Nordlandstrompet et al, We are getting a little theoretical here, but if we have slow air into a venturi - the air speeds up through the throat - and slows again through divergent duct of the backbore (other things happen and it would be prudent to assess them - but elsewhere). Think of it as trying to pass a fixed volume of air through a tube with a narrowing then widening bore - the volume CAN'T change so, as the diameter diminishes the air must speed up to get the 'fixed' volume through, as the diameter icreases the flow slows, but the VOLUME doesn't change. ROWUK's and NickD comments alude to the same thing. For reference see Bernoulli's Theorum. Another confusion that has been introduced with the gas turbine analogy is this, in gas turbines we, in general, manipulate the airflow to reduce airflow 'vibration' and in our trumpets we would rather have a little bit of controlled vibration - otherwise no sound. So, two different properties of air that are not really complimentary. The trumpet needs air column vibration but that doesn't mean large airflow - if we can get the "contents of the trumpet" to vibrate then that gives the sound - flow, it seems to me, has little to do with it.