Determine the current Celsius temperature of the air. Find a shady spot and use your thermometer to take a reading. If you're using a thermometer that only gives readings in Fahrenheit degrees, use the following formula to convert your Fahrenheit reading to Celsius: Subtract 32 from your temperature reading, then divide the result by 1.8.
Calculate the velocity of sound wave travel given the current air temperature. Use the following formula: 331.5 + (0.6 x T) = V. Where T equal air temperature in Celsius degrees; and V equals velocity in meters per second. For example, at 22.2 degrees Celsius sound travels at roughly 344.8 meters per second because 331.5 + (0.6 x 22.2) = 344.82.
Convert your sound wave velocity from meters per second to feet per second (optional). Multiply the result in step two by 3.281.
Determine the time it takes for sound to echo back to you from your chosen location. Use your ultrasonic transmitter and receiver. Aim your ultrasonic transmitter at an object a short distance away; begin your stopwatch as you send out a pulse. Stop your stopwatch when your receiver registers the pulse echo. The pulse transmission range of your ultrasonic transmitter will depend upon its power capacity.
Calculate the distance. Distance equals velocity multiplied by time. But since you measured the time it took for sound to travel to one point and reflect back to you, you need to use the following formula: (Velocity x Time) / 2 = Distance.