[Sound]

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Calculator		 Sound is a longitudinal mechanical wave. Mechanical because sound is, physically, a vibration of air molecules. The expansion and contraction of air creates sound. That's why you can hear the wind or hear musical instruments, etc.

Think of a tuning fork. Once the fork is set into vibration it resonates, creating air waves. The actual sound you hear is determined by the frequency of the tuning fork.

Three Categories of Sound Waves

The human ear can hear only so many frequencies of sound waves but there are many more. Sound waves fall into three categories classified by ranges in frequencies.

  • infrasonic [20 - Hz]
  • audible [20 - 20,000 Hz]
  • ultrasonic [20,000 + Hz]

What's Hz? Hertz (Hz) the unit of frequency is an "inverse second." Frequency by definition is "how many times something happens in one second." In this case, it is how many wavelengths are generated in one second. The inverse of frequency is thus time. Time would be "how many second would it take for something to happen once." Confusing perhaps.

f = 1
t
t = time required to complete one cycle

[f] = Hertz = Hz

Interference of Waves

A cacophany of noises? Noises, sounds, waves don't just travel here or there with without bumping into something. When waves bump into other waves, a phenomenon called interference occurs.

There are two types of interference between waves:

  • constructive interference: the waves combine to amplify
  • destructive interference: the waves cancel each other out

Speed of Sound

The speed of sound in different substances varies for the substance and on the conditions the substance is in (e.g. the temperature of the substance in some cases).

For example, the speed of sound in air a 0C is 331 m/s whereas the speed of sound in air at 20C is 343 m/s.

So now new concepts are introduced to relate conditions to the speed of sound, without having to ponder what affects what. Let's get to the essentials of the matter:

Speed is affected by changes in volume and pressure.

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Key Themes
  • Three Categories
  • Interference
  • Hertz
  • Speed of Sound
  • Practice Questions

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