Source: A. C. Hosey (Ed.), Industrial Noise, A Guide to Its Evaluation and Control, PHS Pub. No. 1572, Department of Health, Education, and Welfare, Washington, DC, 1967.

Source: A. C. Hosey (Ed.), Industrial Noise, A Guide to Its Evaluation and Control, PHS Pub. No. 1572, Department of Health, Education, and Welfare, Washington, DC, 1967.

A noise contribution less than 10 dB lower than the other noise contributions can usually be neglected.

It should be noted that in using the previous formula the following generalization can be made: Any two identical sound levels will have the effect of increasing the overall level by 3 dB and any three will increase the overall level by 4.8 dB.

Intensity Intensity of a sound wave is the energy transferred per unit time (in seconds) through a unit area normal to the direction of propagation. It is commonly measured in W/ml2 or W/cm2. For a pure tone (single frequency), there is a one-to-one correspondence between loudness and intensity. However, almost all sound contains multiple frequencies. The relationship is not simple because of the interference effects of the sound waves.50 For example, increasing the sound pressure level by 3 dB is equivalent to increasing the intensity by a factor of 2. Increasing the sound pressure level by 10 dB is equivalent to increasing the intensity by a factor of 10, and increasing the sound pressure level by 20 dB is equivalent to increasing the intensity by a factor of 100. Expressed in another way, whereas 10 dB is 10 times more intense than 1 dB, 20 dB is 100 times (10 x 10) more intense, and 30 dB 1,000 times (10 x 10 x 10) more intense.

Loudness Loudness, or amplitude, of sound is the sound level or sound pressure level as perceived by an observer. The apparent loudness varies with the sound pressure and frequency (pitch) of the sound. This is illustrated in Figure 4.14. It is specified in sones or phons. For a pure tone, each time the sound pressure level increases by 10 dB, the loudness doubles (sones increase

FIGURE 4.14 Equal loudness contour. (Source: Toward a Quieter City, A report of the Mayor's Task Force on Noise Control, New York, 1970.)

by a factor of 2). Sound levels of the same intensity may not sound the same since the ear does not respond the same to all types of sound.

A 1,000-Hz pure tone 40dB above the listener's hearing threshold (0dB) produces a loudness of 1 sone, which is a unit of loudness. 51 This loudness of 1 sone is equal to 40 phons. Loudness levels are usually expressed in phons. For practical purposes, each doubling of the sones increases the phons by 10—that is, 1 sone = 40 phons; 2 sones = 50 phons; 4 sones = 60 phons. Also for pure tones, a 10-dB increase in sound level would be perceived as a 10-phon increase in loudness by a person with good hearing in the frequency range of 600 to 2,000 Hz.

For example, take a human listener with normal hearing who hears a 100-Hz pure tone with a SPL of 90 dB. What loudness does the listener perceive?

From Figure 4.14, a SPL of 40 dB at approximately 100 Hz equals a loudness of 10 phons. Since a 50-dB increase in SPL is equivalent to a 50-phons increase in loudness, the tone's loudness is 60 phons, or 4 sones.

Noys Noys is a measure of the perceived noise level (PNL) (in decibel) in relation to the noisiness or acceptability of a sound level. Although similar to loudness, the ratings by observers when tested were different.

Procedures for the calculation of loudness and noisiness are given in standard texts.52

Day-Night Average Sound Level (DNL) System The day-night average sound level is the 24-hour average sound level, expressed in decibels, obtained after the addition of a 10-dB penalty for sound levels that occur at night between 10 p.m. and 7 a.m. It is recorded as Ldn. The DNL system has been adopted by the EPA, the Department of Defense, the Department of Housing and Urban Development (HUD), and the Federal Aviation Administration (FAA), specifically for describing environmental impacts for airport actions.53

Effects of Noise —A Health Hazard

Noise pollution is an environmental and workplace problem. Excessive noise can cause permanent or temporary loss of hearing. Loud sounds affect the circulatory and nervous systems, although the effects are difficult to assess. It interferes with speech, radio, and TV listening; disturbs sleep and relaxation; affects performance as reduced work precision and increased reaction time; and causes annoyance, irritation, and public nuisance. There is a hearing loss with age, particularly at the higher frequencies, and in younger people who have been exposed to loud noises. Occupation-related hearing loss has been documented since the sixteenth century and is still a serious problem. An estimated $835 million compensation was paid workers from 1978 to 1987.54 Sonic booms can cause physical damage to structures. David G. Hawkins, assistant EPA administrator reported the following:55

A poll conducted by the U.S. Bureau of the Census showed that noise is considered to be the most undesirable neighborhood condition—more irritating than crime and deteriorating housing.

Criteria for hearing protection and conservation have been established primarily for the worker. The major factors related to hearing loss are intensity (sound pressure levels in decibels), frequency content, time duration of exposure, and repeated impact (a single pressure peak incident). In measuring the potential harm of high-level noise, frequency distribution as well as intensity must be considered. Continuous exposure to high-level noise is more harmful than intermittent or occasional exposure. High- and middle-frequency sounds at high levels generally are more harmful than low-frequency sounds at the same levels. Greater harm is done with increased time of exposure.

Individuals react differently to noise depending on age, sex, and socioeconomic background. The relation of noise to productivity or performance is contradictory and not well established.

For workers, a sound level over 85 dBA calls for study of the cause. A level above 90 dBA should be considered unsafe for daily exposure over a period of months and calls for noise reduction or personal ear protection if this is practical.

An EPA report identified a 24-hour exposure level of 70 dBA as the level of environmental noise that will prevent any measurable hearing loss over a lifetime. Levels of 55 dBA outdoors and 45 dBA indoors are identified as preventing annoyance and not interfering with spoken conversation and other activities such as sleeping, working, and recreation.56 Some common sound levels and human responses are noted in Table 4.12.

Other effects of noise are reduced property values; increased compensation benefits and possible accidents, inefficiency, and absenteeism; and increased building construction costs.

Sources of Noise

Transportation, industrial, urban, and commercial activities are the major sources of noise, plus the contributions made by household appliances and equipment.

TABLE 4.12 Sound Levels and Human Response


Noise Level (dBA)


Carrier deck, jet operation

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