Chapter Notes - Sound

Table of Contents
1. What is Sound?
2. Sound Produced by a Vibrating Body
3. Sound Produced by Humans
4. Propagation of Sound (Sound needs a medium)
5. How We Hear Sound
6. Amplitude, Time Period, and Frequency of a Vibration
7. Loudness
8. Shrillness or Pitch
9. Audible and Inaudible Sounds
10. Noise and Music
11. Noise Pollution
View more

Every day, our ears are greeted with a multitude of sounds produced by humans, birds, bells, machines, vehicles, televisions, radios, and more.

• Sound is very important in our life. It helps us to communicate with one another and to enjoy music and other auditory information.
• In this chapter we will study how sound is created, how it travels, how we hear it, and why some sounds are louder or have a different pitch.

Sound

What is Sound?

Sound is a form of energy produced by the vibration of an object.

When an object vibrates, it disturbs the particles of the medium (usually air) around it and produces sound waves. These waves travel through the medium and reach our ears. The ear converts these mechanical vibrations into electrical signals that the brain interprets as sound.

Real-life examples:

• When a table is beaten, it vibrates and produces sound.
• When a drum is struck, its surface vibrates and produces sound.
• When a stretched rubber band is plucked, it vibrates and produces sound.

Production of Sound

Different musical instruments produce different types of sound because different parts of the instrument vibrate: a flute produces sound by the vibration of the air column inside it, a guitar by the vibration of its strings, and a drum by the vibration of its membrane or diaphragm.

Sound Produced by a Vibrating Body

Vibration is the to-and-fro or back-and-forth motion of an object.

Vibration is to and fro motion

A vibrating object produces sound; a non-vibrating object does not. Sometimes vibrations can be seen (for example, a plucked string), but often they are too small to see although they can be felt.

Different objects vibrate with different rates and amplitudes - some vibrate faster or with larger movement than others.

Activities demonstrating vibration and sound

• School bell test: Touch the bell when it is not ringing and then when it is ringing; during ringing you can feel the bell vibrating.
• Metal plate (or pan) vibration: Strike a metal plate once and feel it; it vibrates while producing sound and the vibrations cease after the sound stops.
• Rubber-band instrument: Stretch a rubber band over a box and pluck it; you will hear sound and feel the band vibrating.
• Metal dish and water: Strike an edge of a metal dish partially filled with water and observe waves on the water; touch the dish to feel vibrations.
• Making a simple instrument: Use a hollow coconut shell or an earthen pot to make a one-string instrument and identify the vibrating part responsible for sound.
• Water-bowl instrument (jal-tarang): Arrange bowls or tumblers with different water levels and strike them in sequence to produce musical notes.

Many musical instruments such as the sitar, mridangam, manjira (cymbals), ghatam, noot (mud pots), and kartal produce sound by vibrations of their body or parts of their body.

Sound Produced by Humans

Human voice is produced by vibrations of the voice box, also called the larynx.

• The larynx is located at the upper end of the windpipe (trachea). Inside the larynx are two stretched membranes called vocal cords, leaving a narrow slit between them.
• When air from the lungs passes through the slit between the vocal cords, the cords vibrate and produce sound.
• Muscles attached to the vocal cords change their tension and thickness, which changes the pitch and quality of the voice.
• When vocal cords are tight and thin, they produce a high-pitched sound; when they are thick and loose, they produce a low-pitched sound.

Human Voice Box

Simple activities showing sound production

• Blowing through stretched rubber strips: Place two rubber strips together and blow through the gap; the air causes them to vibrate and produce sound.
• Blowing through a paper slit: Make a narrow slit in a piece of paper and blow; the air flow makes the paper vibrate and you hear a sound.

These simple activities show how sound can be produced by vibration of a material (like vocal cords) or by vibration of air itself.

Propagation of Sound (Sound needs a medium)

The movement of sound from the source to the listener is called the propagation of sound. Sound cannot propagate in the absence of a medium. The region where air is removed or absent is called a vacuum.

• Sound does not travel through a vacuum.
• Sound travels through solids, liquids, and gases. The human ear normally detects sound travelling through air.
• Aquatic animals communicate because sound travels through water too.

Sound needs a medium to travel

Aquatic animals communicate through water

Activities demonstrating sound transmission through different media

• Sound in a tumbler with a phone: Place a phone inside a dry tumbler and listen to a ringtone. Cover the rim with your mouth and remove air by sucking; the sound becomes fainter as air is removed, showing that sound needs air to travel well.
• Sound in water: Shake a small bell under water (not touching the container) and place your ear near the water surface; you will hear the bell, showing that sound travels through liquids.
• Sound through solids: Hold one end of a metre scale (or metal rod) to your ear while a friend taps the other end; you will hear the tapping clearly through the solid.
• Sound through strings (toy telephone): Connect two cans by a taut string; speak into one can while the other is held by a friend - sound travels along the string.

Conclusion: Vibrating objects produce sound and the vibrations are carried by a medium (air, water, or solid) in all directions.

How We Hear Sound

We hear by means of our ears. The external ear acts like a funnel that collects and concentrates sound waves.

• Sound waves enter the external ear and are directed to the eardrum (tympanic membrane).
• The eardrum is a stretched membrane that vibrates when sound waves strike it. These vibrations pass to the middle ear bones and then to the inner ear.
• In the inner ear, vibrations are converted into electrical signals that travel to the brain through the auditory nerve. The brain interprets these signals as sound.

Activity: Observing sound vibrations with a can

1. Take a plastic or tin can and cut off both ends.
2. Stretch a piece of a rubber balloon across one end and secure it with a rubber band. Place a few dry cereal grains on the stretched rubber surface.
3. Ask a friend to speak close to the open end of the can. Observe the cereal grains; they will jump because the sound waves make the rubber surface vibrate.

Amplitude, Time Period, and Frequency of a Vibration

The to-and-fro motion of an object is called vibration or oscillatory motion. Two important characteristics of a vibration (and hence of sound) are amplitude and frequency.

Sound travels in the form of waves. When a pebble is dropped in pond water, it produces ripples in water. The ripple is called a wave. Sound travels producing similar waves. 

• Amplitude: The maximum displacement from the mean (rest) position to the peak of the wave. For sound, larger amplitude means a stronger vibration.
• Frequency: The number of vibrations or oscillations made per second. Frequency is measured in hertz (Hz). A frequency of 1 Hz means one oscillation per second.
• Time period: The time taken for one complete oscillation. Time period is the reciprocal of frequency.

Every wave can have a different amplitude or frequency 

Frequency Formula

If an object vibrates 40 times in one second, its frequency is 40 Hz.

Loudness

Activity: Exploring sound intensity and vibration

• Striking a tumbler: Strike the rim of a metallic tumbler gently with a spoon and listen. Strike harder and listen again - the louder sound corresponds to larger vibration amplitude.
• Measuring vibration visually: Suspend a small lightweight ball so it lightly touches the rim of the tumbler. Strike the tumbler and notice how far the ball is displaced; larger displacement means larger amplitude and louder sound.

Conclusion: Loudness depends on the amplitude of vibration. Greater amplitude produces louder sound and smaller amplitude produces a feeble sound.

Loudness of sound is approximately proportional to the square of the amplitude of the vibration producing the sound. If amplitude doubles, loudness increases by about four times.

• Loudness is measured in decibels (dB).
• Sound levels above about 80 dB are uncomfortable and prolonged exposure can damage hearing.

Shrillness or Pitch

The frequency of a sound determines its shrillness or pitch. Higher frequency gives a higher pitch (more shrill); lower frequency gives a lower pitch.

Examples:

• Children and women usually produce higher-frequency sounds and hence have a higher pitch than adult males, who typically produce lower-pitched voices.
• A drum produces low-frequency, less-shrill sound; a whistle produces high-frequency, shriller sound.
• A lion produces a low-frequency roar (low pitch) while many birds produce high-frequency chirps (high pitch). Loudness and pitch are independent: a lion's roar can be louder even though its pitch is lower than a bird's chirp.

Shrillness and Loudness 

Audible and Inaudible Sounds

• Audible sounds: Sounds with frequencies between 20 Hz and 20,000 Hz (20 kHz) are audible to the average human ear.
• Inaudible sounds: Sounds below 20 Hz are called infrasound, and sounds above 20,000 Hz are called ultrasound; both are inaudible to humans. Some animals (for example, dogs) can hear higher frequencies that humans cannot.

Audible and Inaudible Sound

Uses of inaudible sounds

1. Certain animals can hear ultrasound; this ability is used in dog whistles that are audible to dogs but not humans.
2. Ultrasound is used in medical imaging and diagnosis (for example, fetal scanning, detecting internal body structures) and in many industrial applications for locating flaws inside objects.

Noise and Music

• Noise: Unpleasant, unwanted or unpleasantly loud sounds are called noise (for example, a persistent horn, construction noise, or aircraft noise).
• Music: Sounds arranged in pleasing patterns of rhythm, melody and harmony are called musical sounds.

Difference Between Noise and Music 

Noise Pollution

Excessive or unwanted sound in our environment is called noise pollution. Common sources include vehicle horns, loudspeakers, industry, machinery, household appliances at high volume, firecrackers, and aircraft.

Unbearable Sound is Noise Pollution

Harms of noise pollution: Continuous exposure to high noise levels can cause sleep disturbance, stress, hypertension, temporary or permanent hearing loss, anxiety and reduced work efficiency. Sounds above about 80 dB can be painful and damaging over time.

Problems due to Noise Pollution Hierarchy

Measures to limit noise pollution

• Keep radios and TVs at moderate volume.
• Use good quality silencers (mufflers) on vehicles and reduce unnecessary horn use.
• Plant trees along roads and around buildings; vegetation helps absorb sound.
• Run awareness campaigns to inform people about the harmful effects of noise and ways to reduce it.

Noise pollution can be limited or controlled by controlling the noise coming from the source. 

Hearing impairment

Complete hearing impairment is rare and usually present from birth. Partial hearing loss can result from disease, injury or age. Children with hearing impairment need special care; many learn sign language and use hearing aids. Modern devices and social support can help hearing-impaired persons lead productive lives.