The Nature of Sound and Waves

• Sound is a form of energy causing a sensation of hearing.
• The law of conservation of energy also applies to sound.
• Sound is produced by vibrating objects, such as a tuning fork or vocal cords.
• Vibration is a rapid back-and-forth motion.
• Sound travels through a medium, which can be solid, liquid, or gas.
• When an object vibrates, it causes air particles around it to vibrate, displacing them from stable positions
• Vibrating air particles exert a force on neighboring particles, causing them to vibrate and move from their rest positions.
• This chain reaction continues until the disturbance reaches ears
• Waves facilitate sound's efficient movement through solids, liquids, and gases.

Waves

• A wave is a disturbance that travels through a medium, carrying energy.
• Sound travels as mechanical waves requiring a medium to propagate.
• Mechanical waves require a medium to propagate: sound waves cannot travel through a vacuum.
• Electromagnetic waves do not require a medium to propagate: light waves can travel through a vacuum.

Transverse Waves

• Particles oscillate perpendicular to the direction of wave propagation.
• Transverse waves have crests (high points) and troughs (low points).
• Examples: light waves, water waves, waves on a string.
• Transverse waves can travel without a medium (e.g., light in space).
• Transverse waves do not require material medium particles to vibrate.
• Transverse waves can be demonstrated using a rope.

Longitudinal Waves

• Particles oscillate parallel to the direction of wave propagation.
• Longitudinal waves have compressions (high-pressure regions) and rarefactions (low-pressure regions).
• Examples: Sound waves, waves in a slinky, seismic P-waves.
• Longitudinal waves always require a medium
• Longitudinal waves are mechanical in nature, requiring vibration of medium particles
• Longitudinal waves can be demonstrated using a stretched slinky

Sound Waves

• Sound waves are longitudinal waves because the particles of the medium vibrate parallel to the direction of wave propagation.
• When sound waves are produced, surrounding particles oscillate back and forth along the direction in which the wave is moving.
• When sound is produced, compressions, where particles are close together, and rarefactions, where particles are spread out, are created.
• These compressions and rarefactions move through the medium, transferring sound energy.
• Particle motion (vibration) and wave direction are aligned for sound waves moving as longitudinal waves

Sound Needs a Medium

• Sound requires a medium, like air, to travel
• In a vacuum, sound cannot propagate.

Mechanical Waves

• Sound waves are mechanical waves because they require a material medium (air, water, or silver) to travel through
• They cannot propagate in a vacuum

Examples of Mediums

• Communication and hearing occur through air.
• Marine sound transmission occurs through water.

Characteristics of Sound Waves

• Frequency, amplitude and speed are characteristics of sound waves

Frequency

• Frequency measures the number of oscillations (compressions and rarefactions) that pass a fixed point in one second.
• The SI unit for frequency is Hertz (Hz).
• Frequency is related to the pitch of sound and how the brain interprets it.
• Higher frequency equals higher pitch.
• The time taken for one complete oscillation from maximum density to minimum and back to maximum defines the time period (T) of the wave.
• Formula used to calculate time period: T = 1/f
• The density of the medium oscillates between a maximum (compression) and minimum (rarefaction) value when a sound wave propagates.

Amplitude

• Amplitude measures the maximum displacement of particles from equilibrium during a wave oscillation.
• Amplitude determines the loudness of the sound
• Loudness measures the response of the ear to the sound
• Larger amplitude creates louder sound, smaller amplitude softens the sound.
• Sound amplitude decreases as sound travels away from its source, reducing loudness.

Wavelength

• The distance between two consecutive compressions (peaks or crests) or two consecutive rarefactions (troughs) in a sound wave
• SI Unit is Meter (m)
• The symbol for wavelength = lambda (λ)

Speed

• Distance traveled by a sound wave per unit time
• SI unit is m/s.
• Speed depends on the nature of the medium (solid, liquid, or gas).
• Speed increases with temperature and density of the medium.
• Sound travels faster in solids, slower in liquids, and slowest in gases.
• The speed of sound is defined as the distance a point on a sound wave travels per unit of time.
• Formula for finding the speed of sound: Speed = Distance / Time
• Relation between speed frequency and wavelength: Speed = wavelength x frequency

Speed of Sound in Different Media at 25°C

• Aluminium is 6420 m/s.
• Nickel is 6040 m/s.
• Steel is 5960 m/s.
• Iron is 5950 m/s.
• Brass is 4700 m/s.
• Glass (Flint) is 3980 m/s.
• (Sea) water is 1531 m/s.
• (Distilled) water is 1498 m/s.
• Ethanol is 1207 m/s.
• Methanol is 1103 m/s.
• Hydrogen is 1284 m/s.
• Helium is 965 m/s.
• Air is 346 m/s.
• Oxygen is 316 m/s.
• Sulphur dioxide is 213 m/s.

Compression

• Region where particles are closely packed, resulting in high pressure
• There is high pressure and density.
• Particles move towards each other when a tuning fork vibrates, it compresses air in front.

Rarefaction

• Region where particles are spread apart, resulting in low pressure
• There is low pressure and density.
• Particles move away from each other as the vibrating object moves.

Supersonic Speed and Sonic Boom

• Supersonic Speed: An object that travels faster than the speed of sound.
• Examples of supersonic objects: bullets, jet aircrafts
• Sonic Boom: A sharp, loud sound produced when a supersonic object generates shock waves in the air.
• The shock waves carry significant energy, causing intense air pressure variations.
• A sonic boom can shatter glass and may damage buildings

Reflection of Sound

• Sound bounces back when it falls on a hard surface in a reflection
• The laws of reflection of light are applicable to sound.
• The incident sound wave, reflected sound wave, and normal lie in the same plane at the point of incidence.
• The angle of reflection of sound is always equal to the angle of incidence of sound

Echo

• The phenomenon where a sound produced is heard again due to reflection
• A distinct echo requires a time interval of at least 0.1s between the original and reflected sound
• The minimum distance of reflective surface to hear an echo should be 17.2 m.
• Multiple echoes can occur due to multiple reflections

Reverberation

• Reverberation occurs when reflected sounds persist in an enclosed space, caused by repeated reflections from surfaces, even after the source stops.
• Excessive reverberation is undesirable because it causes unclear sound which makes speech or music difficult to understand
• Walls and roofs can be covered with sound-absorbing materials to reduce reverberation
• Seats designed with sound-absorbing materials can help minimize reflection

Multiple Reflections of Sound

• They are used in megaphones, horns, musical instruments, and stethoscopes
• Devices like megaphones, horns, and instruments such as trumpets and shehnais are designed to focus sound in a specific direction.
• Stethoscopes are used to listen to sounds produced within the body, such as heartbeats or lung sounds.
• Acoustic Design of Halls: The ceilings of concert halls, conference halls, and cinema halls are often curved to ensure that sound, after reflection, reaches all parts of the space uniformly

Audible Range

• Normal human hearing covers frequencies from approximately 20 Hz to 20,000 Hz (20 kHz).
• Hearing is most sensitive to frequencies between 2 kHz and 4 kHz.

Inaudible Range

• Infrasound: Frequencies below 20 Hz

Ultrasound

• Frequencies above the audible range (above 20 kHz), are referred to as ultrasound
• While humans are unable to hear ultrasound, animals like bats and marine mammals can
• They use these ultrasonic frequencies for various purposes like navigation and communication

Cleaning

• Ultrasound cleans hard-to-reach areas like spiral tubes, odd-shaped parts, and electronic components.
• Ultrasonic waves detach dust, grease, and dirt in a cleaning solution.

Flaw Detection

• It detects cracks and defects in metal blocks used in structures like buildings, bridges, and machinery.
• Ultrasound reflects from defects, indicating their presence.

Medical Applications

• Echocardiography: Ultrasound reflects from heart parts to create images of the heart.
• Ultrasonography: Generates images of internal organs (e.g., liver, kidney, gall bladder, uterus).
• Ultrasonography is also used to detect stones, tumors, and abnormalities in organs, and helps in examining fetal development during pregnancy.
• Kidney Stones: Breaks kidney stones into fine grains, which are expelled through urine.
• Ultrasound provides clear, reliable results for industrial and medical uses.
• Longer-wavelength sounds bend around defects and are unsuitable for precise detection.

SONAR

• The word 'SONAR' stands for 'Sound Navigation And Ranging'.
• SONAR uses ultrasonic waves to measure the distance, direction, and speed of underwater objects.
• The transmitter produces and sends ultrasonic waves.
• The detector receives reflected waves and converts them into electrical signals.
• Applications of SONAR: measuring sea depth determining the depth of oceans and seas, and underwater exploration locating underwater features like hills, valleys, submarines, icebergs, and sunken ships.
• Bats and porpoises also use ultrasound for navigation

Formula for SONAR

• The total distance 2d traveled by the ultrasonic wave is: 2d = v x t
• The rearranged formula to calculate d: d = v x t / 2

Where

• d = Depth or distance of the object
• v = Speed of sound in water
• t = Time interval between transmission and reception

Human Ear

• The ear converts pressure variations in air caused by sound waves into electrical signals the brain interprets
• The ear consists of three main parts: the outer ear, middle ear, and inner ear
• Here are the definitions and functions of different parts of the ear:

Outer Ear (Pinna)

• Collects sound from the surroundings and channels it into the auditory canal

Auditory Canal

• The tube-like structure that directs sound waves toward the eardrum

Eardrum (Tympanic Membrane)

• The thin membrane vibrates in response to pressure variations caused by sound waves (compression and rarefaction)

Middle Ear (Bones)

• Contains three small bones (hammer, anvil, stirrup) that amplify the vibrations from the eardrum

Inner Ear (Cochlea)

• Converts pressure variations into electrical signals using sensory cells

Auditory Nerve

• Transmits the electrical signals from the cochlea to the brain for interpretation.
• When compression is applied, the pressure on the outside of the eardrum increases, pushing the eardrum inwards
• The compressed soundwave then vibrates the eardrum back and forth
• The middle ear then transmits the pressure variations to the inner ear.
• The pressure variations are turned into electric signals in the inner ear, by the cochlea