Lecture_19

# Lecture_19 - 16.8 Decibels Unit of measurement used to...

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16.8 Decibels Unit of measurement used to quantify the intensity level of sound as perceived by uman ears. human ears. Your ears perceive the “loudness” of sound in a very interesting way. For example, let’s say you hear a sound with intensity I 1 . Then you hear a sound with intensity I 2 that you perceive as being twice as loud. When you actually measure the intensity of the second sound, you find that I 2 is about 10 times as big as I 1 ! Not twice! Human ears typically underestimate the sound intensity. Our perception of sound is one in which uniform increases in loudness actually correspond to multiplicative increases in the sound intensity. pp y For this reason, we use a logarithmic scale to relate the loudness of the sound to its intensity. β is the Intensity Level of the sound. It is measured Decibels (dB) × = o I I log dB) 10 ( in Decibels (dB). I is the Sound Intensity you want to quantify. I o is the threshold of hearing (1 x 10 -12 W/m 2 ). Experiments have shown that an increase in intensity level of 10 dB produces a sound twice as loud, thus, a 70 dB sound is twice as loud as a 60 dB sound.

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Some Sound Intensities Sound Intensity (W/m2) Intensity Level (dB) oudest noise achieved in a laboratory 0 9 10 Loudest noise achieved in a laboratory 10 210 Eardrum ruptures 10 4 160 Jet engine (at 30 m) 10 130 Threshold of pain 1 120 Tiger Stadium on a good night 0.5 117 Thunder (loud) 0.1 110 Subway train 10 -2 100 eavy street traffic 0 -5 0 Heavy street traffic 10 70 Normal conversation 10 -6 60 Whisper 10 -10 20 Normal Breathing 10 -11 10 Threshold of hearing 10 -12 4
Example : What happens to the intensity level, if the intensity of the sound is doubled? Let’s say your baby emits a crying sound with an intensity of 8 × 10 -6 W/m 2 . What intensity level does this correspond to? × = o I I log dB) 10 ( β × × × = 12 6 10 1 10 8 log dB) 10 ( ( ) 6 10 8 log dB) 10 ( × × = dB 69 = ow you go and have twins, and both babies cry with an intensity of 8 × 0 -6 /m 2

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Lecture_19 - 16.8 Decibels Unit of measurement used to...

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