Scoop vs Reflex vs Horn Subwoofers
Scoop vs reflex vs horn subwoofers is a trade between efficiency, size and extension. The same 18" driver gives a different result in each because the box changes the acoustic load on the cone. A reflex box uses a port to extend the bottom end; a scoop is a short front-loaded horn; a folded horn uses a long path to raise efficiency, at the cost of size.
Bass reflex
An enclosed box with a port tuned to a set frequency (Fb). Around Fb the port carries most of the output and cone excursion falls to a minimum. Below Fb the driver is unloaded: the response falls at about 24 dB per octave, against 12 dB per octave for a sealed box, and the cone is held only by its suspension. That is why a reflex sub needs a high-pass filter just below Fb. See cabinet tuning.
- For: smallest box for a given extension, simplest build, accurately modelled from the driver's Thiele/Small data.
- Against: no horn gain, so more amp power and more boxes per dB.
- Driver: needs full T/S data to model the box, and enough Xmax because the cone does the work above Fb.
Scoop bin
A short, front-loaded horn: the driver fires forward into a curved or angled flare, with the rear of the cone in a sealed chamber. The flare raises the radiation resistance on the cone, so more of the amp's power becomes sound in the band the horn covers. The short path and modest mouth limit how low a single bin loads, which is why scoops are traditionally stacked in numbers.
- For: more output per watt than a reflex box across the horn's band; well-proven designs exist.
- Against: larger and heavier than a reflex box of similar extension; loading weakens below the horn's cutoff.
- Driver: horn loading favours a strong motor and low Qts. The sealed rear chamber and high cone pressure ask for a robust suspension and coil.
Folded horn and bass walls
A long horn path folded inside the cab. For full loading, a horn's mouth circumference needs to be roughly one wavelength at its cutoff frequency. At 40 Hz that is 343 / 40 = 8.6 m, a mouth of about 5.9 m² in free space. No single box is that big, so folded horns are designed to be stacked: adjacent mouths combine, and the floor and walls act as extensions of the horn.
- For: the highest efficiency of the three when built in enough numbers.
- Against: the largest boxes, hardest to build, and least forgiving of a driver that does not match the design.
- Driver: the same profile as a scoop: strong motor, low Qts, high thermal capacity.
The mechanism is covered in horn loading explained.
Side by side
| Bass reflex | Scoop bin | Folded horn | |
|---|---|---|---|
| Loading | Port resonance | Short front-loaded horn | Long folded horn |
| Efficiency | Lowest | Higher in the horn's band | Highest, in numbers |
| Box size | Smallest | Large | Largest |
| Roll-off below the working band | About 24 dB/oct below Fb | Loading falls below horn cutoff | Loading falls below horn cutoff |
| Build difficulty | Easy | Medium | Hard |
| Driver profile | Modelled from T/S, good Xmax | Strong motor, low Qts | Strong motor, low Qts |
Whatever the box, build to a published design and use the driver it specifies or one with close T/S figures. See 15 vs 18 inch subs for how cone size changes box size.
Which Wavemark driver
- WM18X451 (£345): made for scoops and folded horns. 1800 W RMS / 3600 W max, 115 mm (4.5") copper coil, ferrite motor, double spider, 8 Ω with a 4 Ω option. T/S data on request. 19.2p per rated watt.
- WM1850/8 (£275): 1800 W AES / 3600 W peak, 125 mm coil, vented ferrite motor, Sd 1244 cm², Xmax 12 mm (Vd 1493 cm³). Full T/S published (Fs 42.9 Hz, Qts 0.644, Vas 120.9 L), so you can model a reflex box before cutting wood. 15.3p per rated watt.
Both can be heard in our listening room by appointment.
More on horn drivers: choosing an 18" scoop driver.
Sources: reflex and sealed roll-off rates and horn mouth sizing are standard loudspeaker theory (L. L. Beranek, Acoustics; speed of sound 343 m/s at 20 °C). Wavemark WM1850/8 spec sheet and WM18X451 product data. Calculated values rounded.
Questions
Is a scoop bin a horn?
Yes, a short front-loaded one. The driver fires into a curved or angled flare with its rear in a sealed chamber. The short path raises efficiency in its band but limits how low a single bin loads.
Which is more efficient, a reflex box or a horn?
A horn, in the band where it loads the driver. A reflex box has no horn gain, so it needs more amplifier power and more boxes for the same level, but it is much smaller for the same extension.
Why are bass horns so big?
Full loading needs a mouth circumference of roughly one wavelength at the cutoff frequency. At 40 Hz that is about 8.6 m, a mouth of around 5.9 square metres in free space. Stacking boxes and using the floor and walls builds up that area.
What driver suits a scoop or folded horn?
A strong motor with low Qts, high thermal capacity and a robust suspension. Wavemark's WM18X451 is made for scoops and folded horns, with an 1800 W RMS rating, a 4.5 inch copper coil and a double spider.