The Physics of Sound
Everything we do downstream starts here, so it is worth slowing down for. Sound is mechanical energy travelling through air as waves of pressure. When something vibrates, it pushes and pulls on the air around it, creating little zones of higher and lower pressure that ripple outward — and they travel at roughly 343 m/s in air at 20 C, which works out to about 1 ms for every 34 cm. That speed isn't fixed: it rises about 0.6 m/s for every degree the air warms. It sounds like a footnote, but it is why a room that heats up over the course of a show slowly drifts out of time-alignment and tuning.
A wave has a few properties worth keeping separate in your head. Frequency is how many cycles pass per second, measured in hertz, and we hear it as pitch — humans cover roughly 20 Hz to 20 kHz. Wavelength is just the speed divided by the frequency, and the numbers are surprisingly physical: a 20 Hz tone is about 17 m long, 1 kHz is about 34 cm, and 20 kHz is around 17 mm. That is why low frequencies wrap around obstacles and are hard to aim, while high frequencies stay tightly directional and get soaked up by air and soft surfaces.
Amplitude is the size of the pressure swing, and we perceive it as loudness. Phase is the timing relationship between two waves, and it is the one that catches people out. When two copies of the same sound arrive together they reinforce; when one is delayed by half a wavelength they fight and cancel. That cancellation is comb filtering: if a reflection arrives a little after the direct sound, the first notch in the response lands where the delay equals half a wavelength — at one over twice the delay. A 1 ms reflection, for instance, carves a hole near 500 Hz and then again at every odd multiple above it. Once you can hear that, you stop blaming the EQ for problems the room is creating.
If it sounds thin or hollow, it's phase until proven otherwise.