[nitneylion452]

Originally Posted by electroken

The speed of sound (or ultrasound) varies with the density of air. That means temperature, humidity, and barometric pressure are all working against you in your quest for an accurate distance measurement.

But that wouldn't lead to a difference of 40cm. At 70 deg F, C is ~344 m/s, at 60 it is 340 m/s and at 80 it is 347 m/s. Not a notable difference. Also barometric pressure is fair to assume constant, unless he's somewhere significantly above or below sea level.