SOUND INTENSITY DECIBELS CHECK LAST WEEKS HOMEWORK SOUND

SOUND INTENSITY - DECIBELS

CHECK LAST WEEK’S HOMEWORK

SOUND WAVES – SOME VOCAB Each cycle of a sound wave includes one condensation and one rarefaction The frequency of the sound wave is the number of cycles per second that pass by a given location

THE NATURE OF SOUND A sound with a single frequency is called a pure tone. A 440 Hz tone consists of 440 condensations, each followed by a rarefaction, every second. A healthy young person hears frequencies from 20 – 20, 000 Hz

THE NATURE OF SOUND

COMPARING SOUND INTENSITIES What if we wanted to compare two sound intensities? The simplest comparison would be a ratio of the intensities Ex: compare I=8 x 10 -12 W/m 2 to I 0=1 x 1012 W/m 2 I/I 0= 8 x 10 -12 W/m 2 / 1 x 10 -12 W/m 2 = 8 So I is 8 times as great as I 0 This does NOT mean that I is 8 times louder than I 0

COMPARING SOUND INTENSITIES When a sound wave reaches a listener’s ear, the sound is interpreted by the brain as being loud or soft Greater intensities give rise to louder sounds. But the relationship between loudness and intensity is not a simple linear relationship. Doubling the intensity does not mean the loudness doubles!

DECIBELS

EXAMPLE

TYPICAL SOUND INTENSITIES AND INTENSITY LEVELS Threshold of hearing Whisper Car without a muffler Rock Concert Threshold of Pain Intensity (W/m 2) Intensity level d. B 1. 0 x 10 -12 1. 0 x 10 -10 1. 0 x 10 -2 1. 0 10 0 20 100 120 130

PROBLEM SOLVING

EXAMPLE Humans can discern a change in loudness of about 1 -3 d. B. Doubling the loudness corresponds to a change of 10 d. B. Note that doubling the loudness is not the same thing as doubling the intensity! (to double the loudness, the intensity needs to increase A LOT)

EXAMPLE On a dark and scary night, you hear a sound at 90 d. B. A moment later, you hear it again, this time at 93 d. B. The corresponding intensities are I 1 and I 2. How many times greater is the intensity of I 2 than I 1?

EXAMPLE What is the change in sound intensity needed to double the perceived loudness of a sound? 2 times the loudness means a 10 d. B increase in sound intensity So β 2 -β 1=10 d. B I 2/I 1 = 10 Intensity must increase by a factor of 10 to get a sound that is perceived to be twice a loud. Means that a 200 W stereo is only twice as loud as a 20 W stereo!

ASSIGNMENT P. 504 #63 -69 Due Wednesday.