How To Minimize Horn Edge Diffraction With Felt Damping
Horn loudspeakers can deliver remarkable dynamics, clarity and scale, yet the horn mouth introduces a difficult acoustic transition. When sound reaches the edge of the flare, part of the wave continues into free space while another portion interacts with the cabinet, baffle or nearby surfaces. The resulting interference can add a fine, bright character to voices, cymbals and brass.
Felt damping is a simple, reversible way to soften that transition. Used carefully, it reduces the strength of reflections and delayed edge energy without absorbing the direct output from the compression driver. For a custom system such as a Sunship Audio loudspeaker, the material, thickness, position and visual integration should be treated as part of the acoustic design rather than as an afterthought.
Why Horn Edge Diffraction Matters
Edge diffraction occurs when a sound wave encounters an abrupt boundary. At the mouth of a bi-radial horn, the wavefront expands into the room and can bend around the rim. The reflected and diffracted energy then combines with the direct wave, creating small peaks and dips in the frequency response. These effects are usually most audible through the upper midrange and treble, where wavelengths are short enough to interact strongly with the horn profile.
| Approach | Acoustic effect | Main advantage | Main limitation |
|---|---|---|---|
| Bare horn edge | Strongest boundary reflection | Maximum visual simplicity | Can add glare and comb filtering |
| Thin felt strip | Mild absorption and scattering | Easy to tune and remove | Limited effect at lower frequencies |
| Thick, open-cell felt | Wider-band edge control | Better reduction of reflected energy | May look heavier around the mouth |
| Rounded or extended lip | Smoother wave transition | Works as part of the horn geometry | Requires cabinet redesign |
| Felt plus rounded lip | Broadest control | Combines geometry and absorption | Needs careful construction and measurement |
The goal is not to make the horn acoustically dead. A lively, efficient horn should preserve transient attack and low-level detail. Felt works best when it reduces the first reflection from the rim while leaving the central wavefront, driver loading and intended directivity largely unchanged.
Choosing Felt For The Job
The most useful material is dense enough to absorb high-frequency energy but open enough to avoid behaving like a hard decorative trim. Wool felt, polyester acoustic felt and specialised speaker damping fabrics can all work. Wool often has an attractive tactile finish for timber horns, while synthetic felt offers consistent thickness and colour across a larger production run.
Thickness changes the effective bandwidth. A 3 mm or 5 mm strip may be sufficient around a small horn, while a deeper 8 mm to 12 mm profile can provide more gradual acoustic absorption on a large-format mouth. The edge should not be so soft or loose that it curls into the radiating aperture. A clean, stable boundary is essential.
The felt should cover the region where the wave first meets the rim, rather than extending far into the horn throat. Adhesive-backed products are convenient for trials, but a removable mounting method is preferable during development. Double-sided acoustic tape, small retaining channels or a discreet compression fit allow the installer to compare several arrangements before committing to a permanent finish.
Placing Damping Around The Mouth
A continuous felt ring is usually the logical starting point because it treats the entire perimeter consistently. On a rectangular or bi-radial horn, however, the vertical and horizontal edges may contribute differently. The longer side can produce a stronger reflection at certain listening distances, while the corners can act as concentrated scattering points.
Begin with a narrow strip placed flush with the outer lip. Listen to familiar vocal recordings, then compare the same passage with the felt removed. If the sound becomes smoother without losing presence, increase the width in small steps. A strip that projects too far into the aperture can reduce high-frequency output and make the presentation seem overly polite.
- Keep the inner edge even and free from loose fibres
- Match the damping width on opposite sides of the horn
- Treat corners carefully rather than folding bulky felt over them
- Leave access for later measurement and crossover adjustment
In Australian homes, this process is especially useful in rooms with hard plaster, timber floors or large glass doors. A horn that sounds balanced in a treated Berlin demonstration room may seem more forward in a bright Sydney terrace or a sparsely furnished Melbourne apartment. Felt gives the owner a practical way to refine the speaker-room interface.
Integrating Felt With Horn Geometry
The best felt treatment supports the existing flare rather than disguising a poorly resolved edge. A smoothly rounded wooden lip reduces the abruptness of the transition before the damping material is added. Heavily braced birch plywood cabinets can hold the horn rigidly, while the felt controls the smaller, higher-frequency reflection that remains at the mouth.
Bi-radial horns deserve particular care because their horizontal and vertical coverage patterns are deliberately different. A uniform felt ring may be visually neat but acoustically excessive in one plane. A custom builder can taper the strip, alter its density or recess it into a shallow groove so that the treatment follows the horn’s directivity.
Driver choice matters as well. TAD-Pioneer compression drivers and matching woofers can deliver high output with low compression, so the system may reveal small changes in edge treatment more clearly than a conventional soft-dome loudspeaker. The principles behind that driver heritage are explored in TAD’s Legacy, particularly its emphasis on controlled, intelligible dynamics.
Measuring What The Ear Hears
Listening is valuable, but a microphone helps separate horn-edge effects from room reflections. A gated near-field measurement taken close to the horn mouth can show changes in the upper-mid and high-frequency response before the room dominates. Farther away, measurements reveal how the horn, cabinet and listening environment work together.
Useful comparisons include the untreated horn, a narrow felt strip, and a wider or thicker treatment. Look for reduced ripple rather than a large reduction in overall treble. Waterfall and impulse-response views can show whether delayed energy has decreased. If a felt change produces a broad loss of output, it may be covering too much of the aperture or using a material with excessive flow resistance.
- Measure at the acoustic axis and slightly off-axis
- Record identical microphone position and level for each trial
- Compare impulse response, frequency response and decay
- Check both listening distance and the main seating position
Time-aligned passive crossovers make these checks more meaningful because the driver-to-driver relationship is kept stable while the horn treatment changes. The felt should refine the wavefront, not compensate for incorrect crossover timing, cabinet vibration or an unsuitable listening position.
Avoiding Common Installation Errors
A frequent mistake is placing felt only on the visible front edge while leaving a sharp inner step exposed. That step can remain a strong reflector, particularly on a shallow horn. Another error is using household craft foam or dense rubber, which may reflect high frequencies or produce an unnatural, narrow absorption effect.
The adhesive also deserves attention. Excess glue can harden the felt and create a new reflective strip. Spray adhesive applied unevenly may stain timber, while staples or screws can buzz under high sound pressure. On a valuable custom loudspeaker, test the adhesive on an offcut before applying it to an oiled or lacquered horn.
The material should remain firmly attached at high playback levels. In warm regions such as Brisbane or Perth, heat and humidity can soften some adhesives over time. A mechanical recess or removable trim is safer for long-term reliability, especially where the speakers are transported between homes, studios or demonstration events.
A Practical Finish For Australian Listening Rooms
Felt damping can be both functional and visually coherent. Charcoal, black and deep grey tend to disappear around a dark horn, while natural wool tones can complement oiled birch plywood or richly stained timber. The treatment should follow the horn’s curves cleanly, with no exposed adhesive, uneven corners or loose edges.
Australian listeners often deal with open-plan living areas, reflective kitchens and limited freedom to position large loudspeakers far from walls. In a Brisbane Queenslander, a Perth room with tiled surfaces, or a compact Sydney listening space, a modest reduction in edge energy can improve intelligibility without requiring extensive wall treatment. It also helps maintain a relaxed tonal balance during long listening sessions.
The finished result should preserve the immediacy that makes horn systems compelling: clear vocal articulation, effortless peaks and convincing image scale. A properly chosen felt profile reduces the unwanted halo around those qualities while retaining the horn’s efficiency. Examples of carefully resolved cabinet and horn details can be seen in the Sunship gallery, where construction, finish and acoustic intent are considered together.