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.
Key Concepts
Thiele-Small parameters (Vas, Qts, fs) are the foundation of speaker enclosure design. Vas is the equivalent compliance volume — larger Vas drivers need bigger boxes. Qts (total Q factor) controls the damping behavior: low Qts drivers work best in ported enclosures while higher Qts suits sealed boxes. The f3 point (−3 dB cutoff) determines the lowest frequency the system reproduces at usable levels.
Applications
- Home theater: sizing a sealed or ported subwoofer enclosure to match room size and desired bass extension
- Car audio: fitting a subwoofer box into a trunk or custom enclosure while optimizing low-frequency output
- Professional sound: designing bandpass enclosures for PA systems that need high SPL within a narrow frequency band
- DIY speaker building: comparing sealed vs. ported vs. bandpass alignments before cutting wood
Common Mistakes
- Using online Thiele-Small parameters without verifying — manufacturer specs can vary by ±15% between production runs
- Ignoring port air velocity — undersized ports produce chuffing noise at high volume, but oversized ports waste box volume
- Building a ported box without accounting for driver and brace displacement — net internal volume must match the design, not gross box volume
- Assuming a bigger box always means deeper bass — beyond the optimal volume, response becomes peaky and transient response degrades
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.
Lindeburg, Michael R. 1992. Engineer In Training Reference Manual. Professional Publication, Inc. 8th Edition.