Monacor SPX-21M-4
Best box: 0.11 L sealed · Fc 253.6 Hz · F3 256.0 Hz
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Comparable 2.19" Drivers
Drivers with similar piston area, resonance and Qts — useful alternatives when the Monacor SPX-21M-4 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
- Limited linear excursion (Xmax: 1.2 mm); set a high-pass subsonic filter at 25–30 Hz when driven with high power.
Textbook Bass Reflex — not recommended (4.8 dB bass hump)
PORT BUILDABLE| Net Box Volume (Vb) | 1.5 Liters |
| Tuning Frequency (Fb) | 140.9 Hz |
| F3 (-3 dB Cutoff) | 131.5 Hz |
| F6 / F10 Extension | 125.5 Hz / 117.7 Hz |
| Port Inner Diameter | Ø 5.0 cm |
| Port Physical Length | 16.2 cm |
| First Pipe Resonance | 1062.0 Hz |
| Estimated MOL @ F3 | 94.0 dB SPL |
Recommended Sealed (Acoustic Suspension)
MAX TRANSIENT DAMPING| Net Box Volume (Vb) | 0.11 Liters |
| System Q (Qtc) | 0.707 (Butterworth) |
| System Resonance (Fc) | 253.6 Hz |
| F3 (-3 dB Cutoff) | 256.0 Hz |
| F6 / F10 Extension | 194.3 Hz / 147.1 Hz |
| Air Spring Linearity | High (Zero port compression) |
Thiele/Small Parameters
| Resonance (Fs) | 170.0 Hz |
| Total Q (Qts) | 0.47 |
| Electrical Q (Qes) | 0.523 |
| Mechanical Q (Qms) | 4.66 |
| Equivalent Volume (Vas) | 0.1 Liters |
| Diaphragm Area (Sd) | 13.0 cm² |
| DC Resistance (Re) | 3.74 Ω |
| Efficiency Bandwidth Product (EBP) | 325.2 |
| Linear Excursion (Xmax) | 1.2 mm |
| Thermal Power (Pe) | 12.0 W |
| Nominal Diameter | 2.19" |
| Catalog Source | Load Forge catalog (manufacturer datasheets and retailer listings) |
Frequently Asked Questions
What is the recommended ported (bass reflex) box volume for Monacor SPX-21M-4?
A simple ported box is not recommended for Monacor SPX-21M-4: the textbook alignment (1.5 liters tuned to 140.9 Hz) needs a 16.2 cm port, and no nearby retune stays buildable without a bass hump. A sealed box, a passive radiator or a folded duct is the practical choice.
Can Monacor SPX-21M-4 be used in a sealed box?
Yes. In a sealed (acoustic suspension) enclosure of 0.11 liters with a Qtc of 0.707 (Butterworth), the system exhibits an F3 of 256.0 Hz.
What port dimensions should be used for Monacor SPX-21M-4 to avoid port chuffing?
A straight 5.0 cm port would need 16.2 cm of length, which does not fit a practical cabinet. Use a passive radiator or a folded/3D duct instead of a straight tube.
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.
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