### Theory:

In our daily lives, sound is extremely important. We primarily communicate with one another through sound. We are exposed to a wide range of sounds produced by various sources such as humans, animals, vehicle horns, and so on. As a result, understanding how sound is produced, propagated, and heard from various sources becomes indispensable.
Acoustics is a branch of physics that deals with the production, transmission, reception, control, and effects of sound.
Taking the effects of echo and reverberation into consideration in designing an auditorium and the interior of buildings is called acoustics. The time required to reduce the sound intensity to one-millionth of its initial value is called reverberation. This time plays a vital role in designing an auditorium. Generally, a reverberation time of $$0.5$$  is considered the optimum value for speeches, whereas, for a good acoustic effect in a music hall, a reverberation time of  $$2$$ to $$2.5$$ is considered. Reflecting surfaces like walls and ceiling are covered with good sound-absorbing materials, and the floor is carpeted. Curtains and sound-absorbing hangings are also used for a better acoustic effect. The technique or method to absorb undesirable sounds by the soft and porous surface is called acoustic protection.

In this lesson, we will study the reflection of sound waves, Echo and Doppler effect.

When you think about sound, the following questions come to mind: How is the sound produced? How does sound from various sources reach our ears? What exactly is sound? Is it a force or a source of energy? Let us take a look at all of these questions.

Touching a ringing bell

You can conclude that vibrations produce sound by touching a ringing bell or a musical instrument while producing music. The vibrating bodies produce sound in the form of waves, and they are nothing but sound waves.

Assume you are on the Moon with a friend. Will you be able to hear the sound produced by your friend?

Because the Moon is devoid of air, you will be unable to hear any sounds made by your friend. As a result, you understand that sound produced by the vibration of various bodies requires a material medium for propagation, such as air, water, steel, and so on. As a result, sound can propagate through a gaseous, liquid, or solid medium.
Reference:
https://pixabay.com/vectors/man-bell-school-bell-ringing-swing-311695/