From another thread, you can calculate SPL from driver diameter and peak excursion. (I also divide by 4, 2, 1 and .5 to account for radiation space due to baffle and wavelength.):
p=Sd*Xmax*f^2*pi*rho0/(r*sqrt(2))
where p=sound pressure in Pa, Sd=eqv. piston area in m², Xmax=max peak excursion in m, f=frequency in Hz, rho0=1.2kg/m3, r=speaker-to-mic distance in m.
The sound pressure p can be converted to sound pressure level:
SPL=20*log10(p/0,00002) dB. Note that 0.00002 Pa = 0.0002 dynes/cm²
I had access to a laser vibrometer and tested the equation with an actual driver. The results were within 1 dB, which is easily within the error of my meter and setup.
What I don't understand is that the driver excursion is peak, one half of the peak to peak value, but the 0 dB reference is an RMS number, as is the SPL. How can unit be mixed like that? Can anybody explain why this equation works? Or should the driver excursion really be RMS?
p=Sd*Xmax*f^2*pi*rho0/(r*sqrt(2))
where p=sound pressure in Pa, Sd=eqv. piston area in m², Xmax=max peak excursion in m, f=frequency in Hz, rho0=1.2kg/m3, r=speaker-to-mic distance in m.
The sound pressure p can be converted to sound pressure level:
SPL=20*log10(p/0,00002) dB. Note that 0.00002 Pa = 0.0002 dynes/cm²
I had access to a laser vibrometer and tested the equation with an actual driver. The results were within 1 dB, which is easily within the error of my meter and setup.
What I don't understand is that the driver excursion is peak, one half of the peak to peak value, but the 0 dB reference is an RMS number, as is the SPL. How can unit be mixed like that? Can anybody explain why this equation works? Or should the driver excursion really be RMS?
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