SICA 21 S 4 PL
Best box: 120.8 L bass reflex tuned to 42.9 Hz · F3 44.4 Hz
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Comparable 21.0" Drivers
Drivers with similar piston area, resonance and Qts — useful alternatives when the SICA 21 S 4 PL does not fit your target box volume or tuning. Each entry links to its full enclosure design and Thiele/Small parameters.
⚠️ Engineering Notice for Builders
- Large enclosure volume (120.8 L) requires extensive internal matrix bracing to prevent panel resonance.
Recommended Bass Reflex (Vented)
PORT BUILDABLE| Net Box Volume (Vb) | 120.8 Liters |
| Tuning Frequency (Fb) | 42.9 Hz |
| F3 (-3 dB Cutoff) | 44.4 Hz |
| F6 / F10 Extension | 38.9 Hz / 33.8 Hz |
| Port Inner Diameter | Ø 15.0 cm |
| Port Physical Length | 13.1 cm |
| First Pipe Resonance | 1313.0 Hz |
| Estimated MOL @ F3 | 124.1 dB SPL |
Recommended Sealed (Acoustic Suspension)
MAX TRANSIENT DAMPING| Net Box Volume (Vb) | 56.0 Liters |
| System Q (Qtc) | 0.707 (Butterworth) |
| System Resonance (Fc) | 77.5 Hz |
| F3 (-3 dB Cutoff) | 78.8 Hz |
| F6 / F10 Extension | 59.2 Hz / 44.6 Hz |
| Air Spring Linearity | High (Zero port compression) |
Thiele/Small Parameters
| Resonance (Fs) | 34.6 Hz |
| Total Q (Qts) | 0.3 |
| Electrical Q (Qes) | 0.309 |
| Mechanical Q (Qms) | 9.95 |
| Equivalent Volume (Vas) | 255.0 Liters |
| Diaphragm Area (Sd) | 1661.9 cm² |
| DC Resistance (Re) | 5.7 Ω |
| Efficiency Bandwidth Product (EBP) | 111.9 |
| Linear Excursion (Xmax) | 5.2 mm |
| Thermal Power (Pe) | 1200.0 W |
| Nominal Diameter | 21.0" |
| Catalog Source | Load Forge catalog (manufacturer datasheets and retailer listings) |
Frequently Asked Questions
What is the recommended ported (bass reflex) box volume for SICA 21 S 4 PL?
Load Forge's calculated alignment puts SICA 21 S 4 PL in a 120.8 liter vented enclosure tuned to 42.9 Hz, reaching an F3 cutoff of approximately 44.4 Hz.
Can SICA 21 S 4 PL be used in a sealed box?
Yes. In a sealed (acoustic suspension) enclosure of 56.0 liters with a Qtc of 0.707 (Butterworth), the system exhibits an F3 of 78.8 Hz.
What port dimensions should be used for SICA 21 S 4 PL to avoid port chuffing?
Sized from cone displacement (Sd × Xmax), a minimum port diameter of 15.0 cm with a length of 13.1 cm is suggested; check the port air velocity at your drive level in the Load Forge Studio.
Shareable Acoustic Card for Forums & Social Media
Embed this design card on DIYAudio, AVSForum, Reddit, or Facebook. It shows the calculated F3, MOL (when the power rating is published) and box parameters.
Not sure if 21 S 4 PL fits your physical space?
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