SPL response curves (dB) for sealed, ported, and bandpass enclosures across 10–1000 Hz.
Box Parameters Comparison
Parameter
Description
Sealed
Ported
Bandpass
Units
Vb
Box volume
58.370
96.234
98.048
Liter (L)
Vf
Front chamber
N/A
N/A
42.677
Liter (L)
Vr
Rear chamber
N/A
N/A
55.371
Liter (L)
f3 / fl
-3 dB frequency
49.627
34.249
30.857
Hz
fh
Upper -3 dB freq
N/A
N/A
81.425
Hz
fb
Box frequency
49.123
24.000
50.125
Hz
Pl
Port length
N/A
35.188
14.654
Centimeter (cm)
Share:
How It Works
Subwoofer enclosure design uses Thiele-Small parameters (Vas, Qts, fs) from the speaker manufacturer to predict performance in sealed, ported, and bandpass boxes. The calculator computes optimal box volume, tuning frequency, port length, and -3 dB cutoff for each enclosure type so you can compare them side by side.
Example Problem
A 12-inch driver has Vas = 84 L, Qts = 0.38, fs = 28 Hz. With Qtc = 0.707 the sealed box needs about 42 L, giving an f3 around 40 Hz. A ported SBB4 alignment needs a larger box (~90 L) but extends the -3 dB point down to about 30 Hz.
Frequently Asked Questions
What Qtc should I use for a sealed box?
A Qtc of 0.707 gives the flattest (Butterworth) response with no bump. Values above 0.707 produce a slight peak that adds punch, while lower values roll off gently but need a larger box.
Which is better, sealed or ported?
Sealed boxes are simpler, have tighter transient response, and are more forgiving of build errors. Ported boxes extend bass lower and play louder but need precise tuning. Choose sealed for music accuracy, ported for maximum output.
What are Thiele-Small parameters?
They are a set of measurements (Vas, Qts, Qes, Qms, fs, etc.) that describe a speaker driver's electromechanical behavior. Speaker manufacturers publish them so designers can predict how the driver will perform in various enclosures without building prototypes.
How do I measure port length?
Port length is measured from the inner opening of the port to the outer opening. The calculator computes the required length for a given port diameter and tuning frequency. Use PVC pipe or precision port tubes for accurate results.