Section: 14 | Characteristics of Human Hearing |

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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.

Frequency in Hz

The relation between loudness level and frequency for a typical person is shown in Table 1. Noise levels of various sources are given in Table 2. The recommended maximum exposure to different levels of noise is given in Table 3.

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TABLE 1. Relation between Loudness Level and Frequency

Sound pressure level in dB relative to 20 μPa	125 Hz	500 Hz	1000 Hz	4000 Hz	8000 Hz	10000 Hz
10			10	18
20		16	20	28	11
30	4	27	30	37	21	17
40	17	39	40	45	30	26
50	34	52	50	54	38	35
60	52	65	60	64	47	44
70	70	76	70	73	56	54
80	86	86	80	83	66	64
90	98	96	90	94	77	74
100	108	105	100	106	88	86

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:

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TABLE 2. Noise Levels of Various Sources

Source	Noise level in dBa
Rocket engine	200
Jet aircraft engine	160
Light aircraft, cruising	140
Tractor, 150 hp	115
Electric motor, 100 hp at 2600 rpm	105
Pneumatic drill	100
Subway train	90
Vacuum cleaner	85
Heavy automobile traffic	75
Conversational speech	65
Whispered speech	40
Background noise, recording studio	25-30

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.

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TABLE 3. Recommended Maximum Exposure to Different Levels of Noise

Duration of exposure	Max. level in dBa
24 h	80
8 h	85
4 h	88
1 h	94
30 min	97
15 min	100
2 min	109
28 s	115
0.11 s	139

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

References

Anderson, H. L., Ed., A Physicist’s Desk Reference, American Institute of Physics, New York, 1989, chap. 2.
Gray, D. E., Ed., American Institute of Physics Handbook, Third Edition, McGraw Hill, New York, 1972, chap. 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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CHARACTERISTICS OF HUMAN HEARING

TABLE 1. Relation between Loudness Level and Frequency

TABLE 2. Noise Levels of Various Sources

TABLE 3. Recommended Maximum Exposure to Different Levels of Noise

References

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