Echo-based measurement of the speed of sound
  The speed of sound in air can be measured by popping a balloon next to a microphone a measured distance away from a large flat wall, digitizing the sound waveform, and measuring the time between the sound of popping and return of the echo to the microphone. The round trip distance divided by the time is the speed of sound. 
Alivia Berg and Michael Courtney
physics.pop-ph physics.class-ph