The back waves do not reverse phase and come out of the port, only to cancel the front waves. Below this point, the port acts just like a hole in a sealed box. Above this point (higher frequencies), the mass of air inside the port is too great to respond to the speaker movement, and it acts like a perfectly sealed enclosure. This effect only happens when reaching the resonance frequency of the port. This is where the +3 db efficiency comes from. Because of this, not only it does not cancel the front waves, it reinforces them ( +x +x = 2x ). The port acts like a Helmholtz resonator, and back waves reverse phase and come out the port in phase with the front waves. And just like in math : – x + x = 0.īut wait a minute! If I use a port, doesn’t that mean that the back waves travel through the port and reach the front waves and achieve cancellation ? That is a very good question, because at first glance, that looks like a valid theory. You can make an analogy with math, positive numbers are in phase and negative numbers are out of phase. Because these waves are out of phase, when they meet, they will cancel each other out. We know that the whole point of an enclosure is to separate the waves created by the back of the speaker from the ones generated by the front. Transient response is not as good as sealed equivalent. The vent, if not designed correctly or at high sound levels, can get noisy, as the air escapes the port. After resonance frequency is achieved, the response is degraded with a steep roll-off of 24 db / octave. There are some bad points for a bass reflex speaker as well. This means less distortion and more power handling. The speaker barely moves at the resonant frequency of the box. At this point, the port does most of the work.