Vibration, oscillation and amplitude
When a tanpura string is plucked it moves to and fro and a sound is heard. Touch the string to stop it and the sound stops.
A fast to-and-fro motion of an object makes sound. Motion to and fro about a central position is called oscillation (दोलन). The greatest distance from the central position is the amplitude (आयाम).
Rule 1: No vibration, no sound — A bell, a string, a drum membrane or a speaker cone vibrates and passes the vibration to air molecules, so the sound reaches our ears. Clapping, crackers and thunder also make vibrations, though we do not see them.
- Easy : Place a hand on a speaker and you feel the vibration.
- Medium : Touch a vibrating string and the sound stops.
- Exam-type : A cracker bursting shows no moving object, yet fast vibrations in the air make the sound.
Rule 2: Oscillation of a swing and amplitude — When a swing goes from the middle to one end, to the other end and back to its start, one oscillation is complete. A rubber band pulled harder has a bigger amplitude and gives a louder sound; pulled gently, the amplitude and the sound are smaller.
- Easy : A swing crosses its middle position again and again.
- Medium : The amplitude of a pendulum is the distance from the middle position to one extreme.
- Exam-type : Pull a rubber band harder and the amplitude grows, giving a louder sound.
A thing that oscillates is an oscillator. Easy examples: a swing, a pendulum, a tanpura string.
Thinking sound needs no vibration. Every sound comes from vibration; we just cannot always see it.
Memory clue : "What shakes, speaks" — when the vibration stops, the sound stops.
Spot the difference
| Point | Oscillation | Amplitude |
|---|---|---|
| Meaning | One complete to-and-fro motion | Greatest distance from middle |
| Affects | Time period | Loudness |
Practice questions
- 1What produces sound?
- 2What is an oscillator?
- 3How is one oscillation completed?
- 4What is amplitude?
- 5Why is the sound louder when a rubber band is pulled harder?
- 6What happens if you touch a vibrating string?
- 7Why does a bursting cracker make sound though no vibration is seen?
- 8The ends of a tanpura string are fixed, so how does it vibrate?
Show answers (try them yourself first)
- 1Vibration of an object.
- 2A thing that oscillates.
- 3When the object goes to one end, then the other end and back to the start.
- 4The greatest distance from the middle position.
- 5The amplitude becomes larger.
- 6The vibration and the sound stop.
- 7Fast vibrations occur in the air but cannot be seen.
- 8Its middle part moves on both sides of the central position.
Time period and frequency
The time a swing takes for one trip and the number of trips in a second tell how quickly it swings.
The time for one oscillation is the time period (आवर्तकाल, T). The number of oscillations in one second is the frequency (वारंवारता, n). The unit of frequency is the hertz (Hz).
Rule 1: Frequency = 1 ÷ time period — n = 1 ÷ T. If T is in seconds, n comes in hertz. If 20 oscillations take t seconds, T = t ÷ 20.
- Easy : If T = 0.5 s then n = 1 ÷ 0.5 = 2 Hz.
- Medium : 20 oscillations in 40 s give T = 40 ÷ 20 = 2 s and n = 0.5 Hz.
- Exam-type : A 500 Hz sound has a time period of 1 ÷ 500 = 0.002 s.
Time period and frequency
In the app you can move this figure with your finger.
Open in the appRule 2: Pendulum: length changes the period, amplitude does not — A longer thread gives a longer time period and a lower frequency. Changing the amplitude leaves the frequency unchanged.
- Easy : A short pendulum swings faster.
- Medium : A pendulum pulled a little or a lot has the same frequency.
- Exam-type : On lengthening a pendulum T increases, so n decreases.
The period of a pendulum depends on its length, not on its amplitude. Timing 20 oscillations gives T more exactly.
Thinking a bigger amplitude means a higher frequency. For a pendulum the frequency stays the same.
Memory clue : "n = 1 ÷ T, and T = 1 ÷ n" — one is large when the other is small.
Spot the difference
| Point | Time period (T) | Frequency (n) |
|---|---|---|
| Meaning | Time for one oscillation | Oscillations per second |
| Unit | second (s) | hertz (Hz) |
| If length increases | Increases | Decreases |
Practice questions
- 1What is the unit of frequency?
- 2What is time period?
- 3Write the relation between frequency and time period.
- 4If T = 0.25 s, find the frequency.
- 5How does the period of a pendulum change when its length is increased?
- 620 oscillations take 30 s. Find T.
- 7Find the time period of a 50 Hz sound.
- 8Why does the frequency of a pendulum not change on increasing amplitude?
Show answers (try them yourself first)
- 1Hertz (Hz).
- 2The time taken for one oscillation.
- 3n = 1 ÷ T.
- 41 ÷ 0.25 = 4 Hz.
- 5It increases.
- 630 ÷ 20 = 1.5 s.
- 71 ÷ 50 = 0.02 s.
- 8A pendulum's period depends only on its length.
Pitch of sound
In Sa Re Ga Ma Pa Dha Ni the sound slowly becomes shriller; a mosquito's hum is shrill and a lion's roar is deep.
Pitch (तारत्व) tells how shrill or deep a sound is. It depends on frequency: higher frequency gives higher pitch, lower frequency gives lower pitch.
Rule 1: More tension, higher pitch — Increasing the tension of a stretched wire raises its frequency and the sound becomes shriller. Reducing the tension makes it deeper. Tuning an instrument means adjusting the tension to get the pitch needed.
- Easy : While tuning a tanpura the tension of the strings is changed.
- Medium : The tension of the tabla membrane is changed by tapping the pegs.
- Exam-type : Tightening the wire raises the frequency, so the sound becomes shriller.
Rule 2: Shorter length, higher pitch — Pressing a ruler on a table and shortening its free part makes it vibrate faster and the sound shriller. If the free part becomes very short, the sound may fall outside what we can hear.
- Easy : A short rod vibrates quickly.
- Medium : On a sitar, shortening the free length with a finger raises the pitch.
- Exam-type : Reducing the free part of a ruler from 25 cm to 10 cm makes it vibrate faster and sound shriller.
A lion's roar has low pitch and a mosquito's hum has high pitch. Pitch has nothing to do with loudness.
Thinking a loud sound has a high pitch. Loudness depends on amplitude; pitch depends on frequency.
Memory clue : "High n = shrill sound" — more frequency, shriller sound.
Spot the difference
| Point | Pitch | Loudness |
|---|---|---|
| Depends on | Frequency | Amplitude |
| Example | Lion (low), mosquito (high) | Whisper, shout |
Practice questions
- 1What is pitch?
- 2On what does pitch depend?
- 3Which has higher pitch: a lion's roar or a mosquito's hum?
- 4How does the sound change when the tension of a wire is increased?
- 5What does tuning an instrument mean?
- 6What changes when the free part of the ruler is shortened?
- 7Why does a loud sound not mean a high pitch?
- 8How can a higher-pitched note be played on a sitar?
Show answers (try them yourself first)
- 1The quality that tells how shrill or deep a sound is.
- 2On frequency.
- 3The mosquito's hum.
- 4It becomes shriller.
- 5Changing tension of a string or membrane to get the right pitch.
- 6It vibrates faster and the pitch rises.
- 7Loudness depends on amplitude while pitch depends on frequency.
- 8By shortening the free part of the string or increasing the tension.
Intensity and level of sound (decibel)
The difference between whispering and shouting, or between a quiet room and traffic noise, is the loudness of sound.
Two terms show how loud a sound is: intensity (तीव्रता) and sound level (ध्वनिपातळी). Sound level is measured in decibel (dB). Intensity is proportional to the square of the amplitude.
Rule 1: Double the amplitude, four times the intensity — Intensity is proportional to the square of the amplitude. So doubling the amplitude gives 4 times the intensity and tripling it gives 9 times.
- Easy : Amplitude ×2 → intensity 2 × 2 = 4 times.
- Medium : Amplitude ×3 → intensity 3 × 3 = 9 times.
- Exam-type : Halving the amplitude reduces the intensity to 1/4.
Intensity and level of sound (decibel)
In the app you can move this figure with your finger.
Open in the appRule 2: Ten times the intensity adds 10 dB — When intensity becomes 10 times, the sound level rises by 10 dB. Ordinary conversation is about 60 dB and heavy traffic about 70 dB. Very loud sound can hurt hearing or cause temporary deafness.
- Easy : Normal conversation is about 60 dB.
- Medium : Raising the intensity of a 40 dB sound ten times gives 50 dB.
- Exam-type : If intensity rises 100 times (two steps of 10 times), the level rises by 20 dB.
The decibel is named in honour of Alexander Graham Bell. 0 dB is where hearing begins.
Thinking double the amplitude gives double the intensity. It gives four times.
Memory clue : "Amplitude × k → intensity × k × k" — square the amplitude factor.
Spot the difference
| Amplitude | Intensity |
|---|---|
| 2 times | 4 times |
| 3 times | 9 times |
| Half | 1/4 |
Practice questions
- 1What is the unit of sound level?
- 2What makes a sound loud?
- 3About what is the sound level of normal conversation?
- 4If the amplitude is doubled, by what factor does intensity change?
- 5By how much does the level rise when intensity becomes 10 times?
- 6What happens to intensity when the amplitude is halved?
- 7If the amplitude becomes 3 times, how many times is the intensity?
- 8The intensity of a 70 dB sound becomes 100 times. What is the new level?
Show answers (try them yourself first)
- 1Decibel (dB).
- 2A larger amplitude.
- 3About 60 dB.
- 44 times.
- 5By 10 dB.
- 6It becomes 1/4.
- 79 times.
- 890 dB (two steps of 10 dB).
Audible, infrasonic and ultrasonic sound
A dog hears sounds that we cannot; elephants talk to each other with sounds we cannot hear.
Humans can hear only sounds of frequency 20 Hz to 20,000 Hz, called audible sound. Frequency below 20 Hz is infrasonic (अवश्राव्य); above 20,000 Hz is ultrasonic (पराश्राव्य).
Rule 1: Three frequency ranges — Whales, elephants and rhinoceroses make infrasonic sound. Dogs and bats can hear ultrasonic sound, and bats use it to find their way in the dark. An oscillator making 3-4 oscillations per second is not heard because its frequency is below 20 Hz.
- Easy : A 15 Hz sound cannot be heard by humans.
- Medium : A 25,000 Hz sound is ultrasonic; a dog can hear it.
- Exam-type : 100 oscillations in 20 s give n = 100 ÷ 20 = 5 Hz; this is infrasonic.
Rule 2: Uses of ultrasonic sound — Cleaning delicate watch parts and ornaments, imaging organs inside the body, detecting tumours in the brain, finding faults in metal, killing some microbes and using SONAR to locate the sea bed or a ship.
- Easy : Tiny watch parts are cleaned with ultrasonic sound.
- Medium : In SONAR, ultrasonic sound helps find the distance to the sea bed.
- Exam-type : Ultrasonic waves can be used to find a crack inside metal.
Dogs and some animals are believed to sense ultrasonic signals before an earthquake; research continues on this.
Thinking a bat finds its way by sight. It uses ultrasonic sound.
Memory clue : "Infra = below (under 20 Hz), Ultra = above (over 20,000 Hz)" — 20 to 20,000 is what we hear.
Spot the difference
| Type | Frequency | Who hears/makes it |
|---|---|---|
| Infrasonic | Below 20 Hz | Elephant, whale, rhinoceros |
| Audible | 20–20,000 Hz | Humans |
| Ultrasonic | Above 20,000 Hz | Dog, bat |
Practice questions
- 1What frequency range can humans hear?
- 2What is infrasonic sound?
- 3What is ultrasonic sound?
- 4Name two animals that make infrasonic sound.
- 5State two uses of ultrasonic sound.
- 6What is SONAR used for?
- 7Why can we not hear a 3 Hz sound?
- 8How does a bat fly in the dark?
Show answers (try them yourself first)
- 120 Hz to 20,000 Hz.
- 2Sound with frequency below 20 Hz.
- 3Sound with frequency above 20,000 Hz.
- 4Elephant and whale.
- 5Cleaning delicate parts; finding faults in metal.
- 6To locate the sea bed or a ship.
- 7It is below 20 Hz, so it is infrasonic.
- 8It makes ultrasonic sounds and finds its way from the returning echoes.