Sound — Class 9 Physical Science

"In a quiet forest, a tree falls. Does it make a sound if no one is there? PHYSICALLY, yes — vibrations are produced. But sound as we EXPERIENCE it requires a receiver. Sound is both a physical wave AND a biological sensation."

1. About the Chapter

Sound is a form of ENERGY that travels through a medium as a MECHANICAL WAVE. This chapter covers:

• Production of sound — vibration is the SOURCE of all sound
• Propagation of sound — how sound travels through solids, liquids, and gases
• Longitudinal waves — compression and rarefaction
• Characteristics of sound waves — amplitude, frequency, time period, wavelength, velocity
• Speed of sound — in different media and factors affecting it
• Reflection of sound — echo, reverberation, and their applications
• SONAR — using sound to navigate and detect objects underwater
• Human ear — the amazing biological sound detector

Why This Chapter Matters

• Sound is ESSENTIAL for communication — from human speech to mobile phones to ultrasound imaging
• Understanding echo and reverberation is CRITICAL for auditorium design
• SONAR technology is used by the NAVY for submarine detection and by fishermen for locating fish
• AP Board frequently asks numericals on speed, wavelength, and echo calculations

2. Production of Sound

Sound is produced by VIBRATING objects. A vibrating object moves back and forth — this motion disturbs the surrounding medium, creating sound waves.

Examples of Vibrating Sources

• Human voice: Vocal cords vibrate (two bands of muscle tissue in the larynx)
• Musical instruments: Guitar string vibrates. Drum membrane vibrates. Air column in flute vibrates.
• Tuning fork: Two prongs vibrate when struck — produces a PURE tone
• Loudspeaker: Diaphragm vibrates electrically, producing sound

'If you touch your throat while speaking, you can FEEL the vibration of your vocal cords. If a vibrating object is STOPPED, the sound STOPS immediately.'

3. Propagation of Sound — Need for a Medium

Sound requires a MATERIAL MEDIUM (solid, liquid, or gas) to travel. Sound CANNOT travel through VACUUM.

Bell Jar Experiment

An electric bell is placed inside a glass jar. When air is gradually REMOVED from the jar, the sound becomes FAINTER and finally INAUDIBLE — even though the bell is still ringing. When air is let back in, the sound is heard again. 'This proves conclusively that sound NEEDS a medium to travel.'

Sound in Different Media

How Sound Travels — Particle to Particle

'The source vibrates → pushes the adjacent particles → these push their neighbours → and so on. The particles themselves do NOT travel from source to receiver — only the DISTURBANCE (energy) travels. This is why we say sound is a MECHANICAL WAVE — it transfers energy, NOT matter.'

4. Longitudinal Waves — Compression and Rarefaction

Sound travels through air as a LONGITUDINAL wave — the particles of the medium vibrate PARALLEL to the direction of wave propagation.

Compression (C)

Region where particles are CLOSE together → HIGH pressure and density.

Rarefaction (R)

Region where particles are FAR apart → LOW pressure and density.

'A sound wave is a PATTERN of alternating compressions and rarefactions travelling through the medium. Imagine a SLINKY pushed and pulled at one end — the coils alternately bunch up and spread out, just like compressions and rarefactions.'

Transverse Waves (for comparison — light)

In transverse waves, particles vibrate PERPENDICULAR to the wave direction (e.g., light, waves on a string). Sound is NOT transverse — it is LONGITUDINAL.

5. Characteristics of Sound Waves

Amplitude (A)

Maximum displacement of a vibrating particle from its MEAN position. Unit: metre (m).

• Larger amplitude → LOUDER sound
• 'The amplitude of normal conversation is about 10⁻⁵ m. A jet engine produces amplitude of about 1 m.'

Time Period (T)

Time taken for ONE COMPLETE oscillation (one compression + one rarefaction). Unit: second (s).

Frequency (f or ν)

Number of COMPLETE oscillations per second. Unit: Hertz (Hz). 1 Hz = 1 oscillation per second.

• f = 1/T (frequency and time period are inversely related)
• Audible range for humans: 20 Hz to 20,000 Hz (20 kHz)
• Infrasonic (< 20 Hz): Earthquakes, volcanic eruptions, whales communicate. 'Elephants produce infrasonic sound to communicate over several kilometres.'
• Ultrasonic (> 20,000 Hz): Bats, dolphins, medical ultrasound. 'Dogs can hear up to about 40,000 Hz. Bats use ultrasound for ECHOLOCATION — they emit high-frequency clicks and listen to the echoes to navigate in the dark.'

Wavelength (λ)

Distance between TWO SUCCESSIVE compressions or TWO SUCCESSIVE rarefactions. Unit: metre (m).

Velocity/Speed of Sound (v)

The distance travelled by a sound wave per unit time. v = f × λ (or v = λ/T)

Worked Example — Wavelength Calculation

'A tuning fork of frequency 512 Hz produces sound in air. If the speed of sound in air is 340 m/s, find the wavelength.'

λ = v/f = 340/512 = 0.664 m ≈ 66.4 cm.

'Common mistake: Using the formula v = f × λ and forgetting to convert units. Always ensure frequency is in Hz and speed in m/s.'

6. Speed of Sound — Factors Affecting It

Speed of Sound in Various Media (at 25°C)

Factors Affecting Speed of Sound in Gases

1. Temperature: v ∝ √T (speed increases with temperature). For air: v ≈ 332 + 0.6T m/s where T is in °C.
2. Pressure: NO effect on speed of sound in an ideal gas (density changes cancel out).
3. Humidity: Speed INCREASES with humidity (water vapour is lighter than dry air → lower density → higher speed).
4. Wind: Sound travels FASTER downwind, SLOWER upwind.

Lightning and Thunder

'Light travels at 3 × 10⁸ m/s — essentially INSTANTANEOUS. Sound travels at about 340 m/s — SLOW. Count the seconds between seeing lightning and hearing thunder: divide by 3 to get the distance in km. 6 seconds delay = 2 km away.'

7. Reflection of Sound

Like light, sound BOUNCES off surfaces. The angle of incidence EQUALS the angle of reflection.

Conditions for Reflection

• The reflecting surface must be LARGE compared to the wavelength of sound
• The reflecting surface should be HARD and SMOOTH (soft or rough surfaces absorb sound)

Echo

An echo is the REPETITION of sound caused by the REFLECTION of sound waves from a HARD surface.

Condition for hearing a distinct echo: The distance between the source and the reflecting surface must be at least 17.2 m (at 22°C, speed ≈ 344 m/s). This ensures a time gap of 0.1 seconds (the minimum time for the human ear to distinguish two sounds).

Worked Example — Echo Calculation

'A person shouts in front of a hill. The echo is heard after 2 seconds. If the speed of sound is 340 m/s, how far is the hill?'

Distance = (v × t)/2 = (340 × 2)/2 = 340 m. 'The division by 2 is CRUCIAL — sound travels to the hill AND back. The distance to the hill is HALF the total distance travelled.'

Reverberation

PERSISTENCE of sound in a large enclosed space due to MULTIPLE reflections. In an auditorium, reverberation time must be BALANCED: too much → sound is unclear, too little → sound is dead. 'Sound-absorbing materials (curtains, carpets, acoustic tiles) are used to control reverberation.'

Applications of Sound Reflection

1. Stethoscope: Sound of heartbeat travels through the tube by multiple reflections
2. Megaphones and loudspeakers: Conical shape directs sound waves in a particular direction
3. Soundboards: In concert halls, curved surfaces behind the stage reflect sound to the audience
4. Hearing aids: Reflect and focus sound into the ear canal

8. SONAR — Sound Navigation and Ranging

SONAR uses ULTRASONIC waves to detect and locate objects UNDERWATER.

Principle

Ultrasonic waves are sent from a ship → they travel through water → reflect off objects (submarine, fish, seabed) → the echo is detected by a receiver.

Working

A transmitter sends a SHORT BURST of ultrasonic sound. The time taken for the echo to return is measured. The distance is calculated using: d = (v × t)/2

Applications of SONAR

• Detecting submarines and underwater obstacles
• Locating schools of fish — ESSENTIAL for AP's fishing industry
• Mapping the ocean floor (bathymetry)
• Measuring depth of the sea

AP Context — SONAR in Coastal Andhra

'Andhra Pradesh has a coastline of about 974 km. Fishing is a MAJOR livelihood in coastal districts (Visakhapatnam, East Godavari, West Godavari, Krishna, Guntur, Prakasam, Nellore). SONAR technology helps fishermen locate large schools of fish — improving catch efficiency.'

9. Structure of the Human Ear

The human ear has THREE parts: Outer ear, Middle ear, Inner ear.

Outer Ear

Pinna: The visible part — collects sound waves and FUNNELS them into the ear canal. Ear canal (auditory canal): Directs sound to the eardrum.

Middle Ear

Eardrum (tympanic membrane): Sound waves hit the eardrum → it VIBRATES. Three small bones (hammer, anvil, stirrup / malleus, incus, stapes): These bones AMPLIFY the vibrations about 20 times and transmit them to the inner ear.

Inner Ear

Cochlea: A SPIRAL-SHAPED, fluid-filled structure lined with tiny HAIR CELLS. Vibrations create waves in the fluid → hair cells bend → generate ELECTRICAL NERVE IMPULSES. Auditory nerve: Carries these electrical signals to the BRAIN → we PERCEIVE sound.

Sound Intensity and Hearing Loss

• Prolonged exposure to sounds ABOVE 80-90 dB can cause PERMANENT hearing loss
• 'Noise pollution is a SERIOUS problem in cities like Visakhapatnam and Vijayawada. Ear protection (earplugs) should be used in noisy environments.'
• 'Loudspeakers are restricted after 10 PM in residential areas under the Noise Pollution (Regulation and Control) Rules, 2000.'

10. Common Mistakes to Avoid

1. 'Sound can travel through vacuum' — FALSE. Sound NEEDS a material medium (solid, liquid, or gas). Light can travel through a vacuum — sound CANNOT. This is proven by the bell jar experiment.
2. 'Amplitude and loudness are the same thing' — They are RELATED but not identical. Loudness depends on amplitude AND the sensitivity of the ear. Loudness ∝ (Amplitude)².
3. 'Frequency and pitch are the same' — Pitch is our PERCEPTION of frequency. Higher frequency = higher pitch. But pitch is subjective — a person's ear may perceive pitch differently.
4. 'Wavelength is the distance from crest to trough' — Wavelength is the distance between SUCCESSIVE compressions (or successive rarefactions), NOT from compression to rarefaction.
5. 'SONAR uses audible sound waves' — SONAR uses ULTRASONIC (high-frequency) sound waves because they have SHORTER wavelengths so they give BETTER resolution and can travel LONGER distances in water.

11. AP SSC Exam Focus

Key Tip

'For echo problems: Remember Distance = (v × t)/2. The factor of 1/2 is because sound travels to the obstacle AND returns. In reverberation questions, the time gap for echo is less than 0.1 seconds, so the reflected sound MERGES with the original sound.'

12. Quick Self-Test

1. 'A sound wave has frequency 440 Hz and wavelength 0.77 m. Calculate the speed of sound.' Answer: v = f × λ = 440 × 0.77 = 338.8 m/s (approximately the speed of sound in air).

2. 'Why is an echo not heard in a small room?' Answer: For a distinct echo, the minimum distance to the reflecting surface must be 17.2 m (for sound to travel 34.4 m and return). In a small room, the distance is FAR LESS — the reflected sound MERGES with the original, producing REVERBERATION (not a distinct echo).

3. 'What is the difference between infrasonic and ultrasonic sound? Give examples.' Answer: Infrasonic (< 20 Hz) — elephants, earthquakes, whales. Not heard by humans. Ultrasonic (> 20,000 Hz) — bats, dolphins, medical ultrasound. Not heard by humans.

4. 'A ship sends a SONAR signal to the seabed. The echo returns after 4 seconds. If the speed of sound in seawater is 1500 m/s, find the depth of the sea.' Answer: Depth = (v × t)/2 = (1500 × 4)/2 = 3000 m = 3 km.

5. 'Explain how the human ear converts sound vibrations into nerve impulses.' Answer: Sound → Pinna collects → Ear canal → Eardrum vibrates → 3 bones amplify (20×) → Fluid in cochlea waves → Hair cells bend → Electrical nerve impulses → Auditory nerve → Brain interprets as sound.

6. 'Why is sound heard better on a humid day than on a dry day?' Answer: The speed of sound INCREASES with humidity because water vapour is LIGHTER than dry air → lower density of air → sound travels faster. Also, humid air transmits sound energy MORE efficiently (less absorption).