It is chosen somewhat arbitrarily, but it's not completely random: see e.g. http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wave%20properties/text/Mic

It is chosen somewhat arbitrarily, but it's not completely random: see e.g. http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wave%20properties/text/Microwave_ovens/index.html http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wa...