Download class 8 ncert sound notes and more Study notes Physics in PDF only on Docsity!
SOUND
What is sound?
- Sound is a form of energy that travels as vibrations through air or another medium.
- Sources of sound How is sound produced?
- Sound is produced by vibrating bodies.
- The vibrating body can be anything , for example , a stretched animal hide (as in a drum or tabla ), stretched string (as in a guitar or sitar ) or an air column (as in a flute ).
- Simple pendulum – The pendulum consists of a weight is given a small push , it undergoes to – and fro movements that can be easily observed. These vibrations are also known as oscillations.
Characteristics of Vibrations
- The three important characteristics of vibrations are amplitude , frequency and time period.
- AMPLITUDE – The maximum extent of vibration of a vibrating body from its mean position is known as its amplitude.
- FREQUENCY – The number of vibrations experienced by a vibrating body in one second is known as its frequency. It is denoted by f. The SI unit of frequency is the hertz (Hz). 1 Hz is equal to one vibration per second.
- The multiple unit kilohertz (KHz) is equal to 1000 Hz.
- The time taken by the vibrating body to complete one vibration is known as its time period. It is denoted by T. • The number of vibrations made in 1 s is 1\T.
- The number of vibrations made in 1 s is the frequency f. • f= 1\T and T = 1\f • Time period and frequency are reciprocals.
CHARACTERISTICS OF SOUND
Sound can be characterized by loudness, pitch and quality.
Loudness
Sound can be loud or soft. If the string of a guitar is plucked gently, it produces a soft sound. If it is plucked harder, a louder sound is produced. Loudness depends on the following three factors:
- Amplitude of vibration.
- Area of vibrating body.
- Distance of listener from source of sound. AMPLITUDE OF VIBRATION : A vibration carrying more energy produces a louder sound. Such a vibration or wave has a larger amplitude. Loudness is proportional to the square of the amplitude. If the amplitude becomes twice ,the loudness increase by four times. Loudness ∞ Amplitude AREA OF VIBRATING BODY : The greater the area of the vibrating surface, the louder the sound produced. For example , a big drum produces a louder sound than a small drum. Smaller amplitude ---- soft sound Larger amplitude ---- loud sound DISTANCE OF LISTENER FROM SOURCE OF SOUND : The nearer the source, the louder the sound we hear. Sound waves lose energy as they travel, so the farther away we are from the source, the softer the sound we hear. The loudness of sound is measured in a unit called decibel (dB)
PITCH
- Pitch denotes the shrillness or flatness of sound.
- Sound can be high pitched or low pitched.
- Pitch can distinguish a shrill sound from a grave (hoarse) sound even though the two sound have the same loudness.
- The pitch of the sound produced by an object vibrating with a low frequency is low and the sound is called a grave sound.
- Low frequency ͢ low pitch ͢ Grave sound
- The pitch of the sound produced by an object vibrating with a high frequency is high and the sound is called shrill sound
- High frequency ͢ high pitch ͢ shrill sound
- A sound having a frequency of 512 Hz has a higher pitch than a sound of frequency 256 Hz.
- The position (high or low ) of a sound on a musical scale is related to its pitch.
- There are seven notes in Indian music : sa , re , ga, ma, pa, dha and ni. Each note has a higher frequency than the one before it, with ni being the highest note. • In western music , the seven notes do, re, mi, fa, so, la and ti are used.
QUALITY
long air column. The flute , shehnai are examples. Wind blow in through reeds are known as reed instruments. example : harmonium, mouth organ and nadaswaram
How do sound travels
Sound is a form of energy and is produced by a vibrating body. The vibrating body transfers its energy to the neighbouring molecules in the medium , which in turn pass on their energy to other molecules. Thus, sound travels in the form of waves and reaches our ears. This suggests that sound needs a material medium to travel in.
SPEED OF SOUND
During the thunderstorm , the flash of lightning is seen before the thunder is heard, although both occur at the same time and at the same place. similarly, when we watch a cricket match from afar, we see the bat hitting the ball before we hear the sound. These examples show that sound travels much slower than light. The speed of light is approximately 3,00,000,000 m/s ,or 3×108 m/s while the speed of sound in air at 0 °C is about 330 m/s. The speed of sound is greater in liquids and greatest in solids.
REFLECTION AND ABSORPTION OF SOUND
Reflection of Sound: When sound travels in a provided medium it hits the surface of another medium and returns back in some various way. This process is called a reflection of sound waves. There are things like a wall, a window that prevents sound from moving across them and these things reflect the sound. This is known as sound reflection. The reflection of sound is used in horns, megaphone and in shehnais. Applications of reflection of sound are an echo, hearing aid, soundboard etc. Absorption of Sound: It is defined as the tendency of material absorbs light. When the sound strikes the surface most of them are absorbed and some amount of sound energy is reflected back. There are things like sofa cover, the curtain which can absorb sound and this is known as sound absorption. These are required in concert halls, recording studios.
HOW WE PRODUCE AND HEAR SOUND
In humans, voice is produced by the larynx or voice box present in the upper part of the human body. The larynx is present near the upper end of the windpipe. Two vocal cords present across the larynx stretch in such a way that it leaves a narrow space between them for the passage of air. When human speaks, the muscles present in our larynx get stretched and the opening becomes narrower. When air is made to pass through the slit, the vocal cords vibrate. With more tightening of vocal
cords, vibrations in it increase and hence higher volume of sound is produced. This is the reason why the human voice has different types of pitches.
HOW WE HEAR SOUND
When the sound propagating through air, reaches your ear, you are able to hear it. Our ear is divided into three parts : Outer ear, Middle ear, Inner ear.
- Outer ear : It consists of a flap- like cartilaginous structure called pinna , one on each side of your face. It connects sound waves. The pinna is further connected to ear canal, at the end of which eardrum is present. Eardrum is a stretched membrane that separates the outer ear from the middle ear.
- Middle ear : It consists of a chain of three bones - hammer, anvil and stirrup. Hammer is connected to the eardrum and the innermost bone or stirrup is connected to the inner ear. These three bones are named according to their shapes and are collectively called ossicles. Middle ear is connected to the throat through eustachian tube. 3.Inner ear : The inner tube contains cochlea and semicircular canals. Cochlea is a spirallycolied tube and looks like a snail shell. It is filled with a fluid and has many hair cells inside it. The receptors send impulses through sensory neurons to the auditory nerve that connects inner ear to the brain.
WORKING OF THE HUMAN EAR
Pinna collects sound waves from the surrounding and sends them to the eardrum through ear canal. When sound waves fall on the eardrum, they make the eardrum vibrate. The three bones (hammer, anvil and stirrup) receive the vibrations from the eardrum and amplify them. Cochlea receives the amplified vibrations from three bones. The sensitive hair cells present in the cochlea convert them into nerve impulses. The nerve impulses are carried to the brain through auditory nerve. Brain converts nerve impulses into specific sounds.
Audible and Inaudible sounds
We know that a vibrating object produces sound. But, when you vibrate your hand forward and backward, you do not hear any sound. why? This is because our ears do not respond to sounds having frequency less than 20 Hz. Also, frequency higher than 20,000 Hz cannot be heard by human beings. The sound whose frequency lies between 20 Hz and 20,000 Hz that we are able to hear is called audible sound. so, the audible range of human ear is 20 Hz - 20,000 Hz. The sound (inaudible to humans ) having frequency less than 20 hz are called infrasonic sounds.
