Designing a Crossover for a Bi-Radial Horn
A bi-radial horn can produce exceptional dynamics, wide controlled dispersion, and remarkably low distortion when its acoustic behavior is matched to the right compression driver. The crossover is the point where those advantages either become coherent musical performance or are compromised by poor integration.
This case study follows a representative high-efficiency loudspeaker using a TAD-Pioneer compression driver, a bi-radial wooden horn, and a large woofer in a heavily braced birch plywood cabinet. The objective is not to prescribe one universal circuit, but to show how a passive crossover can be developed from acoustic evidence rather than from nominal driver specifications alone.
A successful network must manage frequency response, directivity, phase, sensitivity, impedance, and power handling at the same time. It also has to preserve the immediacy that makes horn-loaded loudspeakers so compelling. That requires a methodical balance between measurement, simulation, cabinet construction, and extended listening.
Start With The Acoustic Targets
The first decision is the intended handover region. A large compression driver may operate safely lower than a smaller diaphragm, but the horn’s flare geometry and acoustic loading remain decisive. A bi-radial horn controls horizontal and vertical dispersion differently, so its useful bandwidth is defined by coverage consistency as much as by the driver’s advertised response.
For a representative two-way system, the crossover target might fall near 650–900 Hz. The exact point depends on the horn mouth, throat adapter, woofer diameter, and desired listening distance. Crossing too low can expose the compression driver to excessive excursion and distortion. Crossing too high can make the woofer directional before the horn has taken over, producing an uneven power response through the crossover region.
Driver history also matters. The development of modern high-frequency compression drivers is closely tied to cinema systems, where efficiency, controlled coverage, and reliability were essential; this compression driver history provides useful context for understanding why these components behave differently from conventional dome tweeters.
Build A Useful Measurement Model
The raw frequency response is only the beginning. Each driver should be measured in the finished cabinet, with the actual horn, throat components, mounting hardware, damping, and grille arrangement in place. A woofer measured in free air will not reveal the effects of its enclosure, while a compression driver measured without its final horn will provide misleading crossover information.
Measurements should include on-axis response, a horizontal and vertical listening window, impedance, and distortion at several output levels. Gated measurements can establish the midrange and treble response, while nearfield and merged measurements help describe the woofer at lower frequencies. The goal is to identify the region where both acoustic sources are smooth enough to combine.
The acoustic centers of the woofer and compression driver must also be recorded. In a physically deep horn system, the high-frequency diaphragm may sit substantially behind or ahead of the woofer’s acoustic origin. That offset creates a frequency-dependent phase difference. A passive network can compensate for some of it through filter slope and polarity, but cabinet geometry and driver placement should solve as much of the time alignment as possible before components are selected.
Shape The Filter Around The Horn
A textbook electrical filter does not automatically produce a textbook acoustic filter. The driver’s natural roll-off, horn gain, impedance curve, and rising response all become part of the final transfer function. For that reason, a second-order electrical filter may result in a fourth-order acoustic slope, while a nominal fourth-order network may be excessive once the natural behavior of the driver is included.
A practical case study might begin with a low-pass network for the woofer and a high-pass network for the compression driver, followed by a small equalization section. The high-pass circuit protects the compression driver below the intended operating range. A shaping capacitor, resistor, or parallel notch can then reduce a broad horn or diaphragm feature without forcing the main filter to do all the work.
The values below illustrate the design logic rather than a finished production schematic. Real component values must be calculated from measured impedance and then verified in the cabinet.
| Section | Primary role | Typical acoustic aim | Main design concern |
|---|---|---|---|
| Woofer low-pass | Limits upper-band output | Smooth handover into the horn | Cone breakup and directivity |
| Compression-driver high-pass | Protects the diaphragm | Controlled slope near the crossover | Excursion and power handling |
| Attenuation network | Matches sensitivity | Preserve system balance | Resistor noise and heat |
| Notch or contour circuit | Reduces a defined peak | Flatten the combined response | Interaction with impedance |
| Zobel or impedance correction | Stabilizes load behavior | Make filter action predictable | Extra parts and losses |
| Polarity and delay choice | Optimizes summation | Maximize lobe coherence | Acoustic center offset |
Sensitivity matching is especially important in a horn system. The compression driver may produce significantly more output than the woofer for the same amplifier voltage, so an L-pad or series attenuation network is often required. A simple resistor can alter the filter’s expected behavior by changing the impedance seen by the inductors and capacitors. The attenuation circuit therefore has to be included in the simulation from the start.
Control Phase And Time Alignment
The best amplitude response can still sound vague if the drivers do not integrate in time. With a bi-radial horn, phase rotation around the crossover region is influenced by horn depth, diaphragm position, filter slope, polarity, and the acoustic roll-off of both drivers. A reverse-polarity test is a useful diagnostic: a deep null near the crossover often indicates that the relative phase is close to the desired relationship, although it does not by itself prove that the normal-polarity response is optimal.
In a passive loudspeaker, physical alignment is generally preferable to a complicated corrective network. Moving the woofer and horn relative to one another, choosing an appropriate baffle layout, and accounting for the listening axis can reduce the required electrical compensation. This is one reason integrated cabinet design matters. A crossover cannot be separated from the enclosure that determines its acoustic reference points.
The final network may use asymmetrical slopes. For example, the woofer could roll off with a steeper acoustic slope while the compression driver uses a gentler transition, provided the summed response and directivity remain smooth. Filter symmetry is less important than a stable power response and a clean time-domain relationship.
Validate The Network In The Cabinet
Once a prototype crossover has been assembled, measurements should be repeated at realistic output levels. High-efficiency systems can reveal small issues that remain hidden at conventional test volumes. Listen for compression, intermittent rattling, resistor heating, and changes in balance after the network has operated for an extended period.
Component selection should follow electrical necessity and reliability rather than fashion. Air-core inductors avoid core saturation but may have higher resistance or greater physical size. Film capacitors are often appropriate in the signal path, while rugged non-polar electrolytics can be practical where values are large and placement is less critical. Resistors need adequate power ratings, particularly in the compression-driver attenuation section.
Cabinet vibration also influences the result. A heavily braced birch plywood enclosure reduces panel radiation and keeps the acoustic output dominated by the drivers. The crossover should be mounted away from strong magnetic fields and secured against vibration, with short, orderly wiring and clearly separated high-current woofer paths where practical.
Evaluate Tonal Balance And Dispersion
Listening tests should use familiar recordings with vocals, acoustic instruments, dense orchestral material, and abrupt dynamic changes. A crossover that measures flat on-axis may still sound too bright if its off-axis response rises through the presence region. Conversely, a small dip on the reference axis can be preferable if it produces a smoother listening window across a room.
The bi-radial geometry makes spatial evaluation essential. Measurements at several horizontal and vertical angles reveal whether the woofer and horn maintain similar coverage through the handover. If the woofer narrows sharply before the horn becomes dominant, listeners may hear a change in room energy even when the central axis looks excellent.
Fine tuning should proceed in small steps. A fractionally different capacitor value, a small change in attenuation, or a resistor added to a contour circuit can alter both tonal balance and phase. Each revision should be documented with its measured response, impedance, listening notes, and component changes so that progress remains repeatable.
Practical Priorities For The Final Design
A well-developed passive crossover is the result of controlled compromises. These priorities help keep the project focused:
- Set the crossover frequency from horn coverage, driver distortion, and woofer directivity rather than nominal bandwidth alone.
- Measure both drivers in their final mechanical positions before calculating component values.
- Include impedance correction, attenuation networks, and natural driver roll-off in the acoustic simulation.
- Optimize phase, listening-window response, and power response alongside the on-axis frequency curve.
- Use reliable, appropriately rated components and verify the network at sustained high output.
The final assessment should combine measurements with long listening sessions. A crossover that appears technically successful but makes voices sound detached, cymbals aggressive, or bass timing indistinct still requires refinement. The best result is usually the one that disappears as a circuit and leaves a coherent, effortless presentation.
Hear The System In Its Intended Form
A bi-radial horn loudspeaker is a complete acoustic system, not simply a woofer, compression driver, and collection of parts. Its crossover works because the horn profile, driver loading, cabinet geometry, acoustic centers, and component network have been developed together.
To experience how those decisions translate into scale, dynamics, and tonal coherence, arrange a private listening session at the Sunship Audio demonstration room in Berlin. Hearing the finished system provides the clearest measure of whether the crossover has achieved its real purpose: natural integration with the immediacy and control of a properly engineered horn loudspeaker.