Horn Loading Explained
Horn loading explained in one line: a horn is an acoustic transformer. It matches the high mechanical impedance of a driver to the low acoustic impedance of air, so more of the cone's motion becomes radiated sound over the horn's working band. Wikipedia's summary puts the gain at roughly 10 dB more sound power from the same amplifier output. The cost is size, which is set by the lowest frequency you want.
Why a bare cone is inefficient
A direct radiator's reference efficiency follows from its T/S parameters: η0 = (ρ / 2πc) × Bl² × Sd² / (Re × Mms²). For B&C's 18SW115 (Bl 30.3 Tm, Sd 1210 cm², Re 5.3 Ω, Mms 275 g) that comes to about 1.8%. The rest of the amplifier power heats the voice coil. At low frequencies the cone is small relative to the wavelength, and the air presents a light load: the cone pushes it aside rather than driving it.
A horn raises the radiation resistance the cone sees, so a given motion transfers more acoustic power. That is why horn systems reach high sensitivity.
The parts of a horn
- Throat. The narrow end at the driver. In compression-loaded designs the throat area is smaller than Sd, raising pressure at the cone.
- Front and rear chambers. The front (compression) chamber between cone and throat forms an acoustic low-pass with the throat and sets the upper limit. A sealed rear chamber acts as a spring behind the cone and shapes the low end with the driver's parameters.
- Flare. How cross-section grows along the path: conical, exponential, hyperbolic or tractrix. The flare rate sets the lower cutoff.
- Mouth. For full loading, the classic criterion is a mouth circumference of about one wavelength at cutoff.
- Path length. Bass horns are long, so they are folded inside the box.
Cutoff and size
The numbers explain why full-size bass horns are rare. At 40 Hz the wavelength is 343 / 40 = 8.6 m. A circular mouth with that circumference has a radius of 1.37 m and an area of about 5.9 m². Below cutoff, loading collapses: output falls and the cone moves with little control.
Practical bass horns cut that size down:
- Boundaries. A floor mirrors the mouth, halving the physical area needed (about 2.9 m² for 40 Hz). A floor and wall quarter it.
- Stacking. Several horns side by side combine into one larger mouth. This is why horn stacks gain more than individual boxes suggest.
- Short horns. A short flare, as in a scoop, gives horn gain over a narrower band.
Bass horn types
| Type | How it works | Trade-off |
|---|---|---|
| Folded front-loaded horn | Driver fires into a long folded path to the mouth | High efficiency; large and heavy |
| Rear-loaded horn | Front of cone radiates directly, rear feeds a horn | Direct and horn output must be aligned in phase through the overlap |
| Scoop bin | Short front-loaded horn with a sealed rear chamber | Compact for its output; horn gain over a narrower band |
| Tapped horn | Both sides of the cone drive the same horn path | High output for its size over a limited band |
See scoop vs reflex vs horn, tapped horn drivers and how to build a scoop bin.
Horns for mids and highs
At higher frequencies the wavelengths are short and horns are small. A compression driver uses a light diaphragm, a phase plug and a small throat; professional combinations reach sensitivities of 105 to 112 dB/W/m. The horn's geometry also sets coverage, quoted as horizontal by vertical angles such as 90° × 40°.
What horn loading means for the driver
Inside the band, a horn reduces cone excursion for a given output, so the driver runs cooler and cleaner. Below cutoff the cone is unloaded and can be driven past Xmax, so horn and scoop cabinets need a high-pass filter set below the working band. See Xmax and excursion.
Horn bass favours a strong, well-damped motor, low Qts and a large coil for heat. The WM18X451 is built for scoops and folded horns: 1800 W RMS, 115 mm (4.5") copper coil, ferrite motor and double spider, at £345.
Safety
Horn cabinets are large and heavy. Lift with two or more people, use handles and wheels, and stack only on firm, level ground.
Sources: Wikipedia: Horn loudspeaker (10 dB figure, mouth circumference criterion); Wikipedia: Tweeter (105 to 112 dB/W/m); B&C Speakers published data for the 18SW115, checked October 2026; Wavemark WM18X451 and WM1850/8 product data. Efficiency relation: L. L. Beranek, Acoustics. Calculated values rounded.
Questions
What does horn loading do to a speaker?
It improves the coupling between the cone and the air over the horn's working band. Typical figures are around 10 dB more sound power from the same amplifier output than a cone radiating directly, plus control of dispersion.
Why are bass horns so big?
Full loading needs a mouth circumference of about one wavelength at the cutoff frequency. At 40 Hz that is 8.6 m, a circular mouth of about 5.9 m². Floors, walls and stacking reduce the physical size needed.
Is a scoop bin a horn?
Yes, a short one. A scoop is a front-loaded horn with a sealed rear chamber, giving horn gain over a narrower band than a long folded horn.
Do horn cabinets need a high-pass filter?
Yes. Below the horn's cutoff the cone is unloaded and can exceed Xmax at modest power, so a high-pass filter below the working band protects the driver.