Ok, but sometimes when your replace only the chain, an unpleasant noise appears the first hundred kilometers

....I might use BC's for audio route prompts though.

Here's an example from JL

https://www.johnlewis.com/shokz-openmove-bluetooth-wireless-open-ear-headphones-grey/p110575797

Yes, I thought routing instructions might be one use for cyclists.

I don't know anything about the John Lewis item you refer to, but I wouldn't have thought it would be particularly suitable for a cyclist. Mine have a half-loop over the ear to position them positively (photo in my original post) while the one from JL you illustrate looks like backwards cafe-racer sunglasses that will slide off your ears -- also backwards -- at the first sleeping policeman you ride over. More fashion than kinesthetics...

Also, they seem a bit pricey to me for something you intend to use only for the presence frequency range.

BTW, the ones I bought are more than reasonably water-resistant. Earlier tonight they fell off a stack of watchboxes on which they were balanced for charging and into my lime and honey tea. I rescued them, shook them a bit to get the running liquid off, put the captive rubber plug back in the USB-C charging port after blowing into the port, and tried them with music, and allelujah, they worked fine.

Hi
Thanks for the contact.
Photos are too large for here.  Are you on WhatsApp?

Apologies for not replying, I must have missed the notifications. I had the seller photograph the BB and figured the sizing codes out from those photos.
If you are the new owner Binky, hope you enjoy! 

Hello,
On my Thorn Raven Twin, I replace my chain about every 7500kms and have just recently replaced the chainrings and sprocket for the first time after accumulating about 44000kms.
Regards
Hugo