1. Production of Sound

Sound is a form of energy which produces a sensation of hearing in our ears. Just like light and heat, sound is an energy form, but unlike them, it requires a physical movement or vibration to be produced.

• Sound is produced due to the vibration of objects. Vibration is a kind of rapid to and fro motion of an object.
• Examples: Striking a tuning fork, plucking the string of a guitar, vibrating vocal cords in humans, scratching or rubbing an object.
• In humans, sound is produced by the vocal cords in the larynx (voice box) as air from the lungs passes through them, making them vibrate.

2. Propagation of Sound

The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid, or gas. Sound moves through a medium from the point of generation to the listener.

2.1 Mechanism of Propagation (Compressions and Rarefactions)

When an object vibrates, it sets the particles of the medium around it vibrating. The particles do not travel all the way from the vibrating object to the ear. Instead, a particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle, displacing it, and then returns to its original position. This process continues in the medium till the sound reaches your ear. It is the disturbance that travels, not the particles themselves.

• Compression (C): When a vibrating object moves forward, it pushes and compresses the air in front of it, creating a region of high pressure and high density. This region is called a compression.
• Rarefaction (R): When the vibrating object moves backward, it creates a region of low pressure and low density. This region is called a rarefaction.

As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air. These make the sound wave that propagates through the medium.

2.2 Sound Needs a Material Medium to Travel

Sound is a mechanical wave and needs a material medium like air, water, or steel for its propagation. It cannot travel through a vacuum.

3. Sound Waves are Longitudinal Waves

Waves can be categorized based on the direction of vibration of particles with respect to the direction of wave propagation.

• Longitudinal Waves: In these waves, the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but simply oscillate back and forth about their position of rest. Example: Sound waves in air, waves in a slinky pushed and pulled continuously.
• Transverse Waves: In these waves, particles do not oscillate along the direction of wave propagation but oscillate up and down (perpendicular) about their mean position. Example: Light waves, waves on a stretched string. (Note: Light is not a mechanical wave).

4. Characteristics of a Sound Wave

A sound wave can be described graphically by tracking the changes in density and pressure as the wave moves. The peaks (crests) represent maximum compression (C), and the valleys (troughs) represent maximum rarefaction (R).

Every sound wave is characterized by five major parameters:

4.1 Wavelength ($\lambda$)

The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength.

• Symbol: $\lambda$ (Greek letter lambda).
• SI Unit: Metre (m).

4.2 Frequency ($\nu$)

Frequency tells us how frequently an event occurs. When sound propagates, the density of the medium oscillates between a maximum value and a minimum value. The change in density from maximum to minimum and back to maximum makes one complete oscillation.

• Definition: The number of complete oscillations per unit time is called the frequency of the sound wave.
• Symbol: $\nu$ (Greek letter nu) or sometimes $f$.
• SI Unit: Hertz (Hz), named after Heinrich Rudolph Hertz.

4.3 Time Period ($T$)

The time taken for one complete oscillation in the density of the medium is called the time period of the sound wave. Or simply, the time taken for two consecutive compressions or rarefactions to cross a fixed point.

• Symbol: $T$.
• SI Unit: Second (s).
• Relationship with Frequency: Frequency and time period are inversely related.

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

4.4 Amplitude ($A$)

The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.

• Symbol: $A$.
• Unit: Depends on the quantity, generally density or pressure.
• Amplitude determines the loudness or softness of a sound. A higher amplitude produces a louder sound.

4.5 Speed ($v$)

The speed of sound is defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time.

$$ v = \frac{\text{Distance}}{\text{Time}} = \frac{\lambda}{T} = \lambda \times \nu $$

Therefore, Speed = Wavelength × Frequency. The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.

5. Pitch, Loudness, and Quality (Timbre)

Characteristic	Depends On	Description
Pitch	Frequency	How the brain interprets the frequency of an emitted sound is called pitch. Faster vibrations = higher frequency = higher pitch (e.g., a bird chirping, women's voice). Lower frequency = lower pitch (e.g., a lion's roar, men's voice).
Loudness	Amplitude	A measure of the response of the ear to the sound. Higher amplitude = louder sound. When you strike a table hard, you produce a loud sound (high energy, high amplitude).
Quality / Timbre	Waveform shape	The characteristic which enables us to distinguish one sound from another having the same pitch and loudness. A sound which is more pleasant is said to be of rich quality.

6. Speed of Sound in Different Media

Sound propagates through different media at different speeds. The speed of sound depends on the properties of the medium and physical conditions like temperature.

• State of Matter: Generally, the speed of sound decreases as we go from solid to gaseous state. Speed in Solids > Liquids > Gases.
• Temperature: In any medium, as we increase the temperature, the speed of sound increases. For example, the speed of sound in air is $331 \text{ m/s}$ at $0^{\circ}\text{C}$ and $344 \text{ m/s}$ at $22^{\circ}\text{C}$.

7. Reflection of Sound

Sound bounces off a solid or liquid surface just like a rubber ball bounces off a wall. The laws of reflection of sound are the same as light:

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

7.1 Echo

If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo.

• The sensation of sound persists in our brain for about 0.1 seconds.
• To hear a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1s.
• Assuming the speed of sound in air is 344 m/s (at $22^{\circ}\text{C}$), the total distance traveled by sound (going and returning) = $344 \times 0.1 = 34.4 \text{ m}$.
• Therefore, the minimum distance of the obstacle from the source of sound must be half of this, i.e., 17.2 m. (This distance changes with temperature).
• Multiple echoes can be heard due to successive or multiple reflections (e.g., rolling of thunder).

7.2 Reverberation

A sound created in a big hall will persist by repeated reflections from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation.

• Excessive reverberation is highly undesirable in auditoriums.
• How to reduce it: The roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster, or draperies. Seat materials are also selected for their sound-absorbing properties.

8. Uses of Multiple Reflection of Sound

1. Megaphones, Loudhailers, Horns, and Trumpets: These instruments are designed to send sound in a particular direction without spreading it in all directions. The tube followed by a conical opening reflects sound successively to guide most of the sound waves in the forward direction.
2. Stethoscope: A medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. The sound of the patient's heartbeat reaches the doctor's ears by multiple reflection of sound through the tubing.
3. Concert Halls, Cinema Halls: The ceilings are curved so that sound after reflection reaches all corners of the hall. Sometimes a curved soundboard is placed behind the stage so that sound reflects uniformly across the width of the hall.

9. Range of Hearing

The audible range of sound for human beings extends from about 20 Hz to 20,000 Hz (one Hz = one cycle/s). Children under the age of five and some animals like dogs can hear up to 25 kHz (25,000 Hz).

• Infrasonic Sound (Infrasound): Sounds of frequencies below 20 Hz.
  Examples: Pendulum vibrations, rhinoceroses (communicate at 5 Hz), whales, elephants, and earthquakes (produce low-frequency infrasound before main shock waves begin).
• Ultrasonic Sound (Ultrasound): Frequencies higher than 20 kHz (20,000 Hz).
  Examples: Produced by dolphins, bats, and porpoises. Moths of certain families have very sensitive hearing and can hear ultrasonic squeaks of bats, escaping predation.

10. Applications of Ultrasound

Ultrasounds are high-frequency waves. They are able to travel along well-defined paths even in the presence of obstacles. They have immense applications in industry and medicine.

• Cleaning: Used to clean parts located in hard-to-reach places (e.g., spiral tubes, odd-shaped parts, electronic components). The objects are placed in a cleaning solution and ultrasonic waves are passed into it. Due to high frequency, particles of dust, grease, and dirt detach and drop out.
• Detecting flaws in metal blocks: Used in construction of buildings and bridges. Ultrasonic waves are passed through metal blocks and detectors are used to transmit transmitted waves. If there is even a small defect/crack, the ultrasound gets reflected back indicating a flaw.
• Echocardiography: Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart.
• Ultrasonography: An ultrasound scanner uses ultrasonic waves to form images of internal organs like liver, gall bladder, uterus, kidney, etc. It helps detect stones, tumors, or examine fetal development during pregnancy.
• Lithotripsy: Ultrasound is used to break small stones formed in the kidneys into fine grains, which later get flushed out with urine.

11. SONAR (Sound Navigation And Ranging)

SONAR is a device that uses ultrasonic waves to measure the distance, direction, and speed of underwater objects.

• How it works: It consists of a transmitter and a detector, installed in a boat or ship. The transmitter produces and transmits ultrasonic waves. These waves travel through water, strike the object on the seabed, and are reflected back to the detector.
• The detector converts ultrasonic waves into electrical signals, which are appropriately interpreted.
• Formula: Let the time interval between transmission and reception of ultrasound signal be $t$ and speed of sound through seawater be $v$. The total distance $2d$ traveled by the ultrasound is:

$$ 2d = v \times t $$

This method is called echo-ranging. It is used to determine the depth of the sea and to locate underwater hills, valleys, submarines, icebergs, and sunken ships.

12. Structure of the Human Ear

The human ear is an extremely sensitive device that allows us to convert pressure variations in air with audible frequencies into electric signals that travel to the brain.

Three Parts of the Ear:

1. Outer Ear: Called the pinna. It collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the auditory canal is a thin membrane called the tympanic membrane or eardrum. When a compression reaches the eardrum, inward pressure pushes it inwards. Rarefaction pulls it outward. Thus, the eardrum vibrates.
2. Middle Ear: The vibrations are amplified several times by three bones (the hammer, anvil, and stirrup). The middle ear transmits these amplified pressure variations to the inner ear.
3. Inner Ear: The pressure variations are turned into electrical signals by the cochlea (a snail-like structure). These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.