Making the box bigger => lowering the resonant frequency.The internal volume of the box and the internal volume of the port are key to defining the resonance frequency. The air inside the port has its own mass and resonates according to the movement of the air “spring” inside the box (see the closed box article for more information about this “spring”). To achieve a desired resonant frequency of the box, now you have to take into account the dimensions of the port as well. Read more and sign up Achieving a desired f B (resonant frequency) That is why the bass reflex speaker has such a steep roll-off (24 db / octave) below the resonance frequency point. 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. Lower distortion at resonance frequency.Normally after reaching the resonance frequency of the driver, the response will start to go down hill, but that is when the port starts to put in some work and extends the frequency response.
+3 db efficiency compared to the equivalent volume sealed enclosure.Why choose a bass reflex speaker design ? While building a bass reflex enclosure is pretty straight forward, designing it will take a bit more effort and the end result doesn’t tolerate designing errors, like a sealed enclosure does. It gives the best balance between efficiency, low end extension and build difficulty. It’s like a sweet spot, if you will, for woofer enclosures. This type of enclosure is very popular and is the pick of the range when stacked against a sealed enclosure. This vent helps increase the efficiency of the woofer, with substantial contribution on the low frequencies. A bass reflex speaker is just like a normal closed box speaker, but in addition, has a an open tunnel, or port, which allows air to circulate freely in and out of the box.
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