Choosing a subwoofer enclosure is not a taste decision. It is a math decision. Your driver's Thiele-Small parameters tell you whether it wants sealed or ported, and your build goal decides whether bandpass is even on the table. Get those two checks right and the box you cut will sound the way the driver was engineered to sound.
Ported designs produce 3 to 6 dB more output than sealed with the same driver and amplifier across their tuned frequency range (SVS Sound, 2022). Bandpass adds another 5 dB on top of that, but only inside a narrow passband. The decision framework below walks through the three questions that get you to the right answer in under five minutes.
- Qts under 0.4 with EBP above 100 means ported. Qts between 0.4 and 0.7 with EBP under 50 means sealed (Audio Intensity, 2026)
- Ported gains 3 to 6 dB SPL over sealed across its tuned range, roughly equivalent to doubling amp power (SVS Sound, 2022)
- Sealed rolls off at 12 dB per octave below resonance. Ported rolls off at 24 dB per octave below tuning, twice as steep
- Cabin gain in a vehicle adds roughly 12 dB per octave below 80 Hz, which often makes sealed feel louder than spec sheets predict (JL Audio)
- Bandpass gains 5 dB efficiency over sealed but only inside a 1 to 1.5 octave passband, which is too narrow for daily music
Building from scratch? Our subwoofer enclosure design guide covers volume calculations, port tuning, joinery, and material selection for all four enclosure types in one reference.
The Three Questions That Decide the Box
Before you measure a single panel, work through these three questions in order. Each one narrows the field. By the end you should have one obvious answer, not three options to choose between.
- What is the driver's Qts? Below 0.4 the driver wants ported. Between 0.4 and 0.7 it wants sealed. Above 0.7 it was probably designed for free-air or infinite baffle, not a box.
- What is the driver's EBP? EBP equals Fs divided by Qes. Under 50 confirms sealed. Over 100 confirms ported. Between 50 and 100 the driver works in either, so the build goal breaks the tie.
- What is the build goal? SQ daily driver and tight transient response means sealed. Maximum bass per watt means ported. Single-frequency SPL competition is the only case where bandpass earns its complexity.
If question one and question two disagree, trust EBP. Qts is helpful as a first filter, but EBP is the more reliable guide because it factors in both the mechanical and electrical Q. If question three pulls a different direction than the first two, the math wins. A driver tuned for ported will not give you SQ in a sealed box no matter how badly you want it to.
What Each Enclosure Type Actually Does
The three box types are not variations on the same idea. They control cone motion in fundamentally different ways, and that is what gives each one its sound signature. Understand the mechanism and the spec sheet differences stop being abstract.
Sealed
A sealed box is airtight. The trapped air acts as a pneumatic spring that adds to the driver's mechanical suspension. That stiffer system has a higher resonance (Fc) than the driver's free-air Fs and a controlled rolloff of 12 dB per octave below it. The result is the cleanest transient response of any box type and the most forgiving over excursion behavior. Below resonance the air spring still resists cone motion, so the driver does not unload the way it does in a ported design.
Ported
A ported box adds a tuned vent. The port resonates at a target frequency (Fb) and acts as a passive radiator at and just below that point, reinforcing output the cone alone could not produce efficiently. Above Fb the box behaves much like sealed. Below Fb the port becomes acoustically transparent, the air spring vanishes, and cone excursion rises sharply with a 24 dB per octave acoustic rolloff. Build a ported box and you have to add a subsonic filter to protect the driver from over-excursion below tuning.
Bandpass
Bandpass places the driver inside the box on an internal baffle, with a sealed rear chamber and a ported front chamber. Sound only exits through the front port. That structure creates an acoustic bandpass filter: only frequencies inside the port's passband escape, with steep 24 dB per octave attenuation on both ends. The passband is 1 to 1.5 octaves wide, and inside it the driver is roughly 5 dB more efficient than sealed (Crutchfield). Outside it, you hear nothing useful.
How Thiele-Small Tells You Which Box
Every subwoofer ships with a Thiele-Small spec sheet. Three numbers on that sheet decide the box: Qts, Vas, and Fs. Use them in this order and you will not guess.
| Parameter | What It Tells You | Sealed Range | Ported Range |
|---|---|---|---|
| Qts | Total Q at resonance, the damping signature | 0.4 to 0.7 | 0.2 to 0.4 |
| EBP (Fs ÷ Qes) | Single best indicator of intended alignment | Under 50 | Over 100 |
| Vas | Equivalent compliance volume in cubic feet | 0.5 to 1× Vas | 1 to 2× Vas |
| Fs | Free-air resonance, the driver's natural pitch | Higher Fs is sealed-friendly | Lower Fs allows lower port tuning |
Sources: Crutchfield; JL Audio; Audio Intensity (2026)
The Vas number sets your box volume target. Sealed runs roughly half to all of Vas, while ported runs one to two times Vas to leave room for port tuning at the desired frequency. A driver with a Vas of 1.2 cubic feet wants a sealed box around 0.6 to 1.0 cubic feet, or a ported box from 1.5 to 2.5 cubic feet depending on your tuning target. Smaller than that in either direction over-damps the driver and robs efficiency. Larger than ported recommends and the cone unloads at the bottom end.
How Cabin Gain Changes the Math
Free-air physics is not the same as in-car physics. A vehicle interior is a small pressurized cavity, and at frequencies where the wavelength is longer than the cabin's longest dimension, the cabin reinforces output instead of letting it radiate away. The transfer function adds roughly 12 dB per octave of gain below 80 Hz (JL Audio), which is exactly the rolloff slope of a sealed enclosure.
The two slopes cancel. A sealed sub that measures down 12 dB at 30 Hz on a frequency response sweep in free air ends up effectively flat at 30 Hz inside the car. Ported drops at 24 dB per octave below tuning, so cabin gain only cancels half of it. That is why sealed often plays louder in the seat than its anechoic specs suggest, and why SQ competitors lean sealed for response shape predictability.
Pickups, hatchbacks, and SUVs all behave a bit differently. A sedan with a closed trunk loses some cabin gain through the rear deck. A regular cab pickup gains more pressurization than a crew cab. Box-tuning that ignores the vehicle is a guess. We measure cabin gain with an RTA in every install where the response shape matters.
Real-World Output Differences
The 3 to 6 dB output advantage of ported over sealed sounds like a small number. It is not. A 3 dB increase doubles acoustic power. A 6 dB increase quadruples it. Said another way, getting from sealed to ported output with sealed alone would require quadrupling your amplifier wattage and your sub's thermal handling.
Chart: Output Comparison Across Box Types (Same Driver, Same Amp)
Source: SVS Sound (2022); Crutchfield
That output advantage comes with tradeoffs. Group delay measures how long a signal takes to leave the box after the amp sends it. Sealed runs about 8 to 10 ms at 30 Hz. Ported runs 20 to 30 ms at the same frequency (JL Audio). The kick drum stays tight in sealed and gets a little fat in ported. SQ judges hear that difference. Most casual listeners do not, and prefer the higher output.
Common Mistakes That Sink the Build
Most box failures are predictable. Five mistakes account for the bulk of "this sounds wrong" questions we get from DIY builders.
- Picking the box type before reading the spec sheet. "I want ported because ported is louder" is the most common error. If your driver has a Qts of 0.55, ported will sound boomy and uncontrolled no matter how loud it gets.
- Building to the wrong volume. An undersized ported box raises tuning above target and chokes low-end output. An oversized sealed box drops Fc below ideal and turns the response soft. Stay inside the manufacturer's recommended Vb.
- Miscalculating port length or area. Ports under-sized for power compress at high SPL, then chuff. Our ported box design and tuning guide covers the full math, including the port-area-to-cone-area ratio.
- Skipping the subsonic filter on a ported build. Below tuning, the cone is unloaded and excursion goes vertical. A 24 dB per octave high-pass set 5 to 10 Hz below tuning protects the driver.
- Ignoring cabin gain. A box that measures flat in your driveway will not measure flat at the listening position in the seat. Always tune in-car with an RTA, not on a workbench.
Frequently Asked Questions
How do I know if my subwoofer needs a sealed or ported enclosure?
Which is louder, sealed or ported?
Is a bandpass box worth building?
How much bigger is a ported box than a sealed box?
Can the same subwoofer work in both sealed and ported boxes?
Need a Box Cut Right the First Time?
We CNC every Proline X enclosure on ShopSabre routers from 3/4" Langboard Elite MDF (48.5 lbs/ft³), with V-groove and dado joinery, double-layer baffles where the driver demands it, and threaded inserts matched to the bolt pattern.
If you have a driver picked out and want it loaded into a box tuned to its T/S parameters, contact us with the model number and we will spec the enclosure to match.
