1. Sound Intensity & Resonance
12-2
Sound Intensity & Resonance

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

3. 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/M2]
P=Power, A=Area intercepting the sound (4πr²for a spherical wave)

4. 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 kHz sound at 1 x W/m2
I° = human threshold of hearing

6. To recap:
~frequency of sound determines the pitch
~intensity sound determines loudness
~I° = 1x10-12 W/m² threshold of hearing
~Ip = 1.0 W/m² threshold of pain

7. 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

8. The sound level b is defined as:
decibel
10-12 W/m2, human
hearing threshold
Note: this is not an intensity (units aren’t W/m2) this does measure the relative
loudness of a sound.

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

10. 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

11. 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.

12. Tacoma Narrows Bridge