Sunship Audio

Why a slight time delay can align a woofer to a horn

A horn-loaded compression driver and a direct-radiating woofer produce sound from acoustic centres located at different physical positions within the cabinet. Even when the front baffles of both drivers sit on the same plane, the acoustic centre of a woofer typically lies somewhere within its voice coil and former, while the acoustic centre of a horn-loaded compression driver sits deep behind the horn mouth, near the driver diaphragm. The sound leaving each driver therefore travels a different path length to reach the listener's ears.

When a single transient, such as a snare drum hit, leaves both sources simultaneously, it arrives at the seat at two different moments. The ear and brain perceive this as a smeared leading edge, a loss of image focus, and a flattening of dynamic texture. This is the core problem that careful crossover design aims to solve, and it is the reason a small delay network on the woofer channel can transform the presentation of a high-efficiency system.

Why path length differs across drivers

A 38 cm paperwoofer might have its acoustic centre roughly 8 cm behind the front baffle, while a TAD-Pioneer compression driver behind a bi-radial horn can sit 25 to 40 cm deeper into the enclosure. The horn path is longer, but the propagation velocity through the horn throat also differs from free-air propagation at the woofer. The flare geometry of the horn further shapes wavefront timing, and the roll-off rate of the crossover determines how much overlap there is between the two sources.

In typical Australian loungerooms, where the listening seat often sits three to four metres from the loudspeakers across open-plan spaces with polished timber or tile floors, these small timing errors become more audible than they might in a heavily treated studio. Brick-veneer construction common across suburban Sydney and Brisbane adds reflective plasterboard surfaces that emphasise any smearing in the time domain. Good time alignment therefore delivers a larger subjective improvement in a local listening room than the measurements alone might suggest.

The acoustic centres behind each driver

Locating the true acoustic centre of a horn is more involved than simply measuring the depth of the throat. The flare shape, mouth size, and compression ratio all influence the apparent source point. For a well-designed bi-radial horn with a wooden flare, the acoustic centre typically sits a few centimetres in front of the driver diaphragm, while a woofer's acoustic centre sits several centimetres behind its cone. These two reference points rarely align on the baffle, even when both flush-mount designs appear visually co-planar.

When a crossover frequency sits in the 500 Hz to 1 kHz region, the wavelength at the crossover is roughly 34 to 68 cm, similar in scale to the spacing between acoustic centres. This is precisely where phase errors become most disruptive. Below the crossover, the horn attenuates naturally; above it, the woofer rolls off. In the overlap region, both contribute, and any misalignment between them produces the comb-filter dips and peaks that blur the stereo image.

How a small delay restores coherence

The fix is conceptually straightforward: delay the electrical signal going to the closer driver so both acoustic outputs arrive in step at the listening position. A path length difference of, say, 30 cm corresponds to roughly 0.9 milliseconds at sea level. Introducing a passive all-pass delay network — usually a second-order topology built from air-core inductors and high-quality film capacitors — holds the woofer signal back by precisely that interval.

The maths behind it is simple, but the listening benefit is anything but subtle. When the woofer and horn arrive together, the wavefronts merge into a single coherent source rather than two slightly offset sources. Bass lines lock in with vocal sibilants, piano attacks feel unified, and the stereo image gains depth and specificity that was previously blurred. Many visitors to the Sunship Audio demonstration room describe the change as the system suddenly snapping into focus.

Method Typical delay range Complexity Sonic character
First-order all-pass 0.1 – 0.4 ms Low Subtle, natural
Second-order all-pass 0.3 – 1.5 ms Moderate Strong coherence gain
Third-order all-pass 0.5 – 2.5 ms High Maximum integration
Active DSP delay Any value Requires electronics, ADC/DAC Most precise
No alignment 0 ms None Varies with geometry

These approaches each bring a woofer into step with a horn-loaded compression driver in different ways, trading off component count against integration depth. Higher-order networks allow longer delays but introduce more phase shift through the audio band, which must be compensated elsewhere in the crossover.

Implementing delay in passive crossovers

For a fully passive design, the delay network is usually placed immediately after the high-pass section feeding the compression driver, or in series with the low-pass section feeding the woofer, depending on which path is shorter. Component choice matters significantly: air-core inductors avoid the saturation and hysteresis that iron cores impose, while polypropylene or paper-in-oil capacitors preserve transient information better than ceramic or electrolytic types.

Time-aligned passive crossovers require careful voicing because the delay network interacts with the crossover slopes. A second-order Butterworth low-pass combined with a fourth-order Linkwitz-Riley high-pass can produce excellent integration once the delay is dialled in. This is the philosophy behind Sunship Audio's design philosophy, where every cabinet is built around measured acoustic centres rather than nominal driver depths.

For Australian builders using locally sourced components, Jantzen, Mundorf, and ClarityCaps are typically available through specialists in Melbourne and Brisbane. Shipping times from European suppliers can stretch during peak periods, so component selection often happens months before final cabinet assembly.

Listening benefits and setup guidance in Australian rooms

The practical benefit of time alignment shows up most clearly on acoustic material with sharp transients. A well-recorded drum kit, a solo violin, or a close-mic'd vocal all reveal the integration quality of the crossover. Misalignment shows up as a slight double-strike on snare hits, a softening of sibilance, or a vague sense that the bass is detached from the mid-band. At events like the Melbourne International HiFi Show, where dozens of systems are demonstrated back-to-back, the time-aligned pairs always draw longer listening sessions from attendees.

Setting the delay correctly requires either measurement or careful listening. A simple method uses a sine wave at the crossover frequency and adjusts the delay until the acoustic output at the listening seat reaches its peak. More sophisticated approaches use dual-channel FFT measurement to verify the impulse response of each driver and calculate the required offset.

Practical steps for tightening the integration between a horn-loaded compression driver and a direct-radiating woofer:

A few milliseconds of delay in the right place, executed with quality components and careful voicing, turns a hybrid horn-and-woofer system from two competent speakers sharing a cabinet into a single, coherent musical instrument. The improvement crosses the boundary between measurable and felt, and once heard, it becomes very difficult to live without.