🔊 Sound Waves – Quick Revision

• 🔹 Sound is a mechanical wave.
• 🔹 Sound waves are longitudinal waves.
• 🔹 Sound requires a medium for propagation.
• 🔹 Sound cannot travel through vacuum.
• 🔹 Sound travels fastest through solids.
• 🔹 Human hearing range is approximately 20 Hz to 20 kHz.
• 🔹 Frequency below 20 Hz → Infrasonic sound.
• 🔹 Frequency above 20 kHz → Ultrasonic sound.

🌊 What is a Wave?

A wave is a disturbance or vibration produced in a medium that transfers energy from one point to another.

🔄 Vibration

The to-and-fro movement of a body about its mean position is called vibration.

Example: The vibration of a source can produce a disturbance that transfers energy through a medium.

📖 Types of Waves

1️⃣ Mechanical Waves

Mechanical waves require a material medium such as a solid, liquid or gas to propagate.

Examples: Sound waves, water waves and waves on a string.

2️⃣ Electromagnetic Waves

Electromagnetic waves do not require a material medium for propagation.

Examples: Light, heat radiation, ultraviolet rays, X-rays, infrared rays and gamma rays.

↕️ Transverse and Longitudinal Waves

🔵 Transverse Wave

A wave in which the particles of the medium vibrate perpendicular to the direction of propagation of the wave is called a transverse wave.

Examples: Light waves and water waves at the surface of water.

Important Terms

• Crest: The highest point of a transverse wave.
• Trough: The lowest point of a transverse wave.

🟣 Longitudinal Wave

A wave in which the particles of the medium vibrate parallel to the direction of propagation of the wave is called a longitudinal wave.

Example: Sound waves.

Important Terms

• Compression: A region where particles are closely packed, producing relatively high pressure and density.
• Rarefaction: A region where particles are spread apart, producing relatively low pressure and density.

⭐ Important Facts About Sound

1. Sound is a mechanical and longitudinal wave.
2. Sound is produced through vibrations.
3. Sound travels through solids, liquids and gases.
4. Sound cannot travel through vacuum.
5. Sound travels fastest in solids.
6. Sound travels through a medium by successive compressions and rarefactions.

📐 Characteristics of a Sound Wave

The important terms used to describe a sound wave are:

1. Wavelength
2. Amplitude
3. Wave Number
4. Time Period
5. Frequency
6. Velocity of Wave

1️⃣ Wavelength (λ)

Wavelength is the distance between two consecutive compressions or two consecutive rarefactions in a longitudinal sound wave.

It represents the distance covered by one complete wave.

SI Unit: metre (m)

2️⃣ Amplitude (A)

Amplitude is the maximum displacement of an oscillating particle from its mean position.

Greater amplitude generally produces a louder sound.

3️⃣ Wave Number

Wave number is defined as the reciprocal of wavelength.

SI Unit: m⁻¹

4️⃣ Time Period (T)

The time period is the time taken by a vibrating particle to complete one oscillation.

SI Unit: second (s)

5️⃣ Frequency (f)

Frequency is the number of complete oscillations produced by a vibrating body in one second.

SI Unit: hertz (Hz)

6️⃣ Velocity of Sound Wave (v)

The velocity of a wave is related to its wavelength and frequency.

Since f = 1/T:

According to the notes, the speed of sound is approximately 340 m/s at room temperature.

👂 Range of Hearing

The normal range of hearing for human beings is approximately:

🟢 Audible / Sonic Sound

Sound having a frequency between approximately 20 Hz and 20 kHz can be detected by the human ear.

🔵 Infrasonic Sound

Sound having a frequency less than 20 Hz is called infrasonic sound.

Examples mentioned in the notes: Elephants and whales can produce infrasonic sounds.

🟣 Ultrasonic Sound

Sound having a frequency greater than 20 kHz is called ultrasonic sound.

Examples: Dogs, bats and rats can hear ultrasonic sounds.

🔊 Loudness of Sound

Loudness is related to the amplitude of a sound wave. A sound wave with a greater amplitude is perceived as louder.

Loudness is measured in decibel (dB).

🎵 Pitch of Sound

The pitch of a sound depends on its frequency.

• 🎶 Higher frequency → higher pitch.
• 🎶 Lower frequency → lower pitch.

A high-pitched sound has a larger number of compressions and rarefactions passing a fixed point per unit time.

🪞 Reflection of Sound

Sound waves can be reflected from surfaces. The laws of reflection of sound are equivalent to the laws of reflection of light.

Two Laws of Reflection of Sound

1. The incident sound wave, reflected sound wave and the normal at the point of incidence lie in the same plane.
2. The angle of incidence is equal to the angle of reflection.

🔁 Echo

The repetition of sound caused by the reflection of sound waves is called an echo.

A distinct echo can be heard when there is a sufficient time gap between the original sound and the reflected sound.

📐 Formula for Echo

Where:

• S = Speed of sound
• D = Distance between the source and reflecting surface
• T = Time interval between the original and reflected sound

⭐ Important Numerical

If the speed of sound is 340 m/s and the time interval is 0.1 s:

Since sound travels to the reflecting surface and returns, the minimum distance of the reflecting surface is approximately 17 m.

🏛️ Reverberation

The persistence of sound in a large hall due to repeated reflection of sound from the walls, ceiling and floor is called reverberation.

🛠️ Methods to Reduce Reverberation

• Heavy curtains can be placed on doors and windows.
• Carpets or other suitable materials can be used on floors.
• Suitable sound-absorbing materials can be used in halls.

📢 Applications of Reflection of Sound

1️⃣ Megaphones and Loudspeakers

Megaphones and loudspeakers use reflection of sound to send sound in a particular direction without spreading it around.

2️⃣ Stethoscope 🩺

A stethoscope is a medical instrument used to listen to sounds produced inside the human body. Reflection of sound within the tube helps transmit the sound towards the listener.

3️⃣ Sound Board

A sound board can be used behind speakers in large halls to help direct sound towards the audience.

4️⃣ Curved Ceilings

Curved ceilings in concert halls help sound reach different parts of the hall after reflection.

🦻 Hearing Aid

A hearing aid is a battery-operated electronic device used by people who have difficulty hearing.

🔬 Applications of Ultrasonic Sound

Ultrasonic sound has several important applications in science, medicine and industries.

1. 🏭 Used to detect cracks in metal blocks in industries without damaging them.
2. 🧹 Used for cleaning hard-to-reach parts of objects such as spiral tubes and oddly shaped machines.
3. 🩻 Used to investigate the internal organs of the human body.
4. 👶 Used to monitor the development of a fetus inside the mother's uterus.
5. 🪨 Used to break kidney stones into fine grains.

🦇 Echolocation

Echolocation is the ability to locate objects using reflected sound waves or echoes.

How Bats Use Echolocation

• Bats are nocturnal animals and fly in darkness.
• They emit short bursts of ultrasonic sound.
• The sound waves strike nearby objects and return as echoes.
• Bats detect these echoes.
• They can use the echoes to determine the position and distance of objects.

Examples mentioned in the notes: Bats, dolphins and whales.

🌊 SONAR – Sound Navigation and Ranging

SONAR stands for Sound Navigation and Ranging.

SONAR uses sound waves to determine the distance or location of objects underwater.

📍 Applications of SONAR

• Finding the depth of the sea.
• Locating underwater hills and valleys.
• Locating submarines.
• Locating icebergs.
• Locating sunken ships.

🧮 Important Sound Wave Formulas

⚡ One-Minute Revision

❓ Frequently Asked Questions – Sound Waves Class 9

What is sound?

Sound is a form of energy that produces the sensation of hearing.

Why can't sound travel in vacuum?

Sound is a mechanical wave and requires particles of a material medium to propagate. A vacuum does not contain the required material medium.

What is the human hearing range?

The human hearing range is approximately 20 Hz to 20 kHz.

What is the difference between loudness and pitch?

Loudness is related to the amplitude of sound, whereas pitch is related to the frequency of sound.

What is SONAR?

SONAR stands for Sound Navigation and Ranging. It uses sound waves to locate and determine the distance of underwater objects.