Why we varnish the inside of a horn to seal the wood
Sunship Audio treats every horn flare as a precision acoustic instrument rather than a piece of carpentry. From the heavily braced birch plywood cabinets in our Berlin workshop to the time-aligned passive crossovers that pair them with TAD-Pioneer drivers, each decision serves a measurable sonic purpose. Sealing the internal walls of a wooden horn with a carefully chosen varnish is one of those decisions that rarely makes headlines yet quietly determines how a loudspeaker sounds across decades of use.
For listeners in Australia, this detail matters more than it might elsewhere. A system shipped from Berlin to a living room in Brisbane, Adelaide or Perth will travel through seasonal swings that few European climates match. Understanding why the varnish is there explains how a fully custom horn-loaded loudspeaker system retains its character whether it ends up in a fibro beach house on the Mornington Peninsula or a climate-controlled media room in Sydney's inner west.
The acoustic imperative behind a sealed horn
A horn is essentially a controlled acoustic transformer. It converts the high-pressure, low-volume output of a compression driver into lower pressure and higher volume at the mouth, which is what gives horn-loaded designs their effortless dynamics and natural scale. That transformation only works when the air column inside the horn behaves predictably. Any leakage, porosity, or movement in the boundary walls disrupts the standing wave pattern and introduces coloration that no amount of equalisation can fix later.
We treat the horn walls as part of the acoustic circuit. Varnishing the interior stabilises the wall impedance and removes the variability that raw timber introduces. The result is a more consistent load on the driver across the entire bandwidth the horn is asked to reproduce, which translates into tighter imaging and a more believable soundstage in the listening seat.
Wood as a living material in variable climates
Birch plywood is a brilliant cabinet material because of its laminated grain structure, yet it still responds to its environment. Australian homes can swing from dry winter air in Canberra to humid summer evenings on the Gold Coast, and timber expands and contracts across those seasons. Without a sealed internal finish, the horn walls would breathe slightly every time the relative humidity changed, subtly shifting the effective length and flare profile of the horn.
Varnish acts as a moisture regulator as much as a sealer. By slowing the rate at which the wood exchanges water vapour with the air inside the cabinet, the finish keeps the geometry stable. This is one of the reasons a Sunship Audio system that was voiced in Berlin continues to perform as designed years later in a Darwin penthouse or a Hobart townhouse, even when the owners have never seen the inside of their own speakers.
How varnish interacts with birch plywood walls
Birch ply is dimensionally stable, but it is still built from alternating veneers that each carry their own grain direction. When the wood is freshly machined, the exposed edges of those veneers are the most vulnerable points on any horn. They absorb and release moisture faster than the faces, and they can also harbour microscopic fibres that gradually work loose under the constant vibration of high SPL playback.
Applying varnish to the inside of the horn saturates those exposed edges and binds the surface fibres into a unified skin. We use a hard, slow-cure varnish that penetrates the first millimetre or so of the ply and then builds a thin film across the surface. The film is kept thin deliberately, because a thick rubbery coating would damp the horn in the wrong part of the frequency range and rob the design of its intended liveliness.
The same film also locks the surface down for the long term. Untreated wood inside a horn slowly darkens, collects dust, and may release small amounts of aromatic resin that can settle on driver diaphragms over the years. The varnish isolates the driver from anything the wood might emit and gives the interior a clean appearance that makes any future service work in our Berlin workshop straightforward.
Preventing air leakage through micro-pores
Wood is not airtight at the microscopic level. Even a finely machined birch plywood panel contains thousands of micro-pores and resin channels per square centimetre, and the glue lines between veneers can harbour tiny voids. For a sealed-box design, a few leaks might be tolerable. For a horn, they are not, because the horn relies on a precise pressure differential between the throat and the mouth.
Varnish fills those micro-pores and seals the glue lines against any slow bleed of air. The horn then behaves like a continuous, controlled waveguide rather than a leaky wooden pipe. That extra degree of sealing is one reason our demonstration room in Berlin can showcase the same effortless dynamic range that customers later hear in their own homes, thousands of kilometres away.
Damping reflections inside the horn mouth
A horn flare is a transmission line as much as it is an amplifier. Sound waves travel along its length, expand, and partially reflect at every change in cross-section. If the interior surface is too lively, those internal reflections bounce back toward the throat and re-enter the driver's acoustic environment, smearing transients and adding a boxy character to vocals and midrange instruments.
Varnish introduces just enough internal damping to settle the highest-frequency wall vibrations without killing the natural musicality of the wood. We have spent considerable time tuning the number of coats and the cure schedule to land in this sweet spot. Combined with the heavily braced cabinet and the time-aligned crossover, this is one of the quiet reasons the speakers sound coherent from the listening seat rather than just on paper.
Varnish versus other sealing approaches
There are several ways to seal the inside of a wooden horn, and each comes with trade-offs. The summary below outlines how our chosen hard varnish compares with the alternatives we considered during development.
| Sealing method | Air sealing | Humidity stability | Effect on damping | Repairability | Long-term appearance |
|---|---|---|---|---|---|
| Hard varnish (our choice) | Excellent | High | Moderate, tunable | Good with light sanding | Stable film |
| Shellac | Good | Moderate | Light | Very easy | Amber, darkens |
| Two-part epoxy | Excellent | Very high | Heavy, may over-damp | Difficult | Hard, glossy |
| Polyurethane | Very good | High | Moderate to heavy | Moderate | Tough, plasticky |
| Hard wax oil | Fair | Low | Very light | Easy | Natural, matte |
| Unsealed wood | Poor | Very low | Variable | Low | Changes over time |
The hard varnish sits in a balanced middle that suits our bi-radial wooden horns particularly well. It seals without choking the sound, stabilises without over-damping, and remains serviceable decades later.
Practical recommendations for owners
If you already own a Sunship Audio system, or are considering commissioning one, a few habits will help the internal varnish finish last as long as the drivers themselves.
- Keep the listening room within a sensible humidity range, ideally between 40 and 60 percent, which is comfortable for instruments and humans alike.
- Avoid placing the speakers in direct sun, especially near north-facing windows in Melbourne or Brisbane houses where afternoon light is intense.
- Use a soft brush rather than a vacuum nozzle when dusting around the horn mouths to avoid scuffing the surrounding baffle.
- Leave the grilles in place if small children or pets share the room, since the horn mouths are deliberately shaped rather than flat.
- Schedule a periodic check with our workshop every few years, particularly if the system has travelled between climates such as a coastal Sydney apartment and a dry inland property.
- Avoid stacking books or placing heavy objects directly on the cabinet, as the bracing is tuned to the cabinet's intended orientation.
- Treat the system as a long-term instrument rather than furniture to be moved around the house on a whim.
For readers who would like a broader overview of what a fully custom horn-loaded system brings to a listening room, our guide to the rewards of owning a fully custom horn loaded loudspeaker system walks through the design choices in more detail.