Chapter 25: SOUND

Introduction

Sound is produced by vibrating objects. Vibrations cause compressions [high pressure] and rarefactions [low pressure].

Sound waves travel through a medium like this. Sound waves are longitudinal i.e. individual particles of a medium move in a direction parallel to the direction of propagation of the disturbance. The particles don’t move physically but oscillate back and forth.

In humans, the voice box or larynx has two vocal cords that produce sound through vibration when lungs force air through them.

Vocal cords in males are 20mm, women 15mm and children even shorter.

Fig 1: Larynx and Physiology of ear

Eardrum vibrates when sound waves hit it; these vibrations are sent to the brain from the inner ear.

The sound waves are collected by the pinna [outer ear] these then pass through the auditory canal to the ear drum which amplifies it. The middle ear [hammer, anvil, and stirrup] then amplifies it and sends it to the inner ear. In the inner ear [the cochlea] converts it to electrical signals which are taken to the brain by the auditory nerve. The brain interprets them as sound.

Properties of Sound

·         Loudness of sound is proportional to square of its amplitude.

·         Loudness is measured in decibels.

·         Amplitude decides the loudness of the sound.

·         Higher sound has higher energy so travels a longer distance.

·         However as it moves away from the source its amplitude keeps on decreasing.

·         Pitch or shrillness is determined by its frequency.

·         Quality or timber of sound decides the pleasantness of sound.

·         Sound of single frequency is a tone; sound which is a mixture of multiple frequencies is a note.

Speed of Sound

Speed of sound remains almost same for all frequencies in a given medium under same physical conditions. Intensity is the amount of sound energy passing through a unit area it’s different than loudness. Two sounds of equal intensity can be of different loudness.

Speed of sounds is highest in solids and least in gases.

It increases with temperature of the medium. Sound travels through solids, liquids, gases but not through vacuum.

Sonic Boom

When a sound producing object moves faster than sound, it creates shock waves that have high energy. Air pressure variations due to these shockwaves produce a large, sharp sound called sonic boom.

Sound obeys the laws of reflection same as light waves. The sensation of sound persists in our brain for 0.1s. If the obstacle from which sound is reflected is at distance more than 17.2m then we hear echo. Sound absorbent materials absorb sound and prevent multiple reflections.

Stethoscope, horns, trumpets, shehenais, auditoriums are designed to take advantage of multiple reflections of sound.

Human Ear

Human ear can hear sounds only between 20 Hz to 20000 Hz i.e. 20 -20000 oscillations per second. Children below 5 can hear infrasonic sounds i.e. below 20Hz. During earthquakes infrasonic sound are produced which are heard by birds and animals and they are disturbed earlier.

Applications of Sound

Ultrasonic sounds above 20 kHz. They have many applications like in cleaning of instruments, detecting cracks in building, machines, ultrasound machines and echo cardiograph, SONAR. All depend on detection of reflected ultrasonic waves from obstacles, body parts etc.

Doppler Effect is observed when a moving object causes change in frequency of EM waves. In military it’s used to detect enemy aircrafts.

                                                          Fig 2: Doppler effect

In astronomy it’s used to find speed of moving stars and in sonography it’s used to study heart beats and blood flow. In echocardiography it used to study heart beats.

Quiz

Score more than 80% marks and move ahead else stay back and read again!