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# What is sound?
## Introduction: Why do we hear things?
Roughly speaking, sound is molecules bumping into each other back and forth. This back-and-forth oscillation creates a pressure wave that propagates through the medium it is in, such as air (or liquid or solid). And to create sound, something needs to vibrate.
When this vibration of air molecules reaches our ears, it vibrates a membrane in our ear called the eardrum. The vibrating eardrum then further vibrates three tiny bones in our middle ear, amplifying the sound. After that, the vibration reaches the organ called the cochlea in our inner ear, which is filled with fluids that move around. The motion of the fluid stimulates around 25,000 nerve endings, converting them into electrical signals. These signals are then passed to the brain for interpretation.
(To read more you can following this link [https://www.hopkinsmedicine.org/health/conditions-and-diseases/how-the-ear-works](https://www.hopkinsmedicine.org/health/conditions-and-diseases/how-the-ear-works))
The typical human ear, can perceive sound waves oscillating with frequency somewhere between 20 Hz to 20,000 Hz. However, this ability decreases with age. And different species of animals can perceive different ranges of sound frequency. Some breeds of dogs, for instance, can perceive up to 60,000 Hz. Hmm, wait, what does this Hz mean?
This rate of vibration is measured in **Hertz**, with the symbol **Hz**. One Hz is **one full cycle per second.**
For example, 600 Hz is 600 oscillations per second. In other words, a 600 Hz sound wave is describing molecules vibrating back-and-forth 600 times per second.
The typical house-hold AC outlet is operates at 60 Hz. That is, there is an alternating current going back and forth at 60 times per second.
For a long while, many movies are all shot and played at 24 frames per second. That is, to give the illusion of animation, 24 pictures are shown to you every second. That is, it has a refresh rate of 24 Hz.