# Production & Propagation of Sound

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## Sound

Sound is a mechanical wave phenomenon and is normally associated with our sense of hearing. Sound is a property of vibrating objects.

• Sound is produced by vibrating sources in a material medium. Medium can be any gas, liquid or solid.
• The vibrating sources set the particles of a medium in vibration in such a way that sound travels outwards in the form of longitudinal waves.
• Some of the energy of the vibrations are transmitted over a distance.

## Examples Of Vibrating Sources

• Musical instruments, like drums, guitar
• Hitting a piece of iron with a hammer
• Loudspeakers (Consists of a cone which vibrates under the effects of electricity and magnetism)
• Explosion resulting from explosives.

### Video On Production Of Sound

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## Propagation Of Sound Waves

Vibration in the tuning fork produces disturbances in the surrounding air. When the prongs’ movement is outwards, the prongs push the surrounding air molecules away, creating a local compression.

This disturbance of air layers is then passed from molecule to molecule by collisions, causing the local compression to move outwardly.

When the prongs’ movement is inwards, a partial void, or rarefaction is created. Pressure differences causes the air molecules to rush back into the region again. This periodic to-and-fro movement of the prongs will create alternating regions of compressions and rarefactions. The sound waves span outwardly parallel to the direction of the wave propagation (longitudinal nature). We say that sound travels as a progressive longitudinal wave.

• In air, compressions are regions where the pressure is higher than surrounding air and rarefactions are regions where pressure is lower than the surrounding air.
• Similar to transverse waves, sound waves follow the wave equation: $v = f \lambda$ or $$\text{Wavelength} (\lambda) = \frac{\text{Speed of Sound} (v)}{\text{Frequency} (f)}$$

#### Notes:

• The energy of the sound waves is propagated and carried by colliding particles of a material medium. Hence, a (material) medium is required in order to transmit these (energy) waves.
• The speed of energy propagation is dependent on the proximity of these particles in a medium. Hence, given that the proximity of particles in the air, liquids or solids is different, the speed of sound differs in air, liquids and solids. Sound travels faster in denser media. It travels faster in liquids than in gases and fastest in solids.

## Range Of Audible Frequencies

The human ear responds to sounds with frequencies in the range from 20 Hz to 20,000 Hz. This is called the audible range of the human ear. Examples of vibrating sources that produce sound in the audible range of frequencies are drums, guitar strings, tuning fork, human vocal cords and diaphragms of loudspeakers.

In audible sound waves whose frequencies are less than 20 Hz are in the infrasonic range. Sources of infrasonic waves include earthquakes, thunder, volcanoes and waves produced by vibrating heavy machinery. The hearing ranges of elephants and whales extend into the infrasonic region.

Frequencies above 20,000 Hz are in the ultrasonic range. The audible range of dogs, cats, moths and mice extends into ultrasound frequencies. They can hear very high frequencies that humans cannot.

### Video For Audible Frequency

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## Worked Examples

### Example 1

Explain how a sound wave can be generated.

A sound wave is generated when an object vibrates, causing the air around it to move. This vibration can be initiated by anything that moves back and forth or oscillates, such as a guitar string when plucked, the diaphragm of a speaker as it moves in and out, or the vocal cords when air is pushed through them. These vibrations disturb the surrounding air molecules, creating areas of high and low pressure known as compressions and rarefactions, respectively, which propagate through the air as a sound wave.

### Example 2

Detail how the air molecules oscillate when a sound wave travels through the air.

When a sound wave moves through air, it causes the air molecules to vibrate in a direction parallel to the wave’s propagation. This means the molecules move back and forth along the same path that the sound wave is traveling. During compressions, the molecules are pushed closer together, creating areas of higher pressure. During rarefactions, the molecules spread apart, creating areas of lower pressure. Despite this movement, the air molecules do not travel with the sound wave; instead, they oscillate around their original position.

### Example 3

What is the distance between the compressions in a sound wave?

The distance between the compressions in a sound wave is known as the wavelength. The wavelength is a key characteristic of any wave, including sound waves, and it varies depending on the frequency of the sound. Higher frequency sounds have shorter wavelengths, meaning the compressions are closer together. Conversely, lower frequency sounds have longer wavelengths, with compressions farther apart.

The wavelength is given by:

$$\text{Wavelength} (\lambda) = \frac{\text{Speed of Sound} (v)}{\text{Frequency} (f)}$$

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