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Sorry, rilla. Stupid fake-howl from JKDS. I suck at ww, btw.
First off, there is no such thing as negative absolute pressure. Absolute pressure is strictly non-negative.
Contrast with gauge pressure, which is more commonly used in affordable pressure gauges. Gauge pressure is relative to some non-zero pressure (typically 14.7 psi, or 101,325 Pa). A typical gauge can read out a negative value, because it's measuring a pressure difference.
These gauges are cheaper to manufacture because they to not need a robust pressure vessel built into them which maintains a vacuum. They just need a way to vent and reseal a (much weaker) pressure vessel, allowing it to come into equilibrium with whatever the ambient pressure is. This allows for accurate readings on cloudy days.
This negative value on the gauge is not indicative of a negative absolute pressure. It can only be at most as negative as the ambient pressure at the time the vent was sealed on the pressure vessel. This corresponds to an absolute pressure of 0 Pa.
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What is pressure?
Pressure is caused by particles bouncing off of each other. If the particles are held in some vessel, they will bounce off the vessel, too. The higher the particle density, the higher the pressure. The higher the temperature, the more rapid the vibrations of the particles (as I've discussed before), which results in collisions which impart more energy transfer, and more rapidly moving particles, which in turn causes more frequent collisions, and higher pressure.
All that boils down to the ideal gas law. As per usual, this is an approximation, but it's good enough to illustrate my point.
PV = nRT
Pressure, P, times Volume, V, equals the number of particles, n, times {some positive constant}, R, times the Temperature, T.
Reformed to match what I said:
P = {R} * n/V * T
Pressure goes up when particle density (number of particles, n / volume, V) goes up. Pressure goes up when temperature goes up.
For the sake of talking about sound, we can ignore temperature, and, since it's always positive, too, we can just lump it in with R.
P = {blah} * n/V
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What is sound?
Sound is a waving of particles in a medium.
When a particle moves, the space it enters gains higher particle-density, thus higher pressure. The space it leaves has lower particle density, and thus lower pressure.
This change in particle density is self-interacting. The particle collisions are going to favor bounces which send particles to the lower pressure region, so they move there... but now their moving leaves a lower particle density region behind them. So now the collisions favor moving them back. Back and forth; back and forth. This is particles waving.
It's not the particle-waves / wave-particles of QM... it's macroscopic particles moving in non-QM ways... this is classical physics. Sure the wave-particle stuff is going on, but at a much smaller length scale and a much shorter time scale.
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Sound is pressure waves moving though the air. The pressure waves oscillate back and forth around some equilibrium pressure (typically close to 14.7 psi or 101,325 Pa). This equilibrium pressure imposes a maximum wave size, for exactly the reason that pressure can not go negative.
I found this exerpt from a really excellent article about Krakatoa:
[...] there’s a limit to how loud a sound can get. At some point, the fluctuations in air pressure are so large that the low pressure regions hit zero pressure—a vacuum—and you can’t get any lower than that. This limit happens to be about 194 decibels for a sound in Earth’s atmosphere. Any louder, and the sound is no longer just passing through the air, it’s actually pushing the air along with it, creating a pressurized burst of moving air known as a shock wave.
Taken from this article about the volcanic eruption of Krakatoa in 1883.
Physically, a shock wave is a region, or volume, characterized by a dramatic change in pressure, temperature and density.
More on shock waves later.
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Originally Posted by MadMojoMonkey
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I don't understand the fallacy part.
rilla again, FFS.
