Sunship Audio

Steep low-pass filtering for woofers in high-efficiency horn systems

In high-efficiency horn-loaded loudspeakers, the crossover is not an afterthought but a structural element that shapes how every driver behaves. At Sunship Audio, the design process begins with the compression driver and horn geometry, then works outward to the woofer and the network that ties the system together. The choice of crossover slopes, particularly on the low-pass side feeding the woofer, influences everything from cabinet dimensions to amplifier matching. A steeper filter offers tangible mechanical and acoustic advantages, but those benefits only materialise when the cabinet, driver, and listening room cooperate.

A steep low-pass filter is essentially a guard rail for the woofer, deciding what signal energy reaches the cone and at what point the network begins attenuating aggressively. The practical consequences of that decision show up in reliability, distortion behaviour, and the way the system integrates with the horn-loaded midrange. The following sections explore those consequences, including how the choice plays out in everyday Australian listening conditions.

Defining a steep low-pass slope

A filter's steepness is measured in decibels per octave. Common values include 12 dB/octave (second-order), 18 dB/octave (third-order), and 24 dB/octave (fourth-order), with some designs pushing to 36 dB or 48 dB per octave. The higher the number, the more aggressively the filter attenuates frequencies above the chosen crossover point.

In a passive high-efficiency system using TAD-Pioneer drivers and bi-radial wooden horns, a 24 dB/octave slope is frequently the starting point. It provides more than enough attenuation to prevent midrange leakage into the woofer, while keeping the network buildable with quality capacitors and inductors. Shallower slopes can sound attractive on paper but often demand trade-offs elsewhere in the design.

Mechanical protection and excursion control

The most immediate benefit of a steep low-pass filter is mechanical. A woofer asked to reproduce a 1 kHz signal at full power will produce far more heat and cone excursion than the same driver reproducing a 250 Hz signal. By rolling off the energy above the crossover point quickly, the filter limits the band where the cone is asked to move significantly.

This matters in practice. In coastal homes from Sydney to Perth, where summer humidity climbs above 80 percent, drivers run hotter for longer during evening sessions. A steeper filter reduces thermal stress on the voice coil, which in turn reduces the likelihood of thermal compression during long listening periods. The same logic applies in inland areas such as the Adelaide Hills, where summer temperatures push ambient conditions well above 30 °C and air-conditioned listening rooms already struggle to dissipate heat.

Lower distortion at realistic listening levels

When a woofer receives frequencies well above its intended band, the cone approaches breakup modes and the suspension enters non-linear territory. Both effects raise harmonic distortion, often adding a hard, glassy quality to upper bass that overlays the midrange horn. A steep filter keeps those frequencies out of the woofer's working band, so the cone stays in its pistonic range and distortion stays low.

This is particularly noticeable in rooms where listeners sit relatively close to the speakers, which is the norm in many Australian homes where dedicated listening spaces tend to be compact rather than cathedral-sized. The horn-loaded midrange is then free to carry vocals and acoustic instruments without competing with woofer artefacts, yielding a cleaner handover at the crossover frequency.

Cleaner integration with horn-loaded midrange drivers

Sunship's systems pair woofers with TAD-Pioneer compression drivers on bi-radial wooden horns. Both halves of the system have well-defined behaviour in their respective bands, but only if the crossover network keeps each driver inside its operating envelope. A steep low-pass filter makes the lower driver behave more like an idealised source, which simplifies the acoustic summation at the crossover frequency.

Practically, this means the time-aligned passive crossover can be designed with narrower overlap. The horn takes over cleanly, and the woofer stops contributing just before the horn becomes directional. The audience hears a more coherent wavefront, especially off-axis, which matters in irregular rooms such as Queenslander-style homes with high ceilings and open hallways.

Time alignment and phase behaviour

A steeper filter is not free, and one of its costs is increased group delay in the stop-band region. For a 24 dB/octave Linkwitz-Riley network, the delay peaks near the crossover frequency and falls off either side. If the woofer and horn are physically offset, that delay can be used as a tool to bring them into time alignment.

Mechanical alignment of the drivers in the cabinet front baffle reduces the offset, but the remaining phase difference is corrected by choosing the slope that yields the right delay profile. This is where measurement and simulation become non-negotiable, shifting the work from theory to listening. Sunship's Berlin demonstration room is built for exactly this process, with adjustable toe-in and well-controlled acoustics to validate the chosen slopes.

Practical considerations in Australian rooms

Australian domestic architecture offers a few common challenges. Many homes use brick veneer construction with tile or concrete floors, which produces a bright, reflective acoustic. In a Sydney apartment with floor-to-ceiling glass facing the harbour, the woofer's direct sound reaches the listener first, then the reflected sound returns a few milliseconds later. A steeper filter with controlled phase helps the brain lock onto a single source rather than smearing the image with reflections.

Another consideration is the Australian electrical supply. Equipment sold locally must carry the RCM mark and comply with AS/NZS safety standards. While passive networks are not directly regulated, the choice of capacitors and inductors affects both reliability and how the system behaves under higher mains voltages occasionally seen in remote parts of Western Australia or Queensland. Quality components and conservative ratings keep the network stable regardless of where it ends up.

Implementation choices and component selection

Once the slope is chosen, the network must be built. Steeper filters demand more reactive components, raising the bar on parts quality. Foil inductors with low DC resistance, film capacitors with stable dielectric behaviour, and careful point-to-point layout all contribute to a filter that performs as designed. Heavily braced birch plywood cabinets keep the drivers mechanically quiet, so the network does not have to compensate for cabinet talk.

A common misconception is that steeper always means better; in practice, the best slope matches the driver's mechanical behaviour, the horn's directivity, and the room. Most Sunship designs settle on 24 dB/octave Linkwitz-Riley, with 18 dB/octave and 36 dB/octave variants where driver or room geometry justify them. For readers weighing these trade-offs in their own projects, the crossover design FAQ walks through the most common questions asked of the workshop.

Comparing common low-pass slopes

The table below summarises the slopes most often used in high-efficiency woofer sections, with their practical implications.

Slope (dB/oct) Stop-band attenuation one octave above f_c Typical group delay Best suited for
12 -12 dB Low Two-way mini-monitors, low-power systems
18 -18 dB Moderate Compact two-ways with wide-band midwoofers
24 -24 dB Moderate to high High-efficiency horn-loaded systems with dedicated woofers
36 -36 dB High Multi-way designs with narrow overlap regions
48 -48 dB Very high Active electronic crossovers or specialised studio monitors

Practical recommendations for crossover slopes