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To Help Teachers and Students.

Talha's Physics Academy is an exploration environment for concepts in physics which employs free Physics Books and other linking strategies to facilitate smooth navigationThe entire environment is interconnected with thousands of links, reminiscent of a neural network.

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### Difference Between Echo and Reverberation

Like all waves, sound waves can be reflected. Sound waves suffer reflection from the large obstacles. As a result of reflection of sound wave from a large obstacle, the sound is heard which is named as an echo. Ordinarily echo is not heard as the reflected sound gets merged with the original sound. Certain conditions have to be satisfied to hear an echo distinctly (as a separate sound).

The sensation of any sound persists in our ear for about 0.1 seconds. This is known as the persistence of hearing. If the echo is heard within this time interval, the original sound and its echo cannot be distinguished. So the most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound dies off. As the speed of sound is 340 m/s, the distance travelled by sound in 0.1 second is 34 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 17 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly.

A reverberation is quite different than an echo. The distinction between an echo and a reverberation is depicted in the animation below.

A reverberation is perceived when the reflected sound wave reaches your ear in less than 0.1 second after the original sound wave. Since the original sound wave is still held in memory, there is no time delay between the perception of the reflected sound wave and the original sound wave. The two sound waves tend to combine as one very prolonged sound wave. If you have ever sung in the shower (and we know that you have), then you have probably experienced a reverberation. The Pavarotti-like sound which you hear is the result of the reflection of the sounds you create combining with the original sounds. Because the shower walls are typically less than 17 meters away, these reflected sound waves combine with your original sound waves to create a prolonged sound - a reverberation.

### Echo-Reflection of Sound (School Level)

Like all waves, sound waves can be reflected. Sound waves suffer reflection from the large obstacles. As a result of reflection of sound wave from a large obstacle, the sound is heard which is named as an echo. Ordinarily echo is not heard as the reflected sound gets merged with the original sound. Certain conditions have to be satisfied to hear an echo distinctly (as a separate sound).

The sensation of any sound persists in our ear for about 0.1 seconds. This is known as the persistence of hearing. If the echo is heard within this time interval, the original sound and its echo cannot be distinguished. So the most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound dies off. As the speed of sound is 340 m/s, the distance travelled by sound in 0.1 second is 34 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 17 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly.

Further, for reflection of any wave to take place, the size of the reflector should be large compared to the wavelength of the sound, which for ordinary sound is of the order of 1 metre. A large building, a mountain side, large rock formation etc. are good reflectors of sound for producing an echo. Also, for the reflected sound to be heard, it must have enough intensity or loudness. Moreover, if the echo is to be distinguished from the original sound the two should not mix or overlap. For this, the original sound should be of very short duration, like a clap or shout.

So, following conditions could be listed for formation of echo:

The size of the obstacle/reflector must be large compared to the wavelength of the incident sound (for reflection of sound to take place).
The distance between the source of sound and the reflector should be at least 17 m (so that the echo is heard distinctly after the original sound is over).
The intensity or loudness of the sound should be sufficient for the reflected sound reaching the ear to be audible. The original sound should be of short duration.

### Introduction to Nuclear and Particle Physics-A. Das and T.Ferbel

Book Description

This thoroughly revised book, now in its third edition, continues to provide a comprehensive introduction to this increasingly important area of nuclear and particle physics. It combines coverage of basic concepts, principles and applications, along with the latest developments.
Beginning with the historical developments of the subject, properties and constituents of the nucleus, quantitative facts about nucleus, etc., the book moves on to give insights into nuclear models, phenomenon of radioactivity and its applications in various fields, nuclear reactions including reactions in sun and stars, photoelectric and Compton effects, pair production, particle accelerators and types of radiation detectors. The text also presents an extensive discussion on elementary particles and their fundamental reactions, fundamental forces, conservation laws and the quark model. Besides updating and revising the existing text, the new edition amplifies several sections across the book for easy understanding of the topics discussed.
The book contains experimental data, a large number of solved problems, review questions and answers to unsolved problems.
The text is designed for the students of B.Sc. (Physics), though it can also serve as introductory review material for M.Sc. (Physics) students.