Learning Objectives
- Understand how sound is produced and propagated
- Learn the characteristics of sound waves
- Understand the speed of sound in different media
- Learn about reflection of sound, echo, and reverberation
- Understand the human ear and range of hearing
Key Concepts
Production of Sound
Sound is produced by vibrating objects. When an object vibrates, it creates disturbances in the surrounding medium, which propagate as sound waves. Examples: vibrating string of a guitar, vibrating vocal cords, vibrating tuning fork.
Sound as a Longitudinal Wave
Sound is a longitudinal wave — particles of the medium vibrate parallel to the direction of propagation. Sound creates alternate regions of compression (high pressure, particles close together) and rarefaction (low pressure, particles spread apart).
Sound needs a material medium for propagation. It cannot travel through vacuum.
Characteristics of a Sound Wave
Wavelength (λ): The distance between two consecutive compressions or two consecutive rarefactions. SI unit: metre (m).
Frequency (f or ν): Number of oscillations per second. SI unit: Hertz (Hz). 1 Hz = 1 oscillation per second.
Time period (T): Time taken for one complete oscillation. T = 1/f.
Amplitude (A): Maximum displacement of a particle from its mean position. Determines the loudness of sound.
Velocity of sound: v = f × λ (speed = frequency × wavelength).
Speed of Sound in Different Media
Sound travels fastest in solids, then liquids, and slowest in gases.
Speed of sound in air at 20°C ≈ 344 m/s. In water ≈ 1482 m/s. In steel ≈ 5960 m/s.
Speed increases with temperature (in air, increases by about 0.6 m/s per °C rise).
Loudness, Pitch, and Quality
Loudness: Depends on amplitude. Larger amplitude = louder sound. Measured in decibels (dB).
Pitch: Depends on frequency. Higher frequency = higher pitch (shriller). Lower frequency = lower pitch.
Quality (Timbre): Allows us to distinguish between two sounds of the same pitch and loudness. Depends on the waveform.
Reflection of Sound
Sound obeys the same laws of reflection as light: angle of incidence = angle of reflection.
Echo: A reflected sound heard after the original sound. Minimum distance for echo = 17.2 m (at 20°C in air) because the ear needs a gap of at least 0.1 s to distinguish two sounds.
Reverberation: Persistence of sound due to multiple reflections in an enclosed space. Reduced by using sound-absorbing materials on walls.
Range of Hearing
Audible range for humans: 20 Hz to 20,000 Hz.
Infrasound: Frequency below 20 Hz (e.g., earthquakes, elephants).
Ultrasound: Frequency above 20,000 Hz (e.g., bats, dolphins). Applications: SONAR, medical imaging (ultrasonography), cleaning, detecting cracks in metal.
SONAR (Sound Navigation and Ranging): Uses ultrasonic waves to determine the depth of the sea or locate underwater objects. Distance = (speed × time) / 2.
Structure of Human Ear
Outer ear (pinna) collects sound → ear canal → eardrum (tympanic membrane) vibrates → three bones in middle ear (hammer, anvil, stirrup) amplify vibrations → cochlea in inner ear converts vibrations to electrical signals → auditory nerve sends signals to brain.
Summary
Sound is a longitudinal mechanical wave produced by vibrating objects. It requires a medium for propagation and travels fastest in solids. Sound waves are characterised by wavelength, frequency, amplitude, and speed. Reflection of sound produces echoes and reverberation. Humans hear frequencies between 20 Hz and 20,000 Hz; ultrasound has many practical applications including SONAR.
Important Terms
- Longitudinal Wave: Wave where particles vibrate parallel to the direction of propagation
- Compression: Region of high pressure in a sound wave
- Rarefaction: Region of low pressure in a sound wave
- Echo: Reflected sound heard after the original sound
- Ultrasound: Sound waves with frequency above 20,000 Hz
- SONAR: Technique using ultrasound to measure underwater distances
Quick Revision
- Sound is a longitudinal wave; needs a medium; cannot travel through vacuum
- v = f × λ; speed: solid > liquid > gas
- Loudness depends on amplitude; Pitch depends on frequency
- Audible range: 20 Hz to 20,000 Hz
- Echo requires minimum distance of 17.2 m (in air at 20°C)
- SONAR: d = (v × t)/2