The way I did mine was like this:
1 cube box in my truck.
Played 20 to 70 hz, in 1 hz increments
Recorded each score
Placed the box outside, with the meter about 3 feet away.
Played 20 to 70 hz, in 1 hz increments.
Subtracted the ambient score from the in-car score. The frequency with the smallest difference is your resonant frequency.
In my case, it was around 44 hz. Now the exact note depends on my vehicle temperature, meter placement, and what setup I'm using. Funny thing about my truck is that it peaks in pretty much the same range no matter what setup i'm using.
Really, it just comes down to building and testing. You can do all this testing to find your resonant frequency, but there's far too many other variables for that to be considered the only method of testing you need to do.
Also, it is not uncommon for a vehicle to have two peak frequencies.
