Description
The SB Acoustics SB21RDC-C000-4 is a 21 mm ring dome fabric tweeter, designed to combine the clarity of a dome with the dispersion control of a ring radiator. It’s a great option for speaker builders looking for smooth off-axis response, wide soundstage, and non-fatiguing treble, all in a compact, efficient design.
Features
- Non-resonant diaphragm design for minimum high frequency break-up
- Dual balanced compression chamber for improved dynamics
- Copper cap for reduced voice coil inductance and minimum phase shift
- Saturation controlled motor system for low distortion
- Non-reflective rear chamber with optimized damping for improved dynamics
- Flow optimized vented pole piece for optimum coupling to rear chamber
- CCAW voice coil for low moving mass
- Long life silver lead wires
- Low resonance frequency
Specs
- Nominal Impedance 4 Ω
- DC resistance, Re 3.1 Ω
- Voice coil inductance, Le 0.04 mH
- Effective piston area, Sd 4.6 cm2
- Voice coil diameter 20.4 mm
- Voice coil height 1.5 mm
- Air gap height 2.5 mm
- Linear coil travel (p-p) 1.0 mm
- Moving mass incl. air, Mms 0.25 g
- Free air resonance, Fs 760 Hz
- Sensitivity (2.83 V / 1 m) 90 dB
- Mechanical Q-factor, Qms 2.54
- Electrical Q-factor, Qes 1.64
- Total Q-factor, Qts 1.0
- Force factor, Bl 1.5 Tm
- Rated power handling* 40 W
- Magnetic flux density 1.02 T
- Magnet weight 0.13 kg
- Net weight 0.33 kg
* IEC 268-5, high-pass Butterworth, 2600 Hz, 12 dB/oct.
🔍 What’s a Ring Dome?
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Unlike a standard dome, a ring radiator doesn’t vibrate from a central apex. Instead, the annular ring moves and the center remains stationary.
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Result: Improved off-axis dispersion, more linear response, and reduced breakup modes.
✅ Why Choose the SB21RDC
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Smoother treble and off-axis response than many standard domes.
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Natural, airy highs – detailed without being harsh.
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Compact dome size with the footprint of a 1″ tweeter – great for bookshelf or MTM builds.
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Low Fs (~1.0 kHz) – good crossover flexibility, even below 2 kHz with proper slope.
🛠️ Crossover Guidelines
⚖️ Comparison: SB21RDC vs Other 21–26 mm Tweeters
| Tweeter |
SB21RDC-C000-4 |
SB21SDCN-C000-4 |
SB26STCN-C000-4 |
| Dome Type |
Ring Dome (Textile) |
Standard Dome (Textile) |
Standard Dome (Textile) |
| Fs |
~1.0–1.2 kHz |
~950 Hz |
~1.6 kHz |
| Dispersion |
Very wide |
Wide |
Moderate |
| Sensitivity |
~90 dB |
~90 dB |
~91 dB |
| Tonal Quality |
Airy, spacious |
Smooth, full |
Balanced, clear |
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