Building a Precise Crossover for a Bi-Radial Horn
A bi-radial tweeter horn can deliver exceptional clarity, dynamic range, and controlled dispersion, but its performance depends heavily on the crossover network that feeds it. The compression driver, horn geometry, cabinet alignment, and bass section must operate as one acoustic system rather than as isolated components.
A custom crossover is therefore more than a collection of capacitors, inductors, and resistors. It is the point where measured frequency response, acoustic phase, driver protection, sensitivity matching, and listening impressions are brought into balance. Small changes in component values can affect the tonal character, transient behavior, and integration with the woofer.
For a system built around TAD-Pioneer compression drivers and heavily braced birch plywood cabinets, the design process begins with the intended operating range and ends with careful verification in the completed loudspeaker. The result should preserve the immediacy of the horn while avoiding harshness, uneven power response, or an artificial transition between drivers.
Start with the horn and compression driver
The horn determines much of the tweeter’s acoustic behavior. A bi-radial profile controls horizontal and vertical dispersion through two defined axes, helping the system maintain a predictable radiation pattern across its working bandwidth. Its mouth size, throat geometry, and flare rate influence both directivity and the lowest practical crossover point.
The compression driver adds another set of variables. Diaphragm material, phase-plug design, sensitivity, resonance, and thermal capacity all affect how the driver should be filtered. A nominal specification may suggest a crossover frequency, but the actual acoustic response on the chosen horn is more important than a catalog figure.
Measurements should therefore be taken with the exact driver, horn, mounting arrangement, and enclosure intended for production. The Sunship Audio blog provides useful context on the construction choices and design principles that influence this kind of integrated loudspeaker.
Define the acoustic target
A crossover target is usually described by its slope and turnover frequency, but those electrical terms do not tell the whole story. The horn and driver already contribute a natural acoustic roll-off, resonances, and changing directivity. The woofer has its own response, including cone break-up, inductance, and cabinet-related irregularities.
The designer must decide how the two acoustic outputs will sum through the crossover region. A fourth-order acoustic Linkwitz-Riley alignment may be appropriate in one system, while a gentler or asymmetric alignment may better suit another. The phase relationship at the listening axis, the off-axis response, and the desired integration with the woofer all matter.
A good target also includes a protection strategy. The high-pass section should reduce low-frequency energy that could over-excurse or overheat the compression driver, particularly during high-level playback. Protection should remain inaudible during normal operation rather than imposing a heavy sonic signature.
Shape the passive network
In a passive loudspeaker, the crossover must work against real-world impedance. A compression driver is rarely a flat resistive load, and its impedance rises with frequency because of voice-coil inductance. The horn may reveal a strong resonance near the lower operating limit. These effects can shift the intended filter response unless they are included in the design.
A practical network may combine a high-pass filter, padding resistors, impedance equalization, and response-shaping branches. An L-pad can reduce horn sensitivity to match the woofer, while a contour network can correct a broad rise without wasting excessive amplifier power. Components should be selected for their electrical role, current handling, stability, and low parasitic behavior.
The physical layout matters as much as the schematic. Inductors should be spaced and oriented to minimize magnetic coupling. High-current paths need short, secure connections, and resistors require adequate ventilation. Mounting the crossover on a rigid board inside the cabinet reduces vibration-related noise and makes future servicing more reliable.
Balance sensitivity and dispersion
Horn systems often produce much higher sensitivity than conventional direct-radiating tweeters. Matching levels cannot be treated as a simple volume adjustment because attenuation also changes the way the driver interacts with the crossover impedance. The chosen resistor values must be evaluated with the complete network and measured again after assembly.
Directivity is equally important. A bi-radial horn can maintain controlled dispersion over a broad range, but its pattern will narrow as frequency rises. If the woofer radiates more widely at the crossover point, the on-axis response may look smooth while the power response becomes uneven. Listening-room balance can then suffer even when a single microphone trace appears excellent.
For that reason, a custom network should be judged through multiple measurement angles. On-axis response, horizontal and vertical off-axis curves, early reflections, and estimated in-room response reveal whether the crossover supports consistent coverage. Time alignment between the horn acoustic center and woofer is another essential consideration, especially when a passive network is expected to preserve coherent transients.
| Design factor | What to measure | Crossover implication |
|---|---|---|
| Compression-driver resonance | Impedance and near-field response | Set a safe high-pass region and adequate slope |
| Horn directivity | Horizontal and vertical polar data | Choose a crossover point with compatible coverage |
| Driver sensitivity | Level-matched response | Calculate padding without disturbing filter behavior |
| Impedance curve | Electrical response across frequency | Account for rising impedance and resonant peaks |
| Acoustic timing | Relative arrival at the listening axis | Adjust polarity, delay assumptions, or phase shaping |
| Woofer breakup | Response above its clean operating range | Add a low-pass strategy that suppresses unwanted output |
Validate with measurements and listening
Simulation is valuable during the early stages, but a simulated crossover cannot capture every detail of the finished loudspeaker. Cabinet diffraction, horn mounting, grille effects, driver tolerances, and real component values can all alter the result. Measurement with the actual enclosure is the point where the design becomes trustworthy.
The process usually moves through several iterations. First, the designer verifies the raw driver responses and impedance. Next, a prototype network is built using accessible components. Frequency response and phase are measured, the network is adjusted, and the system is tested at realistic output levels. Final listening then helps identify qualities that measurements may not fully describe, such as vocal presence, cymbal texture, image stability, and fatigue over long sessions.
Component tolerances deserve attention in the final version. A capacitor or resistor value that is slightly different from the design target can move the crossover enough to affect summation. High-quality parts are useful when their tolerance, reliability, and dielectric behavior serve the circuit, but expensive components cannot compensate for an incorrect acoustic target.
Refine the system as a whole
The best passive crossover is designed alongside the cabinet, woofer alignment, horn dimensions, and intended amplifier. Changing the woofer, moving the horn forward, or altering the cabinet damping can change the acoustic phase and invalidate an otherwise successful network. A crossover should therefore be treated as part of the loudspeaker architecture.
Time-aligned construction can simplify the task by placing the acoustic centers in a more favorable relationship. It does not remove the need for phase measurements, because the electrical filter and driver behavior still determine the final result. The goal is a stable acoustic blend in which the transition remains convincing across a useful listening window.
For a deeper look at network topology, component choices, and implementation, the dedicated crossover design notes offer a useful reference. In a custom build, these principles are then adapted to the exact horn, driver pair, cabinet volume, and listening environment.
Practical priorities for the final build
A disciplined design sequence prevents the crossover from becoming an exercise in trial and error. Before committing to a finished network, document the measurement conditions, retain the raw driver data, and make changes one variable at a time.
- Measure the compression driver on its actual bi-radial horn rather than relying only on published data.
- Select the crossover point from acoustic response and directivity evidence, not electrical calculations alone.
- Check impedance, phase, sensitivity, and off-axis behavior after every major network revision.
- Use physically stable component mounting with appropriate spacing between inductors.
- Compare the completed loudspeaker at both moderate and high levels to verify tonal balance and driver protection.
A successful custom crossover allows the horn to sound open and effortless while keeping the woofer transition quiet, controlled, and believable. It preserves the compression driver’s speed without exaggerating upper-midrange energy, maintains consistent dispersion, and gives the amplifier a manageable load.
To experience how a measured crossover, bi-radial horn, and time-aligned cabinet work together, arrange a listening session in Sunship Audio’s Berlin demonstration room and hear the completed system in person.