Sound, Doppler Effect, Amplitude Time Period & Frequency | Science & Technology for UPSC CSE PDF Download

Sound Waves: Sound is produced by vibrating bodies. The vibrations produced by a vibrating body produce waves in air.

Transverse and Longitudinal Waves: A wave motion is said to be tranverse if the particles of the medium oscillate about their mean positions in a direction perpendicular to that in which the disturbance or wave is travelling. The wave produced on the surface of water by throwing a stone on it is a transverse wave.
A wave in which the particles of the medium oscillate to and fro about their mean positions in a direction parallel to that in which the wave is travelling is known as longitudinal wave. sound wave is a kind of longitudinal wave.
In transverse waves, the particles of the medium pareallel to their mean position. All electromagnetic waves are transverse in nature.

Wavelength (λ): The wavelength is defined as the distance between the two successive particles which are at exactly the same point in their paths and are moving in the same direction.

Musical Sounds: Musical sounds are produced by periodic vibrations.

Sounds other than musical sounds are called noise and their wave function have no definite regularities. Musical sounds are characterised by three properties: pitch, loudness and quality.

Musical Scale: It consists of a series of notes whose fundamental frequencies have specified ratios. Conventionally, there are eight notes which fix an octave. The frequency ratio of the eighth and the first note is 2:1. The note of the lowest frequency is called the keynote. Musical instruments such as piano or harmonium have many octaves. The lingering of sound after the source has been cut off is called the phenomenon of reverberation. The duration for which the sound can be heard after the source has been switched off is called the reverberation time. If reverberation time is large, the successive syllables uttered cannot be distinctly heard.

This happens if the reverberation time is greater than about 0.8s. If the walls of the hall are curved then there can be focussing effect of sound and even standing waves are generated. These effects lead to distortion of sound. The study of factors which influence the quality of sound in a building helps to determine the acoustic architecture. The reverberation time can be reduced by providing sound absorbing surfaces on the walls and the ceiling. A room with zero reverberation time is called a dead room.

The acoustic properties of a hall are also improved by suitably placing reflectors and absorbers. Reflectors placed behind the speaker will improve the level of loudness inside the hall. Absorbers when placed at the far end of the hall from the speaker reduce the reverberation. If sound is continuously generated at some point in a hall the energy density of sound at any other point inside normally gets stabilized because of the balance between energy generation and energy dissipation due to absorption.
Echoes: Echoes are produced by the reflection of sound from a hard srface such as a wall or cliff.

Refraction of Sound

When successive layers of air have different temperatures, the ability of sound to travel faster in warm air than in cold air causes bending of sound waves. This bending of sound waves is called refraction. On a warm day, the air near the ground is warmer than the air above and so the speed of sound waves near the ground is higher. This causes bending of the sound waves away from the ground. On a cold day or at night, the reverse happens and the sound waves bend toward the earth. Thus on a cold day sounds can be heard over longer distances.

Sounds can be heard at abnormally long distances over water on quiet day. This happens because air next to water is cooler than air above and, therefore, sound waves bend toward the water and can travel long distances. 

The Doppler effect is the change in frequency of a wave (sound or light) due to the motion of the source or observer. The frequency (and hence pitch) of a sound appears to be higher when the source appraoches the listener and lower when the source recedes from him.
Doppler Effect

It is due to the Doppler effect that the whistle of a train appears shriller when it approaches a listener than when it moves away from him. The Doppler effect is very useful in astronomy. It can be used to find out whether a star is approaching us or receding away from us. It can also be used to detect or even measure the rotation of a star, e.g., the sun. The effect can be used to track a moving object, such as a satellite, from a reference point on the earth. The method is remarkably accurate; changes in the position of a satellite 108 m away can be determined to a fraction of a centimetre.

Sonic Boom A suspersonic (faster than sound) aircraft produces a cone of sound called a shock wave. When this shock wave reaches a listener, he hears a sort of loud explosion, called the sonic boom.

Amplitude, Time Period and Frequency of Vibration

Amplitude: The maximum displacement of a vibrating body from its mean position is called its amplitude. If we strike the tuning fork harder against a rubber pad, its prongs will bend more and therefore, the amplitude of the vibration will increase. Time Period: The time taken by a vibrating body to complete one vibration is called its time period.
Frequency of a vibrating body is the number of vibrations it makes in one second. Thus, the frequency of the sound is the frequency of the vibrating body which produces that sound.

Pitch and Loudness. Sounds of two sources differ from one anothe because they have a different pitch or a different loudness.

Pitch: A shrill sound is called a high pitch sound and a soft or less shrill sound is called a low pitch sound. Pitch depends on the frequency of the sound, i.e. the frequency of the vibrating source producing that sound.

Loudness: Loudness is a characteristic of sound which distinguishes a loud sound from a feeble or faint sound, both having the same pitch. The loudness of a sound depends upon the amplitude of the vibrating body producing the sound. The greater the amplitude, the louder will be the sound. If a tuning fork is gently striked against a rubber pad, it produces a faint sound. If we strike harder, it produces a louder sound. The frequency (or pitch) is the same in both the cases. In the second case, the sound is louder because the amplitude of vibrations of the prongs is greater than in the first case. 

Souncd from one source can be distinguished from the sounds of another sources on the basis of—

(a) pitch,which depends on the frequency of vibrations, and

(b) loudness, which depends on the amplitude of vibrations. 

Sounds Can Travel in Gases, Liquids and Solids: Sound can also travel in liquids. Sound cannot travel in the absence of a medium—solid, liquid or gas.

Speed of Sound. The speed of sound in air at 0°C is about 330 m/s. The speed of sound in water is about 1440 m/s and in steel it is about 5000 m/s. The reflection of sound can be used for measuring the depth of a sea at a certain place. The apparatus used for this purpose is called sonar. The apparatus sends out ultrasonic (high frequency) sound towards the bottom of the sea. The sound reflected from the sea-bed is received by the apparatus. By measuring the time taken by the sound to return. to the ship and knowing the speed of sound in sea water, the depth of the sea or ocean at that place can be determined. Materials like metallic sheets, sheets of hard plywood and smooth plastered walls are good reflectors of sound. On the other hand, porous materials such as sheets of cork or thermocole are bad reflectors of sound. They absorb most of the sound striking them.

The walls, ceiling and floor of a big auditorium or cinema hall are covered by sound absorbing materials. The audience do not hear the echoes because there is no reflection of sound from these materials, they hear only one sound coming directly from the source.

Resonance: When a body is maintained in a state of vibration by a periodic force of time period equal to its own free period, it is said to execute resonant vibrations and the phenomenon is called Resonance. Soldiers crossing a suspension on bridge are always asked to break step. If they start marching across the bridge in step, the frequencies of their step may agree with the natural frequency of the bridge. In that case, the bridge structure may set into violent and dangerous resonant vibrations and may even come crashing down. Several toys are constructed in such a way as to respond to a particular word of command. Their action depends upon the phenomenon of resonance.
Sometimes a brisk rattling sound appears in an automobile running at a particular speed but if the speed changes a little, the sound disappears. The sound is due to the resonance taking place between the car engines and the rattling object.