• Subwoofer DIY v1.1

• Sealed Systems
• Frequency Response Calculations
• Power Response Calculations
• Design Notes
• SEALED.XLS

• my home page

Vas = Equivalent air compliance (litres)
Qts = total Q of the driver at Fs
Fs = resonance frequency of the driver (Hz)

Choose a final Qtc (total Q of system at resonant frequency) for your design. Recommended values for Qtc are from 0.6 to 1.0. Transient response degrades with higher Qtc values, but the power handling of the system increases. A Qtc of 0.7 will usually give pretty good results, but you can use a higher figure if the subwoofer has a low resonant frequency (<20 Hz) or if it's being designed for car use.

Then,

Qr = Qtc/Qts
Vr = Qr^2-1
Vb = Vas/Vr
Fb = Qr*Fs
F3 = Fb*((1/Qtc^2-2+((1/Qtc^2-2)^2+4)^0.5)/2)^0.5

If Qtc>(1/2)^.5
then dBpeak = 20*log(Qtc^2/(Qtc^2-0.25)^0.5)
Else dBpeak = 0

where,

Vb = net box volume (litres)
Fb = box resonant frequency (Hz)
F3 = -3dB frequency (Hz)
dBpeak = maximum peak or dip in system response