The first version of this belt carried its audio module in a pocket on the strap, on the lower right edge, and the spec described the module as 22 × 13 × 15 mm sitting in a 42 × 11 × 8 mm dock. Both numbers were written down. Neither of us subtracted one from the other.
It is over by 5 mm in depth and 4 mm in height. It was never going to fit.
The problem underneath the problem
Even if the arithmetic had worked, the idea was wrong. A 13 mm lump behind a 3.6 mm strap sits exactly where a waistband folds when you sit down, and you would feel it every time. Leather also bends, which is the entire point of leather, so it cannot hold a rigid pocket's shape under load.
The fix was already in the design and nobody had noticed: the buckle. It is 11.8 mm thick, it is rigid, and it stands off the body on its own backplate. A cavity inside it adds nothing you can feel, because the outer surface does not move. The strap goes back to being leather and a thin liner along its whole length, which is what makes a belt comfortable in the first place.
What moved
The module now sits in a 21 × 31 × 9.2 mm cavity machined into the buckle, opening only on the lower edge, with a shallow scoop for a thumbnail. It is a real cavity, cut with a boolean, not a dark plate laid over solid metal. That distinction matters more than it sounds: an overlaid plate reads as a sticker in a render and hides the fact that the walls were never checked.
Two things collided with the new cavity and had to move. The mechanical release tab was at x = +10.5 mm, directly inside the bay footprint, and went to −8. The contextual button went to −24.
Then the module was still wrong
The first re-proportioning was 24 × 17 × 7 mm, and it had the axes backwards. The 24 mm ran across the ear and only 17 mm ran along the nozzle. The bowl of the ear is about 20 mm at its widest, so a rigid 24 mm plate spanning it presses on cartilage. Ordinary stemless earbuds are about 17 mm across for exactly this reason.
It is now 18 × 21 × 7 mm. Eighteen across the ear, twenty-one running down the bowl like a short flat stem, and the end that meets the canal is the narrow, thinner end. Crowned, it stands 8.1 mm off your head, which is flusher than most earbuds at 8 to 12 mm. That flushness is the one thing about this module we are confident is genuinely good.
The shell could not be metal
A Bluetooth antenna sealed inside a closed metal box does not radiate. The body and nozzle are a composite and the aluminium is only the outer face plate, which doubles as the whole touch surface. This was an electrical requirement that happened to produce a better looking object than one material would have. We did not choose it for the look.
What is still unresolved
The nozzle leaves the body at 20 degrees. A bud resting in the ear bowl wants closer to 55. Twenty is simply what an 18 mm body allows without widening the cavity again, and letting the enclosure dictate the acoustic axis is the same class of mistake as the original mismatch. It is on the open list, not solved. The options are reshaping the body, offering two tip sizes, or a wider bay, and we have not picked one.
The internal volume, after walls, is about 1 700 mm³. Into that go a driver with its back volume, a cell, a microphone, an antenna and an SoC. The driver's back volume is what produces bass, and there is not enough of it. That is why we say voice-grade and two to three hours of talking, rather than implying a replacement for real earbuds.
The actual lesson
The reason a five-millimetre mismatch survived in a document for weeks is that prose does not fail. A sentence saying a module fits in a pocket is just as readable whether it is true or not.
So the fit checks are now assertions in the parametric model that builds every render: crowned thickness against cavity depth, module width against the gap between the guide rails, the top of the module against the cavity ceiling, and both underside controls against the cavity footprint. Then the wearability numbers went in beside them, because those decide whether it can be worn at all: width across the ear, how far it protrudes, and whether the nozzle assembly stays inside the body's own width.
The build now refuses to produce an image of a product that could not exist. That is a low bar. It is also a bar we were under.