Sound — Class 9 (CBSE)

Sound is everywhere — but you've never seen it. This chapter is about how something invisible can travel from a singer's vocal cord to your eardrum, how a bat hunts in pitch dark, and how ultrasound machines build a 3D image of an unborn baby. It's all wave physics.

1. The story — sound as wave

Sound was, for centuries, a puzzle. People knew it could be loud or soft, high or low, close or distant. But what IS sound? In 1660, Robert Boyle did the decisive experiment: he placed a ringing bell inside a chamber and pumped the air out. As the air thinned, the bell's sound got fainter and fainter, until in a near-vacuum it became inaudible. Conclusion: sound needs a medium to travel.

Today we understand sound as a mechanical wave — a disturbance that propagates through a medium (gas, liquid, or solid) by causing particles to vibrate. Sound CANNOT travel through vacuum (which is why space movies showing explosions with sound are physically wrong — explosions in space would be silent).

This chapter is the wave properties of sound, the math of its speed, and the technology built on it (ultrasonic imaging, SONAR, hearing aids).

2. Sound is a longitudinal wave

A wave is a disturbance that travels through a medium without the medium moving as a whole.

Two types:

• Transverse wave: particles vibrate PERPENDICULAR to the wave's direction of motion. Example: a wave on a stretched string, light, water surface waves.
• Longitudinal wave: particles vibrate PARALLEL to the wave's direction of motion. Example: SOUND, compression of a slinky spring.

Compressions and rarefactions

A sound wave is alternating regions of:

• Compression — particles pushed close together; high pressure, high density.
• Rarefaction — particles spread out; low pressure, low density.

Both move in the same direction as the wave. The particles themselves oscillate forward and backward but don't move with the wave.

3. Properties of a sound wave

Wavelength ($\lambda$)

The distance between two consecutive compressions (or rarefactions). SI unit: metre (m).

Frequency ($\nu$ or $f$)

The number of waves passing a point per unit time. SI unit: hertz (Hz) = 1/s = waves per second.

Time period ($T$)

The time taken for one complete wave to pass a point.

$T=\frac{1}{f}$

If $f=100\text{Hz}$, then $T=0.01\text{s}=10\text{ms}$.

Amplitude ($A$)

The maximum displacement of a particle from its mean position. For sound, also: the maximum compression/rarefaction. Determines loudness.

Wave speed ($v$)

How fast the disturbance travels through the medium. Connects $\lambda$ and $f$:

$\boxed{v=f\times \lambda }$

For air at 20°C: $v_{\text{air}}\approx 343\text{m/s}$ ≈ 1235 km/h.

Pitch and timbre

• Pitch = perceived "highness" or "lowness" of a sound. Determined by FREQUENCY. Higher frequency → higher pitch. A whistle has higher pitch than a tuba.
• Loudness = how loud the sound is. Determined by AMPLITUDE. More amplitude → louder.
• Timbre (also called quality) = what makes a violin and a piano playing the same note sound different. Determined by the WAVE SHAPE (harmonic content).

4. Speed of sound in different media

Sound travels faster in DENSER media (mostly because the particles transmit the disturbance more efficiently).

Key observations:

• Solids > Liquids > Gases (in general).
• Speed depends on the elasticity and density of the medium.
• In air, speed INCREASES with temperature (~ 0.6 m/s per °C of warming). Hot summer day → sound travels slightly faster.

5. Reflection of sound — echoes

When sound hits a hard surface, it bounces back. This is reflection of sound, just like light.

Laws of reflection (apply to sound too)

1. The angle of incidence equals the angle of reflection.
2. The incident wave, reflected wave, and normal at the point of reflection all lie in the same plane.

Echo

An echo is a reflected sound heard distinctly (i.e., separable from the original sound). For a clear echo:

• The reflected wave must reach your ear at least 0.1 s after the original. (Below this, our brain merges them into a single sound — called reverberation.)
• Sound travels at ~340 m/s in air. In 0.1 s, sound covers 34 m. Round trip means the reflecting surface must be at least 17 m away.

So if you stand within 17 m of a wall, you don't hear an echo (you hear reverberation). Beyond 17 m, you hear a distinct echo.

Why concert halls have rounded interiors

To reduce echo (which would muddle the music) but increase reverberation (which "fills out" the sound). Walls and ceilings absorb high-frequency reflections; the audience absorbs additional sound. Master designers balance the two for clarity + warmth.

6. SONAR — Sound Navigation and Ranging

SONAR uses ultrasonic waves to measure distances under water (where light doesn't penetrate well but sound does).

How it works:

1. A SONAR device on a ship sends out an ultrasonic pulse.
2. The pulse hits an object (sea bed, fish, submarine) and reflects back.
3. The device detects the returning pulse and measures the round-trip time $t$.
4. If speed of sound in water = $v$, the distance to the object is:

$d=\frac{v\times t}{2}$

The factor of 2 is because the pulse goes out AND comes back — the total distance traveled is $2d$.

Applications

• Mapping the sea floor (geology, oceanography).
• Finding shoals of fish (commercial fishing).
• Detecting submarines (naval defence).
• Finding sunken ships and aircraft.
• Navigation in foggy or cloudy conditions.

7. Range of hearing

Human ears can perceive sounds with frequencies in a specific range:

• Infrasonic (below 20 Hz) — not audible. Whales, elephants, earthquakes produce infrasonic waves.
• Audible range (20 Hz to 20,000 Hz = 20 kHz) — what humans can hear.
• Ultrasonic (above 20 kHz) — not audible to humans. Used in industry, medicine.

Animals' ranges

• Dogs: up to ~40 kHz (can hear dog whistles).
• Bats: up to ~120 kHz (used for echolocation).
• Dolphins: up to ~150 kHz.

Why old people lose high-frequency hearing first

Inner-ear hair cells degenerate with age. The high-frequency cells (closer to the entrance of the cochlea) are exposed to more noise over a lifetime and degenerate first. This is presbycusis — age-related hearing loss.

8. Ultrasound — applications

Medical imaging

Send ultrasonic pulses into the body. They reflect at boundaries between different tissues (skin/fat, fat/muscle, muscle/bone). The pattern of reflections is converted into an image.

• Pregnancy scans — see the fetus without harmful radiation.
• Cardiac ultrasound (echocardiogram) — see the heart in motion.
• Tumour detection — abnormal tissue reflects differently.

Why ultrasound vs X-rays for pregnancies? Ultrasound is non-ionising — no radiation damage to the fetus.

Industrial cleaning

Ultrasonic baths clean jewellery, surgical instruments, electronic components by creating tiny bubbles (cavitation) that scrub away dirt.

Echolocation (in nature)

Bats and dolphins emit ultrasonic clicks and listen for returning echoes. This gives them a 3D "sound map" of their environment — used for navigation and hunting in darkness or murky water.

Range-finding

Ultrasonic sensors in cars (parking sensors), security systems, and robotic devices use the principle: time × speed = distance.

9. Structure of the human ear

The ear has three parts:

1. Outer ear

• Pinna (visible ear) — funnels sound into the ear canal.
• Auditory canal — channels sound to the eardrum.

2. Middle ear

• Eardrum (tympanic membrane) — vibrates when sound reaches it.
• Three small bones (hammer/malleus, anvil/incus, stirrup/stapes) — amplify the vibrations ~ 20×.
• Eustachian tube — equalises pressure with the atmosphere.

3. Inner ear

• Cochlea (spiral, fluid-filled) — converts vibrations to electrical signals.
• Hair cells lining the cochlea — different cells respond to different frequencies.
• Auditory nerve — sends the electrical signals to the brain.

Why we have two ears

To localise sound. Differences in:

• Arrival time at each ear (sound from the left arrives at the left ear slightly earlier).
• Intensity at each ear (the head shadows one ear).

… allow the brain to compute the sound's direction. Lose one ear and you lose most direction sense.

10. Worked example — SONAR ranging

A SONAR device on a ship emits an ultrasonic pulse. The reflected pulse from a submarine is detected 1.5 s later. If the speed of sound in sea water is 1500 m/s, find the distance to the submarine.

Step 1 — Total distance traveled = speed × time = 1500 × 1.5 = 2250 m.

Step 2 — Pulse went OUT and came BACK, so it traveled twice the distance to the submarine.

Step 3 — Distance to submarine = 2250 / 2 = 1125 m.

11. Closing thought

Sound is a workhorse of nature and technology:

• Music = harmonics of sound waves arranged in pleasing ratios.
• Speech = your vocal cords vibrating at 100–300 Hz, producing the longitudinal pulses we call language.
• Echolocation = millions of years of evolution before humans invented SONAR.
• Earthquakes = enormous infrasonic waves that travel through the Earth, used by seismologists to map the planet's interior.

You can't see a sound wave but you can hear it, feel it (a bass beat in your chest), measure it (oscilloscope), produce it (string, drum, voice), reflect it (echo), focus it (parabolic dish), absorb it (carpet, ceiling tiles), and use it (ultrasound). All from the simple idea that a disturbance can propagate through a medium without the medium moving as a whole. That's wave physics — and after this chapter, you'll see waves everywhere.