it causes pain.

The threshold of hearing is the quietest sound that can typically be heard by a young person with undamaged hearing. This varies somewhat among individuals but is typically in the micropascal range. The reference sound pressure is the standardized threshold of hearing and is defined as 20 micropascals (0.0002 microbars) at 1,000 Hz.

The threshold of pain, or the greatest sound pressure that can be perceived without pain, is approximately 10 million times greater than the threshold of hearing. It is, therefore, more convenient to use a relative (e.g., logarithmic) scale of sound pressure rather than an absolute scale (OTM/Driscoll).

Decibels are measured on a logarithmic scale: a small change in the number of decibels indicates a huge change in the amount of sound pressure and correspondingly the potential for damage to a person's hearing.

The decibel scale is convenient because it compresses sound pressures important to human hearing into a manageable scale. By definition, 0 dB is set at the reference sound pressure (20 micropascals at 1,000 Hz, as stated earlier). At the upper end of human hearing, noise causes pain, which occurs at sound pressures of about 10 million times that of the threshold of hearing. On the decibel scale, the threshold of pain occurs at 140 dB. This range of 0 dB to 140 dB is not the entire range of sound, but is the range relevant to human hearing (Figure 3).

Decibels are logarithmic values, so it is not correct to sum multiple sound values using arithmetic addition. See Appendix B.3 for information on the cumulative effects of multiple sound sources on the decibel level.

The decibel is a dimensionless unit; however, the concepts of distance and three-dimensional space are important to understanding how noise spreads through an environment and how it can be controlled. Sound fields and sound power are terms used in describing these concepts.

Sound fields are categorized as near field or far field, a distinction that is important to the reliability of measurements. The near field is the space immediately around the noise source, sometimes defined as within the wavelength of the lowest frequency component (e.g., a little more than 4 feet for a 25-Hz tone, about 1 foot for a 1,000-Hz tone, and less than 7 inches for a 2,000-Hz tone). Sound pressure measurements obtained with standard instruments within the near field are not reliable because small changes in position can result in big differences in the readings.

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.

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.

In spaces defined by walls, however, sound fields are more complex. When sound-reflecting objects such as walls or machinery are introduced into the sound field, the wave picture changes completely. Sound reverberates, reflecting back into the room rather than continuing to spread away from the source. Most industrial operations and many construction tasks occur under these conditions. Figure 5 diagrams sound radiating from a sound source and shows how reflected sound (dashed lines) complicates the situation.

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.

Far from the noise source--unless the boundaries are very absorbing--the reflected sound dominates. This region is called the reverberant field. If the sound pressure levels in a reverberant field are uniform throughout the room, and the sound waves travel in all directions with equal probability, the sound is said to be diffuse.

Up to this point, this discussion has focused on sound pressure. Sound power, however, is an equally important concept. Sound power, usually measured in watts, is the amount of energy per unit of time that radiates from a source in the form of an acoustic wave. Generally, sound power cannot be measured directly, but modern instruments make it possible to measure the output at a point that is a known distance from the source.

Understanding the relationship between sound pressure and sound power is essential to predicting what noise problems will be created when particular sound sources are placed in working environments. An important consideration might be how close workers will be working to the source of sound. As a general rule, doubling the sound power increases the noise level by 3 dB.

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.

Sound intensity is heard as loudness, which can be perceived differently depending on the individual and their distance from the source and the characteristics of the surrounding space. As the distance from the sound source increases, the sound intensity decreases. The sound power coming from the source remains constant, but the spherical surface over which the power is spread increases--so the power is less intense. In other words, the sound power level of a source is independent of the environment. However, the sound pressure level at some distance, r, from the source depends on that distance and the sound-absorbing characteristics of the environment (OTM/Driscoll).

Most noise is not a pure tone, but rather consists of many frequencies simultaneously emitted from the source. To effectively evaluate the total noise of a most sources, it is usually necessary to measure it across its frequency spectrum. One reason for this is that people react differently to low, mid, and high-frequency sounds. Additionally, for the same sound pressure level, high-frequency noise is much more disturbing and more capable of producing hearing loss than low-frequency noise. Furthermore, engineering solutions to reduce or control noise are different depending on the predominant frequency of the noise. As a general guideline, low-frequency noise is more difficult to control.

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