Railways Physics Sound Sound

Introduction

• Periodic Motion

Any motion that repeats itself in equal intervals of time is called periodic motion. Aperiodic motion can be represented in terms of sines and cosines, so it is called a harmonic motion. The uniformly rotating earth represents a periodic motion that repeats itself at every 24 hours.

• Simple Harmonic Motion (S.H.M.)

Oscillatory motion in which the acceleration of the particle is directly proportional to the displacement and directs towards a fixed point in a direction opposite to displacement is called simple harmonic motion abbreviated as S.H.M. If a particle performs oscillatory motion such that its acceleration (a) and displacement (x) are related as below\[a\propto -x\], then the motion of particle is simple harmonic. An oscillatory motion is always periodic but a periodic motion may not be oscillatory.

Examples of S.H.M.

(i) clock pendulum,

(ii) oscillating liquid in a U-tube,

(iii) oscillating block in a liquid,

(iv) oscillating frictionless piston fitted in a cylinder filled with ideal gas, etc.

• Sound

Sound is a form of energy which produces a sensation of hearing in our ears.

• Source of Sound and its Propagation

A source of vibration (vibration means a kind of rapid to and fro motion of an object) is normally a source of sound. When we pluck a string of guitar or sitar or veena it produces sound. Similarly vibrations of wings of bee or mosquito. Sound is emitted by vibrating source and is transmitted through a material medium producing sensation of hearing in our ears. The motion of a vibrating source sets up waves in the surrounding medium.

• Sound Needs a Material Medium for its Propagation

In the absence of medium (air) around the source, sound is not being propagated and light (electromagnetic) waves travel through the vacuum.

Mechanical Waves

• A mechanical wave is a periodic disturbance which requires a material medium for its propagation. The properties of these waves depend on the medium so they are known as elastic waves, such as sound-waves, water waves, Waves in stretched string. On the basis of motion of particles the mechanical waves are classified into two parts.
• Transverse wave: When the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave, the wave is known as the transverse wave. For example, waves produced in a stretched string, waves on the surface. These waves travel inform of crests and troughs. These waves can travel in solids and liquids only.
• Longitudinal wave: When the particles of the medium vibrate along the direction of propagation of the wave then the wave is known as the longitudinal wave. For example sound wave in air, waves in a solid rod produced by scrabbing etc. These waves travel in the form of compressions and rarefactions. These waves can travel in solids, liquids and gases.

• Electromagnetic Waves

The waves which do not require medium for propagation are called electromagnetic waves. This means that these waves can travel through vacuum also. For example, light waves, X-rays, \[\gamma \]-rays, Infrared waves, radio waves, microwaves, etc. These waves of transverse type.

• Difference between sound waves and electromagnetic waves

(i) Sound waves are longitudinal and electromagnetic waves are transverse.

(ii) Sound waves travel at a speed of 340 m/s whereas electromagnetic waves travel at a speed of \[3\times {{10}^{8}}m/s\]

(iii) Sound waves do not pass through a vacuum but electromagnetic waves (light) do.

• Characteristics of Sound Waves

Sound is characterised by three parameters: (i) Pitch            (ii) Loudness       (iii) Quality

(i) Pitch: Pitch is the sensation (brain interpretation) of the frequency of an emitted sound and is the characteristic which distinguishes a shrill (or sharp) sound from a grave (or flat) sound.

(ii) Loudness: Loudness or softness of a sound wave is the sensation that depends upon its amplitude. The loudness of sound is a measure of the sound energy reaching the ear per second. The loudness of sound is measured in 'decibel dB’ The loudness of sound of people talking quietly is about 65 dB, the loudness of sound in a very noisy factory is about 100 dB,

(iii) Quality (Timber): Quality or timber of a sound wave is that characteristic which helps us in distinguishing one sound from another having same pitch and loudness. We recognize a person (without seeing) by listening to his sound as it has a definite quality. A pure sound of single frequency is called a tone. An impure sound produced by mixture of many frequencies is called a note. It is pleasant to listen.

• Reflection of Sound

When soundwaves strike a surface, they return back into the same medium. This phenomenon is called reflection. Laws of reflection of sound waves

(i) Angle of incidence is equal to the angle of reflection.

(ii) The incident wave, the reflected wave and the normal all lie in the same plane.

• Echo

Phenomenon of hearing back our own sound is called an echo. It is due to successive reflection from the surface of obstacles of large size.

• Conditions for the formation of Echoes

(i)  The minimum distance between the source of sound and the reflecting body should be 17.2 metres.

(ii) The wavelength of sound should be less than the height of the reflecting body.

(iii) The intensity of sound should be sufficient so that it can be heard after reflection.

• Reverberation

Persistence of sound after its production stopped, is called reverberation. When a sound is produced in a big hall, its wave reflect from the walls and travel back and forth. Due to this energy does not reduce and the sound persist. A short reverberation is desirable in a concert hall (where music is being played) because it gives 'life' to sound.

• Speed of sound

Speed of sound through any medium depends upon elasticity and density of medium.

(i) In solids, \[v=\sqrt{\frac{Y}{d}}\]

(ii) In liquids, \[v=\sqrt{\frac{B}{\rho }}\]

(iii) In gases, \[v=\sqrt{\frac{\gamma p}{\rho }},\]\[v=\sqrt{\frac{\gamma RT}{M}};\]\[\gamma =\frac{{{C}_{p}}}{{{C}_{v}}}\]