How sound is produced
Tap a tuning fork lightly and you hear a hum; touch it with a finger and you feel it trembling.
A vibrating object produces sound. The two prongs of a tuning fork (स्वरित्र) move back and forth and make a compression (संपीडन) and a rarefaction (विरलन) in the air around. These together are the sound wave.
Rule 1: Compression and rarefaction — Where air is squeezed, pressure and density are high: that is a compression. Where air is spread out, pressure and density are low: that is a rarefaction.
- Easy : When the prongs swing outwards they squeeze the air outside them: a compression.
- Medium : When the prongs come close, the same place gets more room for the air and a rarefaction forms.
- Exam-type : Hold the fork so that its prongs cannot move and the sound stops. So vibration is the cause of the sound.
Rule 2: The wave travels, not the air — The state of compression and rarefaction passes from one region to the next. The air particles only shake to and fro about their own places.
- Easy : In a stadium wave the pattern goes round, but every person stays in his own seat.
- Medium : When the waves reach the ear they vibrate the ear-drum and a signal goes to the brain.
- Exam-type : The smoke from an incense stick near the fork only shivers; it is not carried away. So the air itself does not travel.
Sound is energy. Energy travels from place to place, not the material.
Thinking that sound means air flowing from the source to the ear. Only the pattern of compression and rarefaction moves.
Memory clue : "Com = crowd, Rare = thin" — in a compression air crowds together, in a rarefaction it thins out.
Spot the difference
| Point | Compression | Rarefaction |
|---|---|---|
| Pressure | High | Low |
| Density | High | Low |
| Air particles | Close together | Far apart |
Practice questions
- 1What produces sound?
- 2How many prongs does a tuning fork have?
- 3What is a region of high pressure called?
- 4Is the density of air high or low in a rarefaction?
- 5What happens to sound when the vibration stops?
- 6How do air particles move when a sound wave goes ahead?
- 7What moves away through the air in a sound wave: the air or the state of compression and rarefaction?
- 8Why does the sound stop when you hold a prong with your finger?
Show answers (try them yourself first)
- 1A vibrating object.
- 2Two.
- 3Compression.
- 4Low.
- 5The sound stops.
- 6They move to and fro about their own places.
- 7The state (compression and rarefaction).
- 8The prong stops vibrating, so no waves form in the air.
A medium for sound to travel
Two astronauts standing side by side on the Moon cannot talk directly, so they use a radio-like device.
Sound needs a medium (माध्यम) such as air, water or a solid to be produced and to travel. With no medium sound cannot go forward.
Rule 1: The bell jar experiment — A working electric bell is kept inside a glass bell jar and a pump removes the air. As the air goes down, the sound heard outside also gets weaker.
- Easy : While the air is pumped out, the sound slowly fades.
- Medium : When almost all air is gone the hammer is still seen striking, but the sound is hardly heard.
- Exam-type : Let the air back in and the sound grows again. So sound needs air.
Rule 2: No medium, no sound — The Moon has no air, so sound cannot travel there. Radio waves need no medium, so astronauts use them.
- Easy : A call on the Moon is not heard even by the person next to you.
- Medium : Space is almost a vacuum, so the sound of an explosion far away never reaches the Earth.
- Exam-type : Two people working outside a spacecraft talk over the radio in their helmets, not directly.
Sound goes through solids, liquids and gases, but not through a vacuum.
Thinking that sounds from space reach the Earth. There is no medium in space.
Memory clue : "No air, no sound" — less medium, less sound.
Spot the difference
| Point | Sound waves | Radio waves |
|---|---|---|
| Medium | Needed | Not needed |
| In vacuum | Do not travel | Travel |
Practice questions
- 1What does sound need to travel?
- 2Name two kinds of medium.
- 3What happens when air is removed from the bell jar?
- 4Why can people not talk directly on the Moon?
- 5What do astronauts use to talk?
- 6What is a vacuum?
- 7If all air is removed from the bell jar the hammer is seen moving but no sound is heard. Why?
- 8State one difference between radio waves and sound waves.
Show answers (try them yourself first)
- 1A medium.
- 2Air and water (or a solid).
- 3The sound becomes weaker.
- 4There is no air (medium) there.
- 5A radio-wave device.
- 6A space with no air or other matter.
- 7There is no medium for the sound to travel in.
- 8Sound waves need a medium; radio waves do not.
Frequency of sound
In a jal-tarang, bowls with different water levels give a different note each time you tap them.
The number of complete waves (one compression plus one rarefaction) made in one second is the frequency (वारंवारता). Its unit is the hertz (हर्ट्झ), written Hz.
Rule 1: Frequency and hertz — One wave per second means 1 Hz. The frequency equals the number of back-and-forth vibrations of the source in one second.
- Easy : A fork that vibrates 100 times in one second has a frequency of 100 Hz.
- Medium : An object makes 1500 vibrations in 5 seconds. Frequency = 1500 ÷ 5 = 300 Hz.
- Exam-type : A source of 440 Hz sends out 4400 complete waves in 10 seconds.
Frequency of sound
In the app you can move this figure with your finger.
Open in the appRule 2: What decides the frequency? — A fork's frequency depends on the length and thickness of its prongs and on the metal used. Changing the height of the air column in a bowl changes the frequency.
- Easy : A bowl with more water has a shorter air column and gives a higher note.
- Medium : Two jal-tarang bowls sound different because their air columns have different heights.
- Exam-type : A fork with shorter, thinner prongs can have a higher frequency, but changing the metal changes it too.
Higher frequency means a higher note, lower frequency a lower note.
Thinking that frequency is the loudness. Loudness is a different property; frequency decides the note.
Memory clue : "Vibrations per second = Hz" — the vibrations in 1 second are the hertz.
Spot the difference
| Point | Low frequency | High frequency |
|---|---|---|
| Note | Lower | Higher |
| Waves per second | Fewer | More |
Practice questions
- 1What is the unit of frequency?
- 2If one complete wave is made in one second, what is the frequency?
- 3A source makes 200 vibrations in a second. What is its frequency?
- 4An object makes 800 vibrations in 4 seconds. What is its frequency?
- 5What does the frequency of a tuning fork depend on?
- 6How does the note change when more water is put in a bowl?
- 7How many complete waves does a 300 Hz source send out in 6 seconds?
- 8Why does the bowl with less water give the lower note?
Show answers (try them yourself first)
- 1Hertz (Hz).
- 21 Hz.
- 3200 Hz.
- 4800 ÷ 4 = 200 Hz.
- 5Length and thickness of the prongs and the metal.
- 6The note becomes higher.
- 7300 × 6 = 1800.
- 8Its air column is longer, so the frequency is lower.
Sound and music
The strings of a sitar and the holes of a flute give different notes, yet both use the same idea.
When the frequency changes, the note (स्वर) changes. Musical instruments change the length of the vibrating string or air column to get different notes.
Rule 1: String instruments — In a sitar, violin or guitar the note changes by changing the tension of the string or the length that vibrates under the fingers.
- Easy : Tighten a string and the note goes up.
- Medium : Pressing a guitar string on a fret shortens the part that vibrates and raises the frequency.
- Exam-type : With equal tension, the thinner string gives a higher note than the thicker string.
Rule 2: Wind instruments — In a flute or shehnai a column of air vibrates. Opening or closing holes changes the length of the column and so the note.
- Easy : Opening more holes of a flute gives a higher note.
- Medium : Even changing the way of blowing gives different notes from a flute.
- Exam-type : A long-tube instrument gives lower notes and a short-tube instrument gives higher notes.
A shorter vibrating part gives a higher frequency and a higher note.
Thinking that every instrument has strings. In flute and shehnai an air column vibrates.
Memory clue : "Short part, high note" — shorten the vibrating length and the note rises.
Spot the difference
| Instrument | What vibrates | How the note changes |
|---|---|---|
| Sitar, guitar | String | By changing tension or length |
| Flute, shehnai | Air column | By opening and closing holes |
Practice questions
- 1Name two string instruments.
- 2Name two wind instruments.
- 3What happens to the note when string tension is increased?
- 4What changes the note in a flute?
- 5Why does pressing a guitar string with a finger change the note?
- 6State the link between frequency and note.
- 7Of a thick and a thin string under equal tension, which gives the higher note?
- 8Of two flutes, will the longer one give a higher or a lower note? Why?
Show answers (try them yourself first)
- 1Sitar and guitar (or violin).
- 2Flute and shehnai.
- 3The note becomes higher.
- 4The length of the air column (by opening and closing holes).
- 5The vibrating length shortens and the frequency increases.
- 6A greater frequency gives a higher note.
- 7The thin string.
- 8Lower, because its air column is longer.
Human voice and the loudspeaker
Touch your throat while speaking loudly and you feel a faint trembling. That is the source of your voice.
Human voice is made in the larynx (स्वरयंत्र), where two vocal cords (स्वरतंतू) vibrate when air from the lungs passes. In a loudspeaker (ध्वनिक्षेपक) electric current makes a coil and a cone vibrate.
Rule 1: Larynx and vocal cords — Two vocal cords leave a narrow slit for air. Muscles tighten or loosen them, which changes the kind of voice.
- Easy : Blow air between two stretched strips of cycle tube and you hear a sound; the larynx works the same way.
- Medium : Men's vocal cords are longer than those of women and children, so their voices differ.
- Exam-type : Say "bow-wow" and then "meow": the tension in your vocal cords feels different.
Rule 2: Construction of a loudspeaker — A loudspeaker has a permanent magnet, a coil around it and a cone attached to the coil. A changing current makes the coil move to and fro; the cone vibrates and makes waves in the air.
- Easy : Gently touch the cone of a working loudspeaker and you feel it trembling.
- Medium : A loudspeaker changes electrical energy into sound energy.
- Exam-type : A larger current moves the coil more and the sound gets louder, but sound above about 100 decibels can harm us.
Very loud sound can be harmful. Loudspeakers in public places need a limit.
Thinking that voice is made in the air. Human voice is made in the larynx, in the throat.
Memory clue : "Lung air shakes the cords" — air passes and the cords vibrate to make the voice.
Spot the difference
| Point | Human voice | Loudspeaker |
|---|---|---|
| Part that vibrates | Vocal cords | Cone with coil |
| Energy source | Air from lungs | Electrical energy |
Practice questions
- 1Where in the body is human voice made?
- 2How many vocal cords are in the larynx?
- 3Which energy changes into sound energy in a loudspeaker?
- 4What changes when the vocal cords are tightened?
- 5Name three parts of a loudspeaker.
- 6Why is a child's voice different from a man's?
- 7What happens if a loudspeaker gets more current? State one danger.
- 8How does the cycle-tube strip model match the larynx?
Show answers (try them yourself first)
- 1In the larynx.
- 2Two.
- 3Electrical energy.
- 4The kind of voice (the note) changes.
- 5Permanent magnet, coil and cone.
- 6A child's vocal cords are much shorter.
- 7The sound gets louder; sound above 100 decibels can harm the ear.
- 8The strips act like vocal cords and the gap like the air passage.