Here is a procedure used in a middle school science class to find the speed of sound in air. A student takes two erasers and begins to bang then together with a regular beat - perhaps at a rate of 20 'claps' in 10 second - in front of the brick wall of the school building. The student begins walking backward until she can't hear the echo of the erasers anymore. Explain, in terms of the principles of waves explored in this Lesson, how this procedure is able to estimate the speed of sound in air