Sound Level Primer
This Sound Level Primer provides information to foster a better understanding of the physical aspects of sound, the subjective nature of noise, some of the issues involved in community noise management and the basic principles of noise mitigation. Noise is defined as unwanted or excessive sound. Sound becomes unwanted when it interferes with normal activities such as sleep, work or recreation. Under extreme conditions sound can cause physical harm such as hearing loss or adverse mental health effects. How people perceive any given sound depends on several measurable physical characteristics of the sound. These factors are:
Intensity: Sound intensity is often equated to loudness. The sound level magnitude (typically measured in decibels) is a measure of sound intensity. A 10 decibel increase in intensity is generally perceived as a doubling in loudness.
Individual human response to noise is subject to considerable variability. There are many factors, both emotional and physical, which contribute to the variation in human reaction to noise. These factors are summarized in Table 1. The existence of numerous emotional and physical variables prohibits defining an exact individual or community response for any given noise level. Community noise criteria are therefore based on statistical averages of human response to noise and applicable health criterion. Table 2 summarizes some generalized effects of noise on people in a residential environment.
Sound levels are most often measured on a logarithmic scale of decibels (dB). The decibel scale compresses the audible acoustic pressure levels which can vary from 20 micropascals (µPa), the threshold of hearing and reference pressure (0 dB), to 20 million µPa, the threshold of pain (120 dB). Because sound levels are measured in dB, the addition of two sound levels is not linear. To add sound levels in dB, the levels (dB) must be converted into “energy” terms, which are added and then converted back to dB. Adding two equal sound levels creates a 3 dB increase in overall level. An example is 50 dB (100,000 in “energy”) added to 50 dB (100,000 in “energy”) equals 53 dB (200,000 in “energy”). If more than a 10 dB difference exists between two sound levels, there is no significant additive effect. An example is 50 dB (100,000 in “energy”) added to 60 dB (1,000,000 in “energy”) equals 60.4 dB (1,100,000 in “energy”). Figure 1 further illustrates the range of acoustic pressures (µPa) and their relationship to the acoustic decibel scale.
Because the human ear does not hear sound energy linearly (on a one-to-one basis), humans do not perceive changes in sound level as equally loud. Research indicates the following general relationships between sound level and human perception:
The human ear does not perceive sound levels from every frequency as equally loud. As part of the hearing process, the human ear will attenuate low and high-frequency sounds. To compensate for these phenomena in perception, the A-weighted decibel scale, referred to as dBA, is used to measure and evaluate environmental noise levels. The A-weighted scale adjusts sound pressure levels by frequency, reducing low and high-frequency sound, similar to the way people hear sound.
TABLE 1 - Factors Involved In the Human Reaction to Noise
EMOTIONAL VARIABLES
PHYSICAL VARIABLES
Source: Aviation Noise Effects, USDOT-FAA, March 1985.
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TABLE 2 - Effects of Noise on People (Residential Land Uses Only) |
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Day-Night Average Sound in Decibels |
Effects1 |
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Hearing Loss |
Speech Interference |
Annoyance2 |
Average Community Reaction4 |
General Community Attitude Towards Area |
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Indoor |
Outdoor |
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Qualitative Description |
Percentage Sentence Intelligibility |
Distance in Meters for 95% Sentence Intelligibility |
Percentage of Population Highly Annoyed3 |
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75 & above |
may begin to occur |
98% |
0.5 |
37% |
very severe |
Noise is likely to be the most important of all adverse aspects of the community environment. |
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70 |
will not likely occur |
99% |
0.9 |
25% |
severe |
Noise is one of the most important adverse aspects of the community environment. |
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65 |
will not occur |
100% |
1.5 |
15% |
significant |
Noise is one of the important adverse aspects of the community environment. |
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60 |
will not occur |
100% |
2.0 |
9% |
moderate |
Noise may be considered an adverse aspect of the community environment. |
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55 & below |
will not occur |
100% |
3.5 |
4% |
slight |
Noise considered no more important than various other environment factors. |
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1
"Speech Interference" data are drawn from the following tables in the United States Environmental Protection Agency's (USEPA) "Levels Document"; Table 3, Figure D-1, Figure D-2, Figure D-3. All other data from the National Academy of Science 1977 report "Guidelines for Preparing Environmental Impact Statements on Noise, Report of Working Group 69 on Evaluation of Environmental Impact of Noise."
2 Depends on attitudes and other factors.
3 The percentages of people reporting annoyance to lesser extent are higher in each case. An unknown small percentage of people will report being "highly annoyed" even in the quietest surroundings. One reason is the difficulty all people have in integrating annoyance over a very long time.
4 Attitudes or other non-acoustic factors can modify this. Noise at low levels can still be an important problem, particularly when it intrudes into a quiet environment.
NOTE: Research implicates noise as a factor producing stress-related health effects such as heart disease, high-blood pressure and stroke, ulcers and other digestive disorders. The relationships between noise and these effects, however, have not as yet been quantified.
SOURCE: Federal Interagency Committee on Urban Noise. 1980. Guidelines for Considering Noise for Land Use Planning and Control.
FIGURE 1 - Indoor and Outdoor Sound Levels
Sound Sound
Pressure Level
Outdoor Sound Levels (mPa) (dBA) Indoor Sound Levels
6,324,555 - 110 Rock Band at 5 m
Jet Over-Flight at 300 m - 105
2,000,000 - 100 Inside
Gas Lawn Mower at 1 m - 95
632,456 - 90 Food Blender at 1 m
Diesel Truck at 15 m - 85
Noisy Urban Area--Daytime 200,000 - 80 Garbage Disposal at 1 m
- 75 Shouting at 1 m
Gas Lawn Mower at 30 m 63,246 - 70 Vacuum Cleaner at 3 m
Suburban Commercial Area - 65 Normal Speech at 1 m
20,000 - 60
Quiet Urban Area -- Daytime - 55 Quiet Conversation at 1m
6,325 - 50 Dishwasher Next Room
Quiet Urban Area--Nighttime - 45
2,000 - 40 Empty Theater or Library
Quiet Suburb--Nighttime - 35
632 - 30 Quiet Bedroom at Night
Quiet Rural Area--Nighttime - 25 Empty Concert Hall
Rustling Leaves 200 - 20
- 15 Broadcast and Recording Studios
63 - 10
- 5
Reference Pressure Level 20 - 0 Threshold of Hearing
Notes:
mPa - MicroPascals describe pressure levels (force/area).