Sunship Audio

Phase-Coherent Crossovers For Horn And Woofer Integration

A horn-loaded loudspeaker can deliver remarkable clarity, dynamics and efficiency, yet its compression driver and woofer do not automatically arrive at the listener in phase. Their acoustic centres may sit at different depths, their slopes may rotate phase, and the room can shift the apparent balance between direct and reflected sound. A crossover must therefore be designed around acoustic behaviour rather than electrical frequency alone.

Designing a Crossover That Matches Horn and Woofer Phase at the Listening Position means treating the complete system as a time-domain instrument. The horn profile, driver mounting, cabinet geometry, crossover topology and listening distance all contribute to the result. This approach is particularly valuable in large-format systems using TAD-Pioneer compression drivers, bi-radial wooden horns and heavily braced birch plywood enclosures.

Why Acoustic Phase Matters

When horn and woofer outputs meet with matching phase, their wavefronts add smoothly through the crossover region. The listener hears a stable vocal image, convincing instrumental texture and consistent energy through the midrange. If their phases oppose each other, the result may be a deep response notch, a vague centre image or a presentation that changes noticeably when the listener moves.

Electrical phase and acoustic phase are different measurements. A nominal second-order filter may appear symmetrical in a schematic, while the actual drivers introduce their own roll-off, delay and resonance behaviour. Horn throat geometry can shift the compression driver’s acoustic origin rearwards or forwards relative to the woofer cone, making physical placement part of the crossover design.

The listening distance also matters. A compact domestic system may be judged at two or three metres, while a larger horn system in a dedicated room may be evaluated farther away. At each distance, the relative arrival time changes slightly, so the design target should reflect the position where the owner normally listens.

Establishing The Acoustic Reference

The first practical step is to measure each way independently, with the microphone at the intended listening position. A reference point on the tweeter or compression-driver axis is useful, but it should not replace measurements taken at the actual ear position. The microphone height, toe-in and distance should be recorded so later tests remain comparable.

Impulse-response data can reveal the leading edge of each driver’s output and estimate the acoustic offset between horn and woofer. Gated measurements help isolate direct sound, although room reflections arrive quickly in Australian living rooms with timber floors, glass and open-plan layouts. Nearfield woofer measurements combined with farfield horn measurements can provide a useful starting point, but final decisions should be checked at the listening seat.

Useful reference information includes:

A crossover designer can then decide whether the alignment should be achieved through physical offsets, electrical delay, filter phase rotation or a combination of all three. In a passive system, the available correction is limited, which makes cabinet geometry and driver selection especially important.

Selecting The Crossover Region

The crossover frequency should fall where both drivers operate comfortably, not simply where their published specifications overlap. A compression driver on a large wooden horn may offer excellent sensitivity and controlled directivity well below the range where a small dome tweeter would remain comfortable. A woofer, however, becomes increasingly directional as its diameter grows, so the crossover must also support a coherent radiation pattern.

Filter slopes determine how quickly each driver hands over energy. Steeper acoustic slopes can protect the compression driver and reduce overlap, but they introduce greater phase rotation and can make a passive network more complex. Gentler slopes may sound open and continuous when the drivers are naturally well behaved, though they demand more careful control of distortion and excursion.

A suitable design process usually compares several acoustic targets rather than assuming one topology is universally correct:

The best result is often an acoustic target that differs from the electrical labels on the components. A “12 dB per octave” section may become a different effective slope once driver response, horn loading and cabinet diffraction are included.

Building Phase Into A Passive Network

A time-aligned passive crossover can use component values to shape both amplitude and phase. Series inductors, capacitors, padding resistors and impedance compensation networks must be calculated from the drivers’ measured impedance, not from their nominal eight-ohm or sixteen-ohm ratings. The compression driver’s impedance curve may rise substantially in the lower midrange, while the woofer can show strong mechanical and cabinet-related features.

Polarity is another variable. Reversing one driver can produce a deeper crossover-region null, which is sometimes evidence that the chosen slopes are approaching the intended acoustic relationship. It is not, however, a substitute for phase measurement. A polarity change that improves the response on-axis may damage the vertical listening window or create an unstable image elsewhere in the room.

The passive network should be auditioned at realistic power levels. Large resistors can change value as they heat, iron-core inductors may approach saturation, and capacitor tolerances can alter the final balance. In the Australian market, where specialist horn systems are often shipped long distances between Melbourne, Sydney, Brisbane and Perth, a robust network and mechanically secure construction are as important as the nominal schematic.

For systems with appropriate amplification and installation control, active operation can offer a cleaner route to delay and slope adjustment. The practical distinctions are outlined in this bi-amp guide, especially when separate amplifier channels are used for the horn and woofer.

Measuring At The Listening Position

Once the crossover is assembled, measure the horn and woofer separately, then together, using identical microphone placement and level calibration. Examine magnitude, excess phase, group delay and the impulse response around the crossover region. A smooth summed response is valuable, but a clean phase trace and a compact, well-defined impulse transition often reveal problems that frequency response alone can hide.

Measurements should include small movements around the main seat. A one-metre-wide listening position is common in a dedicated room, while Australian homes may require a wider sofa-based window. If the response collapses with a small vertical shift, the crossover may be producing excessive lobing. That can result from unequal slopes, excessive acoustic spacing or an incorrect polarity choice.

Listening tests remain essential after the instruments show a promising alignment. Well-recorded speech, a centred vocal and percussion with sharp transients can expose timing errors quickly. Bass guitar and lower piano notes are useful for judging whether the woofer hands over with the right weight, while sustained strings reveal uneven directivity or a narrow response depression.

Refining The System For The Room

Room placement can make a correctly aligned crossover appear incorrect. Moving a horn system closer to a rear wall increases boundary reinforcement, while toe-in changes the horn’s high-frequency energy at the listening seat. In a Sydney terrace or Melbourne apartment, the available distance from wall to sofa may be fixed, so the crossover should be validated within realistic placement limits rather than an idealised laboratory arrangement.

A demonstration room in Berlin can establish a controlled baseline, but the final installation may have different floor construction, ceiling height and furnishing density. Australian rooms often combine hard surfaces with large windows and open kitchen areas, increasing early reflections. These reflections do not necessarily require heavy absorption; they do require careful attention to directivity, toe-in and the listening axis.

The comparison below describes common alignment approaches and their practical implications:

Alignment Approach Main Strength Typical Limitation Suitable Use
Physical driver offset Preserves a simple passive network Cabinet depth and appearance become more complex Purpose-built horn systems
Passive phase shaping Works without dedicated DSP Component values depend heavily on impedance Integrated custom loudspeakers
Active delay and filtering Precise time alignment and flexible slopes Needs extra channels and system setup Bi-amped installations
Broad listening-window tuning More consistent across seats May sacrifice peak on-axis linearity Shared living rooms and sofa listening
Single-seat optimisation Maximum precision at one position Performance changes away from the reference seat Dedicated listening rooms

A successful design is therefore a compromise with a clear priority: coherent arrival at the main listening position, controlled directivity through the handover and stable behaviour around it. When the horn, woofer, cabinet and crossover are developed as one acoustic system, phase alignment becomes audible as natural focus, effortless dynamics and a seamless transition between drivers.