SPL is determined by the volume displacement (Vd) of air it moves, so basically cone area (Sd) times excursion (Xmax, or x in eqns below) at a given frequency.
Assuming radiation in free space and no directionality, the equation for the sound pressure from a point source can be used:
w is the angular frequency in rad/s,
Q is the volume flow in m³/s,
rho0 is 1.2 kg/m³,
r is the distance in metres.
The volume flow Q is the derivative of volume, Q=jwV, and volume is displacement*surface V=x*Sd.
All in all this results in
if x represents the peak value of the displacement, so does p. If it is the RMS value of the displacement, the pressure will also be RMS, and possible to convert to sound pressure level, SPL, in dB by the equation Lp=20*log(|p|/pref), where pref is 20 µPa.
Simplified, if a given driver at a given excursion is producing a given SPL at 80Hz, it will need 4x the Vd at 40Hz and 16 x Vd at 20Hz for the same SPL. The reverse is true as you go up one or two octaves where it will be 1/4 and 1/16th Vd respectively.
This also does not factor in the apparent loudness changes from the Fletcher Munson curves.
Note as the maths above is for free space, ie the enclosure is suspended in free air well above the ground, and the enclosure is small enough that it radiates omnidirectionally, which is basically any dimension is less than 1/4 wavelength at the highest frequency of operation. Put it on the ground and the SPL will increase by 6dB as it's now radiating into 2pi or 1/2 space.
This is all effective for subs, but as you begin to get higher in frequency, the enclosure will become quite large relative to the wavelengths being produced, and directivity will be a factor and change the numbers some, so don't use it there unless you understand.
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