12. Study of Sound

Explanation: sound travels as longitudinal waves, in which the particles of the medium vibrate and form compressions and rarefactions. A vacuum has no particles, so no compressions and rarefactions can form and sound cannot travel. That is why astronauts on the moon, where there is no air, talk to each other by radio.

Explanation: the velocity of sound is highest in solids, lower in liquids and least in gases. At 25°C sound travels at about 5960 m/s in steel but only about 1498 m/s in pure water — roughly four times faster in steel.

Explanation: the flash of lightning and the sound of thunder are produced at the same moment. Yet we see the lightning first and hear the thunder a few seconds later, because light travels at about 3 × 10⁸ m/s while sound in air travels at only about 340 m/s.

Explanation: SONAR (Sound Navigation and Ranging) sends ultrasonic waves into the water from a ship. They are reflected by underwater objects and received back. From the time taken and the velocity of sound in water, the distance and direction of the object are found. It is used to find sunken ships, submarines, icebergs and the depth of the sea.

• In a large hall, sound from the stage or screen must reach every seat clearly and with nearly equal loudness.
• Sound gets reflected from hard surfaces according to the laws of reflection.
• A properly curved roof reflects the sound falling on it and spreads it evenly towards all parts of the hall, including the back rows.
• It also prevents repeated reflections between flat parallel surfaces, which cause echoes and reverberation.

Therefore the roofs of movie theatres and conference halls are made curved.

• In a closed, empty house, sound is reflected again and again by the hard walls, floor and ceiling.
• There is no furniture, curtains, carpets or people to absorb the sound. Soft materials are bad reflectors and absorb sound.
• Since the doors and windows are closed, the sound cannot escape either.
• So the reflected sounds reach us one after another, mix with the original sound and persist for a longer time.

Therefore reverberation is more intense in a closed and empty house.

• Our brain retains a sound for about 0.1 s. To hear a distinct echo, the reflected sound must reach us at least 0.1 s after the original sound.
• For this, the reflecting surface must be at least about 17.2 m away (at 22°C), so that sound travels at least 34.4 m to it and back.
• The walls of a classroom are much closer than this. The reflected sound returns in less than 0.1 s and mixes with the original sound, so it is not heard separately.
• Also, benches, students’ clothes and notice boards absorb part of the sound.

Therefore we cannot hear a distinct echo in a classroom.

An echo is the repetition of an original sound heard after it is reflected by a surface such as a hill, a tall building or a large wall.

Factors for a distinct echo:

• Time interval: the reflected sound must reach the listener at least 0.1 s after the original sound, since the brain retains a sound for 0.1 s.
• Distance: the reflecting surface must be at least about 17.2 m from the source (at 22°C, when sound travels at 344 m/s).
• Temperature: the velocity of sound increases with the temperature of air, so the minimum distance needed also changes with temperature.
• Nature of the surface: the surface should be hard and large, such as a rock, cliff or wall, so that most of the sound is reflected, not absorbed.
• Loudness: the original sound should be loud enough for the reflected sound to be heard.

The Gol Gumbaz (Golghumat) at Vijapur (Bijapur), Karnataka, is the tomb of Sultan Mohammed Adil Shah, built in the 17th century. It is a huge square hall covered by one of the largest domes in the world, with a circular ‘whispering gallery’ running around the base of the dome.

Reasons for multiple echoes:

• Large size: the dome is very large (its outer diameter is about 44 m), so the reflecting surfaces are far more than 17.2 m from the speaker. Each reflected sound returns after more than 0.1 s and is heard as a separate echo.
• Hard, smooth surfaces: the walls and the dome are made of stone and plaster, which are good reflectors and absorb very little sound.
• Curved dome: the circular, concave inner surface of the dome reflects sound again and again and sends it back towards the gallery and the hall, instead of letting it spread away.
• Empty hall: there is no furniture, carpet or curtain to absorb the sound.

Because of these continuous multiple reflections, a single clap or word is repeated several times (it is often said, seven times), and even a whisper at one side of the gallery can be heard clearly on the opposite side.

• Dimensions: the distance from any point in the classroom to the walls and ceiling should be less than about 17 m. Then the reflected sound returns in less than 0.1 s and merges with the original sound, so no separate echo is heard.
• Shape: a simple rectangular shape is best. Large bare dome-shaped or concave walls and ceilings should be avoided, because they focus reflected sound and can produce echoes.
• Materials: soft boards, curtains, and furniture help absorb sound and reduce reverberation.

Where: in auditoriums, cinema halls, conference halls, recording studios, music rooms, radio and TV studios, large classrooms and libraries, and in engine rooms of factories.

Which materials: thick curtains, carpets, soft boards, fibreboard and thermocol panels, perforated or false ceilings, cushioned seats, glass wool and jute or felt coverings on walls.

Why:

• Hard walls, floors and ceilings reflect sound many times, causing echoes and reverberation that make speech and music unclear.
• Soft and porous materials absorb sound instead of reflecting it, so reverberation and echoes are reduced.
• As a result, sound is heard clearly and pleasantly in every part of the hall.
• In studios they also keep outside noise out, and in factories they reduce noise pollution.

Answer: the velocity of sound will be 344 m/s at 20°C.

Answer: the lightning was 1360 m (1.36 km) away from Nita.

Answer: velocity of sound = 330 m/s; distance between the two walls = 1650 m.

Answer: sound travels faster in bottle A, twice as fast as in bottle B.

Answer: since the density in bottle B is 4 times that in A, the speed can be the same only if the temperature of B is 4 times the temperature of A (temperature on the absolute scale).

Jaltarang (‘waves in water’) is an ancient Indian melodic percussion instrument. It consists of a set of porcelain or metal bowls of different sizes, arranged in a semicircle in front of the player, each filled with a different amount of water. The player strikes the edges of the bowls with two thin sticks, and each bowl gives a musical note.

How different notes are produced: when a bowl is struck, the bowl and the water in it vibrate and produce sound. The frequency of this vibration, and so the pitch of the note, depends on the size of the bowl and the amount of water in it.

• A larger bowl, or a bowl with more water, vibrates more slowly (lower frequency) and gives a lower note.
• A smaller bowl, or one with less water, vibrates faster (higher frequency) and gives a higher note.

The player tunes the instrument by adding or removing water until each bowl gives the correct note (sa, re, ga, ma, pa, dha, ni). Striking the bowls in different orders produces a melody.