Sunship Audio

Cutting A Smooth Horn Mouth With A Router

A smooth horn mouth begins with accurate geometry, a stable workpiece and a routing method that removes timber gradually. In a loudspeaker horn, the flare profile controls how the acoustic wave expands from the throat into the room, so a small hollow or step can affect both appearance and performance.

For a custom horn-loaded loudspeaker, the mouth is usually formed from laminated timber or carefully shaped plywood rather than treated as a simple decorative opening. Bi-radial profiles, curved transitions and large radiused edges demand more control than a standard round-over operation. A router can deliver that control when the template, cutter and feed direction are planned together.

This approach suits the kind of careful construction used in high-efficiency systems based around compression drivers. Sunship Audio’s horn design philosophy places the acoustic shape, time alignment and cabinet construction in the same design conversation. The router is one tool within that larger process, not a substitute for accurate modelling or patient finishing.

Geometry Before Timber

Start with the horn profile, not the router. A CAD drawing or full-size paper pattern should show the throat, flare rate, mouth dimensions and the transition into the front baffle. For a rectangular or bi-radial horn, mark both axes and identify the centreline clearly. Symmetry is critical because a mouth that looks acceptable from one angle can still be acoustically uneven.

A practical method is to create a full-size template from MDF, acrylic sheet or stable plywood. The template can represent the outer mouth, the inner boundary, or a series of stepped contour lines. For a deep flare, several shallow templates often provide better control than trying to remove the entire shape with one cutter.

The router should follow a reference edge that is genuinely accurate. If the template has a slight bump, every pass will copy it into the horn. Check the profile with a long straightedge, flexible ruler and calipers before fixing it to the workpiece.

Prepare The Workbench And Jig

The timber must be held securely without placing clamps in the cutter’s path. A broad sacrificial base with double-sided routing tape, low-profile clamps and a few screws outside the cutting area can prevent movement. Heavy birch plywood is a useful substrate because it resists flexing, while laminated hardwood can provide the visual warmth associated with a finished wooden horn.

Australian workshops often deal with fast humidity changes, particularly around Brisbane, Sydney and coastal Western Australia. Allow solid timber and plywood to acclimatise before machining, and avoid routing a freshly delivered panel that may still be adjusting. A clean, shaded workspace is preferable to a hot garage where adhesive joints and tape can soften.

Before switching on the tool, work through this preparation list:

A 240-volt plunge router with variable speed is usually more versatile than a small trim router for a substantial horn mouth. Use a sharp carbide bit, preferably with a bearing when the template is above the workpiece. A blunt cutter creates heat, tear-out and a ragged edge that becomes difficult to repair.

Route In Controlled Passes

Set the plunge depth for a light cut, commonly around 2–4 mm depending on the material and cutter diameter. Several shallow passes are safer and cleaner than one ambitious plunge. They also make it easier to inspect the surface as the flare develops. On a large mouth, remove the centre waste first with a straight bit, leaving a narrow perimeter for the template-guided operation.

Feed the router against the cutter’s rotation in a conventional cut. With the bit turning clockwise when viewed from above, this normally means moving the router anticlockwise around an outside edge. A climb cut can pull the machine forward unexpectedly and should be avoided except for carefully controlled, very light clean-up work by an experienced operator.

Keep the base of the router flat and move at a steady pace. Pausing in one location can burn the timber, while rushing produces scallops. Listen to the motor: a strained pitch usually indicates excessive depth, a blunt cutter or feed speed that is too slow. On dense hardwood laminations, reduce the pass depth and make a separate finishing cut.

For a curved horn mouth, do not force the router around a tight turn. The cutter diameter sets a minimum radius, and a large bit may bridge a concave section rather than follow it accurately. Use a smaller guided bit where necessary, or leave the tight transition for a sanding drum, spindle sander or hand rasp.

Refine The Surface Without Losing Shape

Routing should leave a consistent surface, but it will not remove every witness mark. Begin with a firm sanding block or shaped sanding pad that matches the flare. Flexible abrasive wrapped around a foam or rubber former can work well on a convex curve, while a rigid block helps preserve a straight section.

Avoid aggressive freehand sanding at the throat. That area is small, and a few strokes can enlarge it or change the flare rate. Mark the surface with light pencil lines and sand until the marks disappear evenly. This simple method reveals low spots without relying on touch alone.

These finishing choices help protect the acoustic contour:

For a natural timber horn, oil can emphasise the grain but may slightly darken end grain. A hardwax or satin clear finish offers a more uniform appearance, while a painted horn can hide colour variation but will still reveal poor surface preparation. Test the finish on an offcut made from the same laminate.

Compare Router Strategies

Different horn constructions call for different cutting arrangements. A single template-guided pass may suit a shallow plywood waveguide, whereas a laminated solid-wood horn often benefits from rough shaping followed by several contour passes. The best method is the one that maintains the intended throat-to-mouth transition and keeps the workpiece stable.

The table below compares common approaches for a workshop making a custom loudspeaker horn.

Router strategy Best suited to Main advantage Main risk
Template-guided flush trim Consistent outer mouth profile Fast, repeatable edge Bearing or template errors are copied
Plunge routing with stepped templates Deep flare and layered construction Good control over material removal More setup and alignment time
Circle-jig routing Round or elliptical mouth Accurate concentric geometry Less suitable for complex bi-radial shapes
Freehand shaping after roughing Final transitions and small corrections Flexible access to difficult areas Easy to alter symmetry or flare rate
Router plus sanding former Smooth visible acoustic surface Blends machining marks effectively Requires patience and profile checking

A stepped-template process is often the safest compromise for a large horn. Route each contour slightly proud, inspect the result under strong side lighting, then make a final shallow pass. In Australia, where specialist hardwood and acoustic components may involve long freight distances, this cautious workflow also reduces the chance of ruining an expensive panel that is inconvenient to replace.

Once the mouth is smooth, check the horn from the driver mounting face, the listening position and the side profile. The finished edge should flow continuously into the baffle, with no abrupt flat, hollow or change in symmetry. Accurate routing, measured sanding and disciplined dust control turn a rough opening into a precise acoustic surface ready for assembly.