Choosing an 18" Driver for Scoop Bins

Choosing an 18" driver for scoop bins means choosing for a horn, not a reflex box. A scoop is a short front-loaded horn with a sealed rear chamber, so the cone works into a heavier acoustic load than a direct radiator. What matters is motor strength, low Qts, thermal capacity and a suspension that stays centred, then whether it physically fits.

1. Motor strength: Bl and Bl²/Re

Bl (force factor, in tesla-metres) is the force the motor makes per amp. In a horn the driver is coupled to a larger air mass, and a strong motor keeps it controlled and converts more amplifier power into output. A useful comparison figure is Bl²/Re, which sets the motor's electrical damping. Two B&C 18s with the same Sd show the spread:

• 18SW115: 30.3²/5.3 = 173 N²/W
• 18TBW100: 26.4²/5.3 = 132 N²/W

Same cone area, 1.1 dB apart in sensitivity, but about 31% more motor damping on the 18SW115.

2. Low Qts

Qts is the driver's total damping at resonance. Horns supply the loading; the driver should be tightly controlled, not resonant. Horn and scoop designs generally use low-Qts drivers; higher values suit sealed and reflex boxes. For reference, the 18SW115 is 0.30 and the 18TBW100 is 0.39. The WM1850/8 is 0.644, which suits reflex boxes most directly; model it in a horn simulator before building a scoop around it. See Thiele/Small parameters.

3. Power and heat

Scoops run hard for long sets, and voice coil resistance rises as it heats, which costs output (power compression). Compare continuous ratings (AES or RMS), not peak or program, and note that standards differ: B&C quote "nominal" and "continuous", Wavemark the AES or RMS figure. A larger coil has more surface area to shed heat. See voice coils and heat and power ratings explained.

4. Suspension

Inside the horn's band, excursion for a given output is lower than in a direct radiator, but the suspension still has to keep the coil centred in the gap under sustained load. A double spider adds lateral stability against rocking. Below the horn's cutoff the cone is unloaded, so set a high-pass filter. See Xmax and excursion.

5. Fit

For existing bins, check before anything else:

• Cut-out diameter: WM1850/8 392 mm; B&C 18SW115 and 18TBW100 425 mm. Not interchangeable without work.
• Bolt circle: so existing T-nuts line up.
• Mounting depth: rear chambers are tight. WM1850/8 202 mm; 18SW115 242 mm; 18TBW100 241 mm.
• Impedance: two 8 Ω drivers in parallel give 4 Ω. See impedance and wiring.

More in will it fit: driver swaps.

6. Ferrite or neodymium

Neodymium saves weight per unit of motor strength: the 18SW115 is 11.9 kg. Ferrite costs less for the same motor and suits rigs that live in a van. See ferrite vs neodymium.

Power, build and price

The WM18X451 is the Wavemark driver built for scoops and folded horns: 1800 W RMS on a 4.5" copper coil with a double spider, at £345. T/S data is available on request for modelling. Both drivers can be heard in our listening room by appointment.

Sources: B&C Speakers published data for the 18SW115 and 18TBW100 (8 Ω), checked October 2026; Wavemark WM1850/8 spec sheet and WM18X451 product data. Rating standards differ between makers. Calculated values rounded.