Science Class 9 — Exploration
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Reflection of Sound, Echo, SONAR and the Human Ear

🎓 Class 9 Science CBSE Theory Ch 10 — Sound Waves: Characteristics and Applications ⏱ ~17 min
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This MCQ module is based on: Reflection of Sound, Echo, SONAR and the Human Ear

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10.10 Reflection of Sound

When sound waves hit a hard surface like a wall, a cliff or a ceiling, they bounce back. This bouncing back is called the reflection of sound. Sound, like light, follows two simple laws on reflection:

  • The angle of incidence equals the angle of reflection.
  • The incident wave, the reflected wave and the normal to the reflecting surface all lie in the same plane.

Sound reflects best from large, hard, smooth surfaces. Soft surfaces such as curtains, carpets and padded furniture absorb a major part of the sound rather than reflecting it.

Echo

An echo is a clearly heard repetition of a sound caused by reflection from a distant obstacle. For an echo to be distinguished, the reflected sound must reach the ear at least 0.1 second after the original sound — that is the time the human brain needs to register the two as separate. Taking the speed of sound as 344 m/s, in 0.1 s sound covers 34.4 m. But the sound has to travel to the wall and come back, so the obstacle must be at least

\(d_{\min} = \dfrac{v \times t}{2} = \dfrac{344 \times 0.1}{2} = 17.2\) m.

This is why you cannot hear an echo of your own voice in a small room — the wall is simply too close.

Echo — sound goes, reflects and returns Wall / Cliff Original sound → ← Echo For echo: distance ≥ 17.2 m so total time gap ≥ 0.1 s.
Fig 10.7: Sound reflects from a distant wall and returns as an echo.

Reverberation

In an enclosed hall, sound bounces back and forth between the walls, floor and ceiling many times. Each reflection adds slightly to the original sound. The persistence of sound after the source has stopped, due to repeated reflections, is called reverberation. Excessive reverberation makes speech indistinct in a large auditorium. To control it, the ceilings and walls of concert halls and cinemas are lined with sound-absorbing materials such as compressed fibreboard, rough plaster, drapery, and even special perforated panels. Seats are upholstered with absorbent fabric.

Worked Example — distance from echo time

A boy claps his hands at a distance of 200 m from a high wall. After how many seconds does he hear the echo? Take v = 340 m/s.
The sound travels 2 × 200 = 400 m.
t = d/v = 400/340 ≈ 1.18 s.

10.11 Uses of Reflection of Sound

Megaphone and loudhailer

A megaphone is a hollow tube that opens out into a wide cone. The walls of the cone repeatedly reflect sound forward, so that very little energy spreads sideways or backwards. The voice reaches the audience much louder than from an open mouth.

Stethoscope

The doctor's stethoscope picks up faint sounds of the heart and lungs from the chest, and the long rubber tubes carry these sounds to the doctor's ears by repeated reflection from the inner walls of the tubes — almost no energy is lost.

Speaking tubes and curved ceilings

Old buildings used long speaking tubes between rooms — a soft whisper at one end is reflected along the tube and reaches the other end clearly. Many concert halls and Mughal-era audience chambers have curved domed ceilings; they focus the reflected sound towards the listeners.

Megaphone → louder, focused Stethoscope Heart sounds reflect along tubes
Fig 10.8: Megaphone and stethoscope both rely on multiple reflections of sound.

10.12 SONAR

SONAR stands for Sound Navigation and Ranging. It is mounted on ships and submarines and is used to detect underwater objects, measure ocean depth, locate fish shoals, sunken vessels and submerged mountains. A SONAR consists of a transmitter that sends out ultrasonic pulses and a detector that picks up the echoes reflected from underwater objects.

If the time between sending the pulse and receiving its echo is \(t\), and the speed of sound in sea water is \(v\), then the distance \(d\) of the object below the ship is

\(2d = v \times t \quad\Rightarrow\quad d = \dfrac{v \times t}{2}\)
SONAR — measuring depth using ultrasound pulse echo Object d d = v × t / 2
Fig 10.9: A SONAR on a ship detects an object on the seabed by timing the echo.

Worked Example — SONAR

A SONAR pulse returns to a ship 5 s after being sent. The speed of sound in sea water is 1500 m/s. How deep is the seabed?
Total path = v × t = 1500 × 5 = 7500 m.
Depth d = 7500/2 = 3750 m.

10.13 Activity — Listening to Reflections

Activity 10.3 — The Two-Tube EchoL3 Apply
Predict first: Two cardboard tubes are placed in a "V" shape against a smooth wall. A ticking watch is held at the open end of one tube. Will you hear the watch through the other tube only when the angles are equal?
  1. Stand two long cardboard tubes on a table so that their open ends both face a smooth, hard wall — making a "V" with the wall at the top.
  2. Place a small ticking watch at the open end of the first tube and put your ear at the open end of the second tube.
  3. Adjust the angles slowly until the ticking is sharpest.
  4. Measure the angle each tube makes with the wall surface.
Observations: The ticking is heard most clearly when the angle of the listening tube equals the angle of the source tube. If you tilt one tube more, the ticking grows fainter.

Conclusion: Sound obeys the same law of reflection as light — angle of incidence = angle of reflection.

10.14 Range of Hearing — Recap and Audible Use

From Part 2 we know that healthy human ears respond from about 20 Hz to 20 000 Hz. Below 20 Hz lies the infrasonic band; above 20 000 Hz lies the ultrasonic band. We now look at how the ear actually catches sound, and how ultrasound finds practical use.

10.15 Structure of the Human Ear

The ear is the body's microphone. Anatomically it has three parts that work together to transform pressure waves in air into nerve signals the brain interprets as sound.

Outer ear

  • Pinna (auricle): The visible flap on the side of the head. It collects sound waves and funnels them inwards.
  • Auditory canal (ear canal): A narrow tube about 2–3 cm long that channels sound to the eardrum.
  • Eardrum (tympanic membrane): A thin, taut membrane at the inner end of the canal. Compressions of air push it inward; rarefactions pull it outward, so it begins to vibrate at the same frequency as the sound.

Middle ear

Three tiny bones — the hammer (malleus), anvil (incus) and stirrup (stapes), collectively called ossicles — form a lever system that mechanically amplifies the eardrum vibrations several times before passing them to the inner ear.

Inner ear

The stirrup taps on a fluid-filled spiral organ called the cochlea. The vibrations create pressure waves in the cochlear fluid, which bend tiny hair cells lining its inner walls. The hair cells convert the mechanical motion into electrical impulses, which the auditory nerve carries to the brain. The brain finally interprets these impulses as the sound we "hear".

👂 Ear Anatomy Tour — Click each part to name it L1 Remember

Click the pinna, auditory canal, eardrum, an ossicle, the cochlea, or the auditory nerve to recall what each part does in turning sound into a brain signal.

Structure of the Human Ear Pinna Auditory canal Eardrum Hammer Anvil Stirrup Cochlea Auditory nerve → brain Sound waves Outer Middle Inner
Fig 10.10: The three regions of the human ear and their main parts.
Click any part of the ear above to recall its name and role in hearing.

10.16 Applications of Ultrasound

Because of their high frequency and short wavelength, ultrasonic waves can travel along well-defined paths and can be focused on small objects. They are also unaffected by ordinary background noise. These properties make ultrasonics extremely useful in industry and medicine.

In medicine

  • Echocardiography: Ultrasound is bounced off the moving walls of the heart to build a real-time image of the heart's working. This helps doctors check valves, chambers and blood flow without surgery.
  • Ultrasonography (sonography): A safe scanning method to examine internal organs (liver, kidneys, uterus). Pregnant women undergo ultrasound scans to monitor the growth of the foetus and to detect any abnormalities.
  • Breaking kidney stones: Focused ultrasonic pulses (lithotripsy) shatter small stones into fragments that the body can pass naturally.

In industry

  • Cleaning hard-to-reach parts: Spiral tubes, electronic components, jewellery and dental tools are placed in a cleaning solution that is set vibrating at ultrasonic frequencies. The high-frequency waves dislodge dust, grease and grime from the deepest crevices — far better than ordinary scrubbing.
  • Detecting flaws and cracks in metals: Ultrasonic waves are sent through metal blocks and ship hulls. Where there is an internal crack or air pocket, the waves are partially reflected. The reflection pattern reveals the size and depth of the flaw without cutting open the material.
  • SONAR: Already discussed above — uses the reflection of ultrasound in water.

Competency-Based Questions

A research vessel uses a SONAR transmitter to map the seabed. The pulse takes 4 s to return. The speed of sound in sea water is 1500 m/s. On a separate test, the captain claps near a 30 m vertical cliff to find out whether his crew can hear an echo. The temperature is 25 °C, where v = 346 m/s in air.
Q1. Calculate the depth of the seabed below the vessel. L3
d = v × t / 2 = (1500 × 4)/2 = 3000 m.
Q2. Will the captain hear an echo from the 30 m cliff? L4
Time gap = 2d/v = 60/346 ≈ 0.173 s. This is greater than 0.1 s, so an echo will be heard.
Q3. Why does a stethoscope tube transfer faint heart sounds so efficiently to the doctor's ears? L2
  • (a) The rubber generates sound
  • (b) Sound undergoes repeated reflections inside the smooth walls of the tubes with little loss
  • (c) The chest piece amplifies sound electrically
  • (d) The sound is converted to ultrasound
(b) The waves bounce along the smooth inside of the tube without spreading sideways, so almost all the energy reaches the ear.
Q4. Identify the part of the ear that converts mechanical vibrations into electrical signals. L1
The hair cells in the cochlea of the inner ear. The auditory nerve then carries these signals to the brain.
Q5. Describe one industrial use of ultrasound and explain why ordinary audible sound would not work. L4
Ultrasonic cleaning of intricate parts: high-frequency waves have very short wavelengths, so they reach the smallest crevices and shake off dirt. Audible sound has wavelengths of metres and cannot be focused into such small spaces; it would also disturb workers.

Assertion–Reason Questions

Options: (A) Both A and R are true and R is the correct explanation of A. (B) Both true but R is not the correct explanation. (C) A true, R false. (D) A false, R true.

A: An echo cannot be heard from a wall that is 10 m away.
R: The minimum distance for hearing a distinct echo in air is 17.2 m, because the brain needs at least 0.1 s gap between the original and reflected sound.
(A) Both true and R correctly explains A.
A: Walls of cinema halls are made rough and lined with curtains and absorbent panels.
R: This reduces excessive reverberation and keeps speech and music clear.
(A) Both true and R correctly explains A. Soft surfaces absorb sound and prevent multiple reflections.
A: SONAR uses ultrasonic waves to find the depth of the sea.
R: Light waves travel faster than sound and are therefore preferred in deep water.
(C) Assertion is true. Reason is false — light is absorbed quickly by water and cannot reach great depths, so sound (specifically ultrasound) is used.
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