In response to this column on cooling coffee with your breath, reader Michael Godfrey wrote:
Cecil replied dismissively:
I have trouble understanding how anyone could believe that you can’t alter the pressure of your breath by puckering your lips. But maybe that’s because I played trumpet for six years.
The only way to make a sound on a trumpet, or any other brass instrument, is to do what Michael Godfrey describes: The player flexes his diaphragm to blow out violently, just as if you were blowing out birthday candles. At the same time his flexes and puckers his lips, to hold back most of the air and increase the pressure. He leaves only a small gap between his lips to allow a thin jet of high-speed air to escape. That jet causes his lips to vibrate, kinda like blowing a raspberry, and the sound of his vibrating lips, passing through a metal pipe, is what you hear as a trombone or trumpet.
And it’s not enough just to pressurize the breath. Brass players practice pressurizing their airstream to just the right extent. Lower pressures produce lower notes, and higher pressures produce higher notes.
That’s why, if you watch a trumpet player in action, you can usually see his face scrunch up and even change color as he hits the high notes. He’s straining to both 1) flex his diaphragm to super-pressurize the air in his mouth, and 2) flex the muscles of his lips and cheeks to contain that super-pressurized air and form it into the correct stream.
That’s also why novice trumpet players often puff their cheeks out. They haven’t yet learned to flex their cheeks to control the intense air pressure inside their mouths.
So Cecil really goofed with that off-handed comment. And now we can safely assume that he never played a brass instrument.
You have to read the comment in context, which you obviously didn’t.
He’s talking specifically about pressurizing air to the extent that the pressure is causing significant cooling. I’m sure you blow a mean trumpet and all, but have you ever noticed your trumpet get cold or have condensation form on the outside? Didn’t think so.
But isn’t there a significant difference in temperature between puckered blowing and the “huuuuuh” kind of blowing that you would do to warm your hands? It certainly feels like it here in my climate controlled office.
The cooling effect that you feel is some combination of decompression, convection, and evaporation. Somebody who knew the physics could do the math. As a starting point, this chart puts the maximum pressure from human lungs at 1.9psi.
And another interesting point: “decompression cooling” is adiabatic: the total heat energy content of the mass of gas is the same, but spread out over a greater volume of less density, so per-volume energy content goes down: gets cooler.
But the flipside is that in this specific case, everyone’s talking about having compressed this air in the mouth first. That would also be adiabatic: the air would have to gotten hotter by the precise ratio of compression. Does a trumpeter’s mouth get hotter when holding embrasure but not blowing? I very sincerely doubt it.
And the cooling effect (as strictly attributable to “decompression”) cannot be greater in magnitude than the original compression heating, and the baseline of the temperature change in both directions will be the original (relatively warm) temperature of exhaled air.