How can I calculate how far sounds/frequencies travel in open air

how i can measure how far a sound will travel.
I have two subwoofers in my car and i want to know how far the low frequencies travel.

There is no definable cut off point, the sound decreases in intensity as distance increases, until it reaches the point where it is inaudible. That point depends on the other background noise, and also the surroundings, buildings, highway, etc.

Turn them on, close the doors and walk away and note where the noise is not audible.

Speed=frequency in to lamda