With the development of digital technology, plentiful sensors are embedded in the smartphone, which makes it a valuable tool for teaching elementary experimental physics. For instance, the magnetic field sensor of the smartphone has been used to measure gravitational acceleration by measuring the magnetic pendulum's oscillation period. The pressure sensor has helped explore the relationship between atmospheric pressure and altitude. The speed of sound in air can be easily measured using two smartphones with a sound sensor. Recently, Kim et al. put forward a smartphone "multiple tasking" method to analyze free-falling motion and measure the acceleration due to gravity; however, it involved a complex error analysis. To solve the problem, this experiment uses the acoustic stopwatch sensor of the smartphone, which can record the time of free-falling motion accurately.