Sunship Audio

Cabinet Baffle Width and Horn Directivity Behaviour

The width of a loudspeaker's front panel governs how a horn-loaded driver projects sound into a room. For compression drivers coupled to wooden biradial horns, this single dimension shapes the dispersion patterns, the smoothness of the off-axis response, and the way a system integrates with a time-aligned crossover.

When a horn's mouth occupies only part of the available baffle, the unused regions begin to matter. Edges within a fraction of a wavelength from the mouth radiate and scatter energy, tilting the polar response. Above 3 kHz these effects become visible in measurements, and designers at the Sunship Audio workshop model them before committing a cabinet to ply.

For Australian listeners, the practical consequences appear in rooms with widely varying geometry, from the long, low-ceiling layouts common in Brisbane Queenslander homes to the wider open-plan volumes in newer Melbourne developments. Baffle width is one of the variables that determines how a system integrates with the listening seat.

Treating baffle width as an acoustic variable rather than a cabinet-making afterthought is the design starting point. Each horn mouth is positioned relative to the surrounding plywood edges with knowledge of the wavelengths, the crossover frequencies, and the intended seating axis. The sections that follow walk through the behaviour and the construction choices.

Baffle behaviour Narrow panel Wide panel
Edge diffraction severity High above 2 kHz, combing in polars Low, smooth off-axis roll-off
Horizontal dispersion Broader mid, narrower HF due to edges Matches horn flare angle
Vertical dispersion Often tilted by asymmetric placement Symmetric about horn axis
Image focus in reflections Diffuse, slightly smeared Stable, anchored to driver
Cabinet mass requirement Moderate, smaller front panel Higher, larger panel needs bracing

Wavelengths, baffle width, and edge diffraction

A compression driver on a horn behaves as if the baffle extends its aperture downward in frequency. The classic criterion is that the baffle should be at least one-half wavelength wide at the lowest frequency of interest. Below that threshold, the front and rear wavefronts wrap around the edges and cancel.

For a horn crossing over at 800 Hz, the half-wavelength is roughly 215 mm. Any baffle narrower than this produces progressive loss of bass energy and a tilted power response. Above the transition, the horn's flare angle dominates the polar pattern, and the crossover frequency becomes the design target for the lower edge of the baffle.

Edge diffraction adds another layer. Sound at the baffle boundary bends around the corner and re-radiates as a delayed secondary source. In a time-aligned passive crossover, this adds an extra time smear the crossover cannot cancel. The relationship between phase plug design and baffle geometry becomes part of the same conversation.

Narrow baffles and their acoustic signature

Narrow baffles suit compact floor-standers and studio nearfields where footprint matters. They work well when the listener sits within a metre or two of the speaker, as the widened polar response at mid frequencies fills the nearfield evenly. Many Australian audiophiles in inner Sydney and Adelaide apartments favour this format for its smaller stands and tighter placements.

The signature is distinctive. Mid frequencies disperse broadly, giving room-filling ease, but above the baffle-step transition the edges act as secondary sources, narrowing the effective horizontal pattern unevenly. The result can sound spacious yet slightly hollow in the upper midrange, with a polite HF roll-off that is actually a diffraction artefact rather than true smoothness.

In horn-loaded systems this compounds. A biradial horn produces a tight pattern, but edge diffraction broadens it below cutoff and narrows it above, producing a frequency-dependent dispersion that is hard to tame. Felt or foam along the inner edges helps but does not cure the issue.

Wide baffles and vertical pattern control

Wide baffles behave like a continuation of the horn. The flare continues into the baffle surface, and diffraction is pushed to frequencies low enough to lie outside the crossover band. The horn's directivity index becomes the dominant factor.

Vertical pattern control benefits in particular. Listeners often sit with a vertical offset relative to the driver; if the baffle is asymmetric vertically, the off-axis response becomes uneven. A wide, symmetric baffle restores the intended lobe. This matters in open-plan living spaces common in Perth or on the NSW Central Coast, where the sofa faces a unit at a non-negotiable height.

The trade-off is cabinet mass. A wide front panel needs heavier birch plywood and additional corner braces to stay rigid, and the finished cabinet is heavier. In older Australian houses with timber floors, mass anchors the speaker; in newer apartments on suspended slabs, it requires a sturdier amplifier stand.

Flush mounting and recessed horns

Recessing the horn into the front surface delays edge diffraction by increasing the path length from the mouth to the nearest edge. A 20 mm recess can shift the first problematic diffraction null by a noticeable margin, smoothing the upper midrange.

Time alignment requires attention. The acoustic centre of the horn moves backward relative to the woofer, and the passive crossover's delay compensation must be adjusted. A 15 mm recess misaligns a crossover that was correct for a flush mount, producing a comb-filter artefact in the critical 1–3 kHz region where the ear is most sensitive.

Construction precision becomes critical. Recess walls must be parallel, smooth, and free of resonances, which is easier in heavily braced birch plywood than in MDF. Australian cabinetmakers familiar with marine-grade birch ply used in boatbuilding often produce excellent results, as the lamination techniques transfer directly.

Practical measurements: polars and curves

Predicting baffle effects requires polar measurements at multiple frequencies. A complete polar map at 200 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 8 kHz reveals the pattern's frequency dependence. Diffraction shows up as lobes that shift angle with frequency and as dips in the off-axis curve.

Comparison between narrow and wide baffles in the same horn is informative. The wide baffle maintains smoother off-axis curves and a more stable directivity index, while the narrow baffle shows a wider main lobe at low frequencies and a narrower, erratic lobe at high frequencies. Designers use these measurements to choose crossover points where polar behaviour is most stable across both baffle widths.

Local audio clubs in Melbourne and Sydney run measurement sessions, and ACMA-compliant RF-quiet rooms make this easier. Hobbyists can replicate the work with a measurement microphone and software, comparing results with published manufacturer curves.

Listening room considerations in Australia

The Australian listening room varies enormously. A fibro cottage in Wollongong with plasterboard-lined walls behaves very differently from a rendered brick extension in suburban Adelaide. Room volume, ceiling height, and floor construction all interact with the speaker's dispersion.

Houses of representative Australian construction tend to have moderate ceiling heights (2.4–2.7 m), hard floors, and mixed soft and hard furnishings. Speakers with wide baffles and controlled directivity integrate more cleanly because they direct less energy into reflective surfaces. In dry winter months in southern Australia, when humidity drops and absorptive materials stiffen slightly, dispersion control matters even more.

Local market realities also matter. Specialist dealers in capital cities carry a curated range, and many Australians audition at manufacturer demonstration rooms or events such as the Australian Hi-Fi Show. Distances are large, so auditioning at home is common, and baffle width choices that suit the showroom may not suit the home.

Choosing baffle width for real rooms

A few practical guidelines help match baffle width to the intended use.