Sunship Audio

The Effect of Horn Mouth Geometry on Off-Axis Response

Horn loudspeakers are often judged from the listening seat, yet their character across a room depends heavily on what happens away from the central axis. The shape and size of the horn mouth determine how sound spreads, how quickly directivity narrows with frequency, and how much energy reaches nearby walls, ceilings and furniture.

This matters particularly with high-sensitivity systems using compression drivers. A TAD-Pioneer driver can deliver exceptional detail and dynamic headroom, but the driver’s output must be managed by the horn profile. The mouth is the transition between the controlled wave inside the horn and the wider acoustic environment of the room.

A circular, rectangular or bi-radial mouth can produce quite different coverage even when the throat, crossover point and overall cabinet volume are similar. Rounded profiles may encourage smoother expansion, while a broad rectangular mouth can preserve horizontal coverage and limit unwanted vertical energy.

For Australian listeners, these differences become practical in rooms ranging from compact Melbourne terraces to large Brisbane living areas. Hard floors, glass, open-plan layouts and close neighbouring properties all influence whether reflected sound is welcome, excessive or difficult to control.

Why The Mouth Controls Dispersion

A horn guides acoustic energy from the small throat around the compression driver to the much larger mouth. As the wavefront expands, the horn’s flare rate affects loading, efficiency and directivity. The larger the mouth relative to the wavelength, the more confidently the system can maintain a defined radiation pattern.

At low frequencies within the horn’s operating range, the mouth may be acoustically small. Sound then bends around its edges and spreads widely. At higher frequencies, the same mouth becomes acoustically large, so the output becomes more directional. This transition is central to off-axis response: a listener moving sideways may hear a gradual tonal change, or a sharp reduction in treble.

Mouth geometry also determines edge diffraction. A sudden termination can create secondary wavefronts that interfere with the direct sound, producing ripples in the frequency response. Carefully shaped edges, rounded corners and a controlled flare help reduce those disturbances. The result is usually a smoother power response, meaning the total energy radiated into the room changes less dramatically as frequency rises.

Bi-Radial Profiles And Room Coverage

A bi-radial horn uses different curvature in the horizontal and vertical planes. This allows the designer to keep a wider horizontal pattern for several seats while restricting vertical spread towards the floor and ceiling. In a domestic room, that can reduce strong early reflections without sacrificing useful coverage across a sofa or listening row.

Sunship Audio’s design philosophy and construction details are explored in the Sunship Audio blog, including the relationship between horn geometry, cabinet integration and time alignment. With a heavily braced birch plywood enclosure, the horn can be positioned as part of a unified acoustic system rather than treated as an isolated component.

Useful consequences of a well-chosen bi-radial mouth include:

This approach can suit Australian homes where a loudspeaker may need to cover a dining area, open kitchen or second seating position. In a Sydney apartment, restrained vertical radiation may also help reduce sound transfer through ceilings and floors, while a Melbourne listening room with heavy carpet may benefit from retaining sufficient horizontal energy.

Comparing Common Mouth Geometries

The dimensions of a horn mouth should be considered alongside the crossover frequency, driver exit angle and intended listening distance. A broad mouth does not automatically guarantee wide dispersion. Its proportions, flare law and edge treatment determine how effectively that area is used.

The following comparison describes general tendencies rather than fixed rules. Actual results depend on the complete acoustic design, including the compression driver, crossover slope, cabinet placement and room response.

Mouth geometry Typical dispersion tendency Off-axis tonal behaviour Suitable application
Circular Similar coverage in both planes Can narrow evenly with frequency Symmetrical rooms and focused listening
Wide rectangular Broad horizontal, narrower vertical Good lateral consistency when carefully designed Multiple seats and long rooms
Short rectangular Moderate horizontal control Earlier loss of pattern control at lower frequencies Compact systems
Exponential flare with sharp edges Strong loading but possible diffraction Greater response irregularity near the mouth Designs where efficiency is prioritised
Bi-radial with rounded termination Separately controlled horizontal and vertical spread Smoother transition off axis High-end domestic and studio systems

The mouth’s apparent width is especially important in the horizontal plane. A speaker with a wide opening may keep voices and instruments tonally credible for listeners sitting well to the side. By contrast, a narrow opening can create a precise central image but cause the treble to fall away quickly outside the sweet spot.

Vertical geometry deserves equal attention. Excessive ceiling radiation can make a room sound bright and aggressive, while too little vertical spread may make the presentation feel constrained. A time-aligned passive crossover cannot correct every geometric limitation, so the horn profile and crossover region must be developed together.

Practical Placement And Listening Checks

Off-axis performance is experienced through reflected sound as much as through direct sound. A loudspeaker that measures smoothly on axis but loses energy abruptly at 30 degrees may produce a strong tonal contrast between the direct wave and side-wall reflections. A smoother decline can make the room sound more natural and stable.

Placement remains important even with controlled directivity. Toe-in changes the angle at which the listener meets the horn, while distance affects how completely the drivers integrate before the sound reaches the ears. In an Adelaide room with bare timber flooring, modest toe-in and a rug may produce a more balanced result than simply reducing treble at the amplifier.

Useful listening checks include:

Australian rooms often combine reflective glazing with absorbent furnishings, creating uneven acoustic behaviour. A Perth home with polished concrete may expose mouth-edge diffraction and side-wall reflections more clearly than a heavily furnished Canberra room. Listening tests should therefore be performed in the actual space, with the loudspeakers at their intended height and distance from the walls.

Designing For A Natural Power Response

The best off-axis behaviour is not necessarily the widest possible coverage. The aim is controlled, predictable radiation that maintains a consistent relationship between direct and reflected energy. When the power response is smooth, the room contributes ambience without imposing large spectral changes on every recording.

For a custom loudspeaker, the designer can select the mouth proportions according to the room and listening habits. A broad horizontal pattern may be valuable for family listening or two-channel systems shared by several people. A narrower pattern can work well in a dedicated room where the listener sits centrally and wall reflections need to be minimised.

The local high-end market also makes practical considerations important. Large horn systems may travel considerable distances from a workshop to buyers in Sydney, Brisbane or regional New South Wales, so cabinet rigidity and protection during freight matter alongside acoustic performance. A carefully designed mouth must survive transport, integrate with the woofer section and remain mechanically stable over years of use.

Horn mouth geometry ultimately shapes the speaker’s relationship with the room. By controlling how energy expands, reflects and decays away from the listening axis, it influences tonal consistency, imaging and perceived scale. That is why a refined horn system treats the mouth as a primary acoustic component rather than a decorative opening in the cabinet.