Choosing a Driver for a Tapped Horn

Picking a driver for a tapped horn is less forgiving than for any other sub cab. Two 18s that look alike on a spec sheet can give very different results in the same fold. Use a driver the plan was designed around, or simulate the exact geometry with your driver's full parameter set before cutting wood.

How a tapped horn works

In a front- or rear-loaded horn, one side of the cone drives the horn and the other works into a sealed chamber. In a tapped horn both sides are used: the driver sits part way along the folded path, one face driving the throat and the other "tapping" in further along, near the mouth. The two outputs sum in the horn.

The result is high output from a compact box over a fairly narrow band, often with deep extension for the size. The costs: narrow usable bandwidth, a response that depends heavily on the driver, and excursion that climbs fast below the horn's lower limit. Background: horn loading explained and scoop vs reflex vs horn.

Why the driver matters so much

In a sealed or reflex box, small parameter differences give small response differences. In a tapped horn, the driver and the path form one tightly coupled system: path length, throat area, tap position and chamber volumes are all chosen around a particular driver. Change it and the peaks, dips and excursion curve all move.

Parameters that matter

• Fs: where the driver resonates relative to the horn's tuning shapes the low end.
• Qts: sets damping around resonance. A design optimised for one Qts range can turn peaky or weak with a driver outside it.
• Vas: suspension compliance expressed as an air volume; it interacts with the chamber volumes.
• Xmax: below the passband excursion rises steeply. Xmax and the high-pass filter decide how hard you can drive it.
• Bl: motor force against the air load the horn presents.

No single figure decides it; the combination in the specific design does. See Thiele/Small parameters and Xmax and excursion.

Simulate before you build

Hornresp, written by David McBean and developed openly on the diyAudio forum, is widely used for this; it models tapped horns directly and includes a tapped horn wizard. Enter the driver's parameters and the path geometry and it predicts response, excursion and impedance. Check:

1. Response across the band you actually want the sub to cover.
2. Excursion at your intended amp power against the driver's Xmax, especially below tuning.
3. The same design with the plan's recommended driver. If yours looks much worse, it is.
4. Where the high-pass filter has to sit to keep excursion inside Xmax.

Simulation is a guide, not a guarantee: leaks, panel flex, build tolerances and boundary placement all change the real result.

Follow the plan

Published tapped horn plans normally list the drivers they were developed with. That list is the designer's simulation and measurement work; stick to it unless you have simulated an alternative and accept the risk. Buy plans from the designer rather than copying them.

Wavemark drivers and tapped horns

We do not claim either Wavemark driver suits a specific tapped horn design. The WM18X451 (1800 W RMS, 115 mm copper coil, ferrite motor, double spider) is built for scoops and folded horns; its T/S data is available on request so you can run the simulation yourself. The WM1850/8's published figures are Fs 42.9 Hz, Qts 0.644, Vas 120.9 L, Sd 1244 cm² and Xmax 12 mm; compare them against the plan's drivers before considering it. Ask us for data.

Sources: Hornresp development thread by David McBean, diyAudio, checked October 2026; Wavemark WM1850/8 spec sheet and WM18X451 product data.