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What is Bass-reflex enclosure?

What is a bass reflex subwoofer?




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. This vent helps increasebass reflex subwoofer the efficiency of the woofer, with substantial contribution on the low frequencies. This type of enclosure is very popular and is the pick of the range when stacked against a sealed enclosure. It’s like a sweet spot, if you will, for woofer enclosures. It gives the best balance between efficiency, low end extension and build difficulty. 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.


Why choose a bass reflex speaker design?


Here are some pros :


+3 db efficiency compared to the equivalent volume sealed enclosure.

Lower cutoff 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.

Lower distortion at resonance frequency. At this point, the port does most of the work. The speaker barely moves at the resonant frequency of the box. This means less distortion and more power handling.

There are some bad points for a bass reflex speaker as well. After resonance frequency is achieved, the response is degraded with a steep roll-off of 24 db / octave. The vent, if not designed correctly or at high sound levels, can get noisy, as the air escapes the port. Transient response is not as good as sealed equivalent.



How does bass reflex work ?

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. Because these waves Bass reflex helmholtz resonatorare out of phase, when they meet, they will cancel each other out. You can make an analogy with math, positive numbers are in phase and negative numbers are out of phase. And just like in math : – x + x = 0.


But 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. In reality, something else happens. The port acts like a Helmholtz resonator, and back waves reverse phase and come out the port in phase with the front waves. Because of this, not only it does not cancel the front waves, it reinforces them ( +x +x = 2x ). This is where the +3 db efficiency comes from.


This effect only happens when reaching the resonance frequency of the port. 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. Below this point, the port acts just like a hole in a sealed box . The back waves do not reverse phase and come out of the port, only to cancel the front waves. That is why the bass reflex speaker has such a steep roll-off (24 db / octave) below the resonance frequency point.

Additional factors to consider

Subsonic filtering. Below the resonant frequency of the box-port assembly, the vent acts just like a hole in a sealed box. As a result, the compliance of the box is really high and the speaker moves unrestricted. In the absence of the internal air “spring”, the woofer is not damped anymore and can reach impressive excursions, over the quoted Xmax, if push too hard. In conclusion, use a passive or active filter, to reduce the chance of speaker damage.


Damping material. Like with sealed enclosures, damping material can be used to reduce panel resonances and standing waves. Unlike the sealed enclosure, you don’t stuff the box with damping material, because you will obstruct the port. Just place 1-2″ absorbent material on the the walls of the enclosure (on one of each opposite sides or all of the walls).


Conclusion

The bass reflex speaker design, while not difficult to build, will take a bit more time to design. However, we will cover more about designing the bass reflex box in a future article, because we only mentioned the basic principles (check out the bass reflex alignments). This should not discourage you to try something more challenging. Building and designing a sealed enclosure is pretty straight forward and you should definitely start with one of those. Ported boxes are not so tolerant to design and build errors. So, when you are ready to take a step forward, and think that the advantages of a bass reflex subwoofer are something you want, you should definitely give it a try.


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