Sound travels in waves. When the waves hit your ear, you hear a sound. Have you ever noticed the waves in the ocean? They go up and down, up and down. Sound waves act the same way. The number of times they go up and down is called the frequency. People measure frequency in units called hertz. One hertz is equal to one complete wave (up and down) in one second. People can hear sounds between 20 and 20,000 hertz.

Another property of a waves is called amplitude. Amplitude is how strong or weak a wave is. In a lake, the amplitude of waves is usually measured in feet or meters. The amplitude of a wave switches between up and down (positive and negative). The waves switch back and forth and are called periodic functions because they repeat in a certain period of time. The amplitude is a measure of the height of the wave. Intensity is directly related to amplitude. Sound wave intensity is measured using a logarithmic scale with units of decibels. One bel is defined as the logarithm (base 10) of the ratio of two intensities. The scale is actually a ratio; the lower number is chosen arbitrarily. Zero decibels is set as the lower limit of our hearing. Bels are too large for most uses, so instead we use decibels (deci=1/10). The unit is named after Alexander Graham Bell, inventor of the telephone.

When waves run into each other, they usually don't reflect. Instead, they combine. If the amplitudes of two waves point in the same direction (either up or down), then the new wave has a larger amplitude. This is called constructive interference. Sound waves with a higher amplitude sound louder. If the waves had opposite amplitudes (one pointed up and the other pointed down), then the new wave has a smaller amplitude. This is called destructive interference. Constructive interference will make a sound louder while destructive interference will make a sound quieter.

	When two waves of the same wavelength and frequency occur in the same place, they will have an effect on each other. If two waves are in sync, (the crest from one wave coincides with the crest from the other), they add up: this is a constructive interference.
	If two waves are out of sync (the crests from one wave coincides with the troughs from the other), they cancel out, and the resulting wave will be zero;this is a destructive interference,.

Two waves that add together might have different frequencies. That means that the peaks won't always line up the same way because one wave is moving faster than the other. When this happens there are times when the waves interfere constructively and times when they interfere destructively. In music, musicians call this a beat. You can hear beats when two instruments are almost playing the same note, but not quite. Musicians use beats to tune instruments. Piano tuners will strike a tuning fork (which vibrates with a constant frequency) and then play a note on the piano. If they hear a beat, they know that they need to loosen or tighten the string for that note. When you can't hear beats anymore, the instruments are tuned. This is how Doppler could tell that the frequency of the trumpets on the train moving had changed--he heard the wave beats.

Click here to see an Example of Constructive/Destructive interference

Click here for Waves Terminology

Click here to learn more about the Doppler Effect

Here are a few examples of a 440Hz tone combining with a slightly different tone to form a beat: