12 2 Sound Intensity Resonance Sound waves carry

12 -2 Sound Intensity & Resonance

�Sound waves carry energy. �The power of the wave is the energy transported per second, measured in J/S or Watts

• Sound Intensity: The rate at which energy flows through a unit area perpendicular to the direction of wave motion (pressure amplitude) • Sound Intensity is given by: • I=P/A measured in [W/M 2] • P=Power, A=Area intercepting the sound (4πr²for a spherical wave)

Audibility • The human ear picks up sound only if it is within the audible range AND has sufficient intensity • The lowest intensity we can hear is for a 1 k. Hz sound at 1 x 10 -12 W/m 2 • I° = human threshold of hearing

To recap: • ~frequency of sound determines the pitch • ~intensity sound determines loudness • ~I° = 1 x 10 -12 W/m² threshold of hearing • ~Ip = 1. 0 W/m² threshold of pain

• Relative intensity is found by comparing the intensity of two sounds • Ip/Io = 1 x 1012 (a trillion times the intensity) • Due to the huge range we compare sounds on a logarithmic • The relative intensity is also called decibel level

The sound level b is defined as: decibel 10 -12 W/m 2, human hearing threshold Note: this is not an intensity (units aren’t W/m 2) this does measure the relative loudness of a sound.

Ranking of common sounds Scale starts at 0 d. B Each 10 d. B increase in sound level, means 10 x the intensity of the sound.

Forced Vibrations • When a vibration of one object causes another to vibrate. • Examples: 1) sounding board of a musical instrument 2) the box of a music box

• When the frequency of a forced vibration (driving frequency) matches the object’s natural frequency, a dramatic increase in amplitude occurs. • This is known as RESONANCE.

Tacoma Narrows Bridge