Sound And Ultrasound

Hold a finger against your throat and hum. What you feel is the whole of this topic in one sensation: something is shaking, and because it is shaking you can hear it. Every sound that has ever reached you began as a vibration and arrived as a vibration, carried across the room by air that squashes and stretches hundreds of times a second without ever travelling anywhere itself. Take the air away and the vibration has nothing to ride on, which is why the loudest explosion in space would reach you in complete silence.

This lesson takes that idea and turns it into physics you can calculate with. You will learn why a note is high or low, why it is quiet or deafening, and where the two limits of your own hearing sit. Then you will make sound do two things that every wave does: bounce off a hard surface, and spread as it comes through a gap. The first of those turns a clap and a stopwatch into a way of measuring how fast sound travels; the second explains why you can hear a conversation in the next room without being able to see anyone having it.