1. Physics Section 12.2 Apply the sound properties of intensity and resonance Intensity is the rate at which energy flows through a unit area perpendicular to the direction of the wave motion. Intensity = power I = P P = power (watts) Area A A = area (m 2 ) I = intensity (W/m 2 ) Sound radiates outward in all directions creating a spherical wavefront. I = __P__ I = intensity (W/m 2 ) 4πr 2 P = power (W) r = distance to source (m)

2. problems What is the intensity of a sound, at a distance of 5.0 m, whose power output is.75 W ? The intensity of a sound is 8.2 X 10 -6 W/m 2, at a distance of 3.4 m. Find the power output of the sound.

3. The intensity of a sound determines its perceived loudness. The relationship between intensity and loudness is based on a logarithmic scale. dB = 10 log I_ I= intensity of the sound (W/m 2 ) I o I o = 1 x 10 -12 (W/m 2 ) dB = relative intensity/loudness (decibels) The softest sound that can be heard by the average human ear has an intensity of 1 x 10 -12 W/m 2. It is called the threshold of hearing. The loudest sound that can be discerned by the average human ear has an intensity of 1 x 10 0 W/m 2. It is called the threshold of pain.

4. An increase by a factor of 10 in intensity increases the loudness(relative intensity) by 10. illustration: Sound A has an intensity of.5 W/m 2 and a decibel level of 115 dB. Sound B has an intensity of 5 W/m 2 and a relative intensity of of 125 dB.

5. examples Find the relative intensity of a sound with an intensity of 4.5 X 10 -5 W/m 2. Find the intensity of a sound if its relative intensity is 72 dB.

6. Sympathetic vibration occurs when one vibrating object causes another object to vibrate at the same or a related frequency. http://www.youtube.com/watch?v=3mclp9QmCGs Resonance (a type of sympathetic vibration) occurs when the frequency of an applied force matches the frequency of a system resulting in a large amplitude of the vibration.

7. assignment Page 420 problems 1 - 5