Sound is a form of energy that gives us the sense of hearing. We hear sounds from birds, musical instruments, vehicles, people talking and many other sources. But what exactly is sound, and how does it travel from one place to another? In this chapter we study the production and propagation of sound, its characteristics such as loudness and pitch, reflection of sound, echo, and the human ear.
Sound is produced when an object vibrates. Whether it is the string of a guitar, the membrane of a drum, or the vocal cords in our throat, sound always begins with vibration. These vibrations disturb the surrounding medium, and the disturbance travels outward as a wave.
Sound needs a material medium to travel; it cannot travel through a vacuum. In this chapter we will understand how sound waves propagate, how we hear, and how sound is used in technologies like sonar and ultrasound.
Sound is produced by vibrating objects. When we strike a tuning fork, it vibrates and produces sound. When we pluck a guitar string or beat a drum, the vibrations produce sound. The vibration of an object causes the particles of the surrounding medium to vibrate, and this disturbance travels as sound.
To show that vibrating bodies produce sound, we can bring a lightly touching suspended ball near a struck tuning fork; the ball moves because of the fork's vibration. The human voice is produced by the vibration of the vocal cords in the throat.
Vibration: The back-and-forth or to-and-fro motion of an object is called vibration. The number of vibrations (oscillations) per second is called frequency.
Sound is a mechanical wave that requires a material medium for its propagation. It cannot travel through a vacuum. When the source vibrates, it disturbs the particles of the medium around it. These particles, in turn, disturb their neighbours, and the disturbance travels outward in the form of a wave, although the particles themselves only vibrate about their mean positions and do not travel with the wave.
For example, when a stone is dropped into a pond, ripples spread outward on the surface. Similarly, sound waves spread outward from the source in all directions.
Sound can travel through solids, liquids and gases. It travels fastest in solids, slower in liquids and slowest in gases, because the particles are closest together in solids. The speed of sound in air at 20 degree Celsius is about 343 m/s.
Proof that sound needs a medium: If an electric bell is placed inside an airtight glass jar and the air is gradually pumped out, the sound of the bell becomes fainter and finally inaudible, even though the hammer can still be seen striking the bell. This shows that a material medium is necessary for the propagation of sound.
Sound waves are longitudinal waves in air. In a longitudinal wave, the particles of the medium vibrate parallel to the direction of propagation of the wave. Such a wave consists of compressions and rarefactions.
In a transverse wave, the particles vibrate perpendicular to the direction of propagation of the wave, forming crests and troughs. Light waves are transverse, but sound waves in air are longitudinal.
Sound has three main characteristics:
Loudness: Loudness is the sensation produced in the ear which depends on the intensity (energy) of the sound wave. Loudness depends on the amplitude of the vibration - greater amplitude produces louder sound. The unit of loudness is the decibel (dB). Loudness also depends on the area of the vibrating body, the distance from the source, and the density of the medium.
Pitch: Pitch is the characteristic of sound which distinguishes a shrill sound from a flat sound. Pitch depends on the frequency of the vibration - higher frequency produces higher pitch. A woman's voice has a higher pitch than a man's voice because it has a higher frequency. The voice of a mosquito is shriller (higher pitch) than that of a lion.
Quality (Timbre): The quality of sound distinguishes two sounds of the same loudness and pitch coming from different sources. It depends on the waveform. For example, we can distinguish the sound of a violin from that of a flute even at the same pitch and loudness because of their different quality.
Note: A sound produced by a regularly vibrating source is called a musical note. Noise is an unpleasant, irregular sound.
Sound travels at different speeds in different media. The speed of sound depends on the nature of the medium, the temperature and the humidity of the medium.
Sound travels faster in solids than in liquids, and faster in liquids than in gases. The speed of sound increases with an increase in temperature. In solids, the speed of sound is much higher than in air; that is why in a train accident or distant event, we often hear sound through rails faster than through the air.
The speed of sound (v), frequency (f) and wavelength (lambda) of a sound wave are related by:
v = f x lambda
Like light, sound also reflects from surfaces. When sound strikes a hard, smooth surface, it bounces back; this is called the reflection of sound. The laws of reflection of sound are similar to those of light - the angle of incidence equals the angle of reflection.
Echo: An echo is the repetition of sound produced by the reflection of sound waves from a distant reflecting surface. For an echo to be heard distinctly, the reflected sound must reach the ear after at least 0.1 second after the original sound, because the human ear can distinguish two sounds separated by 0.1 second.
Since the speed of sound in air is about 340 m/s, the distance of the reflecting surface for a distinct echo must be at least 17 m. This is calculated as: the total distance travelled by sound in 0.1 s is 340 x 0.1 = 34 m, so the reflecting surface must be at least 34/2 = 17 m away.
Conditions for echo: (1) The minimum distance of the reflecting surface should be 17 m. (2) The reflected sound should be loud enough to be heard.
Reverberation: When sound is reflected many times from multiple surfaces in a closed room, the sound persists; this is called reverberation. In big halls and auditoriums, excessive reverberation is undesirable, so sound-absorbing materials are used on walls and ceilings.
The multiple reflection of sound is used in many practical devices:
The human ear can hear sounds with frequencies from about 20 Hz to 20,000 Hz (20 kHz). This range is called the audible range of hearing.
Infrasonic sounds: Sounds with frequencies below 20 Hz are called infrasonic sounds. Elephants, whales and rhinoceroses produce and hear infrasonic sounds. Earthquakes produce infrasonic waves.
Ultrasonic sounds: Sounds with frequencies above 20,000 Hz are called ultrasonic sounds. Dogs can hear up to about 40,000 Hz, bats produce and hear ultrasonic sounds to navigate in the dark, and dolphins and whales use ultrasonic sound.
Uses of ultrasound: 1. Ultrasound is used to clean parts located in hard-to-reach places. 2. Ultrasonic waves are used to detect flaws in metal blocks (non-destructive testing). 3. Ultrasound is used in medicine to image internal organs (sonography) and to break kidney stones. 4. SONAR (Sound Navigation and Ranging) is used in ships and submarines to locate objects under water using ultrasonic waves.
The human ear is a remarkable organ that converts sound waves into nerve impulses. It has three parts:
| Quantity | Definition | Unit |
|---|---|---|
| Frequency | Number of vibrations per second | hertz (Hz) |
| Amplitude | Maximum displacement from mean position | metre (m) |
| Loudness | Sensation depending on intensity | decibel (dB) |
| Pitch | Depends on frequency | - |
| Wavelength | Distance between two consecutive compressions/crests | metre (m) |
| Medium | Speed of Sound |
|---|---|
| Air (0 C) | 331 m/s |
| Air (20 C) | 343 m/s |
| Water | about 1500 m/s |
| Steel | about 5000 m/s |
In this chapter we learned that sound is produced by vibrating objects and propagates as a longitudinal mechanical wave through a material medium, consisting of compressions and rarefactions. Sound cannot travel through a vacuum, and its speed depends on the medium, being fastest in solids. We studied the three characteristics of sound - loudness, pitch and quality - which depend on amplitude, frequency and waveform respectively. The reflection of sound produces echoes, and reverberation affects the acoustics of halls. We learned about the audible range of human hearing and the many uses of ultrasonic waves, from cleaning and flaw detection to SONAR and medical sonography. Finally, we traced the path of sound through the human ear - from the pinna, through the eardrum and three tiny bones, to the cochlea where nerve impulses are generated. Sound is not just a sense; it is a vital form of energy used across science, medicine and technology.