1. R. Sutton, Demonstration Experiments in Physics (American Association of Physics Teachers, College Park, MD, 2003), p. 169.
2. Determining the speed of sound with stereo headphones
3. GCSE Physics, “Measuring Speed of Sound Using Echoes,” YouTube, https://www.youtube.com/watch?v=1wrD4JLgb1c.
4. This method is based on the Britain’s National Physical Laboratory’s experiment “Measuring the speed of sound using toilet rolls,” https://www.npl.co.uk/measurement-at-home/measuring-sound-using-toilet-rolls.
5. A Google search for free signal (tone) generator applications will yield many options.