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.
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.