Horn mouth curvature and the ten-degree directivity window
Horn mouth curvature hides in plain sight among loudspeaker design variables. A horn can look impressive on a stand, with its flared wooden mouth promising effortless dynamics, yet the precise contour from throat to rim quietly decides how sound behaves once it leaves the cabinet. Directivity is the result, and at narrow off-axis angles around ten degrees, small geometric decisions show up as audible differences. A horn that beams too tightly can sound spectacular on axis yet hollow a few rows back. A horn that spreads too loosely loses its dynamic authority.
For hi-fi listeners, the ten-degree band is particularly revealing because it corresponds closely to the angular spread a single seated listener experiences when speakers are toed in slightly. Reflections from side walls, ceiling edges, and furniture arrive from angles beyond this zone, so the ten-degree window is where direct sound and early-reflected sound meet. Designers at Sunship Audio spend considerable time studying this transition when tuning the bi-radial wooden horns fitted to their TAD-Pioneer compression drivers.
Australia adds another layer to the question. Sydney terraces, Melbourne bungalows, Brisbane queenslanders, and Perth coastal homes vary enormously in volume, ceiling height, and reflective behaviour. A horn that performs one way in a damped inner-city apartment may behave quite differently in a high-ceilinged Adelaide weatherboard house. Curvature is also a tuning knob for the local domestic landscape.
Why horn geometry shapes the wavefront
A compression driver produces sound at a very small diaphragm. Without a horn, that sound would radiate in a near-spherical pattern and lose much of its energy within a metre. The horn acts as an acoustic transformer, gradually expanding the wavefront so that it couples efficiently with the air in the room. The shape of that expansion is everything.
A straight conical horn gives a roughly conical radiation pattern whose beam width changes with frequency. An exponential flare produces a smoother impedance match with the throat but tends to beam more sharply at higher frequencies because the mouth appears electrically small to short wavelengths. A tractrix flare maintains a constant beam width right up to its cutoff. The measured behaviour at ten degrees off-axis depends heavily on the radius of curvature at the mouth, where the wavefront finally detaches from the wood. When the rim curvature is gentle, the mouth behaves like a large radiator, spreading energy into the room. When sharp, the mouth behaves like a small radiator even though its physical size is large, and energy is channelled forward. Designers call this mouth diffraction, and it is the principal reason two horns of identical mouth size can sound quite different in the same seat.
Curvature profiles and their beam patterns
Five curvature profiles dominate horn design: exponential, tractrix, hyperbolic, conical, and the family of oblate-spheroidal or Le Cléac'h flares favoured by some modern high-end builders. Each behaves differently at ten degrees off-axis, and each carries different trade-offs between efficiency, bandwidth, and dispersion control.
A side-by-side comparison helps clarify how these choices shape the ten-degree window.
| Curvature profile | On-axis sensitivity | Behaviour at 10° off-axis | Side-lobe control | Common use |
|---|---|---|---|---|
| Conical | Moderate | Uneven, peaks and dips above 4 kHz | Weak | Utility PA |
| Exponential | High | Narrow beam, level drops sharply | Strong forward throw | Cinema horns |
| Tractrix | High | Wide, smooth roll-off | Good | Studio monitors |
| Hyperbolic | High | Tight, frequency-dependent | Very strong | Long-throw PA |
| Le Cléac'h / oblate | High | Broad and even up to cutoff | Excellent | High-end domestic |
The exponential flare, despite its efficiency, tends to lose too much energy into the side walls for a typical Australian lounge room. The Le Cléac'h flare sustains a smooth response at ten degrees off-axis and then falls away predictably beyond fifteen degrees, which suits a single seated listener. Sunship Audio builds around this profile because the Berlin demonstration room rewards the same off-axis behaviour that a Melbourne or Brisbane audiophile would expect at home.
Why ten degrees off-axis is a critical measurement point
The choice of ten degrees is not arbitrary. Microphone-based directivity measurements are typically taken at zero, five, ten, fifteen, twenty, thirty, and forty-five degrees, and the ten-degree band sits at the transition between direct sound and early reflections. In a typical three-metre equilateral listening triangle, speakers slightly toed-in place the listening axis at zero to five degrees, and the side-wall first reflection at around thirty to forty degrees. Anything at ten degrees is therefore the first response to behave differently from the on-axis sound, often revealing colouration the ear latches onto even when it cannot name it.
A horn whose ten-degree response is several decibels down from its on-axis response will appear to throw a tight beam, useful in a cinema but constraining in a domestic room. A horn whose ten-degree response is essentially flat with on-axis will present a wider, more enveloping stereo image. This is the difference between a Sydney sound-room demonstration and a Brisbane open-plan family room, where closer walls and a tighter beam leave the room feeling under-energised.
Translating theory into birch and brass
In practice, building a horn with controlled curvature at the mouth requires careful joinery. Sunship Audio cuts each side of its bi-radial horns from heavily braced birch plywood, glued and clamped so the flare maintains its shape over decades. The curvature is not a single smooth arc but a compound shape combining a tractrix throat with a controlled oblate mouth, producing a predictable ten-degree response.
The crossover must agree with this directivity behaviour. A time-aligned passive crossover with gentle slopes preserves the wavefront that the horn has carefully shaped; a steep crossover reintroduces phase shifts that undo the careful geometry. When pairing horns with vintage amplifiers, designers find the amplifier's harmonic profile and damping factor influence the perceived ten-degree behaviour as much as the horn itself, which is why the company's vintage amplifier pairing guide is worth studying.
Australian rooms and climate considerations
Australian domestic architecture presents a wider acoustic palette than many overseas markets. A Brisbane timber home in summer sits at humidity levels that can swell birch plywood, subtly shifting the curvature at the rim. A Hobart listening room in winter can run dry enough for the same horn to feel slightly tighter in its dispersion. Sydney apartments with polished concrete floors and large glass doors throw early reflections differently from Melbourne carpets-and-curtains lounges.
These variations matter because the ten-degree response is highly sensitive to boundary conditions close to the horn mouth. Sunship Audio's Berlin demonstration room is treated to a known acoustic standard; Australian customers often commission horns to suit a specific room, considering ceiling height, side-wall distance, and floor treatment. Curvature at the mouth becomes a per-build parameter rather than a catalogue number.
Choosing the right curvature for your system
For listeners weighing their options, the practical starting point is the room. A large, reflective space benefits from a tighter beam because too much lateral energy muddies the image. A small, well-treated room benefits from a broader ten-degree response because reflections are already controlled. Horns built around tractrix or Le Cléac'h profiles tend to suit the latter; exponential and hyperbolic profiles suit the former.
Australian audio shows in Sydney and Melbourne often showcase horn systems at their best. Listening to the same piece from a fixed seat while switching between two horns of identical sensitivity but different curvature reveals the ten-degree difference clearly: tighter horns focus the soloist at the expense of air, broader horns paint a wider acoustic around the voice.
The curvature at the mouth of a horn is the designer's last opportunity to shape how a loudspeaker speaks to a particular room. At ten degrees off-axis, that final decision shows itself, audible, measurable, and a matter of taste.