Moving sound
In this chapter:
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Sound is all around us, at different volumes and in many different forms
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Sound is transmitted through various mediums
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Sound travels as regions of high pressure, the particles are pushed closer together
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Water is less compressible than air, so sound waves travel a lot further in water
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SONAR is a technique that uses sound propagation under water to navigate or detect other vessels
Introduction
Sound is all around us. We cannot close our ears as easily as we can our eyes, so it is hard to block out sound in the same way we block out light. We are affected by sounds, whether they are as loud as a jumbo jet or as quiet as a sob. Sound can be a very enjoyable thing, such as children's laughter or music, or a painful thing. Think of someone crying, nails on a blackboard, or a jackhammer early in the morning.
The transmission of sound is understood by scientists as beginning with the production, or propagation, of sound, followed by its travelling through a medium (air, water), and then being received by human ears or a technological device like a microphone.
SONAR
Sound is a form of energy. Compared to other forms of energy like nuclear or electrical energy, sound is often thought of as a 'weak' form of energy. It would take one person screaming for several thousand years to heat a pot of water using sound energy alone. Sound, however, can also be a very powerful form of energy. SONAR (SOund Navigation And Ranging) is a technique, used by marine vessels, that uses sound propagation under water to navigate or to detect other vessels.
See Image 1
To locate a target, a submarine emits pulses of sound waves that travel through the water, reflect off a target and return to the ship. By calculating the speed of sound in water and the time for the sound wave to travel to the target and back, the location and distance of the target can be quickly calculated. Whales, dolphins and bats use this same technique for locating prey. In some military contexts, the SONAR waves used are so powerful that they have been known to kill whales and other marine life.
How sound travels
Traditionally, we think of sound as travelling in waves, but more accurately sound is understood to travel as regions of high and low pressure. Think about when you quickly open the door to a room, and a window on the other side of the room rattles. This happens because of a change in pressure travelling across the room. Sound travels like a much faster version of this.
When there is no sound, particles in the medium (for humans, usually air) are evenly spread out. Sound travels as regions of high pressure, where the particles are pushed closer together, alternating with regions of low pressure, where they are further apart. In this sense sound travels in 'waves' somewhat like the waves on a beach. As the waves spread apart again the sound disperses (fades away).
How a speaker works
By thinking about how a sound system's speaker works, we can get a sense of the way pressure is involved in the transmission of sound. The speaker moves back and forth very rapidly, and as it moves forward it pushes out a region of high pressure. As it moves back it makes space for a low pressure region. This happens very quickly - the speaker vibrates hundreds or thousands of times a second, and the result is music or sound of some sort.
See Image 2
In a stereo system, the high frequency sound is pushed out by a small speaker called a tweeter. The sound that comes out of a tweeter is very focused and directional, so it is important to have the tweeter placed in the right area. The low frequency or bass sounds are dealt with by a sub-woofer. As they spread out more than the high frequencies, a woofer can be placed anywhere in a room.
Doppler effect
You may have noticed that when an ambulance drives past with its siren blaring, its sound changes as it passes you. What happens is that the siren's sound waves become compressed as the ambulance comes towards you. This increases the frequency of the waves, which makes the pitch of the siren sound higher. When it passes and starts moving away from you, the sound waves from the ambulance are stretched out. This decreases the frequency of the waves, which we hear as a decrease in the pitch of the siren. This is called the Doppler effect. It is named after Christian Andreas Doppler, an Austrian mathematician and physicist who first proposed the reason for the effect in 1842.
See video 'Doppler effect'
Sound also travels through other mediums, such as water, wood and the ground. Water is less compressible than air so sound waves travel a lot further in water. This is why whales' 'songs' can travel up to 3000 km through the water. This is also why, if you were on safari, you could hear a massive trampling herd of wildebeest through the ground long before you saw them.
See Image 3
