I've been thinking about the Vectrix gearbox whine. Whilst it is obviously caused by the straight-cut gears, I think it is amplified by resonance in the empty spaces at the back of the bike. Whilst the gearbox on my bike is not painfully loud or anything, it does whine and I've been trying to reduce it.
The swinging arm is hollow and particularly large on the left hand side. I removed the right side of the swingarm and had a good look around inside with a boroscope. There's a lot of space in there to resonate. I put mineral wool insulation inside the swinging arm crossmember, quite tightly packed, as far as the motor wires which run through the left side but not ON them (in case they get warm/hot in operation). Also, up the front part of the swinging arm which goes towards the bearing end of things.
The chromed plastic cover on the left of the motor has quite a gap under it and I filled it with a pad of mineral wool... not enough to require squashing and without any pressure being exerted on the encoder cover underneath... just enough to fill the space.
I ran the bike on blocks in my workshop and found there was a definite reduction of volume of the whine, with the added bonus of certain frequencies disappearing altogether. This made the overall noise far less obvious. I ran the bike for several minutes and then removed all the mineral wool and ran the bike again. It was a lot more pronounced without the wool.
The shape of the back end of the bike, with its bulbous panels and spaces everywhere, is like a giant soundbox. It even looks like two lutes stuck together with a seat on top (if you're a musician and have a fertile imagination). With the bike on blocks, I removed the rear wheel and ran the motor at various speeds throughout the range. I could actually feel the bulbous panels resonate at the lower frequencies (in the 20-30mph range especially) I used a stethoscope on the big panels and the gearbox whine is clearly audible through them. So, I'm going to remove the panels and stick foam blocks on the panels, making sure of course that the cooling airways etc are not blocked or disrupted. I've not been able to road test the bike, as it's throwing it down with rain again. However, with a noticeable sound reduction in the workshop, I think the road test will be encouraging. I'll do that next and post again with the results.