Measuring and adjusting acoustic damping in a horn throat
A horn-loaded loudspeaker is only as good as the energy management at its narrowest point. The throat, where the compression driver meets the expanding flare, concentrates acoustic pressure more intensely than almost any other region in a hi-fi system. Too little damping leaves ringing and upper-mid glare, while too much steals the dynamic speed that drew the listener to horn topology in the first place. Australian hobbyists, many of whom gather in Melbourne and Sydney listening rooms or attend events run by clubs such as the Melbourne Audio Club, often discover this trade-off only after a driver has already been mounted.
Adjusting damping is not a one-size-fits-all process. The goal is a frequency response that stays smooth through the driver's passband while the time-domain behaviour remains clean. Sunship Audio has spent years refining passive crossover alignments and bracing techniques in Berlin, and the same attention to throat loading applies whether the cabinet lives in a Bondi apartment or a weatherboard home in suburban Adelaide. The steps below combine acoustic measurement, material science, and a few habits that suit local conditions.
Why horn throat damping matters
The horn throat acts as an acoustic transformer. It converts the high-impedance output of a compression driver into a controlled wavefront that the flare can amplify. Any reflection, resonance, or turbulence at the entrance introduces a delayed return signal that interacts with the diaphragm. The result is a comb-filter effect in the upper midrange and a smearing of transients that the ear reads as congestion. In a typical TAD-Pioneer based system, this band sits right where vocals and acoustic instruments live, so the cost of a poorly damped throat is immediate.
Practical consequences include a hollow or honky colouration, a loss of image focus, and, in extreme cases, audible chuffing at high listening levels. Because horn systems often run with high sensitivity, the listener may also notice fatigue during longer sessions. Australian summers, where afternoon temperatures in Perth and Brisbane regularly push past thirty-five degrees, can dry out natural fibres used for damping and shift their behaviour over time, making periodic re-evaluation a useful habit.
The physics at the throat
Acoustic damping at the throat is governed by the impedance mismatch between the driver and the horn entrance. Porous materials placed near the throat convert a portion of the acoustic energy into heat through viscous and thermal losses. The wavelength of sound at two kilohertz, roughly seventeen centimetres, is much larger than a typical throat diameter of two to four centimetres, so the damping material sits in a region of predominantly longitudinal particle motion. That geometry favours materials with high flow resistivity.
Material thickness, density, and placement distance from the driver all influence the result. A thin, low-density layer will treat only the highest frequencies, while a thicker, denser pack will absorb a broader band. The trick is to absorb enough to kill the offending resonance without adding so much mass-loading that the driver's transient response dulls. This balance is why casual experiments with household foams often disappoint.
Measurement tools and setup
A measurement chain built around a USB microphone calibrated against a reference source, a laptop running Room EQ Wizard or ARTA, and a calibrated sweep generator covers most needs. A microphone stand placed at the listening seat, at ear height and angled toward the horn, provides repeatable data. The room should be quiet, with HVAC and refrigerators switched off where possible. In Australian households, ceiling fans are often left running during long listening sessions, so it pays to switch them off during capture.
Sweeps from two hundred hertz to ten kilohertz are usually sufficient for evaluating throat behaviour, because the horn itself shapes lower frequencies and the driver's own breakup modes sit above. Capture at multiple microphone positions, then average the traces to separate room modes from horn behaviour. A near-field measurement at the mouth of the throat, with the probe microphone a few millimetres from the entrance, offers a cleaner view of what the material is doing before the flare takes over.
Reference materials compared
The right filling for a horn throat depends on density, fire rating, and acoustic absorption profile. The table below compares common choices used in DIY and small-batch builds.
| Material | Typical density (kg/m³) | Flow resistivity | Fire behaviour | Cost in AUD (per sq m) |
|---|---|---|---|---|
| Natural lambswool | 25–35 | Medium | Smoulders, treat for compliance | 45–70 |
| Polyester acoustic batting | 20–40 | Low to medium | Self-extinguishing grades available | 18–30 |
| Mineral wool (slag or rock) | 30–80 | High | Non-combustible, AS/NZS 1530.1 rated | 25–45 |
| Open-cell acoustic foam (25 mm) | 25–32 | Low | Varies, check AS 5637.1 | 35–55 |
| Recycled denim batts | 30–50 | Medium | Low flame spread | 40–60 |
Mineral wool and treated natural fibres remain the most common choices in serious horn builds because of their predictable flow resistance and high temperature tolerance. Australian fire safety standards under the National Construction Code treat built-in absorbers differently from freestanding treatments, so any permanently installed cavity fill should be checked against AS 5637.1 and local council requirements before the cabinet is sealed.
Adjustment procedure
Begin with the horn and driver mounted, the crossover connected, and the cabinet positioned in its final spot. Run a baseline measurement and note any peak between one and five kilohertz that exceeds the surrounding trend by more than three decibels. A common offender is a narrow spike accompanied by a slower-decaying waterfall trace, which indicates ringing at the throat.
Apply a single layer of the chosen material to the throat walls, leaving the central passage clear. Re-measure and compare. If the peak drops but a smaller one appears at a higher frequency, add a second thinner layer. Work in small increments and resist the urge to over-stuff: a throat that is heavily packed sounds dull and loses the airiness that makes a well-made horn special. Allow the material to settle for at least twenty minutes before each measurement, because some fibres shift density as they relax.
If a resonance persists, vary the distance between the material and the driver face. A displacement of ten to fifteen millimetres can move the absorption peak enough to target a stubborn mode. For bi-radial horns with asymmetric side walls, pay attention to whether the offending resonance is broadband or narrow. Broadband ringing usually means the material needs to be denser, while narrow spikes often respond to a smaller, more targeted patch.
Fine-tuning and long-term stability
Once the response is smooth, lock in the configuration by photographing the throat interior, recording the material type, layer count, and the distance from the driver. Keep this in the system file alongside crossover schematics and wire gauges. The horn speaker wire guide covers a related part of the build that benefits from the same record-keeping.
Recheck the throat every twelve to eighteen months, especially in homes with high humidity swings or where air-conditioning runs heavily. Compressors and humidity control in Australian coastal cities can push moisture into untreated wool and encourage compaction. If the trace has shifted, a quick removal, fluffing, and refit usually restores the original behaviour without needing fresh material.
For builders sourcing components locally, retailers such as Atomic Bands supply hardware that suits custom horn projects, while the broader design philosophy behind integrated systems is documented on the Sunship Audio site. Combining measured damping with a well-braced cabinet and a time-aligned crossover is what turns a horn system from a curiosity into a reference-grade instrument, regardless of whether it ends up in a Brisbane studio or a converted warehouse in Fremantle.