What are Sound Waves? - Definition, Types & Uses

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  • 0:00 What is Sound?
  • 0:37 Mediums and Wave Types
  • 1:35 Features of Waves
  • 3:10 Uses of Sound
  • 4:00 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain what sound waves are, discuss features of sound waves, and provide some examples and uses they have in everyday life. A short quiz will follow.

What is Sound?

It's the age-old question: If a tree falls in the woods, and no one is there to hear it, does it make a sound? Well, first of all, what exactly is a sound?

Sound is a wave, but so are lots of things. More specifically, sound is a wave made of vibrations in the air. When something makes a sound, it vibrates the air molecules, which sends a chain reaction through the air until it reaches our ear drums. When our ears pick up that sound, signals are sent to our brain so that we can interpret what we're hearing. So based on that physics definition, the tree really does make a sound whether someone is there to hear it or not!

Mediums and Wave Types

In space, no one can hear you scream! And as scary as that statement may be, that shouldn't be altogether surprising. If sound waves are vibrations in the air, then take away that air... and bye-bye vibrations! Sound waves need a medium (or material) to travel through. These kinds of vibrational waves have another name: longitudinal waves. In fact, there are two main types of waves: transverse and longitudinal.

Longitudinal waves are where the vibration moves parallel to the direction the wave is traveling. A Slinky can help us understand this, because a longitudinal wave can be created by pushing a Slinky along its length, sending a pulse across it. Transverse waves, on the other hand is where the vibration is at 90 degrees to the motion of the waves. This time we have to move the Slinky side-to-side. Light waves and water ripples are transverse, but sound waves are longitudinal.

Features of Waves

Scientists love to name things. It's not enough to name the wave itself, we also need to name some of the wave's features. Two of these are called compressions and rarefactions, as shown here (see video). A compression is a high-density part of the wave (the part where the slinky is compressed). It is the peak of the wave. A rarefaction is a low-density part of the wave (the part where the slinky is most spread out). This is the trough of the wave.

One difficult part of understanding this topic is the question of why these compressions and rarefactions are considered to be a wave at all. After all, it doesn't look very... wavy. But, if we plot particle density across the previous image, we can see how we know that it is a wave. Much more wavy, don't you think?

Compression & rarefaction
rarefractionwaves

We can also use this diagram to measure the so-called wavelength of the wave. This is the distance between two identical parts—from a compression (peak) to the next compression, or from a rarefaction (trough) to the next rarefaction. A wavelength is the length of one full wave, in meters.

Last but not least, frequency. A sound wave's frequency is a number that tells you how many waves pass by each second. Frequency is measured in Hertz (Hz). So for example, 60 Hz, the frequency of most TVs, is 60 waves per second. Or in the case of television, this means that the picture refreshes 60 times a second. A high-frequency sound is high pitch, like a high note on a piano. A low-frequency sound is low pitch, like a tuba.

Uses of Sound

So what are some uses of sound?

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