Reflection of Sound and echo calculation method

Like light, sound also obeys the laws of reflection when sound is reflected from hard smooth surfaces. The following experiment is given below

Take two metal tubes and place them inclined to each other such that they point towards a vertical flat surface of a drawing board. At one end of the tube place a ticking watch. At the other end place your ear by adjusting the position we hear the maximum loudness of the sound.

Absorption of sound (sound insulation)

It has been found that shining objects like mirrors, metals or hard objects like buildings or stones reflect sound. However, materials with loose textures absorb sound. For example, curtains, gunny bags, straws, carpets, etc., absorb sound.

Big cinema half or auditoriums are carpeted and their walls are coated with some rough materials so that, they do not reflect any sound. It is because, such places are very big and therefore, echoes are formed. Thus, the quality of sound heard by people becomes poor.

However, in ordinary rooms in which we live, we do not have this problem. It is because they are seldom 17m or more long or wide. Thus, the original sound and reflected sound reach the ears almost at the same time, and hence, no echo is formed.

Echoes

If you stand in front of a high building or a hillock, such that you are at a distance of more than 20m and shout loudly, you will notice that you can hear your own sound. It is because the sound is reflected from the building. This hearing of reflected sound is called echo.

The human ear can hear two sounds separately only, if they reach the ear after an interval of 1/10 of a second. This is a natural feature of any human ear. The average speed of sound is 332 m s^-1. Thus, in 1/10 of a second, the sound will travel a distance of 332 m/s ÷ 1/10 so that s = 33.2 m.

This means if a sound that starts from a point, on reflection from some hard object comes back to the same point, then the distance between the reflecting body and the source of sound should be 33.2 m ÷ 2 = 16.6 m or 17 m. This hearing of a reflected sound from a reflecting body is called an echo.

For instance

If you have to address a small gathering inside a room everyone can hear you in your normal voice. However, if you address the same gathering in the open air, you have to speak loudly so that everyone can hear you.

It is because, when you speak inside a room, the sound waves spread through the air in all directions. Some sound waves reach our ears directly and some strike the walls of the room and get reflected.

These reflected waves also reach our ears almost at the same time. As more waves reach the ear, therefore, everyone is here, even if you speak softly. However, in the open, only direct sound waves reach the ear, as there are no reflected waves. Thus, the ear receives less energy, and hence, you have to speak loudly.

Characteristics of sound

The human ear distinguishes the characteristics of sound. These are loudness, pitch and quality or timbre where each refers to a sensation in the consciousness of the listener.

Loudness

It is easy to distinguish between loud and soft sounds. The term loudness of a sound is self-descriptive. It depends primarily on the amplitude of the sound wave.

Pitch

The pitch of a sound refers to whether it is high (sharp), like the sound of a violin, or low, like the sound of a bass drum. The physical quantity that determines pitch is the frequency.

Quality or Timbre

The property, due to which two notes of the same pitch and loudness produced by two different vibrating bodies can be distinguished, is called quality of sound.

This quality of sound is due to the formation of other notes which are produced due to the vibration of various parts of musical instruments and their shapes.