Speed of sound
Measuring the speed of sound with a smartphone
Author:
Title 4
Learning objectives :
This activity allows students to measure the speed of sound using two synchronized acoustic stopwatches. It reproduces a historic scientific experiment with modern tools.
Concepts covered
Sound propagation; Speed of sound waves; Differential timing; History of science; Metrology
What students will do :
The student uses two smartphones equipped with the FizziQ sound stopwatch to determine the time it takes for the sound to travel a known distance. After programming both devices to start and stop at the same sound signals the student separates them by a measured distance and then calculates the speed of the sound from the difference between the recorded times divided by the distance.
What is required :
Two smartphones with the FizziQ application; A clear space of at least 5 meters; A tape measure to measure distance; FizziQ experience notebook
Scientific background :
Measuring the speed of sound has fascinated scientists for centuries. In 1635, Marin Mersenne was the first to determine a reasonably precise value (around 316 m/s) by measuring the time elapsed between the observation of a cannon flash and the perception of the associated sound. This modern experiment reproduces this principle but with much greater precision thanks to digital technology. FizziQ's sound timer automatically triggers when sound amplitude exceeds a certain threshold, eliminating errors due to human reaction time. Both timers start simultaneously with the first clap, but stop at slightly different times during the second clap: the timer near the source stops immediately, while the one further away records a longer time, precisely corresponding to the sound propagation delay. This delay Δt, compared to the distance d between the two devices, makes it possible to directly calculate the speed of sound: v = d/Δt. Theoretically, the speed of sound in air depends mainly on temperature according to the formula: v = 331.3 + 0.606×T (m/s), where T is the temperature in °C. At 20°C, this speed is approximately 343 m/s. The accuracy of this experiment depends on several factors: 1) The accuracy of the distance measurement; 2) The sensitivity of the microphones and the consistency of their trigger thresholds; 3) The absence of parasitic sound reflections; 4) Atmospheric conditions (temperature, humidity, pressure). By carrying out several measurements and calculating their average, we can achieve an accuracy of around 1-2%, much better than that obtained by Mersenne. This experiment illustrates not only a fundamental physical phenomenon, but also the evolution of scientific methods: the principle has remained the same for four centuries, but the precision has considerably improved thanks to technological advances.