1. Sound Intensity and Resonance

2. What is Sound Intensity?

3. Textbook Definition:
Sound waves traveling in air are longitudinal waves. As the sound waves travel outward from the source, energy is transferred from one air molecule to the next. The rate at which this energy is transferred through a unit area of the plane wave is called the intensity of the wave.

4. Example 1
What is the intensity of the sound waves produced by a trumpet at a distance of 3.2 m when the power output of the trumpet is 0.20 W? Assume that the sound waves are spherical.

6. Range of Human Hearing Human hearing depends on both the frequency and the intensity of sound waves. Sounds in the middle of the spectrum of frequencies can be heard more easily (at lower intensities) than those at lower and higher frequencies.
The human threshold of hearing is at 1000 Hz and 1.0x10-12 W/m2. The threshold of pain is 1.0 W/m2.

7. Decibels (dB) – Relative Intensity
Relative Intensity is the ratio of the intensity of a given sound wave to the intensity at the threshold of hearing. The decibel scale is logarithmic (base 10). Random Fact: The original unit of decibel level is the bel, named in honor of Alexander Graham Bell, the inventor of the telephone. The decibel is equivalent to 0.1 bel

8. On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB. A sound 1,000 times more powerful than near total silence is 30 dB.
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9. Intensity Level (Decibel Level)
𝛽=10 log 𝐼 𝐼 𝑜 Where: I is the intensity of the sound, Io is the threshold of hearing (1.0x10-12 W/m2) B = the intensity level in decibels

10. Practice Problem
An 80 W guitar amplifier is played at maximum volume. If you are standing 12.0 m from the amplifier, how many decibels of sound are you hearing?

11. Resonance (Forced Vibrations)
Textbook Definition: a phenomenon that occurs when the frequency of a force applied to a system matches the natural frequency of vibration of the system, resulting in a large amplitude of vibration Resonance Explained Resonance Demo