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REET L2 · Science

Sound: Waves, Pitch, Echo and Noise Pollution — REET Notes

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This page covers the production of sound, the properties of a sound wave, pitch-loudness-quality, speed, echo, ultrasound, the human ear and noise pollution — all at the Class 8-9 NCERT level. REET 2024 Level 2 asked direct, one-line questions from exactly this range, which makes this a reliably scoring topic.

  • 4 questions in REET 2024
  • Class 8-9 NCERT level
  • 8 min read
Verified 17 September 2026 · Source: NCERT Class 9 Science — Chapter 10: Sound Waves: Characteristics and Applications

How REET asks this topic

REET 2024 Level 2 (Maths-Science) carried 4 questions from Sound: what pitch depends on, working out frequency from a mosquito wing's time period (f = 1/T), and the WHO-recommended safe noise level for a city (repeated across two shifts). In other words, the level is Class 8-9 NCERT/RBSE textbook — definitions, one-line formula questions, and facts around noise pollution. There is no long calculation, but you do need a solid grip on units and formulas.

How sound is produced and how it travels

Sound is a form of energy produced by the vibration of an object. A vibration is a to-and-fro or up-and-down motion that repeats itself. In humans, sound is produced at the larynx (voice box) — at the upper end of the windpipe sit two vocal cords, and when air from the lungs passes between them, they vibrate. NCERT notes that vocal cords in men are roughly 20 mm long, while in women they are about 5 mm shorter — which is why men's, women's and children's voices sound different.

Sound needs a medium — solid, liquid or gas — to travel; it cannot travel through a vacuum. This is exactly why astronauts on the Moon talk to each other over radio rather than directly. In air, sound travels as a longitudinal wave, meaning the particles of the medium vibrate parallel to the direction the wave travels, creating regions of compression (higher pressure/density) and rarefaction (lower pressure/density) one after another.

Characteristics and formulas of a sound wave

QuantityMeaningSI unit / formula
Wavelength (λ)Distance between two consecutive compressions or rarefactionsmetre (m)
Frequency (ν or f)Number of vibrations/oscillations completed in one secondhertz (Hz)
Time period (T)Time taken to complete one full vibrationsecond (s); T = 1/ν
Amplitude (A)Maximum displacement from the mean positiondetermines loudness
Wave speed (v)Distance covered by the wave in one secondv = ν × λ (m/s)

Worked example: if T = 0.005 s, then ν = 1/0.005 = 200 Hz (this was the actual REET 2024 question).[Source 1]

Pitch, loudness and quality — what each depends on

CharacteristicDepends onExample
PitchFrequency — higher frequency = higher (sharper) pitchA child's or woman's voice generally has a higher pitch than a man's; a whistle is sharp, a drum is deep
LoudnessAmplitude — loudness is proportional to the square of the amplitudeBeating a drum harder increases its amplitude and hence its loudness; measured in decibels (dB)
Quality / timbreThe shape of the waveformEven at the same pitch and loudness, a sitar and a flute sound recognisably different

[Source 1]

Speed of sound in different media

MediumSpeed (m/s)
Air (0 °C)331
Air (25 °C)346
Distilled water (25 °C)1498
Sea water (25 °C)1531
Steel (25 °C)5960

Sound travels fastest in solids, then liquids, then gases, and its speed in air increases as temperature rises. The speed of light (about 3 × 10^8 m/s) is far greater than the speed of sound, which is why lightning is seen before thunder is heard.[Source 1]

Echo, the audible range, ultrasound and the human ear

Echo: a reflected sound heard separately from the original sound. The sensation of sound persists in the brain for about 0.1 s, so a reflected sound must reach the ear at least 0.1 s after the original to be heard as a distinct echo. At a speed of 344 m/s (at 22 °C), the minimum distance of the reflecting surface works out to (344 × 0.1)/2 = 17.2 m. When sound keeps reflecting repeatedly inside a large hall, the persistence of sound is called reverberation; sound-absorbing materials (fibre board, curtains) are fitted on walls and ceilings to reduce it. Megaphones, stethoscopes and the curved ceilings of some halls all rely on multiple reflection of sound.

The human audible range is roughly 20 Hz to 20,000 Hz (20 kHz). Frequencies below 20 Hz are infrasonic — used by elephants and whales; frequencies above 20 kHz are ultrasonic — used by bats and dolphins, and audible to dogs. Uses of ultrasound include ultrasonography, echocardiography, breaking small kidney stones, detecting cracks in metal, and cleaning hard-to-reach machine parts. In SONAR (Sound Navigation And Ranging), an ultrasonic pulse is sent out to measure ocean depth or the distance to a submarine: 2d = v × t.

The human ear: the outer ear (the pinna and ear canal) collects sound; the eardrum vibrates; three tiny bones in the middle ear — the malleus (hammer), incus (anvil) and stapes (stirrup) — amplify this vibration and pass it to the inner ear; the cochlea in the inner ear converts it into electrical signals that travel to the brain via the auditory nerve. The eardrum should never be touched with a sharp object.

Noise and noise pollution

Unit of loudness
decibel (dB)
Normal conversation
about 60 dB[Source 1]
Painful level
noise above 80 dB is physically distressing (NCERT)[Source 1]
Safe city noise level (REET 2024 key)
45 dB — this is the answer accepted in BSER's final answer key
Effects
loss of sleep, high blood pressure, anxiety, temporary or permanent hearing loss[Source 1]
Control measures
planting trees along roadsides, fitting silencers on vehicles and machines, keeping noisy industries away from residential areas[Source 1]

How to teach this to children

Classroom activity (Class 6-8)

Place grains of rice or sand on a plate or drum and beat it — the grains jump, showing that a sounding object is vibrating. Then have children make a 'toy telephone' from matchboxes and thread, which shows that sound can travel through a solid (the thread) too.

Teaching aids

Push a slinky spring back and forth on a table to show compression and rarefaction directly. Tap 6-8 bottles or glasses filled with different amounts of water with a spoon to build a simple 'water xylophone' that demonstrates the link between pitch and frequency.

Common misconceptions

Children often believe sound travels fastest in air, that 'loud' means high pitch, or that sound can travel through a vacuum. An activity where one child places an ear at one end of a table and listens for a faint tap at the other end, combined with a comparison table of pitch versus loudness, clears these up effectively.

Assessment idea

Have students list 10 sounds from around them and classify each as 'pleasant/noisy' and 'high/low pitch'; pair this with two easy numerical questions using f = 1/T and v = νλ.

Memory trick / exam tip

Remember the pairing: pitch links to frequency, loudness links to amplitude, and the third one, quality, links to waveform shape. Never treat 'loud' (loudness) and 'high-pitched/thin' (pitch) as the same thing. In a frequency question where the time period is given as a decimal, convert 1/T to a fraction: 0.005 = 5/1000, so 1/0.005 = 1000/5 = 200 Hz.

REET 2024 questions (solved)

Question 139

How many decibel (dB) is safe noise level fixed by the World Health Organisation for a city ?

  1. A140 dB
  2. B90 dB
  3. C65 dB
  4. D45 dB

Explanation

BSER की उत्तर कुंजी के अनुसार विश्व स्वास्थ्य संगठन (WHO) द्वारा शहर के लिए सुरक्षित शोर स्तर 45 dB माना गया है। इससे अधिक शोर लंबे समय तक रहने पर नींद, एकाग्रता और सुनने की क्षमता पर बुरा प्रभाव पड़ सकता है। 90 dB या 140 dB जैसे स्तर तो हानिकारक और पीड़ादायक श्रेणी के हैं।

Question 146

Pitch of a given sound depends upon

  1. Aतरंगदैर्घ्य पर
  2. Bआवृत्ति पर
  3. Cआयाम पर
  4. Dतीव्रता पर

Explanation

ध्वनि का तारत्व (pitch) उसकी आवृत्ति (frequency) पर निर्भर करता है – आवृत्ति अधिक होने पर ध्वनि तीक्ष्ण (ऊँचे तारत्व की) और कम होने पर मोटी होती है। ध्वनि की प्रबलता (loudness) आयाम पर निर्भर करती है।

Question 122

How many decibel is safe noise level fixed by the World Health Organisation for a city ?

  1. A45 dB
  2. B90 dB
  3. C140 dB
  4. D65 dB

Explanation

प्रतियोगी परीक्षाओं की पुस्तकों में WHO द्वारा शहर के लिए सुरक्षित शोर स्तर 45 dB बताया जाता है, और BSER की अंतिम कुंजी में यही उत्तर (A) है। WHO के दिशानिर्देश स्थान और समय के अनुसार अलग-अलग मान देते हैं, जैसे रात में आवासों के बाहर 45 dB तक। 70 dB से ऊपर लगातार शोर श्रवण क्षमता के लिए जोखिम माना गया है और 140 dB तो वयस्कों के लिए भी अधिकतम शिखर-सीमा है, इसलिए 90 और 140 dB सुरक्षित स्तर नहीं हैं।

Question 135

If the time period of vibrations of the wings of a mosquito is 0.005 seconds, then the frequency of vibrations of its wings will be

  1. A200 Hz
  2. B2000 Hz
  3. C500 Hz
  4. D5000 Hz

Explanation

आवृत्ति = 1/आवर्तकाल। अतः आवृत्ति = 1/0.005 = 1000/5 = 200 Hz।

Practice questions

Question 1

Sound travels through air as which type of wave?

  1. Aअनुप्रस्थ तरंग
  2. Bअनुदैर्ध्य तरंग
  3. Cविद्युतचुंबकीय तरंग
  4. Dस्थिर तरंग

Explanation

Sound in air is a longitudinal wave — particles vibrate parallel to the direction of the wave, forming compressions and rarefactions. Sound is not an electromagnetic wave, which is exactly why it cannot travel through a vacuum.

Question 2

Sound cannot travel through

  1. Aजल
  2. Bइस्पात
  3. Cवायु
  4. Dनिर्वात

Explanation

Sound needs particles of a medium to travel. A vacuum has no particles, so sound cannot travel through it.

Question 3

A sound wave has a frequency of 500 Hz and a wavelength of 0.68 m. Its speed is

  1. A340 m/s
  2. B680 m/s
  3. C170 m/s
  4. D735 m/s

Explanation

v = ν × λ = 500 × 0.68 = 340 m/s. The other options come from misapplying the formula.

Question 4

The audible range of a normal human being is

  1. A2 Hz से 2,000 Hz
  2. B20 Hz से 2,000 Hz
  3. C20 Hz से 20,000 Hz
  4. D200 Hz से 2,00,000 Hz

Explanation

A normal human can hear sounds from about 20 Hz to 20,000 Hz. Frequencies below this are infrasonic and frequencies above this are ultrasonic.

Question 5

When a drum is struck harder, which property of its sound mainly increases?

  1. Aआवृत्ति
  2. Bआयाम
  3. Cतरंगदैर्घ्य
  4. Dआवर्तकाल

Explanation

Beating a drum harder increases the amplitude of its vibration, which increases loudness. Frequency depends on the drum's construction, not on how hard it is struck.

Question 6

Bats detect obstacles while flying in the dark by using

  1. Aअवश्रव्य ध्वनि
  2. Bपराध्वनि
  3. Cप्रकाश तरंगें
  4. Dश्रव्य ध्वनि

Explanation

Bats produce high-frequency ultrasound (above 20 kHz) and locate obstacles by listening to the waves reflected back to them.

Question 7

A person claps in front of a cliff and hears the echo after 2 s. If the speed of sound is 340 m/s, the distance of the cliff is

  1. A680 m
  2. B340 m
  3. C170 m
  4. D1360 m

Explanation

In 2 s, the sound travels to the cliff and back, so 2d = v × t = 340 × 2 = 680 m, giving d = 340 m. 680 m is the total distance travelled, not the distance to the cliff.

Question 8

Rice grains placed on a plate start jumping when the plate is struck. This activity mainly helps learners understand that

  1. Aध्वनि उत्पन्न करने वाली वस्तु कम्पन करती है
  2. Bध्वनि निर्वात में चलती है
  3. Cध्वनि की चाल प्रकाश से अधिक है
  4. Dतारत्व आयाम पर निर्भर करता है

Explanation

The jumping grains show that the beaten plate is vibrating — in other words, sound is produced by vibration. The other three statements are scientifically incorrect.

Question 9

The most suitable Class 8 activity to show that sound also travels through solids is

  1. Aधागे और डिब्बियों से खिलौना टेलीफोन बनवाकर बात करवाना
  2. Bपरिभाषा को पाँच बार लिखवाना
  3. Cकेवल पाठ्यपुस्तक का अनुच्छेद पढ़कर सुनाना
  4. Dध्वनि की चाल की तालिका रटवाना

Explanation

In a toy telephone, sound travels from one end to the other through the taut thread (a solid), an effect children experience directly. Rote-learning a definition or just listening to a paragraph read aloud gives no such direct experience.

Question 10

A learner believes that sound travels fastest in air. Which correct fact should the teacher use to address this misconception?

  1. Aध्वनि जल में सबसे तेज़ चलती है
  2. Bध्वनि निर्वात में सबसे तेज़ चलती है
  3. Cध्वनि इस्पात जैसे ठोस में वायु और जल से अधिक तेज़ चलती है
  4. Dध्वनि की चाल सभी माध्यमों में समान है

Explanation

At 25 °C, sound travels at about 346 m/s in air, about 1498 m/s in water and about 5960 m/s in steel, so it is fastest in a solid. Sound does not travel at all in a vacuum.

Frequently asked questions

How many questions come from Sound in REET Level 2?+

The number isn't fixed, but REET 2024 Level 2's two shifts of Maths-Science carried 4 questions from Sound in total, including the WHO noise-level question, which was repeated across both shifts.

What does the pitch of a sound depend on?+

On frequency. The higher the frequency, the higher the pitch.

What is the human audible range?+

Roughly 20 Hz to 20,000 Hz. Frequencies below this are infrasonic, and frequencies above this are called ultrasonic.

What is the minimum distance needed to hear an echo?+

About 17.2 m, at a speed of 344 m/s, because the sensation of sound persists in the ear-brain system for roughly 0.1 s.

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Next topic: current, potential difference and Ohm's law

REET Syllabus

The complete official syllabus for Level 1 and Level 2

REET 2024 Solved Paper

Questions, answer key and explanations for all shifts

Official Sources

  1. NCERT Class 9 Science — Chapter 10: Sound Waves: Characteristics and Applications

Change Log

  • 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked