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Physics of Sound



     What is sound? How does sound work? We hear sounds everywhere all the time. Sounds are produced when things vibrate. When the thing vibrates it causes the medium such as air or water around it to vibrate as well. This is what we hear. Sound waves are comprised of high and low pressure areas called compressions and rarefactions. Traveling longitudinal waves are sound waves that we hear in the air. 
Properties of a wave
     This picture shows the areas of high and low pressure as shown by the shaded bar. The lighter the color the lower the pressure while the darker areas indicate higher pressures. 
     The part of the wave in red is called the wavelength and it repeats indefinitely. The wavelength and the speed of the wave determine the pitch, or frequency of the sound. Wavelength, frequency, and speed are related by the equation speed = frequency * wavelength. The longer the wavelength the lower the pitch. The amplitude of the wave is the height of the wavelength. The amplitude determines how loud the sound will be. The greater the amplitude the louder the sound. 
      When two sounds meet there is interference. There are two kinds of interference: constructive and destructive. Constructive interference is when two waveforms are added together. The peaks add with the peaks, and the troughs add with the troughs, creating a louder sound. Destructive interference occurs when two waves are out of phase (the peaks on one line up with troughs on the other). In this, the peaks cancel out the troughs, creating a diminished waveform. 
Constructive and destructive interference 
     Sound in a tube creates standing waves. This happens when the sound reflects off the end of the tube and it interferes with itself. The longest wave that can fit in the tube is the fundamental while the other ones are overtones. Overtones are multiples of the fundamental. The areas of highest vibration are called antinodes, while the areas of least vibration are called nodes. In an open pipe, the ends are antinodes. In a closed pipe, only half the harmonics are created. 
     The physics of sound is interesting and amplitude, frequency, and the length of the wave indicated how loud the sound will be.  


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