Calculate Pre Delay by Speed of Sound

– Sound Travels at about 1 Foot (approx. 30 cm) per Millisecond (same for All Frequencies)

1) Time = Distance / Speed of Sound

2) and the Speed of Sound is approx. 333 Meters per Second (1000ms)

3) So if Sound Travels from Source to Listener for say 3 Meters, it will Take about 1/100 of a Second to Reach the Listener 

4) i.e. 333:3 = 111 (approx. 1/100 of a Second)