Shhh! Don't Make Waves

The speed of sound is affected by many factors including the humidity and temperature of air, density of material through which the sound wave is passing, and the elasticity of the material. In general, the speed of sound is greater in more elastic materials and less in more dense materials. Our iron example may seem odd, then, since iron is more dense than water. However, it is far more elastic, resulting in greater speed.

The Frequency of Sound

As you know or can imagine by now, if there are thousands of sounds filling the space where you exist, you must not be hearing some or most of them. There are a number of reasons why we may not hear a given sound, not the least of which is the "frequency" of that sound.

The human ear is sensitive to frequencies ranging from about 20 cycles per second to 20,000 cycles per second. Waves with greater frequencies are called "ultrasonic waves" and it's enough to make your dog run around in circles and howl even if you can't hear it, since Fido's range of hearing is sensitive to higher frequencies. "Subsonic waves" are so low you can't hear them, either, but they sure can make you upchuck your dinner if you get hit by one! Just imagine a sound that's lower than that boom box driving down your street and you'll understand what I'm talking about.

Sound Intensity and the Decibel Scale

The intensity of a sound wave is defined as the amount of sound power passing through a unit area, or the amount of energy passing through 1 square meter in 1 second. Actually, a sound wave doesn't carry much energy, but it can sure seem powerful when you're locked in a car with the windows closed.

Sound intensity is noted as watts/m2; however, it has historically been expressed in decibels (dB). The least intense sound that can be heard by a good ear has an intensity of 0 decibels. A sound carrying 10 times as much energy is rated at 10 dB, with 100 times as much energy it is 20 dB, and at 1000 times as much energy it is rated at 30 dB. Karl Kuhn (Basic Physics, 1979, 1996, John Wiley & Sons Inc.) puts this in perspective:

• On the decibel scale a whisper measures about 20 dB.

• Ordinary conversation is about 60
  The copyright of the article Shhh! Don't Make Waves in Physics is owned by Linda Bond. Permission to republish Shhh! Don't Make Waves in print or online must be granted by the author in writing.

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