Sound Waves

Sound is a mechanical wave which we observe by detecting vibrations in the inner ear. Typically, we think of sound as traveling through air, therefore the particles vibrating are air molecules. Sound can travel through other media as well, including water, wood, and even steel.

The particles of a sound wave vibrate in a direction parallel with the direction of the sound wave's velocity, therefore sound is a longitudinal wave. The speed of sound in air at standard temperature and pressure (STP) is approximately 331 m/s.

Question: At an outdoor physics demonstration, a delay of 0.50 second was observed between the time sound waves left a loudspeaker and the time these sound waves reached a student through the air. If the air is at STP, how far was the student from the speaker?

Answer: Speed of Sound Solution

When we observe sound waves through hearing, we pick up the amplitude, or energy, of the wave as loudness. The frequency of the wave is perceived as pitch, with higher frequencies observed as a higher pitch. Typically, humans can hear a frequency range of 20Hz to 20,000 Hz, although young observers can often detect frequencies above 20,000 Hz, an ability which declines with age.

opera singer

Certain devices create strong sound waves at a single specific frequency. If another object, having the same "natural frequency," is impacted by these sound waves, it may begin to vibrate at this frequency, producing more sound waves. The phenomenon where one object emitting a sound wave with a specific frequency causes another object with the same natural frequency to vibrate is known as resonance. A dramatic demonstration of resonance involves a singer breaking a glass by singing a high pitch note. The singer creates a sound wave with a frequency equal to the natural frequency of the glass, causing the glass to vibrate at its natural, or resonant, frequency so energetically that it shatters.

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Question: Sound waves strike a glass and cause it to shatter. This illustrates what phenomenon?

Answer: Resonance.




Question: A car traveling at 70 kilometers per hour accelerates to pass another car. When the car reaches a speed of 90 kilometers per hour the driver hears the glove compartment door start to vibrate. By the time the speed of the car is 100 kilometers per hour, the glove compartment door has stopped vibrating. This vibrating phenomenon is an example of

  1. destructive interference
  2. the Doppler effect
  3. diffraction
  4. resonance

Answer: (4) resonance