Why we place a foam slug behind every compression driver
In every Sunship Audio horn-loaded loudspeaker, a small cylinder of open-cell acoustic foam sits inside the back chamber of the compression driver. It looks like an afterthought, almost invisible once the cabinet is closed, yet it shapes the sound more than any crossover component we fit. The reason is rooted in the way compression drivers breathe, and in what happens to the air column behind the diaphragm when a waveform passes through it.
We started experimenting with this slug while refining the Berlin demonstration room, after listening sessions with visitors from places like Sydney, Melbourne, and Brisbane kept pointing to the same kind of subtle glare in the upper midrange. A cone driver behaves quite differently to a compression driver, and the treatment that works behind a paper cone does almost nothing useful behind a titanium or beryllium diaphragm pumping into a phasing plug. That mismatch is what the foam slug is designed to fix.
The backwave problem in a horn-loaded system
A compression driver moves a relatively small diaphragm across a wide frequency range, and the phasing plug at its throat lets the front wave expand into the horn mouth with great efficiency. The rear of the diaphragm, however, still has to go somewhere. That rearward energy bounces off the inside of the magnet assembly, the back cover, and any wiring or bracing that happens to be in the way, and it returns through the diaphragm as a delayed, out-of-phase copy of the original signal.
In a sealed or ported cabinet the backwave is absorbed by the air spring, the stuffing, or the vent. In a compression driver the back volume is tiny, often less than half a litre, so the air inside behaves more like a stiff spring with a high resonant frequency. Without damping, that resonance rings at a few hundred hertz and bleeds forward through the diaphragm as colouration, particularly in the critical 1 to 4 kHz octave where the human ear is most sensitive to distortion.
The fix is straightforward in principle: introduce a lossy element into the back chamber so that acoustic energy is converted to heat before it can return. The challenge is doing that without choking the driver's midrange efficiency or shifting its fundamental resonance.
Why foam rather than wool or fibreglass
We tested three common damping materials over several prototypes: long-fibre sheep's wool, dense fibreglass batting, and reticulated polyurethane foam of the sort used in studio monitor construction. Each behaved differently in the back chamber, and the comparison below summarises how they measured up on the metrics that mattered most to our design goals.
| Material | High-frequency absorption | Airflow resistance | Particle shedding | Compatibility with magnet structure |
|---|---|---|---|---|
| Sheep's wool | Moderate | Low to medium | Low | Good |
| Fibreglass batting | High | High | High | Poor (loose fibres migrate) |
| Reticulated foam | High | Precisely tunable | None | Excellent |
Sheep's wool is a beautiful material, and we still use it generously inside the main cabinet for the woofer section, where its loose fibre structure breathes with the box. Inside the compression driver's back chamber it packed unevenly, created pockets of differing density, and shifted over time as the driver heated up. Fibreglass absorbed very well but shed fine particles that found their way onto the diaphragm surround and onto the voice coil former. The reticulated foam won on every practical count: a single shaped slug sits in the chamber without migration, and we can tune its density per driver model so the damping curve lands exactly where we want it.
Geometry of the slug and how it sits
The foam is not just stuffed into the back chamber. Each slug is cut to a precise diameter and length so that it forms a loose collar around the voice coil former, leaving a small gap of around four to six millimetres between its outer edge and the chamber wall. The rear face of the slug is bonded to the back cover with a thin bead of neutral-cure silicone, which keeps it centred during assembly and during the temperature swings a cabinet sees in a sun-facing lounge room in Adelaide or a humid coastal apartment in Noosa.
The open-cell structure presents a graded acoustic impedance to the rearward wave. Near the diaphragm the foam is sparse and lets the highest frequencies pass through with minimal loss, while deeper into the slug the cell count rises and absorption increases smoothly across the spectrum. This gradient is the key to broadband damping without smearing the transient response, and it is also why we machine the slug to length rather than cutting it with scissors at the bench.
We also chamfer the entry face of the slug so that the air column does not see a sudden impedance step. A sharp edge here causes a small reflection of its own, and the whole point of the exercise is to prevent reflections from anywhere in the back chamber.
What we hear and what we measure
On the measurement bench the foam slug drops the driver's harmonic distortion in the 2 to 5 kHz region by between three and six decibels, depending on drive level, and it flattens the impedance peak associated with the back-chamber resonance by a useful margin. The Thiele-Small parameters of the driver shift slightly, which is why we re-tune the crossover after fitting the slug rather than designing the network first and the damping second.
In the listening room the effect is the one we chase most often: a quieter, more dimensional upper midrange, with female vocals in particular gaining presence without edge. Visitors from the Australian hi-fi community who have heard the system at audio meets in Brisbane and at dealer demonstrations in Melbourne regularly comment on this before any measurement is mentioned, which is the sort of confirmation we value more than any graph. Anyone thinking through the practicalities of fitting or replacing these slugs in the field will find the answers to common questions on our FAQs page.
Recommendations for builders working with compression drivers
A few practical points for anyone fitting horns, building their own back chambers, or refurbishing older drivers:
- Cut the foam slug to length on a band saw or with a sharp knife rather than tearing it, so the cells at the cut face stay open.
- Match the foam density to the driver's back-chamber volume; a one-size approach rarely works across different TAD and Beyma models.
- Bond the rear face only. Gluing the cylindrical surface against the chamber wall creates an air gap that defeats the damping.
- Allow a four to six millimetre radial gap so the slug can breathe thermally without deforming the foam against the wall.
- Re-measure the driver's impedance after fitting. If the resonance peak has not moved and dropped, the slug is either too short or sitting in the wrong position.
- Avoid any foam with closed cells, which reflects rather than absorbs and can actually increase distortion.
- Keep a spare slug in the toolbox. Foam degrades slowly over decades, and a driver that suddenly sounds brighter than it used to often needs nothing more than a fresh slug.
These small habits, picked up over many prototype cycles, save hours of chasing gremlins later. The slug is cheap, the fitting is forgiving, and the sonic improvement is usually obvious the moment the first note plays.