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Sound waves obey the same laws of reflection as those of light.
Sound waves obey the same laws of reflection as those of light.
Echo
Echo
- An echo is the repetition of the original sound because of reflection from some rigid surface at a distance from the source of sound.
- An echo is the repetition of the original sound because of reflection from some rigid surface at a distance from the source of sound.
- If we shout in a hilly region, we are likely to hear echo.
- If we shout in a hilly region, we are likely to hear echo.
- Why can’t we hear an echo at every place? At 220C, the velocity of sound in air is 344 m/s.
- Why can’t we hear an echo at every place? At 220C, the velocity of sound in air is 344 m/s.
- Our brain retains sound for 0.1 second.
- Our brain retains sound for 0.1 second.
- Thus for us to hear a distinct echo, the sound should take more than 0.1s after starting from the source (i.e., from us) to get reflected and come back to us.
- Thus for us to hear a distinct echo, the sound should take more than 0.1s after starting from the source (i.e., from us) to get reflected and come back to us.
- distance = speed × time = 344 × 0.1 = 34.4 m.
- distance = speed × time = 344 × 0.1 = 34.4 m.
- To be able to hear a distinct echo, the reflecting surface should be at a minimum distance of half of the above distance i.e 17.2 m.
- To be able to hear a distinct echo, the reflecting surface should be at a minimum distance of half of the above distance i.e 17.2 m.
- As velocity depends on the temperature of air, this distance will change with temperature
- As velocity depends on the temperature of air, this distance will change with temperature
Reverberation
Reverberation
- If the reflecting surface is nearer than 15 m from the source of sound, the echo joins up with the original sound which then seems to be prolonged.
- If the reflecting surface is nearer than 15 m from the source of sound, the echo joins up with the original sound which then seems to be prolonged.
- Sound waves get reflected multiple times from the walls and roof of a closed room which are nearer than 15 m.
- Sound waves get reflected multiple times from the walls and roof of a closed room which are nearer than 15 m.
- This causes a single sound to be heard not just once but continuously.
- This causes a single sound to be heard not just once but continuously.
- This is called reverberation.
- This is called reverberation.
- It is this the persistence of sound after the source has switched off, as a result of repeated reflection from walls, ceilings and other surfaces.
- It is this the persistence of sound after the source has switched off, as a result of repeated reflection from walls, ceilings and other surfaces.
- Reverberation characteristics are important in the design of concert halls, theatres etc.
- Reverberation characteristics are important in the design of concert halls, theatres etc.
- If the time between successive reflections of a particular sound wave reaching us is small, the reflected sound gets mixed up and produces a continuous sound of increased loudness which can’t be heard clearly.
- If the time between successive reflections of a particular sound wave reaching us is small, the reflected sound gets mixed up and produces a continuous sound of increased loudness which can’t be heard clearly.
- Reverberation can be decreased by making the walls and roofs rough and by using curtains in the hall to avoid reflection of sound.
- Reverberation can be decreased by making the walls and roofs rough and by using curtains in the hall to avoid reflection of sound.
- Chairs and wall surfaces are covered with sound absorbing materials.
- Chairs and wall surfaces are covered with sound absorbing materials.
- Porous cardboard sheets, perforated acoustic tiles, gypsum boards, thick curtains etc. at the ceilings and at the walls are most convenient to reduce reverberation.
- Porous cardboard sheets, perforated acoustic tiles, gypsum boards, thick curtains etc. at the ceilings and at the walls are most convenient to reduce reverberation.
Acoustics
Acoustics
- The branch of physics which deals with the study of production, transmission and reception of sound is called acoustics.
- The branch of physics which deals with the study of production, transmission and reception of sound is called acoustics.
- This is useful during the construction of theaters and auditorium.
- This is useful during the construction of theaters and auditorium.
- While designing an auditorium, proper care for the absorption and reflection of sound should be taken.
- While designing an auditorium, proper care for the absorption and reflection of sound should be taken.
- Otherwise audience will not be able to hear the sound clearly.
- Otherwise audience will not be able to hear the sound clearly.
- For proper acoustics in an auditorium the following conditions must be satisfied.
- For proper acoustics in an auditorium the following conditions must be satisfied.
1) The sound should be heard sufficiently loudly at all the points in the auditorium. The surface behind the speaker should be parabolic with the speaker at its focus; so that the distribution of sound is uniform in the auditorium. Reflection of sound is helpful in maintaining good loudness through the entire auditorium.
1) The sound should be heard sufficiently loudly at all the points in the auditorium. The surface behind the speaker should be parabolic with the speaker at its focus; so that the distribution of sound is uniform in the auditorium. Reflection of sound is helpful in maintaining good loudness through the entire auditorium.
2) Echoes and reverberation must be eliminated or reduced. Echoes can be reduced by making the reflecting surfaces more absorptive. Echo will be less if the auditorium is full.
2) Echoes and reverberation must be eliminated or reduced. Echoes can be reduced by making the reflecting surfaces more absorptive. Echo will be less if the auditorium is full.
3) Unnecessary focusing of sound should be avoided and there should not be any zone of poor audibility or region of silence. For that purpose curved surface of the wall or ceiling should be avoided.
3) Unnecessary focusing of sound should be avoided and there should not be any zone of poor audibility or region of silence. For that purpose curved surface of the wall or ceiling should be avoided.
4) Echelon effect : It is due to the mixing of sound produced in the hall by the echoes of sound produced in front of regular structure like the stairs. To avoid this, stair type construction must be avoided in the hall.
4) Echelon effect : It is due to the mixing of sound produced in the hall by the echoes of sound produced in front of regular structure like the stairs. To avoid this, stair type construction must be avoided in the hall.
5) The auditorium should be sound-proof when closed, so that stray sound can not enter from outside.
5) The auditorium should be sound-proof when closed, so that stray sound can not enter from outside.
6) For proper acoustics no sound should be produced from the inside fittings, seats, etc. Instead of fans, air conditioners may be used. Soft action door closers should be used.
6) For proper acoustics no sound should be produced from the inside fittings, seats, etc. Instead of fans, air conditioners may be used. Soft action door closers should be used.
Acoustics observed in nature
Acoustics observed in nature
- The importance of acoustic principles goes far beyond human hearing.
- The importance of acoustic principles goes far beyond human hearing.
- Several animals use sound for navigation.
- Several animals use sound for navigation.
(a) Bats depends on sound rather than light to locate objects. So they can fly in total darkness of caves. They emit short ultrasonic pulses of frequency 30 kHz to 150 kHz. The resulting echoes give them information about location of the obstacle.
(a) Bats depends on sound rather than light to locate objects. So they can fly in total darkness of caves. They emit short ultrasonic pulses of frequency 30 kHz to 150 kHz. The resulting echoes give them information about location of the obstacle.
(b) Dolphins use an analogous system for underwater navigation. The frequencies are subsonic about 100 Hz. They can sense an object of about the size of a wavelength i.e., 1.4 m or larger.
(b) Dolphins use an analogous system for underwater navigation. The frequencies are subsonic about 100 Hz. They can sense an object of about the size of a wavelength i.e., 1.4 m or larger.
Medical applications of acoustics
Medical applications of acoustics
(a) Shock waves which are high pressure high amplitude waves are used to split kidney stones into smaller pieces without invasive surgery. A shock wave is produced outside the body and is then focused by a reflector or acoustic lens so that as much of its energy as possible converges on the stone. When the resulting stresses in the stone exceeds its tensile strength, it breaks into small pieces which can be removed easily.
(a) Shock waves which are high pressure high amplitude waves are used to split kidney stones into smaller pieces without invasive surgery. A shock wave is produced outside the body and is then focused by a reflector or acoustic lens so that as much of its energy as possible converges on the stone. When the resulting stresses in the stone exceeds its tensile strength, it breaks into small pieces which can be removed easily.
(b) Reflection of ultrasonic waves from regions in the interior of body is used for ultrasonic imaging. It is used for prenatal (before the birth) examination, detection of anamolous conditions like tumour etc and the study of heart valve action.
(b) Reflection of ultrasonic waves from regions in the interior of body is used for ultrasonic imaging. It is used for prenatal (before the birth) examination, detection of anamolous conditions like tumour etc and the study of heart valve action.
(c) At very high power level, ultrasound is selective destroyer of pathalogical tissues in treatment of arthritis and certain type of cancer.
(c) At very high power level, ultrasound is selective destroyer of pathalogical tissues in treatment of arthritis and certain type of cancer.
Other applications of acoustics
Other applications of acoustics
(a) SONAR is an acronym for Sound Navigational Ranging. This is a technique for locating objects underwater by transmitting a pulse of ultrasonic sound and detecting the reflected pulse. The time delay between transmission of a pulse and the reception of reflected pulse indicates the depth of the object. This system is useful to measure motion and position of the submerged objects like submarine.
(a) SONAR is an acronym for Sound Navigational Ranging. This is a technique for locating objects underwater by transmitting a pulse of ultrasonic sound and detecting the reflected pulse. The time delay between transmission of a pulse and the reception of reflected pulse indicates the depth of the object. This system is useful to measure motion and position of the submerged objects like submarine.
(b) Acoustic principle has important application to environmental problems like noise control. The design of quiet mass mass transit vehicle involves the study of generation and propagation of sound in the motor’s wheels and supporting structures.
(b) Acoustic principle has important application to environmental problems like noise control. The design of quiet mass mass transit vehicle involves the study of generation and propagation of sound in the motor’s wheels and supporting structures.
(c) We can study properties of the Earth by measuring the reflected and refracted elastic waves passing through its interior. It is useful for geological studies to detect local anomalies like oil deposits etc.
(c) We can study properties of the Earth by measuring the reflected and refracted elastic waves passing through its interior. It is useful for geological studies to detect local anomalies like oil deposits etc.
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