Section: 14 | Characteristics of Human Hearing |
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John R. Rumble, ed., CRC Handbook of Chemistry and Physics, 102nd Edition (Internet Version 2021), CRC Press/Taylor & Francis, Boca Raton, FL.
If a specific table is cited, use the format: "Physical Constants of Organic Compounds," in CRC Handbook of Chemistry and Physics, 102nd Edition (Internet Version 2021), John R. Rumble, ed., CRC Press/Taylor & Francis, Boca Raton, FL.

CHARACTERISTICS OF HUMAN HEARING

The human ear is sensitive to sound waves with frequencies in the range from a few hertz to almost 20 kHz. Auditory response is usually expressed in terms of the loudness level of a sound, which is a measure of the sound pressure. The reference level, which is given in the unit phon, is a pure tone of frequency 1000 Hz with sound pressure of 20 µPa (in cgs units, 2·10–4 dyn/cm2 ); loudness level is usually expressed in decibels (dB) relative to this reference level. If a normal observer perceives an arbitrary sound to be equally loud as this reference sound, the sound is said to have the loudness level of the reference. The sensitivity of the typical human ear ranges from about 0 dB, the threshold loudness level, to about 140 dB, the level at which pain sets in. The minimum detectable level thus represents a sound wave of pressure 20 µPa and intensity (power density) 10–16 W/cm2.

The following figure illustrates the frequency dependence of the threshold for an average young adult.

figure

                                                              Frequency in Hz

The relation between loudness level and frequency for a typical person is expressed by the following table:

Relation between Loudness Level and Frequency



Sound pressure level in dB relative to 20 μPa125 Hz500 Hz1000 Hz4000 Hz8000 Hz10000 Hz
101018
2016202811
3042730372117
40173940453026
50345250543835
60526560644744
70707670735654
80868680836664
90989690947774
1001081051001068886


Thus, a 10,000 Hz tone at a pressure level of 50 dB seems equally loud as a 1000 Hz tone at a pressure of 35 dB.

The term noise refers to any unwanted sound, either a pure tone or a mixture of frequencies. Since the sensitivity of the ear is frequency dependent, as illustrated by the above table, noise level is expressed in a frequency-weighted scale, known as A-weighting. Decibel readings on this scale are designated as dBa. Typical noise levels from various sources are illustrated in this table:

Noise Levels of Various Sources



SourceNoise level in dBa
Rocket engine200
Jet aircraft engine160
Light aircraft, cruising140
Tractor, 150 hp115
Electric motor, 100 hp at 2600 rpm105
Pneumatic drill100
Subway train90
Vacuum cleaner85
Heavy automobile traffic75
Conversational speech65
Whispered speech40
Background noise, recording studio25-30


Recommended noise thresholds in the workplace have been established by the American Conference of Government Industrial Hygenists. Some examples of the maximum safe levels for different daily exposure times are given below.

Recommended Maximum Exposure to Different Levels of Noise



Duration of exposureMax. level in dBa
24 h80
8 h85
4 h88
1 h94
30 min97
15 min100
2 min109
28 s115
0.11 s139


No exposure greater than 140 dBa is permitted. Further details may be found in Ref. 3.

References

  1. Anderson, H. L., Ed., A Physicist’s Desk Reference, American Institute of Physics, New York, 1989, chap. 2.
  2. Gray, D. E., Ed., American Institute of Physics Handbook, Third Edition, McGraw Hill, New York, 1972, chap. 3.
  3. Threshold Limit Values for Chemical Substances and Physical Agents; Biological Exposure Indices, 2008 Edition, American Conference of Governmental Industrial Hygienists, 1330 Kemper Meadow Drive, Cincinnati, OH 45240-1634; <www.acgih.org>.
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