SOUND INTENSITY SOUND INTENSITY Sound waves carry energy

SOUND INTENSITY Sound waves carry energy that can be used to do work, like

SOUND INTENSITY The amount of energy transported per second by a sound wave is

SOUND INTENSITY When sound leaves a source the power spreads out. At further distances

SOUND INTENSITY: EXAMPLE 12 x 10 -5 W of sound power passes perpendicularly through

SOUND INTENSITY: EXAMPLE I 1 = 3. 0 x 10 -5 W/m 2 I

SOUND INTENSITY Young people can hear sound frequencies from 20 to 20, 000 Hz.

SOUND INTENSITY Continuous exposure to intensities greater than 1 W/m 2 can be painful

SOUND INTENSITY: EXAMPLE 2 During a fireworks display, a rocket explodes high in the

SOUND INTENSITY The equation for sound intensity is only valid when the sound is

ASSIGNMENT Focus p. 500 #15 Problems p. 503 #52 -58

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SOUND INTENSITY

SOUND INTENSITY Sound waves carry energy that can be used to do work, like forcing the eardrum to vibrate. The energy can be great enough to cause damage to windows and buildings (sonic boom)

SOUND INTENSITY The amount of energy transported per second by a sound wave is the power of the wave Units joules/second (J/s) or watts (W)

SOUND INTENSITY Demo – sound intensity

SOUND INTENSITY When sound leaves a source the power spreads out. At further distances from the source, the same power is spread out over a larger area.

SOUND INTENSITY

SOUND INTENSITY: EXAMPLE 12 x 10 -5 W of sound power passes perpendicularly through the surfaces labeled 1 and 2 above. These surfaces have A 1=4. 0 m 2 and A 2=12 m 2. Determine the sound intensity at each surface and discuss why listener 2 hears a quieter sound than listener 1.

SOUND INTENSITY: EXAMPLE I 1 = 3. 0 x 10 -5 W/m 2 I 2 = 1. 0 x 10 -5 W/m 2 The sound intensity is less at the more distant surface because the same power passes through a much larger area. The listener’s ear intercepts less of the sound power, so the sound is quieter.

SOUND INTENSITY Young people can hear sound frequencies from 20 to 20, 000 Hz. For a 1, 000 Hz tone, the smallest sound intensity that the human ear can detect is ~1 x 10 -12 W/m 2. This is called the threshold of hearing.

SOUND INTENSITY Continuous exposure to intensities greater than 1 W/m 2 can be painful and can result in permanent hearing damage.

SOUND INTENSITY

SOUND INTENSITY: EXAMPLE 2 During a fireworks display, a rocket explodes high in the air above the observers. Assume that the sound spreads out uniformly in all directions. When the sound reaches listener 2, who is r 2=640 m away from the explosion, it has an intensity I 2=0. 10 W/m 2. What is the sound intensity detected by listener 1, who is r 1=160 m away from the explosion?

SOUND INTENSITY The equation for sound intensity is only valid when the sound is emitted from a source uniformly and does not reflect off any surfaces. If the sound reflects off of surfaces, then the equation will underestimate the intensity of the sound, because the reflected sound will also be passing through the surface.

ASSIGNMENT Focus p. 500 #15 Problems p. 503 #52 -58