Sound Wave Beats

Two waves of slightly different frequency create beats when added together Click on image for full size (11K GIF) 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. 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. 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. 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 had changed--he heard the wave beats.

Hear what wave beats sound like

Sound Wave Beats

Two waves of slightly different frequency create beats when added together Click on image for full size (11K GIF) Sound travels in waves. You hear sound because waves hit your ear. Sound waves are similar to ocean waves. They both have a certain frequency. The frequency is measured in hertz, which is one cycle per second. One wave cycle is a complete wave; both the up half (crest) and down half (trough) are part of one cycle. People can hear sounds between 20 and 20,000 hertz. Waves also have a certain amplitude. Amplitude is a measure of how strong the wave is; the higher the amplitude, the higher the crests and deeper the troughs. The amplitude switches between positive and negative during each cycle. Waves are called periodic functions because they repeat in a certain period, or time. Period and frequency are related. Waves don't usually reflect when they strike other waves. Instead, they combine. If the amplitudes of two waves have the same sign (either both positive or both negative), they will add together to form a wave with a larger amplitude. This is called constructive interference. If the two amplitudes have opposite signs, they will subtract to form a combined wave with a lower amplitude. This is called destructive interference. Sound waves with higher amplitudes sound louder than sound waves with lower amplitudes. Constructive interference will make a sound louder while destructive interference will make a sound quieter. Two waves that add together may have different frequencies. That means that the crests and troughs won't add up the same way with each new wave because one is moving faster than the other. Part of the waves will interfere constructively and part will interfere destructively. This is known as a beat in music. You can hear beats when two instruments are almost playing the same note but are not quite together. Musicians use beats to tune instruments. Piano tuners strike a tuning fork and then play a note on a piano. If they hear a beat then they know they need to tighten or loosen the string for that note. When the beat disappears, the note is in tune. This is how Doppler could tell that the frequency of the trumpets on the moving train had changed--he could hear the wave beat.

Hear what wave beats sound like

Wave Beats

Two waves of slightly different frequency create beats when added together Click on image for full size (11K GIF) Sound travels in waves. These waves have both a frequency and an amplitude. The frequency is measured in hertz, which is one wave cycle per second. A cycle is a repeated pattern of positive and negative amplitudes. These cycles are repeated with a certain period, or time interval. This is why waves are called periodic functions. 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 interact, they don't reflect off each other--they combine. If the amplitudes of the waves are both positive or both negative, the combined wave will have a larger amplitude. This is called constructive interference. If the waves have opposite amplitudes, the resulting wave will have a smaller amplitude. This is called destructive interference. Two waves that add together may have different frequencies. If this happens, at one point the waves will interfere constructively while later they will interfere destructively. These changes in constructive/destructive interference are also periodic functions. In music this is known as a wave beat. You can hear beats when two instruments are trying to play the same note but one is off by a little bit. Musicians use beats to help tune their 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 the beat disappears, the note is in tune. This is how Doppler could tell that the frequency of the trumpets on the moving train had changed--he could hear the wave beat.

Hear what wave beats sound like

Last modified prior to September, 2000 by the Windows Team

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