I've been practising off and on most of the day after a week off, mainly pedals and a bit of multiple tonguing practice. Then just now I was working up some aimless tunes around the bottom of the stave when I realised that I was not alone: there was another distinct voice playing what I was playing but transposed about a minor third down. Resisting the obvious immediate impulse to fire up ebay and bid for a dozen mouthpieces and a couple of new trumpets, I had a bit of a think about it. Then an idea dawned. We had a new generator installed while I was away and this one has some soundproofing so its a quietish, relatively clean hum. Grid power went down a couple of hours ago, so this was the first time I'd heard trumpet and new gen together. Checking the numbers, 50 Hz off the low stave notes would be around a minor third, so I guess that's probably it. A resultant tone with the new gen. What was that Robin said about not being the best judge of your own sound? Anybody experienced something similar?