"Once again, grasshoppers, the "simplified" formula for determining the length of a Helmholtz quarter-wave chamber dealing with the elimination of a known resonance frequency is as follows:
The speed of sound in feet per second (say, 1072feet/second)
divided by
The frequency you wish to eliminate (say, 128cycles/second)
equals
8.375feet/cycle (numbers divide/seconds cancel out)
This is the length of a full wave form; much too big to work with under a vehicle.
To get a quarter-wave length, divide 8.373feet/cycle by 4 to get 2.09373feet/quarter-wave length.
Multiply 2.09373feet/quarter-wave by 12inches/foot to get the chamber length of 25.125".
Tube diameter does not affect frequency, only the amount of attenuation.
This worked back in 2009, and it still works today.
Be sure to use your own target frequency in the formula.
The speed of sound figure is not going to vary enough to worry about. We're not rocket scientists.
REMEMBER: Be sure to use your own target frequency in the formula."