Class 9th Science: Chapter 12 Sound


Production and Propagation of Sound

Sound is mechanical energy which causes a sensation of hearing. As the sound propagates in the form of a mechanical waves it needs a medium with the properties of inertia and momentum.Sound travels through gases, liquids and solids. The speed of sound is the maximum in solids, less in liquids and the least in gases. Sound cannot travel through vacuum because there are no particle to exhibit the properties of elasticity and inertia in vacuum.

In humans, sound is produced by the voice box or the Larynx.Vocal cords in the larynx vibrate and produce sound. We hear sound with our ears.
Working of Human Ear
Sound waves are collected by the pinna and then are directed through ear canal sound waves on striking the eardrum make it vibrate exactly the same way as the given sound emitting object. The bones in thge middle ear start vibrating when eardrum  vibrates. It helps in magnifying the vibrations. When the magnified vibrations reach the cochlea in the inner ear, the fluid in it starts vibraTING. These vibrationsare picked up by sensory receptors and are converted into electrical signals. These electrical signals then travel to the brain which interprets sound.
Musical Instruments
To produce pleasant sounds, a number of musical instruments have been developed. Some instruments produce sound due to the vibration of membranes, some due to the vibration of strings, and some others produce sound due to the vibration of an air column. The to and fro or back and forth motion of an object is called vibration. The sitar, veena, violin, guitar and ektara are some stringed instruments. The tabla, cymbals, ghatam, kartal and manjira are some instruments that work on the vibration of a membrane, and instruments like the flute and the trumpet produce sound due to the vibration of an air column present in them.
Longitudinal Waves
In longitudinal waves, particles of the medium vibrate parallel to the direction of propagation of the waves.Wavelength (l) is the distance between two consecutive compressions or two consecutive rarefactions in  longitudinal waves. Time period (T) is the time interval between two consecutive compressions or two consecutive rarefactions in  longitudinal waves.
Transverse Waves
In transverse waves, particles of the medium vibrate perpendicular to the direction of  propagation of waves.Wavelength (l) is the distance between two consecutive crests or two consecutive troughs in transverse waves. Time period (T) is the interval between two successive compressions or two successive rarefactions in  longitudinal waves. Time period (T) is the interval between two successive crests or two successive troughs in  transverse waves.
•  Frequency (n) is the number of waves produced by the source of sound per unit time.
•  Pitch is the interpretation of the frequency of a sound by the brain or pitch is the number of the waves percieved by the listener per second.
•  Pitch is that characteristic of sound by which a shrill ( or an acute) note can be distinguished from a grave of a flat note.
•  Pitch referes only to musical sounds and each musical note has a definite pitch.
•  Pitch of a note depends on its wavelength or frequency.
•  A tone is a sound wave of a single frequency.
•  A note is a sound produced due to a blend of several frequencies.
•  Amplitude (A) is the maximum displacement of the vibrating particles from their mean position.
•  Speed of the sound waves is the ratio of distance to time. It is denoted by the letter ‘v’
The speed of a sound wave depends on the properties of the medium through which it propagates. If the temperature of a gas increases, its density decreases, resulting in an increase in the speed of sound and vice versa.
When an object travels at a speed greater than the speed of sound in air, it is said to be travelling with a supersonic speed. Enormous energy is produced when an object travels at a supersonic speed. This energy is propagated as a sharp and loud sound, called a sonic boom.
The Difference Between Longitudinal Waves and Transverse Waves
              LONGITUDINAL
            TRANSVERSE
1. The particles of medium vibrate in the same direction.

2. They are possible in all kinds of media.

3. They consists of regions of compression and rarefaction.

4. They cannot be polarised.

5. Sound waves in air is an example of longitudinal waves.
1. The particles move at right angles to the direction of wave propagation.

2. They are possible only in solids.


3. They consists of crests and troughs.


4. They can be polarised.

5. Vibrations in a string is an example of transverse waves.

The Difference between Sound Waves and Light Waves :
           Sound Waves
            Light Waves
1. Longitudinal waves travel with a speed of 330 m s-1

2.Require a medium to propagate.

3. Sounds waves are produced by oscillating particles of medium.
1. Transvere waves travel with a speed of 3 x 108 m s-1

2.Do not require a medium to propagate.

3. Light waves are produced by oscillating charged particles of medium.

Musical Sound and Noise
Music is a pleasant sound, whereas noise is un pleasant to hear. The waveform of musical sound is periodic and regular whereas the waveform of of noise is irregular and sudden.
The frequencies that make up a musical sound are in the ratio of small whole numbers whereas this is not the case for noise.

Loudness and Intensity
Loudness is the property by virtue of which a loud sound can be distinguished from a faint one, both having the same pitch and quantity.
Loudness is:
i) directly proportional to square of amplitude.
ii) inversely proportional to square of distance.
iii) directly proportional to surface area of vibrating body.
iv) directly proportional to the density of medium.
v) more the resonant bodies nearby , more will be the loudness.
•  Unit of loudness is phon. Unit of intensity level is decibal.

Intensity of Sound
Intensity of sound is the amount of sound energy incident per unit time per unit area.Intensity is a measurable quantity whereas loudness is sensation.
L = K log I , Where
L = Loudness
I = Intensity
K = Constant
•  Intensity is proportional to (i) square of amplitude (ii) square of frequency (iii) density of air.

Pitch(Shrillness) and frequency
The characteristic of sound by which an acute or shrill note can be distinguished from a flat or grave note is called Pitch. The frquency of note produced by a string in stringed instruments can be changed by changing the place of plucking or by increasing the tension on the string or by using the string of less or more thickness.
Pitch of sound depends on:
(i) frequency (ii) relative motion between sourse and listner. Pitch is sensation only.

Quality (or Timbre) and wave form of Sound
Quality or Timbre of a sound is that characteristic which distinguishes the two sounds of same pitch and loudness but emitted by two different instruments. It depends on waveform. The waveform of a sound from an instrument depends on the presence of subsidiary vibrations along with the principal vibration and the relative amplitudes of various subsidiary vibrations in relation to principal vibration.

Different instruments emit different subsidiary notes,e.g., a note played on a piano has a large number of subsidiary notes while flute contains only a few subsidiary notes.


Reflection of Sound and Echo

When a sound wave is incident on a solid or liquid surface, it bounces back onto the same medium. This is known as the reflection of sound. Like light, sound too undergoes reflection from any hard surface and obeys the same laws of reflection, which light obeys.
Laws of Reflection of Sound
The direction in which the sound wave is incident and the direction, in which it is reflected, make equal angles with the normal to the reflecting surface, at the point of incidence.
The incident sound wave, the reflected wave and the normal at the point of incidence are in the same plane.
Sound waves need polished or rough surfaced obstacles to get reflected. The incident sound wave, the reflected sound wave and the normal, all lie in the same plane.
Echo
An echo is the repetition of sound that results as a reflection from a surface. Multiple reflection of sound is the successive reflection of sound from various reflecting surfaces. We cannot hear an echo unless the reflecting surface is beyond a specified distance from the source of the sound. The sensation of sound persists for 0.1 second. The minimum distance required to hear an echo is 17.2 metres.
Reverberation
The persistence of sound in big enclosures like auditoriums is the result of repeated reflections of sound and is called reverberation.
Reverberation can be minimized by using sound absorbent materials like cardboard, thick curtains and fibre. The process of multiple reflection of sound is applied in:
•  Stethoscope
•  Megaphones
•  Horns
•  Trumpets
A human ear can hear sounds with a frequency of 20 hertz to 20000 hertz. This range of frequency is called the audible range. The audible range for different species is different.
Working of Human Ear in the Transmission of Sound Waves to the Brain
The outer ear is called pinna. It collects the sound from the surroundings. The collected sound passes through the auditory canal, there is a thin membrane called eardrum. When a compression of the medium reaches the eardrum, the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward where a rarefaction reaches it. In this way eardrum vibrates.
The vidrations are amplified several times by three bones in middle ear. The middle ear transmits the amplified pressure variations receieved from the sound wave to the inner ear, the pressure variations are turned into electrical signals by the cochlea.
These electrical signals are sent to the brain via the auditory nerve and the brain interprets them as sound.
Sound waves that have a frequency less than 20 hertz are called infrasonic sound waves or infrasonics. Sound waves that have a frequency greater than 20000 hertz are called ultrasonic sound waves or  ultrasonics.
Ultrasound waves have a wide range of applications in several fields such as health care and industry. Ultrasonography is a technique used to diagnose any defects in internal organs such as liver, uterus and kidney.
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.
Working of SONAR :
A sonar which stands for Sound Navigation and Ranging is technique used for determining the depth and also locating underwater objects such as reefs, submarines etc.
In this method, to find the depth of ocean, a strong ultrasonic wave is sent from the ship towards the bottom of the ocean. This ultrasonic is received back after it is reflected from the bottom of the sea. The time interval t for travel of sound waves from the source to the receiver after reflection is noted.
Then depth d of the ocean is given by the velocity of D = V×t2  , where V is the velocity of Sound.
Echo-ranging method is used to find the distance travelled by the ultrasound ray, which is given by the equation:  2d = vt
Bats and Porpoises like dolphin are examples of species that uses its personal sonar to obtain its food. The ear converts audible frequencies in air into electric impulses that reach the brain. The human ear is categorised into three main parts, the outer ear, middle ear and the inner ear.
Echo versus Reverberation
  • 1. Echo is the reflection of a sound or some other wave off a surface. Reverberation is the sound or the pattern created by the superposition of such echoes.
  • 2. An echo can be heard only when the distance between the source of sound and the reflecting body is  at least 17 m. A reverberation can occur when sound wave is reflected by a nearby wall also.
  • 3. An echo is usually clear and can be clearly distinguished. A reverb is not a clear replica of the original sound sample.
  • 4. Echo can be used to determine the distance of a reflecting object such as a large building or a mountain, if the ambient temperature is known. Reverberation cannot be utilized for distance measurement applications.
  • 5. An echo can be heard both in open and closed spaces. Reverberation is usually experienced in closed spaces with multiple reflecting objects.
  
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