Question

Determine the intensity of a 120-dB sound. The intensity of the reference level required to determine the sound level is 1.0

Answer 1

Answer Intensity is 1.00 W/m²

The Threshold of Hearing and the Decibel Scale

Humans are equipped with very sensitive ears capable of detecting sound waves of extremely low intensity. The faintest sound that the typical human ear can detect has an intensity of 1*10-12 W/m2. This intensity corresponds to a pressure wave in which a compression of the particles of the medium increases the air pressure in that compressional region by a mere 0.3 billionth of an atmosphere. A sound with an intensity of 1*10-12 W/m2corresponds to a sound that will displace particles of air by a mere one-billionth of a centimeter. The human ear can detect such a sound. WOW! This faintest sound that a human ear can detect is known as the threshold of hearing (TOH). The most intense sound that the ear can safely detect without suffering any physical damage is more than one billion times more intense than the threshold of hearing.

Since the range of intensities that the human ear can detect is so large, the scale that is frequently used by physicists to measure intensity is a scale based on powers of 10. This type of scale is sometimes referred to as a logarithmic scale. The scale for measuring intensity is the decibel scale. The threshold of hearing is assigned a sound level of 0 decibels (abbreviated 0 dB); this sound corresponds to an intensity of 1*10-12 W/m2. A sound that is 10 times more intense ( 1*10-11 W/m2) is assigned a sound level of 10 dB. A sound that is 10*10 or 100 times more intense (1*10-10 W/m2) is assigned a sound level of 20 db. A sound that is 10*10*10 or 1000 times more intense (1*10-9 W/m2) is assigned a sound level of 30 db. A sound that is 10*10*10*10 or 10000 times more intense (1*10-8 W/m2) is assigned a sound level of 40 db. Observe that this scale is based on powers of 10. If one sound is 10xtimes more intense than another sound, then it has a sound level that is 10*x more decibels than the less intense sound. The table below lists some common sounds with an estimate of their intensity and decibel level.

Source	Intensity	Intensity Level	# of Times Greater Than TOH
Threshold of Hearing (TOH)	1*10-12 W/m2	0 dB	100
Rustling Leaves	1*10-11 W/m2	10 dB	101
Whisper	1*10-10 W/m2	20 dB	102
Normal Conversation	1*10-6 W/m2	60 dB	106
Busy Street Traffic	1*10-5 W/m2	70 dB	107
Vacuum Cleaner	1*10-4 W/m2	80 dB	108
Large Orchestra	6.3*10-3 W/m2	98 dB	109.8
Walkman at Maximum Level	1*10-2 W/m2	100 dB	1010
Front Rows of Rock Concert	1*10-1 W/m2	110 dB	1011
Threshold of Pain	1*101 W/m2	130 dB	1013
Military Jet Takeoff	1*102 W/m2	140 dB	1014
Instant Perforation of Eardrum	1*104 W/m2	160 dB	1016