Infrasound: Definition, Effects & Uses

Infrasound: Definition, Effects & Uses
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  • 0:00 Sound, Infrasound, Ultrasound
  • 1:32 Where Does Infrasound…
  • 2:24 How Does Infrasound Affect Us?
  • 2:53 Can We Use Infrasound?
  • 4:03 Lesson Summary
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Lesson Transcript
Instructor: Mary Ellen Ellis
Humans can't hear infrasound, but we have created devices to detect these low frequency waves. By observing them, we can learn more about the natural world and even monitor nuclear testing.

Sound, Infrasound, Ultrasound

You'd think defining 'sound' would be easy, right? After all, it's what we hear with our ears. And yet, there are sounds we can't hear. Which means we need a broader definition for what sound is.

Sound is actually a kind of wave, sometimes referred to as a pressure wave. It is a wave that requires a medium through which to travel. In other words, it requires matter. This means there is no sound in the vast emptiness of outer space. A sound wave is created when some kind of force disturbs matter and causes it to ripple outwards in all directions.

Sound waves that humans can hear are the ones that the ears can collect and transmit to the brain for interpretation. Only sound waves of just the right size and frequency fit into the human ear canal. Humans hear sounds in the mid-frequency range. Those sounds that are higher in frequency are called ultrasound, while sounds that are in the lower frequency range are infrasound.

Frequency is the number of cycles that occur during a specified period of time. For sound waves, the cycle is one wavelength, and the time period is one second; the lower the frequency, the fewer wavelengths in the sound wave per second. The frequency of sound waves is measured in hertz. This is the equivalent of number of wavelengths per second. Hertz are represented using the abbreviation Hz. Infrasound waves are generally considered to be any sound waves with a frequency of 20 Hz or less.

Where Does Infrasound Come From?

All sound waves have a source. Something exerts a force and causes matter to vibrate. For instance, when someone is talking, that person's vocal cords cause the air to vibrate, and someone else's ears pick up on the vibrations in the air. Human vocal cords are not capable of producing the low frequency of infrasound, but other things are.

Other types of animals that are larger and have larger vocal cords, like elephants and whales, can produce infrasound waves. Large natural events on the earth can also produce these low-frequency vibrations. Earthquakes, avalanches, volcanoes, and extreme weather events can all produce infrasound. Human-created events can also make infrasound: explosions, wind turbines, engines, aircraft breaking the sound barrier, and certain speakers.

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