Sound

Production of Sound

Introduction to Sound

• Sources: Humans, birds, bells, machines, vehicles, TVs, radios, etc.
• Definition: Sound is a form of energy that creates a sensation of hearing.
• Energy Forms: Includes mechanical energy, light energy, etc.
• Conservation of Energy: Energy cannot be created or destroyed, only transformed.
• Example: Clapping hands converts mechanical energy to sound energy.

Production of Sound

Conclusion

• Sound is produced by vibrating objects.
• Methods: Striking, plucking, scratching, rubbing, blowing, shaking.
• Human Voice: Vibrations in vocal cords produce sound.
• Bird Wings: Flapping wings produce sound.
• Bee Buzz: Produced by the vibration of wings.
• Rubber Band: Plucked rubber band vibrates and produces sound.

Propagation of Sound

How Sound Travels

• Medium: Sound needs a medium (solid, liquid, or gas) to travel.
• Particle Vibration: Vibrating objects cause nearby particles to vibrate, creating a chain reaction.
• Wave Movement: Particles don’t travel; the disturbance moves through the medium.
• Mechanical Waves: Sound waves move by causing neighboring particles to vibrate.

Air as a Medium

• Compression: High-pressure region created when the object moves forward.
• Rarefaction: Low-pressure region created when the object moves backward.
• Sound Wave: A series of compressions and rarefactions moving through the air.
• Pressure Variation: Sound is the propagation of pressure changes in the medium.

Key Points to Remember

• Sound is a form of energy produced by vibrations.
• It requires a medium to travel.
• Sound waves are mechanical waves characterized by particle motion.
• The human voice, musical instruments, and everyday noises are all examples of sound production and propagation.

Sound Waves are Longitudinal Waves

Longitudinal Waves

• Direction: Particles move parallel to the wave propagation.
• Particle Movement: Particles oscillate back and forth about their rest position, not moving from one place to another.
• Example: Sound waves.

Transverse Waves

• Direction: Particles oscillate perpendicular to the wave propagation.
• Example: Water waves when a pebble is dropped in a pond.
• Light Waves: A type of transverse wave, but not involving medium particles, pressure, or density.

Summary

• Longitudinal Waves: Sound waves where particles move parallel to the wave direction.
• Transverse Waves: Waves where particles move perpendicular to the wave direction, like water waves and light waves.

Characteristics of a Sound Wave

Key Characteristics

• Frequency: How often an event occurs.
• Amplitude: The maximum disturbance from the mean value.
• Speed: How fast the sound travels through a medium.

Sound Wave Graph

• Graph Representation: Shows changes in density and pressure as the sound wave moves.
• Compressions (C): High-density regions (particles close together), shown as peaks.
• Rarefactions (R): Low-density regions (particles spread apart), shown as troughs.
• Wavelength (λ): Distance between two consecutive compressions or rarefactions. SI unit is meter (m).

Frequency

• Definition: Number of oscillations per unit time.
• Unit: Hertz (Hz).
• Time Period (T): Time for one complete oscillation. SI unit is second (s).
• Relation: Frequency (ν) = 1 / Time Period (T).

Pitch

• Definition: How the brain interprets the frequency.
• High Pitch: Fast vibration, more compressions and rarefactions per unit time.
• Vibration: Objects of different sizes produce different pitches.

Amplitude

• Definition: Maximum disturbance from the mean value.
• Effect on Sound: Determines loudness. Higher amplitude means louder sound.
• Energy: Louder sounds carry more energy and travel farther.

Quality (Timber) of Sound

• Definition: Characteristic that helps distinguish different sounds.
• Tone: Single frequency sound.
• Note: Mixture of several frequencies, pleasant to hear.
• Noise: Unpleasant sound.

Speed of Sound

• Formula: Speed (v) = Wavelength (λ) × Frequency (ν).
• Example: A sound wave with frequency 2000 Hz and wavelength 0.35 m travels at 700 m/s. To travel 1.5 km, it takes 2.1 seconds.

Intensity of Sound

• Definition: Amount of sound energy passing per second through a unit area.
• Loudness vs. Intensity: Loudness is how we perceive the sound; intensity is the actual energy. Even sounds with equal intensity can be perceived differently.

Speed of Sound in Different Media

• Speed: Sound travels at different speeds in different media.
• Example: Sound of thunder is heard after lightning because sound travels slower than light.
• Factors: Depends on the properties of the medium and temperature.
  • In Air: 331 m/s at 0°C, 344 m/s at 22°C.
  • Mediums: Speed decreases from solid to gas.

Reflection of Sound

• Behavior: Sound bounces off solid or liquid surfaces, similar to light.
• Laws of Reflection: Sound follows the same reflection laws as light.
  • Angles: Incident and reflected sound make equal angles with the normal to the surface.

Echo

• Definition: Reflected sound heard after a delay.
• Condition: To hear an echo, the time interval between the original and reflected sound must be at least 0.1 seconds.
  • Distance: Minimum obstacle distance for a distinct echo is 17.2 meters at 22°C.
• Example: Clapping near a mountain and hearing the sound return.

Reverberation

• Definition: Persistence of sound due to repeated reflections.
• Example: Sound in a big hall lasting longer due to multiple reflections.
• Control: Reduced by using sound-absorbent materials like fiberboard, plaster, or draperies in auditoriums.

Example Calculation

• Problem: A person claps near a cliff and hears the echo after 2 seconds. Find the distance to the cliff if the speed of sound is 346 m/s.
• Solution:
  • Distance Calculation: Sound travels 692 meters in 2 seconds (346 m/s * 2).
  • Result: The distance to the cliff is 346 meters (692 m / 2).

Uses of Multiple Reflection of Sound

1. Megaphones/Loudhailers: Designed to direct sound waves forward using a tube and conical opening.
2. Stethoscope: Uses multiple reflections to amplify the sound of a heartbeat for doctors.
3. Concert Halls: Curved ceilings and soundboards ensure sound is evenly distributed across the hall.

Range of Hearing

• Audible Range: Humans can hear sounds from 20 Hz to 20,000 Hz.
• Children and Animals: Children under five and animals like dogs can hear up to 25 kHz.
• Aging: Older people have reduced sensitivity to high frequencies.
• Infrasonic Sound: Below 20 Hz.
  • Examples: Vibrations of a pendulum, rhinoceroses (5 Hz), whales, and elephants.
  • Earthquakes: Some animals sense infrasound before earthquakes.
• Ultrasonic Sound: Above 20 kHz.
  • Examples: Dolphins, bats, porpoises, and rats.

More to Know: Hearing Aid Devices

• Purpose: Helps people with hearing loss.
• Components:
  • Microphone: Receives sound and converts it to electrical signals.
  • Amplifier: Increases the strength of electrical signals.
  • Speaker: Converts amplified signals back to sound for the ear.

Applications of Ultrasound

• High Frequency Waves: Travel along well-defined paths even with obstacles.

Industrial Uses

• Cleaning:
  • Objects: Spiral tubes, odd-shaped parts, electronic components.
  • Process: Objects are placed in a solution, and ultrasonic waves remove dirt and grease.
• Detecting Cracks:
  • Structures: Buildings, bridges, machines.
  • Process: Ultrasound waves pass through metal blocks; detectors find flaws by reflected waves.

Medical Uses

• Echocardiography: Forms images of the heart using reflected ultrasound waves.
• Ultrasound Scanner:
  • Uses: Imaging internal organs (liver, gall bladder, uterus, kidney) to detect abnormalities.
  • Process: Ultrasonic waves reflect off tissue density changes, converted to images.
  • Pregnancy: Detects congenital defects and growth abnormalities in the fetus.
• Breaking Kidney Stones: Ultrasound breaks stones into fine grains, which are flushed out with urine.

Chapter Summary:

• Sound is produced due to the vibration of different objects.
• Sound travels as a longitudinal wave through a material medium.
• Sound travels as successive compressions and rarefactions in the medium.
• In sound propagation, it is the energy of the sound that travels, not the particles of the medium.
• The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.
• The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, λ.
• The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T.
• The number of complete oscillations per unit time is called the frequency (ν), 1/T = ν.
• The speed v, frequency ν, and wavelength λ of sound are related by the equation, v = λν.
• The speed of sound depends primarily on the nature and the temperature of the transmitting medium.
• The law of reflection of sound states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three lie in the same plane.
• For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 s.
• The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation.
• Sound properties such as pitch, loudness, and quality are determined by the corresponding wave properties.
• Loudness is a physiological response of the ear to the intensity of sound.
• The amount of sound energy passing each second through unit area is called the intensity of sound.
• The audible range of hearing for average human beings is in the frequency range of 20 Hz – 20 kHz.
• Sound waves with frequencies below the audible range are termed “infrasonic” and those above the audible range are termed “ultrasonic”.
• Ultrasound has many medical and industrial applications.