Acoustic resonance:

Sounds are waves in the air, specifically longitudinal waves. These type of waves cause pressure waves (90 degrees
out of phase with wave itself) that are directly proportional in strength to the amplitude of the sound wave. It is
possible to utilize this effect to increase intake air pressures within the intake tube. In most instances the
amplitude of a sound wave is not that large. During resonance however the sound wave amplitude becomes much larger
(and so does the pressure wave following it). During this resonance it is possible to gain significant amounts of
torque and actually create positive manifold pressure (above 1 atm). This effect is sometimes called the AEM HUMP,
where power spikes ~10hp for a couple hundred RPM around 4500 RPM in their cold air intakes. The resonance frequency
of a pipe depends on the length and the diameter of the tube.

Fn= n (v/4L)

n = wave number, v = velocity of sound (~340m/s), L length of pipe
Note: v the speed of sound changes with temperature.

The diameter of a pipe has a small effect on the resonance frequency as well. The sound wave behavior remains consistent
with that of a pipe for ~.6 times the diameter (D). So

Fn= n (v)
___________
4(L+.6(D))

For a 1-meter intake pipe this translates into a resonance frequency of 85 hz or 5100 RPM. The intake pipe reaches
resonance frequency when the driving force of the system (the engine) is also at that frequency. This is RPM/60;
rotations per second.