Sound – Production and Propagation of Sound
* Sound is form of energy that produces the sensation of hearing in our ears.
* Sound is produced due to the vibration of objects.
* Intensity of sound is the amount of sound energy incident per unit time per unit area.
* Sound propagates through a medium.
* A wave is disturbance that moves through medium set neighbouring particles into motions
Wave form are categorised into two types:
* Transverse waves
* Longitudinal waves
In a longitudinal wave:
* A compression is a region of high pressure.
* A rarefaction is a region of low pressure.
* Sound waves in air are longitudinal waves.
* The wavelength is the distance between two consecutive compressions or two consecutive rarefactions. It is denoted byλ.
* The time period is the interval between two successive or two successive rarefactions. It is denoted by T.
* The frequency is the number of waves produced in a unit, ‘T’. It is denoted by ‘n’: n = 1/T
* Pitch is the interpretation of frequency of a sound by brain.
* The amplitude is the maximum displacement of the vibrating particles from their mean position. It is denoted by A.
* The speed of a wave is the ration of its wavelength to its time period or the product of its frequency wavelength. It is denoted by v.
v = λ/T= nλ
* The quality or timbre is the characteristic that distinguishes a sound from other sounds with the same pitch or loudness.
* A tone is a sound of a single frequency.
* A note is a sound produced due to blend of several frequencies.
* The speed of sound of sound in a medium depends on the properties of the medium through which it propagates.
* Supersonic speed is the speed of an object when it travels in air at speed faster than that of sound in air. The sharp and loud sound produced by a supersonic object is called a sonic boom.
Sound- Reflection of sound
* When a sound wave is incident on a hard surface, it bounces back into the same medium. This is known as the reflection of sound.
* Echo is the repetition of sound that results when a sound wave reflects from a surface.
* The minimum distance required to hear an echo is 17.2 metres (velocity of sound = 344 m s-1).
* Multiple reflection of sound is the success reflection of sound from various reflecting surfaces.
* The phenomena of persistence of sound in a closed enclosure, due to multiple reflections are called reverberation.
* The ear converts audible frequencies in air into electric impulses that reach the brain.
* Hearing aids helps us compensate for reduction in our auditory capabilities.
* A human ear can hear sounds ranging from 20 hertz to 20000 hertz.
Sound waves are classified into three categories based on their frequencies.
* Infrasonic sounds (0 to 20 hertz)
* Audible sounds (20 hertz to 20000 hertz)
* ultrasounds (above 20000 hertz)
* Ultrasound waves have a wide range of applications in the medical and industrial field.
* SONAR stands for Sound Navigation and Ranging. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.
* The distance travelled by the ultrasound ray is given by the equation: 2d = v x t
Where:
d = Distance from the ship to the seabed
v = Speed of sound waves
t = Time taken by the sound wave to get back.
Free, Damped and Forced Vibrations
* The oscillations of a body when set into vibration and left to itself are called Free Vibration or Natural Vibrations.
* The frequency of a body executing natural vibration is called its Natural Frequency.
* The periodic vibrations of a body of decreasing amplitude are called Damped vibrations.
* When a body vibrates under the influence of an external periodic force with a frequency equal to that of the external periodic force, then the vibrations of the body are called Forced Vibrations.
Sound – Characteristics of Sound
* Sound propagates in the form of waves.
* The time taken to complete one oscillation is called the time period.
* The maximum displacement of a vibrating object from its rest position is equal to the amplitude of its vibration.
* Frequency is equal to the number of oscillation per second. The unit of frequency is hertz (Hz).
* The loudness of sound is proportional to the square of the amplitude of vibration. The loudness of sound is expressed in decibel (db).
* The shrillness or pitch of a sound is determined by its frequency.
* Human beings can only hear sounds of frequencies between 20 and 20,000 Hz. such sounds are called audible sounds.
Resonance
* When a body is set into vibrations and left to itself, it executes vibration called natural vibrations or free vibrations.
* The periodic vibrations of a body with decreasing amplitude are called damped oscillations.
* When a body vibrates under the influence of an external periodic force, it is said to execute force vibrations.
* When a body vibrates under the influence of an external periodic force whose natural frequency is equal to its own natural frequency, it executes vibrations of increasing amplitude. This phenomenon is called resonance.
Sounds – Noise and Music
* The distinction between sound and noise differ from person to person.
* Any intolerable and irritating sound is called noise.
* The term “Music” refers to any sound that you like listening to.
* Undesirable sounds and disturbance cause noise pollution.
* Noise pollution may cause high blood pressure, panic attacks and lack of sleep among those exposed to it.
* Continuous exposure to loud noise may cause temporary or even permanent hearing impairment.
* People suffering from total hearing disability are usually born with it.
* Children who are born hearing impaired need special care as they find it difficult to communicate. They may also suffer from defective speech.
* People with hearing disabilities use sign language and various devices to communicate.
To minimise noise pollution
* Silencer devices should be installed in air craft engines, vehicles, industrial machines and home appliances.
* Trees should be planted along roads and in residential areas.
* Factories should not be set up in residential areas.
* Vehicles should not blow horns around schools, hospitals and residential areas.
* TVs and music systems should not be played at high volumes.