noise

Sound levels are measured with sound level meters (see decibel), generally with a weighting (the A scale) that mirrors human sensitivity to different frequencies. In most noisy environments, however, the sound level is constantly changing. To describe a sound level that changes over a period of time, acousticians can use the A-weighted noise equivalent level (L_eq), which is that level of a steady sound which has the same energy as the actual, ever-changing noise.

Some Examples of Typical Noise Levels in dBA
Sound Level in dBA	Example
200	200 meters from Saturn V rocket at liftoff.
160	Peak level at ear of a person firing a 30-30 rifle.
140	25 meters from jet aircraft.
120	Submarine engine room. On stage at a rock concert.
100	Noisy factory. Jackhammer (unsilenced).
90	7 meters from large diesel truck.
85	Upper limit of comfort.
80	1 meter from ringing alarm clock. Conversation is difficult. After a 1-hour exposure, thought is difficult and the stomach contracts.
75	Railroad carriage. Normal conversation not possible. Consensus of experts is that sound levels below 75 dBA "are unlikely to cause permanent hearing damage."
70	Small car at 30 mph; 3 meters from a vacuum cleaner.
65	1 meter from normal conversation. Busy office. About half the people in a large sample will have difficulty sleeping.
55	Recommended upper limit for large open offices, restaurants, gymnasiums, swimming pools.
45	Recommended upper limit for homes, hotels, laboratories, libraries, private offices, court rooms.
40	Quiet office. Recommended upper limit for classrooms, churches, motion picture theaters (without the film soundtrack).
35	Quiet bedroom.
25	Countryside on windless day, away from traffic.

Robyn R. M. Gershon, Richard Neitzel, Marissa A. Barrera and Muhammad Akram.
Pilot Survey of Subway and Bus Stop Noise Levels.
Journal of Urban Health: Bulletin of the New York Academy of Medicine, vol 83, no. 5 (Sept-October 2006).

Community Noise Levels

More than 22 different noise rating scales have been devised. Perhaps the most widely used in legislation is the Community Noise Equivalent Level (CNEL), first developed by the California Department of Airports, now incorporated in state (California Administrative Code Title 25 Art. 4, Sec. 1092) and municipal law. CNEL measurements take into account the fact that evening noises annoy people more than daytime noises do, and nighttime noises are worst of all. The CNEL is a 24-hour A-weighted equivalent sound level with a 5-dB penalty applied to sound levels between 7 and 10 pm and a 10-dB penalty applied to sound levels between 10 pm and 7 am. Another measure, used by the Environmental Protection Agency and many cities, is the day-night sound level (L_dn), which is similar to the CNEL, but without the penalty on noises between 7 and 10 pm.

The U.S. Environmental Protection Agency identified "Yearly L_dn Values That Protect Public Health and Welfare with a Margin of Safety".¹ They state that "To protect against hearing damage, one's 24-hour noise exposure at the ear should not exceed 70dB[L_eq]." (Actually, they estimate that at this level 4% of the population would still suffer some hearing damage.) The other values are based on avoiding "interference and annoyance" rather than hearing impairment:

Outdoor	Residential areas, farms, "And other places where quiet is a basis for use."	L_dn no more than 55 dB
Outdoor	Where people spend limited amounts of time, such as school yards, playgrounds	24-hour L_eq no more than 55 dB
Indoor	Indoor residential	L_dn no more than 45 dB
Indoor	Schools, etc.	24-hour L_eq no more than 45 dB

Where?	Why?	Leq
School playground (outdoors)	Avoid annoyance.	55
Hospital rooms	To avoid disturbing sleep.	30
Classrooms	To ensure that speakers can understand one another.	35
Factories, traffic, shopping areas, both indoors and outdoors	To avoid hearing impairment.	70

1. U.S. Environmental Protection Agency, Office of Noise Abatement and Control.
Protective Noise Levels. Condensed Version of EPA Levels Document.
Report EPA 550/9-79-100. (November 1979)

A pdf is available on the web. Go to www.epa.gov/ncepihom/. Select "Simple Search". Enter 550979100 in the search dialog. In the results screen, click on the Adobe Acrobat icon toward the right of the navigation bar.

2. Birgitta Berglund, Thomas Lindvall and Dietrich H. Schwein, editors.
Guidelines for Community Noise.
Geneva: World Health Organization, 1999.

Occupational Noise Levels

In the United States, noise levels in the workplace are regulated by the Occupational Safety and Health Administration. If the 8-hour time-weighted average sound level at a workplace is 85 decibels or more, the employer must institute a program to monitor sound levels.

To determine whether exposure to steady noise at different levels during the day exceeds standards, for each noise level make a fraction whose numerator is the total exposure time at that level, and whose denominator is the total permitted exposure time at that level, from the table below. Add the fractions; regulations require that the total not exceed 1. For example, exposing a worker to a 90 dBA noise for 4 hours and to 105 dBA for 0.5 hour would be barely permissible (4/8 + 5/1 = 1).

OSHA Permissible Daily Noise Exposure²
Sound level in dBA	Duration in hours
90	8
92	6
95	4
97	3
100	2
102	1.5
105	1
110	0.5
115	0.25

U.S. Department of Labor, Occupational Safety and Health Administration.
Hearing Conservation.
OSHA 3074 2002 (revised).

U.S. Dept. of Health and Human Services. Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health.
Criteria for a Recommended Standard. Occupational Noise Exposure. Revised Criteria 1998.
Cincinnati (OH): June 1998.

For Further Reading