• As you play louder, your timbre changes. A loud horn has a different tone quality. Although all the partials in the tone get louder, the higher frequencies partials increase far more.

  If you make a recording of yourself playing quietly, and play the recording back with the volume up, it does not sound like a horn played loudly.

• When you double the number of players, but don't change anything the players are doing, then the sound pressure level goes up 3 decibels.

  I just got out my horn and watched a sound level meter while I played loudly and quietly. I found a 25 to 30 decibel difference, depending on which note I was playing. This corresponds to what you find in the research literature: see Rossing's The Science of Sound.

  If you believe that there are six distinguishable dynamic levels (pp,p,mp,mf,f,ff) then you might expect a 5 or 6 decibel difference from one level to another.

  So if there is a 5 or 6 decibel difference between forte and fortissimo, you will not get fortissimo by doubling the number of players playing forte, since that will get you only 3 decibels.

I propose that you take a sound level meter to your next rehearsal and see if you can show the players the difference.