Sound is vibration in air, which travels as waves of compression and rarefaction of the air. The air itself does not travel, but the wave travels through it. When the waves arrive at your ear, you hear the sound.
In class, I will show simulations of sound waves in air.*
A wave is often depicted by a rising and falling curve, like this graph:
For sound waves, imagine the curve moving from left to right. The peaks on the graph represent points where the air is compressed, and the valleys show where the air is rarified. Here is a comparison of the curve depiction and the compression depiction:
At bottom left, the dark vertical band represents compressed air,
and the white vertical band represents rarefied air.
(Calculations at the end of this video contain errors, but
the video itself and the explanations are very good.)
The website Physics of Sound contains several nice animations of wave behavior, helping you to see how vibrations travel through air, but the air itself does not travel -- it simply vibrates; that is, molecules move back and forth, but stay pretty much in the same place. Like this:
Think of the gray vertical line at left as a loudspeaker which vibrates and makes the air molecules vibrate. A few molecules, randomly chosen, are colored red so that you can easily see how they move. Although the sound wave peaks move left to right, and eventually off the image, the red molecules stay in the image. Once more for emphasis: sound waves travel through air, but the air does not travel. The vibration moves, but the air does not move. When you hear even the loudest sound, it does not feel like wind. (Question: Why do the molecules nearest the speaker move back towards it after being pushed away?)
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Wavelength is the distance from one point on the curve to the next identical point, such as from the top of a peak to the top of the next peak. The sine curve at the top of the page shows three full wavelengths.
Frequency is the number of complete waves that pass a point in space (say, the entrance to your ear) in each second. This depends on the wavelength and on how fast the wave is traveling. Sound in air travels at about 1125 feet per second (ft/sec). (Light travels almost a million times faster, and unlike sound, can travel through a vacuum. Sound and light are different. Different.)
Frequency (f) and wavelength (l) for sound in air are related like this:
f x l = 1125 ft/sec
For example, if a sound wave is 2.56 feet long, it has a frequency of 440 peaks per second (a unit called hertz, or Hz or /sec)
We hear frequency as the pitch of sound. A frequency of 440 Hz corresponds to the note A above middle C on the piano, which is called "concert A". It's the note to which the orchestra tunes up. (See video at bottom of page.)
Intensity is the height, from peaks to valleys, in the wave. In the next figures, the length of the vertical arrows show the intensity of each wave -- the distance between the green lines along the peaks and valleys. Intensity of the wave determines loudness of the sound. High peaks and low valleys means a loud sound.
Vertical arrows show intensity.
Top: loud sound; bottom, soft sound.
Standing Waves
In a closed space, waves reflecting off of surfaces sometimes interfere with each other to produce peaks and valleys that, while vibrating, stay in the same place. These are called standing waves, and they are important to making sound with musical instruments (later). To try to hear this effect, play a 100-Hz test tone through the speaker of your TV or through your sound system. Listen to it as you walk across the room, keeping about the same distance from the speaker. You might find spots where the sound is much softer. These dead spots are the result of wave cancellation in standing waves. This might be one reason that the frequency response of your speakers does not sound smooth in your living room, especially if it has large bare wall spaces that reflect sound well.
Readings and More
Sound, at Wikipedia, including a good animation of propagating sound waves.
Physics of Sound
Table of Frequencies of Musical Notes (as shown in class)
Speaking of tuning up, Woody Herman's band played a song called Tunin' In, based on the sound of a band tuning up. For many years, this was the theme song on the Saturday night jazz program on the local station formerly known as W-Bach.
* Click HERE to obtain the classroom simulation of a loudspeaker producing sound waves in air. It requires a Java Runtime Environment. Setting it up can be tricky.
