The Subwoofer DIY Page
Ported Systems - Power Response Calculations
last updated: 13 April 2011
The Subwoofer DIY Page

To calculate the power response of a ported enclosure system, you will need to know the following:
Vas = equivalent compliance of driver (litres)
Qes = electrical Q of driver at resonance
Fs = resonance frequency of driver (Hz)
PEmax = maximum input power for driver
Fb = resonance frequency of the system
F3 = frequency at which response is down by -3dB
Dia = effective driver diameter driver (cm)
Xmax = peak linear displacement of driver cone (mm)

then,

Sd = pi*(Dia/100)^2/4
Vd = Sd*Xmax/1000
n0 = 9.64*10^(-10)*Fs^3*Vas/Qes
SPL = 112 + 10*LOG(n0)
K1 = (4*pi^3*Ro/c)*Fs^4*Vd^2
K2 = 112+10*LOG(K1)
Par = 3*F3^4*Vd^2
Per = Par/n0
PeakSPL = SPL+10*LOG(PEmax)

where,

pi = 3.14159265359
c = speed of sound in air (345 m/s)
Ro = density of air (1.18 kg/m^3)
n0 = free-air efficiency of driver
SPL = SPL of driver @1W/1M
Par = maximum linear power output
Per = electrical input required to produce Par
PeakSPL = Thermally-limited SPL in passband

Maximum output at a given frequency F can be calculated as
follows:

Fn2 = (F/Fs)^2
Fn4 = Fn2^2
A = (Fb/Fs)^2
B = A/Qts+Fb/(Ql*Fs)
C = 1+A+(Vas/Vb)+Fb/(Fs*Qts*Ql)
D = 1/Qts+Fb/(Fs*Ql)
E = (97/49)*A
dBmag = 10*LOG(Fn4^2/((Fn4-C*Fn2+A)^2+Fn2*(D*Fn2-B)^2))
Pmax = (K1/n0)*((Fn4-C*Fn2+A)^2+Fn2*(D*Fn2-B)^2)
        /(Fn4-E*Fn2+A^2)
SPLmax = K2+10*LOG(Fn4^2/(Fn4-E*Fn2+A^2))
SPLtherm = PeakSPL+dBmag

where,

SPLmax = displacement-limited SPL at frequency F (dB/1M)
SPLtherm = thermally-limited SPL at frequency F (dB/1M)

My thanks to Bill McFadden, for providing these equations for calculating the power response of ported systems.