Understanding the measurement of sound intensity in decibels (dB)

Sound intensity is frequently expressed in terms of decibels (dB). The decibel (1/10 of a Bel) was named in honour of Alexander Graham Bell. It expresses the logarithm to the base 10 of
the ratio between two sound intensities or (sound pressures)2.

The
internationally recognised reference standard intensity for sound is 10-12
watts per m2 which corresponds to a sound pressure of 2.10-5
N/m2 (or Pascal). For a pure tone at 1000 Hz this is close to the
threshold of hearing for young healthy adults.

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dB

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Sound pressure (N/m2;
Pa)

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Relative sound
Pressure

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Jet aeroplane, 80 ft from tai; hair cell damage

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120 dB

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20

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10-6

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Busy traffic, shouting

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80 dB

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2.10 -1

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104

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Conversational speech

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60 dB

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2.10 -2

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103

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Residential area at night

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40 dB

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2.10 -3

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102

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Whisper at 5ft

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20 dB

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2.10 -4

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101

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Threshold for hearing 1000 Hz young adult

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0 dB

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2.10 -5

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1.0

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