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ModeratorSeptember 18, 2020 at 10:39 pm
When you look at how a speaker works, pushing air in first one direction and then the other in order to make the sound wave, and then knowing that the way the port works is to reinforce the target frequency/frequencies by being in phase with the sound coming from the front face of the speaker, and also realizing that no matter where the port is placed (front, rear, sides, top, bottom) the speaker is always acting the same on the port. That is, when the cone moves outward, it pulls air into the port from outside and when moving inward it will force air back into the port from inside. This is a constant fact. But where you place the port on the enclosure determines the phase shift the port needs to accomplish in order to have the port energy match the front face energy to actually reinforce each other. So, if the port is on the front, you want it to be in phase at the resonant frequency so they both push and pull in sync (technically the port is 180° since the back of the cone is pulling while the port is pushing). When you rear port, you want the rear port to be 180° (and again, technically this is 0° since the port is tracking the back of the cone) so that by the time it wraps around the speaker it becomes in phase with the front. The tuning of the port / length determines the output phase of the tuned frequency.
Short enough, yet long enough to give a basic idea, I hope. It’s a deep subject that actually can require quite a few chapters to really nail down.