O Level Notes : Physics - Sound

Sound is a longitudinal wave. We have already studied about transverse and longitudinal waves  in one of the previous lectures on Waves.

Hey again everyone! Hope you are all good. This is an important topic which we are going to learn about today and that is sound.



Okay so have all been learning that sound is a form of energy. And how is this energy transmitted or passed? And the answer is: as a wave.

Sound is a longitudinal wave. We have already studied about transverse and longitudinal waves  in one of the previous lectures on Waves.


Transmission of Sound:

How is sound produced? Have you ever slammed a door when you are angry, or..or shouted? Probably yes. That’s exactly how sound is produced. In more scientific terms, it is produced by vibrating sources placed in a medium, usually air. Sound needs a medium to travel, it cannot be transmitted in vacuum. Why? Let’s get in detail.

As sound is a longitudinal wave, it travels in form of compressions and rarefactions. Compressions form high air pressure as molecules are close together while rarefactions form low air pressure as the molecules are far apart.

As sound is transmitted when air molecules vibrate, it cannot travel in vacuum as no particles are present to transfer the vibrations.

Sound travels at an approximate speed of 300 to 340 metres per second.

Task: Google the experiment to determine the speed of sound.


Reflection of Sound:

Echos refer to the repetition of a sound resulting from reflection of the sound waves.

An echo is formed when a sound is reflected off hard, flat surfaces such as a wall/cliff.

You would have witnessed it if you ever shout out loud in an empty hall or room. The laws of reflection are applied when sound is reflected.

There are some uses of echoes such as determining the depth of a sea. A pulse of sound is sent and the time interval before the reflected signal arrives is noted. We know the speed of sound, the time taken, the distance is then calculated.



Audible sounds are those which we are able to hear. Humans are only able to detect sounds of frequencies between 20 to 20 000 Hz. The sounds of frequency less than or higher than this cannot be heard by us.

The sounds which have frequency greater than 20 000 Hz are called ultrasounds while those of frequency less than 20 Hz are called infrasounds.

These are used in quality control and for medical applications. Search these for further details.


Pitch and Loudness:

Loudness is a factor distinguishing between various sounds. The larger the amplitude of vibration, the louder the sound.

Pitch is a factor distinguishing various sounds. The higher the frequency of a note, the higher the pitch.

So, loudness depends on amplitude while pitch depends on frequency.

Now in the above diagram you can see that the wave representing sound of higher amplitude is louder. While sound wave with higher frequency has a higher pitch. That’s the whole thing.