# WBBSE Class 9 Physical Science Notes Chapter 7 Sound

Comprehensive WBBSE Class 9 Physical Science Notes Chapter 7 Sound can help students make connections between concepts.

## Sound Class 9 WBBSE Notes

Wave motion and wave : When a disturbance produced in one portion of space travels to another portion, without involving the transfer of any material with it, the motion of the disturbance is called wave motion.

The disturbance itself is called a wave.

• Longitudinal wave : If the particles of a medium move along the direction of the motion of the wave, the wave is called a longitudinal wave.
• Transverse wave : If the particles of a medium move perpendicular to the direction of the motion of the wave, the wave is called a transverse wave.

Characteristics of sound :

• Sound is produced by a vibrating source.
• Sound travels as a longitudinal wave through a material medium.
• A vibrating source produces compression and rarefaction pulses, one after the other in the medium. These pulses travel one behind the other as a sound wave.
• In sound propagation, it is the energy of the sound that travels and not the particles of the medium.
• Sound cannot travel in vacuum.

Oscillation : The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.

Periodic motion : Any motion which repeats itself after a fixed interval of time is called periodic motion.

All oscillatory motions are periodic motions but all periodic motions are not oscillatory motions.

Examples of oscillatory motion :

• Motion of a pendulum
• To and fro motion of a loaded spring.
• Vibrations of the wire of a stringed musical instrument.
• Motion of liquid contained in U-tube when it is compressed once in one limb and left to itself etc.

Compression pulses correspond to regions of high density and pressure, whereas rarefaction pulses correspond to regions of low density and pressure in the medium.

Some definitions :
(a) Displacement : The displacement of a particle in oscillatory motion at any instant is the distance of the particle from its equilibrium position at that instant.

(b) Amplitude : The maximum displacement of a particle in oscillatory motion an either side of its equilibrium position is called amplitude of its motion.

(c) Time period : The time required for a complete oscillation by a particle in oscillatory motion is called its time period. It is usually denoted by T and is measured in second (s).

(d) Wavelength : The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength (A,). It is measured either in centimetre (cm) or in metre (m).

(e) Frequency : The number of oscillations of the density of the medium at a place per unit time is called the frequency (v).
$$v=\frac{1}{T}$$ (V = frequency, T = time period)

Frequency is measured in per second (s-1), which is also known as hertz (Hz).
Relation among wavelength, frequency and velocity of a wave : In a medium, the distance travelled by the wave in unit time is called the velocity of the wave in the medium.

The number of complete vibrations made per unit time by a particle in the path of wave is called the frequency of the wave.
Velocity of sound = Frequency × wave length
i..e v = vγ

Reflection of sound : When sound waves hit on the boundary separating two homogeneous media, a portion of sound changes its direction from the surface of separation and returns to the first medium. This is known as reflection of sound.

Laws of reflection of sound :

• The incident sound, reflected sound and the normal on the surface of separation through the point of incidence remain on the same plane.
• The angle of incidence is equal to the angle of reflection.

Mach number : The ratio of speed of a body to the speed of sound is called mach number of the body. If the mach number of a body is greater than 1, then the velocity of the body is supersonic velocity.

Echo : When a sound after reflection from some reflector is again heard separated from the original sound, then this reflected sound is known as an echo.

Condition for echo : For occurrence of an echo the minimum distance between the source of sound and a reflector of sound producing echo should be around 16.6m.

Reverberation : The persistence of sound due to repeated reflection and its gradual fading away is called reverberation of sound.

Subsonic sound : We cannot hear the sound generated from the sources having frequency less than 20Hz. These sounds are known as subsonic sound or infrasonic sound.

Ultrasonic sound : We can not hear sound generated from the sources having frequency 20,000 Hz (20 kHz) or more. These sounds are known as ultrasonic sound.

Audible sound : We can hear only the sounds of the sources producing about 20 Hz to 20,000 Hz in frequencies. The sounds are known as audible sound.

Applications of ultrasound :

• Ultrasound is generally used to clean parts located in hard-to- reach places, for example spiral tube, odd shaped parts, electronic components etc.
• Ultrasound is used for drilling holes or making cuts of desired shape.
• Ultrasounds can be used to detect cracks and flaws in megtal blocks.
• Ultrasonic waves have given doctors powerful and safe tools for imaging human organs.

(a) Echocardiography is a technique in which ultrasonic waves, reflected from various part of the heart, form an image of the heart.

(b) Ultrasonography is routinely used to show image of patient’s organs such as the liver, gall bladder, uterus etc. to doctors.

(c) A technique called phacoemulsification (phaco stands for the lens of the eye) uses the power of ultrasound to break the hardened gel into tiny pieces.

(d) Ultrasound is also employed to break small ‘stones’ that form in the kidneys into fine grains.

• Sonar : The acronym SONAR stands for SOund Navigation AND Ranging. This is a method for detecting and finding the distance of objects under water by means of reflected ultrasonic waves.
• Note : A sound with a number of frequencies is called a note.
• Tone : A sound with a single frequency is called a tone.

Characteristics of Musical Sound :

• Loudness or intensity : Loudness is the measure of how intense or loud a sound is, loudness of a sound is measured by energy contained per unit volume of the medium through which sound passes.
• Pitch : The pitch is that physical cause by which we can distinguish a shrill sound from a flat one of same intensity. It depends on the frequency of sound.
• Quality or timbre : Quality is that characteristic of a musical sound by which we can distinguish a note sounded on a musical instrument from a note of same pitch and intensity produced.

Structure of human ear : The structure of human ear can be divided into three parts :

(i) Outer ear : The outer ear consists of pina and ear canal. Pina is the part which is visible from the outside. The function of the outer ear is to guide sound waves to the middle ear.

(ii) Middle ear : The middle ear is separated from the ear canal by a tightly stretched membrane called eardrum. Beyond it, three interconnected bones called the hammer, anvil and stirrup are situated.
The function of the middle ear is to pick up, amplify and transmit sound waves to inner ear.

(iii) Inner ear : The inner ear consists of a liquid-filled coiled tube called cochlea, which is shaped like a snail. This tube is connected to the stirrup. The cochlea has special sensory cells called hair cells, which are so called because of the hairlike structures that stick out of them. The hair cells are connected to the auditory nerve, which is connected to the brain.

Sound pollution : Any unpleasant and unwanted sound is noise. Sound pollution means creation of discomfort, disturbance and irritation which result to ill effects to mental and physical health.

Sources of sound pollution :

• The sound vehicle causes sound pollution.
• The machinery used in industry creates sound of intensity more than tolerance limit and thereby causes sound pollution.
• Loud-speakers, amplifiers, air horn played at top volume on the roads, are major sources of sound pollution.
• The sound crackers cracked in different functions, particularly marriages and Puja Festivals causes sound pollution.

Harmful effects of sound pollution :

• Sound pollution may cause hindrance in the growth of nervous system of a child in the womb, That is yet to be born.
• One may become deaf due to sound pollution.
• The sound of high intensity and pitch may cause increase of blood pressure, nervous breakdown, heart disease, loss of memory etc.

Remedial measures of sound pollution :

• Factories, smaller or big, should be prohibited in residential areas.
• Sound resistant measures may be taken in industry.
• Unnecessary use of horn in the vicinity of hospital, school etc. should be restricted.
• Airports should be away from residential area.
• Sound pollution awareness programme should be taken up at regular interval.