The nominal diameter is only a size category. Actual frame, cutout, mounting, and depth dimensions determine whether the driver fits the baffle; Thiele/Small parameters, the target bass alignment, and cabinet construction determine enclosure volume and final outside dimensions.
A full-range loudspeaker cabinet is not scaled directly from the cone diameter. The label “4 inch” is useful for product classification, but it does not specify the air volume behind the cone, the desired low-frequency response, the port tuning, or the panel thickness. Those decisions are made from the driver's measured electro-mechanical behavior and the intended use.
Figure 1: Markaudio CHR-70 Gen.3 full-range drivers.
1. Why a 4-Inch Label Does Not Determine Box Size
The driver's frame diameter sets a minimum baffle size. Its Thiele/Small parameters set the range of useful acoustic alignments. A sealed box, bass-reflex box, transmission line, and horn can therefore use the same driver yet have very different external dimensions.
The correct sequence is: choose the acoustic target, calculate or simulate the required net air volume, arrange that volume around the driver and port, then add construction thickness to obtain the external size.
Thiele's vented-box analysis showed that low-frequency behavior can be predicted from resonance frequency, equivalent compliance volume, and damping, while Small described a closed box as a second-order high-pass system. In practical design language, Fs, Vas, and Qts do not output one compulsory box; they allow a family of trade-offs among cabinet size, cutoff frequency, damping, efficiency, and excursion.[3][4]
2. The Driver Parameters That Matter
Markaudio publishes the following representative data for the CHR-70 Gen.3. Values should be checked against the datasheet supplied with the exact production version before a final build.[1][2]
| Parameter | CHR-70 value | How it affects the enclosure |
|---|---|---|
| Fs | 65.4 Hz | The driver's free-air resonance; it helps set the practical bass-alignment region. |
| Vas | 5.17 L | An equivalent air-compliance volume. It is an input to box calculations, not an automatic box-volume recommendation. |
| Qts | 0.55 | Total damping around Fs. Together with the target system Q or vented alignment, it strongly influences required volume and response shape. |
| Sd | 50.2 cm² | Effective radiating area; it combines with excursion to indicate low-frequency displacement capability. |
| Xmax | 4.3 mm one way | Linear travel limit used to check bass output and safe power, especially below a vented box's tuning frequency. |
| Nominal power | 20 W | A thermal/mechanical constraint. It does not independently determine cabinet volume. |
| Mechanical size | 124 mm frame, 112 mm mounting-hole circle, 102 mm cutout, 56 mm front-to-back depth; 69.5 mm motor diameter | Sets the minimum front-panel area, cutout, fastener positions, and rear clearance. |
Figure 2: CHR-70 mechanical dimensions. The drawing identifies 56 mm as the front-to-back dimension and 69.5 mm as the motor diameter; it also notes a 2 mm gasket that should be considered when routing a rebate. Verify the current production drawing before cutting the baffle.
3. From Acoustic Volume to Outside Dimensions
Step 1: Select the system type and response target
A sealed box needs a target system Q, usually written as Qtc. A bass-reflex design needs both net box volume Vb and tuning frequency Fb. These are acoustic choices: compactness, bass extension, maximum output, amplifier power, and intended placement must be considered together.
Step 2: Calculate net air volume
Net volume is the working air space seen by the rear of the cone. For a sealed enclosure, the ideal small-signal relationship is:
Fc = Fs × (Qtc / Qts)
This simplified relationship is useful for a transparent first calculation. It does not include leakage, damping losses, temperature, production tolerance, or the acoustic effect of stuffing.[3]
Step 3: Convert net volume to gross internal volume
The driver basket, magnet, port tube, braces, terminal cup, and any internal partitions displace air. A cabinet drawn to the net target without adding these items will finish undersized.
Step 4: Choose internal proportions and add panel thickness
Wout = Win + 2t; Hout = Hin + 2t; Dout = Din + 2t
The last line applies to a simple six-panel box with panel thickness t. Rebates, inset backs, doubled baffles, curved walls, and non-rectangular cabinets require their own construction drawing.
4. Worked Sealed-Box Example: Markaudio CHR-70
For a calculation that can be checked by hand, choose a sealed target of Qtc = 0.707. Using Vas = 5.17 L and Qts = 0.55:
Vb = 7.92 L net
Fc = 65.4 × (0.707 / 0.55)
Fc = 84.1 Hz
At Qtc 0.707, Fc is also the idealized -3 dB frequency for the second-order response. The result illustrates the trade-off clearly: the sealed box is manageable, but it does not provide the same nominal bass extension as a larger, tuned system.
For this geometry exercise only, reserve a combined 0.58 L illustrative allowance for the driver, brace, terminal cup, and other solid parts. This is not a Markaudio-published displacement value. It represents the Vdriver + Vbracing + Vhardware terms in the gross-volume equation above; before building, replace it with measured or CAD-derived displacements for the exact parts used.
One possible 18 mm panel layout
- Target net air volume: 7.92 L
- Gross internal volume: about 8.50 L, using the illustrative 0.58 L allowance
- Internal W × H × D: 170 × 244 × 205 mm = 8.50 L
- External W × H × D: 206 × 280 × 241 mm
This is a worked geometry example, not a Markaudio cabinet recommendation. The 206 mm baffle easily accommodates the nominal 124 mm frame, but the response still needs simulation and measurement with the actual driver, damping material, amplifier source impedance, and listening position.
5. Comparison with Markaudio's Official Vented Cube
Markaudio also publishes a vented-cube stand-mount drawing for the CHR-70.3. It uses an external cube of 268 × 268 × 268 mm, assumes 18 mm material, doubles the front and top baffles, and specifies a 35 mm diameter by 87 mm total-length vent. The drawing lists Fb = 50 Hz and a nominal anechoic F6 = 40 Hz.[5]
Figure 3: Markaudio CHR-70.3 vented cube. The plan shows how a lower tuning target, a vent, double panels, and damping produce a different exterior from the sealed calculation.
The marked inner dimensions imply roughly 10.6 L of rectangular gross interior space before subtracting the driver, vent, and other occupied volume. That inferred figure is not a printed net-volume specification. The important lesson is that the same CHR-70 parameters support more than one rational enclosure: the sealed example prioritizes simplicity and controlled roll-off, while the official vented design uses more space and a tuned resonator to extend bass.
6. Baffle, Port, Panel, and Proportion Checks
Baffle width is acoustic as well as mechanical
A wider baffle changes the transition from approximately full-space to half-space radiation and changes edge-diffraction timing. Rectangular proportions and an off-center driver can spread diffraction features, but the final on-axis and off-axis response should be measured rather than assumed from one rule of thumb.[6]
Port size must satisfy tuning and airflow
In a bass-reflex box, port area, effective length, box volume, and end correction interact. A smaller port can be shorter but may become noisy; a larger port reduces air velocity but may be too long to fit. Port turbulence, flow separation, compression, resonance, and clearance from internal walls also matter. This is why Fb and vent behavior should be simulated with the actual geometry instead of copied from an unrelated “4-inch speaker” plan.[7]
Panel thickness changes both inside and outside
If the target internal dimensions remain fixed, moving from 12 mm to 18 mm material adds 12 mm to each external axis. Doubling a front baffle adds still more depth. Bracing can allow a lighter panel to behave more rigidly, but the brace itself reduces net volume.
Cabinet proportions control internal modes
Two boxes can have equal volume but different resonant-mode distributions because rectangular-cavity mode frequencies depend on the three internal dimensions. Avoid making all three internal dimensions identical unless the design specifically addresses the resulting coincident modes. Driver position, lining, stuffing, and braces are part of the acoustic layout, not afterthoughts.[8]
7. What Must Be Verified Before Cutting Wood
- Confirm the exact driver version. Production data and measured T/S parameters can vary.
- Model the intended alignment. Check frequency response, impedance, group delay, cone excursion, and port velocity at realistic amplifier power.
- Build net volume from actual displacements. Include the driver, port, braces, terminal cup, and partitions.
- Check physical clearances. Leave room behind the magnet, around the terminals, and at both ends of the vent.
- Prototype and measure. Verify impedance tuning, near-field bass response, leakage, and audible port or panel noise before applying the final finish.
The driver parameters therefore determine the acoustic starting point, not the final silhouette by themselves. Outside dimensions appear only after the response target, topology, net air volume, occupied volume, baffle layout, and construction method have all been specified.
Frequently Asked Questions
Is Vas the recommended enclosure volume?
No. Vas describes the driver's suspension compliance as an equivalent volume of air. It is used with Qts and the chosen alignment to calculate or simulate Vb.
Can every 4-inch full-range driver use the same cabinet?
No. Drivers with the same nominal diameter can have different Fs, Vas, Qts, excursion, frame dimensions, and frequency response. Reusing a cabinet without modeling can change bass extension, damping, and safe output.
Which CHR-70 enclosure is better: sealed or vented?
Neither is universally better. A sealed design is simpler and rolls off more gradually. A properly modeled vented design can extend bass and increase output around tuning, but it requires more careful control of port geometry and excursion below Fb.
Does damping material count as solid displacement?
Porous lining or loose fill does not behave like a solid block, and it can change apparent acoustic compliance. Dense boards, braces, ports, and hardware do physically displace air and should be included explicitly.
Can I build directly from the 206 × 280 × 241 mm sealed example?
Treat it as an educational starting point. Verify your driver, material thickness, component displacement, damping, expected listening level, and measured response before committing to a finished pair.
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References
- Markaudio. “CHR-70.” Product description, parameters, images, and cabinet-plan index. https://www.markaudio.com/online_shop/ch/chr-70/
- Markaudio. “CHR-70A Gen.3” datasheet, linked from the manufacturer's product page. https://www.kjfaudio.com/wp-content/uploads/2016/12/CHR-70A_Gen.3_champ_Eng.pdf
- Small, Richard H. “Closed-Box Loudspeaker Systems—Part 1: Analysis.” Journal of the Audio Engineering Society, Vol. 20, December 1972, pp. 798–808. AES E-Library record.
- Thiele, A. N. “Loudspeakers in Vented Boxes: Part 1.” Journal of the Audio Engineering Society, Vol. 19, May 1971, pp. 382–392. AES E-Library record.
- Markaudio. “Vented Cube Standmount for Markaudio CHR-70.3.” Cabinet drawing, August 2024. Official drawing.
- Linkwitz, Siegfried. “Diffraction from Baffle Edges.” Linkwitz Lab. https://www.linkwitzlab.com/diffraction.htm
- Bezzola, Andri; Devantier, Allan; and McMullin, Elisabeth. “Loudspeaker Port Design for Optimal Performance and Listening Experience.” AES Convention 147, Paper 10311, October 2019. AES E-Library record.
- Smedley, Jack. “Sound Waves in a Cavity.” Physics 104 laboratory, Bates College. https://abacus.bates.edu/~jsmedley/phys104/cavity.htm
