Sound

Sound What causes sound? Sound Waves Speed of Sound Frequency and Pitch The Doppler Effect

What Causes Sound? Every sound is produced by an object that vibrates. For example, your friends’ voices are produced by the vibrations of their vocal cords, and music from a carousel and voices from a loudspeaker are produced by vibrating speakers.

Sound Waves Sound waves are compressional waves. A compressional wave is made up of two types of regions called compressions and rarefactions. You’ll see that when a radio speaker vibrates outward, the nearby molecules in the air are pushed together to form compressions.

Sound Waves (2) As the figure shows, when the speaker moves inward, the nearby molecules in the air have room to spread out, and a rarefaction forms. As long as the speaker continues to vibrate back and forth, compressions and rarefactions are formed.

Sound Waves (3) Compressions and rarefactions move away from the speaker as molecules in the air collide with their neighbors. A series of compressions and rarefactions forms that travels from the speaker to your ear. This sound wave is what you hear.

Moving through Materials Most sounds you hear travel through air to reach your ears. If you’ve ever been swimming underwater and heard garbled voices, you know that sound also travels through water. Sound waves can travel through any type of mattersolid, liquid, or gas. Remember the matter that a wave travels through is called a medium. Sound waves cannot travel through empty space.

The Speed of Sound in Different Materials The speed of a sound wave through a medium depends on the substance the medium is made of and whether it is solid, liquid, or gas. In general, sound travels the slowest through gases, faster through liquids, and even faster through solids.

The Speed of Sound in Different Materials (2) Sound travels faster in liquids and solids than in gases because the individual molecules in a liquid or solid are closer together than the molecules in a gas. However, the speed of sound doesn’t depend on the loudness of the sound. Loud sounds travel through a medium at the same speed as soft sounds.

Pitch If you were to sing a scale, your voice would start low and become higher with each note. Pitch is how high or low a sound seems to be. The pitch of a sound is related to the frequency of the sound waves.

Frequency and Pitch Frequency is a measure of how many wavelengths pass a particular point each second. For a compressional wave, such as sound, the frequency is the number of compressions or the number of rarefactions that pass by each second. Frequency is measured in hertz (Hz).

Frequency and Pitch (2) When a sound wave with high frequency hits your ear, many compressions hit your eardrum each second. Your brain interprets these fast vibrations caused by high-frequency waves as a sound with a high pitch. As the frequency of a sound wave decreases, the pitch becomes lower.

Frequency and Pitch (3) This figure shows different notes and their frequencies. A healthy human ear can hear sound waves with frequencies from about 20 Hz to 20,000 Hz. The human ear is most sensitive to sounds in the range of 440 Hz to about 7,000 Hz.

The Doppler Effect The change in pitch or wave frequency due to a moving wave source is called the Doppler effect. The Doppler effect occurs when the source of a sound wave is moving relative to a listener.

The Doppler Effect (2) As a race car moves, it sends out sound waves in the form of compressions and rarefactions. The race car creates a compression, labeled A. Compression A moves through the air toward the flagger standing at the finish line.

The Doppler Effect (3) By the time compression B leaves the race car, the car has moved forward. Because the car has moved since the time it created compression A, compressions A and B are closer together than they would be if the car had stayed still.

The Doppler Effect (4) As a result, the flagger hears a higher pitch. The Doppler effect happens any time the source of a sound is changing position compared with the observer. It occurs no matter whether it is the sound source or the observer that is moving. The faster the change in position, the greater the change in frequency and pitch.

Using the Doppler Effect The Doppler effect also occurs for other waves besides sound waves. For example, the frequency of electromagnetic waves, such as radar waves, changes if an observer and wave source are moving relative to each other.

Using the Doppler Effect (2) Radar guns use the Doppler effect to measure the speed of cars. Weather radar also uses the Doppler shift to show the movement of winds in storms, such as a tornado.

The Doppler Effect (5) The animation to the left shows a stationary sound source. Sound waves are produced at a constant frequency and the wave fronts move away from the source at a constant speed. The distance between wave fronts is the wavelength. All observers will hear the same frequency.

The Doppler Effect (6) In the animation at the left the same sound source is radiating sound waves at a constant frequency in the same medium. However, now the sound source is moving to the right. The wave fronts are produced with the same frequency as before. However, since the source is moving the center of each new wave front is now slightly displaced to the right. As a result, the wave fronts begin to bunch up on the right side (in front of) and spread further apart on the left side (behind) the source. An observer in front of the source will hear a higher frequency and an observer behind the source will hear a lower frequency.

Mach 1 – Breaking the Sound Barrier Now the source is moving at the speed of sound in the medium (at sea level approximately 750 mph). The wave fronts in front of the source are now all bunched up at the same point. As a result, an observer in front of the source will detect nothing until the source arrives. The pressure front will be quite intense (a shock wave), due to all the wave fronts adding together, and will not be perceived as a pitch but as a “thump” of sound as the pressure wall passes by.

Mach 1 – Breaking the Sound Barrier (2) The figure to the left shows a bullet travelling at Mach 1.01. You can see the shock wave just ahead of the bullet.

Mach 1.4 - Supersonic The sound source has now broken through the sound speed barrier, and is travelling at 1.4 times the speed of sound. Since the source is moving faster than the sound waves it creates, it actually leads the advancing wave front. The sound source will pass by a stationary observer before the observer actually hears the sound it creates. As you watch the animation, notice the clear formation of the Mach cone. It is this intense pressure front on the Mach cone that causes the shock wave known as a sonic boom as a supersonic aircraft passes overhead.

Mach 1.4 – Supersonic (2) The figure at the left shows a bullet travelling at Mach 2.45. The Mach cone and show wave fronts are very noticeable.

Mach 1.4 – Supersonic (3) The picture shows the shock wave front created by a T-38 Talon, a twin-engine, high-altitude, Super Sonic jet trainer.

Mach 1.4 – Supersonic (5) This picture shows the sonic boom created by the THRUST SSC team car as it broke the land speed record (and also broke the sound barrier on land).