Choose an enclosure by modeling the specific driver’s Thiele–Small (T/S) parameters against your desired bass response and available cabinet space—not by speaker diameter or a single Qts cutoff. Use the sealed-box equations as a first estimate, then compare sealed and vented alignments in a model that accounts for the relevant driver data and practical constraints.
Start with the exact driver’s T/S data
Thiele–Small parameters describe a driver’s low-frequency behavior and provide the starting inputs for enclosure design. Find the published specifications for the exact driver model, or use reliable measurements for that unit; do not transfer figures or a box recommendation from a similarly sized model. Kicker describes using measured and calculated or verified T/S data for enclosure design in its driver documentation.
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Three parameters are central to the simplified sealed-box calculation: Fs, the driver’s free-air resonance frequency; Vas, its equivalent compliance volume; and Qts, its total Q. Use the manufacturer’s definitions and units, especially for Fs and Vas. For a sealed enclosure, you also choose a target system Q called Qtc; it describes the combined driver-and-box system and is not the driver’s Qts.
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Qts can help identify candidate alignments, but it does not determine the enclosure by itself. MTX’s subwoofer guide gives Qts ranges of 0.3–0.9 for sealed designs and 0.1–0.40 for vented designs, along with Fs guidance. Those are MTX’s rules of thumb, not universal eligibility limits. The ranges overlap, and a value at or near a boundary is a reason to model the design rather than rule it in or out.
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Guides do not always give identical ranges, and Celestion cautions that simplified theory may predict an alignment unsuitable for a particular driver. Treat any parameter threshold as an initial screen; judge the candidate using the full modeled response and the application you have in mind.
Estimate a sealed-box volume
Celestion’s simplified sealed-box method uses Fs, Vas, and Qts, plus a chosen target Qtc. It defines the compliance ratio α and estimates net box volume Vb and the system’s -3 dB frequency f3 as follows:
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- α = (Qtc / Qts)² − 1
- Vb = Vas / α
- f3 = Fs × Qtc / Qts
Here, α is the ratio used in the box calculation, Vb is the estimated net enclosure volume, and f3 is the calculated -3 dB point for this simplified alignment. Keep units consistent: Vb follows the volume units used for Vas, while f3 follows the frequency units used for Fs. For a second-order Butterworth target, Celestion’s handbook uses Qtc = 0.7. That is a target choice for the calculation, not a required value for every sealed design.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallThe equations provide a first-pass estimate, not a complete cabinet design. They do not establish whether the result fits your physical constraints or suits your driver, room, and intended use. Celestion’s cabinet-design handbook warns: “Since not all drivers are suitable for a given alignment (design) predicted by this theory, the less experienced reader is cautioned not to build his own design solely on the basis of calculations taken from this theoretical approach.”
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Compare sealed and vented designs as complete systems
A sealed design is not automatically better or worse than a vented one. Compare plausible candidates using the factors that affect both the predicted result and the build:
- Response: How much low-frequency extension do you want, and what response shape suits the application?
- Cabinet space: Does the modeled net volume fit the space available?
- Vented-design details: What tuning frequency and port dimensions does the model require, and are those practical for the cabinet?
- Excursion and power: What does the model predict over the intended operating range?
- Model scope: Which driver parameters does the tool use, and are its response plots normalized or adjusted for room effects?
A vented design adds tuning and port decisions; a suitable driver Qts alone does not establish that the cabinet and port will work. Compare the modeled response and practical requirements together, rather than selecting a box from one parameter.
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Check what the calculator actually models
A cited enclosure calculator accepts Fs, Qts, and Vas, as well as Qes, Qms, Xmax, and Sd for broader modeling. It provides response, group-delay, and impedance plots, but describes the response as normalized and says room gain is not included. Those limits matter when interpreting the plots: a normalized response is not a prediction of in-room output, and a model’s available inputs and outputs determine what it can help you check.
Use the appropriate full inputs when evaluating response, excursion, and practical feasibility. Do not assume the short sealed-box equations check every one of those issues. Before building, verify the calculator’s assumptions and compare its predicted cabinet volume, tuning, and performance with the space and operating range you actually need.
A practical enclosure-selection workflow
- Identify the exact driver. Record its published or measured Fs, Vas, and Qts, and collect Qes, Qms, Xmax, and Sd if available for the model you will use.
- Set the design constraints. Define the low-frequency response you want and the cabinet volume you can accommodate.
- Estimate a sealed candidate. Choose a target Qtc and apply the simplified equations to estimate net volume and f3.
- Model alternatives. Compare sealed and vented candidates using a calculator with suitable driver inputs. For vented options, examine tuning and port dimensions as well as response.
- Check practical behavior. Review predicted excursion and power behavior across your intended operating range, and confirm that the modeled cabinet and port are buildable.
- Revisit the assumptions. Confirm that the model’s plots and stated limits answer the question you care about; in particular, do not treat normalized response as in-room response when room gain is excluded.
Once an alignment looks plausible, use its modeled net volume and other design details as the basis for the physical cabinet. The driver’s T/S data narrows the choices; it does not replace checking the finished design against your space and intended use.
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