Some loudspeakers are made in the form of the closed box, also known as the acoustic suspension, design. In this design, the air inside the cabinet is completely sealed. There are several good reasons why this works well.
But next step in cabinet design after the closed box is the bass reflex enclosure. You will occasionally hear of this as a ported or vented cabinet.
The bass reflex cabinet borrows the theory of the Helmholtz resonator.
A Helmholtz resonator is nothing more than an enclosed volume of air connected to the outside world by a narrow tube, called the port. The port can stick out of the enclosure as in a beer bottle - a perfect example of the principle - or inwards. The small plug of air in the port bounces against the compliance (springiness) of the larger volume of air inside and resonates readily.
Try blowing across the top of the beer bottle (when pleasurably empty) and you will see.
The Helmholtz resonator can be designed via a relatively simple formula to have any resonant frequency you choose.
In the case of the bass reflex enclosure, the resonant frequency is set just at the point where an equivalently sized closed box would be losing low end response. Thus, the resonance of the enclosure can assist the drive unit just at the point where its output is weakening, thus extending the low frequency response usefully.
There is of course a cost to this. Whereas an closed box loudspeaker can be designed with a low-Q resonance, meaning essentially that when the input ceases the diaphragm returns straight away to its rest position, in a bass reflex loudspeaker the drive unit will overshoot the rest position and then return.
Depending on the quality of the design, it may do this more than once creating an audible resonance. This can result in so-called 'boomy' bass, which is generally undesirable.
Additionally, a loudspeaker with boomy bass will tend to translate any low frequency energy into output at the resonant frequency.
This a carefully tuned and recorded kick drum will come out as a boom at the loudspeaker's resonant frequency.
The competent loudspeaker designer is in control of this and a degree of boominess will be balanced against a subjectively 'good' - if not totally accurate - bass response.
In short, the closed box is more accurate, but the bass reflex has more bass.
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