The sound

Do you know what sound is?

When you speak, people who are in various positions close to you can usually hear you.

You might try staying in the middle of the school yard, shouting, and checking the location of those who heard your shout.

This “scattering” of sound occurs because sound is a wave that propagates similarly to those that form on the smooth surface of a lake when a rock falls there. The big difference is that it propagates in space, in all directions.

Sound production is related to material vibrations: as we speak we vibrate our vocal cords; we vibrate the strings of a guitar when playing it, the "skin" of a drum is vibrated when we drum it, etc.

From sound sources to our ears vibrations produce waves that propagate in the material environment: solid, liquid and gaseous.

Sound moves air molecules and they hit each other, thereby transferring their energy to another molecule. The transmitted vibrations are called sound waves.

Sound waves are mechanical waves. Sound needs the medium (a, water, etc.) to be produced. Beyond the atmosphere, in space, silence is absolute, because in the vacuum (where there is no matter) sound does not propagate.

Every body capable of oscillating or vibrating has its natural frequency of vibration. This is because the body is made up of vibrating molecules. These molecules vibrating together determine a natural frequency of vibration in the body.

A bamboo stick, a cup, a bridge. All bodies have their natural frequency of vibration.

Now imagine what will happen if, near these bodies, a sound is emitted at exactly the natural frequency of vibration of the body? The vibration amplitude of the molecules is increasing, increasing, increasing… And we have the resonance.

What happens to the body as it resonates? If it is a rigid structure it will end up cracking!

Resonance is responsible for tuning to radio stations and heating food in the microwave oven.: Food molecules resonate with microwaves, increasing their thermal agitation and thus their temperature.