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Topics to be learn :
- Production of sound
- Propagation of sound and medium
- Frequency of sound Waves
- Sound and Music
- Sound produced by the humans
- Sound generation by a loudspeaker
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Sound : Sound is a form of energy which produces sensation of hearing in our ears.
Production of Sound : Sound is produced by the vibrations of an object such as a stretched membrane or a string under tension.
Production of sound by using a tuning fork :
Production of sound by using a tuning fork : A tuning fork is made of a metal. It has a stem and two prongs see fig.
- In below figure, fig.(a) shows the state of air in the absence of propagation sound through it.
- Equally spaced vertical lines show that the average distance between the air molecules is the same everywhere. It means average pressure and average density are the same in the regions A, B and C.
- Suppose the tuning fork is held vertical, its stem is fixed in a stand (not shown in the figure) and its prongs are struck lightly using a light hammer with a piece of rubber at the top. The prongs then start vibrating.
- Figure (b) shows what happens when the prongs move away from each other. The air outside the prongs is compressed (layers in the region A). The pressure and density increase in this region.
- Figure (c) shows what happens when the prongs move close to each other. The air molecules near the prongs move away from each other resulting in lower pressure and lower density. This region is called rarefaction.
- Meanwhile, the compression produced earlier moves forward, i.e., away from the prongs because the air molecules in this region transfer their energy to the air molecules in the region B producing a compression there. The periodic formation of compression and rarefaction results in propagation of sound waves away from the prongs. '
- Eventually, these sound waves reach our ears, the ear-drum vibrates, and we get a sense of hearing a sound as the specific signals reach the brain.
- Propagation of sound and medium
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Propagation of sound and medium : Sound needs a medium, such as air, for propagation.
- As sound travels through air, there occur changes in pressure and density in air.
- The region in which air is at high pressure and high density is called compression.
- The region in which air is at low pressure and low density is called rarefaction.
Sound needs a material medium for propagation- Prove :
Sound needs a material medium for propagation- Prove :
- Above figure shows part of the set up used in this experiment.
- A vacuum tight bell jar contains an electric bell connected to a power supply through the lid of the jar. The jar is placed on a smooth horizontal surface such as that of glass.
- Initially the vacuum pump is off and the jar contains air. The circuit containing the bell is completed using the key or the switch so that the bell starts ringing. This can be heard outside the jar.
- Then the vacuum pump is switched on so that it starts removing the air from the jar.
- We find that the level of ringing sound heard goes on decreasing as the quantity of air in the jar becomes less and less.
- When the pump is operated for a sufficiently long time interval, the quantity of air in the jar becomes so less that the level of ringing sound becomes very low; sound is hardly audible. But we can see the striker in the bell hitting the gong.
- By extrapolation, we conclude that sound generation and propagation needs a medium.
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Frequency : Sound travels in the form of waves. A wave consists of cycles of alternate compression and rarefaction. The number of cycles produced in the air (or any other medium) per second is called the frequency of the sound wave.
It is expressed in hertz (Hz). 1 hertz = 1 cycle per second or 1 vibration per second or 1 oscillation per second.
Sound and Music :
- In musical instruments like sitar and violin (string instruments), vibrating strings produce sound. In these string based instruments, the frequency of vibration of the string is changed by changing the tension on the string and/or by changing the vibrating length of the string using fingers. This results in generation of different notes.
- In musical instruments like flute and shehnai (wind instruments), vibrating air column produces sound. In musical instruments like flute, the holes on the flute are opened or closed to change the length of vibrating air column in the flute. The frequency of waves, therefore, changes and it results in the production of different notes. In flute, different notes can be generated by changing the way of air-blowing also.
- In musical instruments like Tabla (Percussion instrument), vibrating skin produces the sound. The greater the tension in the skin, the higher is the frequency of the sound. The greater the thickness of the skin, the lower is the frequency of the sound. -
- In the humans, sound is produced in the larynx.
Sound produced in the human larynx :
Sound production in the human larynx : In the humans, sound is produced in the voice box called the larynx.
- It is located at the upper end of the windpipe.
- Two vocal cords (chords) are stretched across it with a narrow slit between them for the passage of air (Fig.).
- When the lungs force air through the slit, the cords start vibrating. The frequency of the sound produced depends upon the length and thickness of the cords, and the tension in the chords.
- The frequency increases with the increase in tension and the more the length or the thickness of the cord, the less is the frequency.
- Muscles attached to the cords can make the cords tight (more tension) or loose (less tension).
Here, production of sound occurs due to vibrations of the stretched vocal cords (E chords in Anatomy).
[chord = a variant spelling of cord; used in Anatomy]
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Sound produced in a loudspeaker :
Sound produced in a loudspeaker : Figure shows the internal construction of a loudspeaker.
- Here, a coil is wound around a permanent magnet. The conical screen of the loudspeaker is attached to the coil.
- When a current is passed through the coil, a magnetic field is produced. Its interaction with the permanent magnet results in the back and forth motion of the coil.
- The frequency and the amplitude of the motion of the coil depends on the variation in the current through the coil.
- As the coil moves, the conical screen also moves back and forth. The vibrations of the screen produce sound waves in air.
- Very loud sound can be produced by changing the current.
Very loud sounds can be produced by using a loudspeaker. If sound level is around 100 decibels, the sound can be harmful to us.
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