Sound is that form of energy which needs medium to propagate and also make some-one to hear. We can hear different kind of sounds in our daily life like if we take common example of sound coming from a smart phone or television or from a running motor vehicle. When we clap sound is produced even when we slap sound is produced. So here is few important notes for sound chapter answering many of questions and getting many of your doubts cleared.
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Conservation of Energy
Production of Sound (or) how sound is produced?
- Sound is produced by vibrating objects or when any object vibrates, sound is produced.
For example, the sound coming from vibrating tuning fork, like a train running on a track or when a ball strikes a bat. In the mentioned examples sound is produced by these vibrating objects.
Propagation of sound (or) how sound travel?
- The matter of substance through which sound travels is called a medium. Medium can be of any form solid, liquid or gas. Medium acts as propagation of sound.
- Sound waves moves as a mechanical waves which are characterised by the motion of particles in a medium. Air is the most common medium through which sound travels.
- Sound is produced through a vibrating object in the form of Compression and Refraction.
Compression:
When a vibrating object moves forward it pushes and compresses the air in front of it, creating a region of high pressure, this region is called Compression.
Rarefaction
When compression starts to move backward or away from the vibrating object, it creates a region of low pressure, this region is called Rarefaction.
Sound needs a medium to travel
Sound is a mechanical wave and needs a material medium to travel, like air, water and gas. A particle of the medium in contact with the sound producing object is first displaced from it's rest position and exerts a force on the adjacent particle. After displacing the adjacent particle the first particle comes to rest or original position. This process continues in the medium till the sound reaches our ears.
Sound cannot travel through vacuum. To demonstrate this experiment has been mentioned in NCERT Science book.
Sound-waves are longitudinal waves
- Longitudinal waves are those waves in which particles of the medium move in a direction parallel to the direction of propagation of sound. The particles of medium do not actually move from one place to another but they simply oscillate back and forth about their position of rest.
This is exactly how a sound wave propagates and hence sound waves are longitudinal waves.
Note:- There is also another type of wave called transverse waves, in this the particles do not move along the propagation of wave but oscillates up and down about their mean position as the wave travels.
Characteristics of Sound Waves
A sound wave in a graphic form is represented, which shows how density and pressure change when the sound waves moves in the medium.
In the graph, compression are the region where particles are crowded together and represented by the upper portion of the curve, the peak represents maximum compression. It is the region where density as well as pressure is high. Similarly rarefaction are the regions of low pressure where particles are spread apart are represented by the valley.
Wavelength
Frequency
-Frequency tells us how frequently an event occurs or how may many times a event occurred in per unit time. Suppose you are beating a drum and how many times you are beating the drum per unit time is called the frequency of your beating the drum.
-The change in density of the particles of medium from the maximum value to the minimum value, and again to the maximum value, makes one complete oscillation. The number of such oscillations per unit time is the frequency of the sound wave. It is usually represented by Greek letter (nu). It's SI unit is Hertz (Hz).
Time-Period
- The time taken for one complete oscillation in the density of the medium is called the time period of the sound wave.
(or)
The time taken by the two consecutive compression and rarefaction to cross a fixed point is called the time period of the sound wave. It is represented by the symbol T. Its SI unit is second (s).
-Frequency and time-period are related as follows: F is proportional to 1/T.
Pitch
Amplitude
- The amplitude of a sound wave depends upon the force with which an object is made to vibrate.
- The magnitude of the maximum displacement in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter A. For sound it's unit will be that of density or pressure.
- The loudness or softness of sound is determined basically by it's amplitude. If we strike any wooden surface lightly we hear a soft sound and if we strike with high energy we will hear a loud sound.
- The sound which is more pleasant to hear is said to be of rich quality. A sound of single frequency is called a tone.
Speed
- The speed of a sound is defined as the distance traveled by a point on the compression and rarefaction per unit time.
We know, speed,v = Distance/time
v = lambda/T
Here, lambda is the wavelength of the sound wave. It is the distance traveled by the sound wave in one time period (T) of the wave..
Thus, v = (lambda)(nu) , where frequency (nu) = 1/T
That is speed = wavelength x frequency
Note:- In a given medium the speed of sound remains the same for all the frequencies under the same physical condition.
Intensity
- The amount of sound energy which passes through a unit area per second is called the Intensity of the sound. Even when two sounds are of equal intensity, we may hear one as louder than the other.
Speed of sound in different media
Sonic-Boom
When the speed of any object exceeds the speed of sound, it is said to be traveling at a supersonic speed. When a sound producing source moves with speed higher than speed of sound, it produces shock waves in air. These shock waves carry a large amount of energy, which produces a very sharp and loud sound called the "sonic-boom".
Reflection of sound
Like a rubber ball bounces off a wall after striking, same as sound bounces off after striking a wall or water surface and follows the same laws of reflection. Therefore for the reflection of sound waves to take place, an obstacle of large size which may be polished or rough may be needed. The direction in which the sound is incident or reflected make equal angles with the normal to the reflecting surface and the three are in the same plane.
Echos
The repeated hearing of sound near suitable reflecting objects such as tall buildings or mountains are called echos. Echos may be heard more than once because of successive or multiple reflections.
- the sensation of sound persists in our brain for about 0.1 s.
- to hear a distinct echo the time interval between the original sound and the reflected one must be at-least 0.1 s.
Reverberation
A sound which when produced in a big hall will persist by repeated reflection from the walls until it is reduced to value when the sound will no longer will be audible. This repeated reflection of sound which results in this persistence is called reverberation.
- In a big hall or auditorium reverberation is common and undesirable. To reduce this, the roof and walls off the auditorium are generally covered with sound absorbent materials like fibre-board, rough plaster or draperies. Even the seat materials of the hall or auditorium are selected on the basis of their sound absorbing property.
Uses of multiple reflection of sound
- Megaphones or loudspeakers, musical instruments such as trumpets and shehanais are all designed to send sound in particular direction without spreading it in all direction.
- We all know about stethoscope which is used to listen to sound produced within body. Using stethoscope the doctor listen to patient's heart beat, the sound of heart beat comes to doctor's ear after multiple reflection.
- Ceilings of auditoriums are curved so that audio reaches all corners of the hall.
Range of hearing
- The audible range of sound for human beings extends from 20 Hz to 20,000 Hz. Children under the age of five or dogs can even hear sound of up to 25 kHz (1 kHz = 1000 Hz). As people grow older, their ear become less sensitive to higher frequencies.
Infrasonic sound
Sound of frequency below 20 Hz are called infrasonic sound or infrasound. If we hear infrasonic sound we will hear the vibrations of the wings of a bee. Few more examples are, rhinoceroses communicate using infrasound, whales and elephants produce sound in infrasonic range.
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