How does sound really work

1. In electronic and electromechanical devices, a small diaphragm is vibrating back and forth. The traditional way to cause the vibration was to position a permanent magnet next to a an electromagnet, and then rapidly change the polarity in the electromagnet. This causes the two magnets to alternately attract and repel each other many times per second, causing a vibration. Another common technolgy, especially in small devices, is a "piezoelectric speaker"; it consists of a little crystal that slightly expands or contracts when you run electricity through it. By rapidly changing the current, you can make the crystal expand and contract many times per second, producing a vibration.

2. When you shake an object, it does set up vibrations in the air; but your ear can't detect vibrations that are slower than about 20 vibrations per second; that's a lot faster than you can shake something with your hands.

> "...digital sound IS a pure note. For example, an electric guitar tuner would make a pure note..."

Your example is correct; but these days the term "digital sound" refers to any sound generated by a digital device (which means basically any sound imaginable), and most of those aren't "pure" in the sense your teacher means. A "pure" sound is best described in terms of a graph of the sound wave. If the sound is "pure," its graph is a smooth curve called a "sine wave," that gently rises and falls. In contrast, something like a guitar note (whether produced by a guitar or simulated digitally) has a "bumpy" graph with peaks and valleys of various heights and depths (like the red curve here: (http://www.jimloy.com/physics/sum02.gif )); its graph represents the sum of the various different "pure" pitches contained inside a single guitar note.

1: What is vibrating in a electronic device is obviously the membrane of the speakers. They are being pulled and pushed away by the magnet on the back, to create the vibrations, that is the music or sound we hear from electric devices.

2: Why you don't hear it when you are waving hand is because you don't generate enough force trough impact with the air, for us humans to beeing able to hear it. I can't say for sure, but most likely a dog might be able to hear it. Let's say you slam your open hand onto a table, that will generate a frequency that we can hear.

We can only hear a small window of frequencies, therefore there are alot more sounds around us than we can hear. For instance, old people can't hear high frequencies very well.

Hope that helps.