ABSTRACT
Next we examine ways to describe the propagation of a wave through both time and space. This entails understanding how the spatial and temporal aspects are in a sense the same.
3.1 Speed of Propagation
Consider the propagation of a single pulse. As it moves through a medium, one can compute its speed in the usual way as the ratio of the distance travelled to the time required, that is,
f I distance travelled in time t speed o pu se = 1 (5)
3.1.1 Speed of Sound. It is fairly easy to estimate the speed of a pulse on a wire. In air we can get a good approximation as follows: If a source, such as lightening, produces both light and sound, then because light travels essentially infinitely fast (300,000 km/s) relative to sound (less than 1 km/s) we can neglect the time it takes the light to go from the source to the observer. So the time difference between the arrival of the light and the sound is an excellent estimate of the time it takes the sound to travel the distance. If we know the distance to the source, we can compute the velocity. Using such a technique, the estimated speed of a sound pulse in normal air at sea level and at 20° C is
I 344 m/s (meters per second). I This is roughly 1,000 ft/s or about one mile in 5 s. You should remember these numbers. Their exact values varies somewhat with both temperature and humidity.
