How is sound produced?

Sound waves are a very important part of most of our daily lives even though not all of us know how it is produced. First, let me show you that sound is produced through vibrations via an experiment.

Hold an inflated balloon to your mouth and speak into it. You if you put a finger on the other side of the balloon you can actually feel the vibrations but if you put your ear to the balloon it gets amplified a lot.

Now this proves that sound is made by vibrations, but the sound is not always made by a uniform vibration. For an example- take a pin and burst the balloon you just inflated. It is going to suddenly burst. But why not slowly deflate? Take a moment to think about this.

When we blow air inside the balloon the air is actually way more compressed than the outside air. When we prick the balloon only some of the air gets to go out while all of the air wants to (non-living beings don't have emotions- no offence AI but for the sake of the topic let it be) go out. Suddenly all the layers of air around the balloon get pushed back and such a noise is created. The same thing applies to firecrackers. 

Thus we can prove that any fast motion produces sound.

But when I swing a pendulum once really fast it does not make a sound. Why so? The human ear can only hear the sound of the frequency 20 to 20,000 (or 21 to 21,000 depending on the sources). Frequency is the number of oscillations (vibrations) in one second. 

Now another way to visualise this is by taking a small speaker (I used the side port of a JBL Charge 2) and putting a vibrating membrane with a mirror on top of it. You can make this by taking a tube like a container and tightly putting a membrane (in this case a balloon's top side). Stick a small mirror(preferably a circular) in the centre of the membrane. Then using a laser pointer put a beam of light on the mirror and play some music (some music with some vibration- I did it using 'Rudimental - These Days (feat. Jess Glynne, Macklemore & Dan Caplen)' which is nonexplicit by the way).

You can see the patterns forming on the roof where the laser light getting reflected.