Two waves of slightly different frequency create beats when added together.
Click on image for full size

Related links:

Hear what wave beats sound like

Wave Beats

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.

You might also be interested in:

Science, Evolution, and Creationism

How did life evolve on Earth? The answer to this question can help us understand our past and prepare for our future. Although evolution provides credible and reliable answers, polls show that many people turn away from science, seeking other explanations with which they are more comfortable....more

Christian Doppler

Christian Doppler was an Austrian mathematician who lived between 1803-1853. He is known for the principle he first proposed in Concerning the coloured light of double stars in 1842. This principle is...more

Wave Beats

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...more

Chasing Tornadoes

Storms chasers are different than storm spotters. Chasers travel around Tornado Alley looking for severe storms and tornadoes. This area in the Great Plains is the best for chasing. Besides having a lot...more

Energy Density

A tornado is the most destructive force in nature; that doesn't mean it has the most energy. Thunderstorms which produce tornadoes can have 40,000 times as much energy as a tornado! Tornadoes are so destructive...more

The Doppler Effect

The Doppler effect was named after Christian Doppler, who first came up with the idea in 1842. He determined that the frequency of sound waves would change if either the source of the sound or the observer...more

Tornado Forecasts

The short duration and complicated nature of tornadoes make them nearly impossible forecast. Meteorologists don't really know the specifics of how they form, but they do know what atmospheric conditions...more

Common Tornado Myths

Scientists once thought that you should open your windows during a tornado so your house won't explode. The thinking behind this idea is that the extreme low pressure in a tornado would cause the air in...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA