Externally, bass reflex boxes are characterized by the fact that, in contrast to closed speaker systems, they have a defined opening on the housing on which a tube sits on the inside.
This trick extends the frequency range downwards with the same housing size. This is because the bass reflex box uses the sound radiated backward by the bass driver and does not – as with closed loudspeakers – convert it into heat inside with damping material.
Physically, this is based on the following principle: the air behind the bass driver exerts a resilient force on its membrane (you can feel this spring effect when you press on a chassis with your fingers). If, on the other hand, the membrane itself emits impulses on the back, the elasticity of the air volume means that the enclosed air and the air at the outlet of the bass reflex tube are excited by a spring effect – generated by the air itself. Ultimately, the air and driver form a resilient and thus oscillating system (oscillating circuit), also called a Helmholtz resonator.
A spring (think of the chassis suspension in a car) usually transmits impulses in a milder way and more or less with a time delay. However, it can also have an impulse-intensifying effect with a certain impulse sequence (resonance frequency: the spring gets a further “push” at exactly the right point when it wants to swing back anyway).
Back to the speaker: The sound impulses that the membrane emits to the rear are passed on to the outside through the bass reflex opening with a time delay due to the spring principle of the enclosed air. This leads to phase shifts between 0 and 180 degrees. (At 180 degrees: the membrane moves inwards; simultaneously, air is pushed out of the bass reflex tube in the opposite direction). Exactly in the middle of these extremes lies the mentioned resonant frequency; it is found at 90 degrees phase shift instead (tuning frequency). At this point, the sound from the rear supports the sound from the front to the maximum: Higher frequency ranges are more or less “sprung”, although the phase shift would be less here. Lower frequencies cause the phase shift to increase to 180 degrees, which, due to the sound signals working against each other (= destructive interference ), also leads to weaker amplitudes.
When developing loudspeakers, it is ultimately crucial that the tuning frequency is correctly tuned and placed where a corresponding closed box would no longer emit any sound due to its limitation in the lower frequency range. Of course, this technology can also artificially inflate the existing upper bass of a closed system.
What are the practical advantages and disadvantages of bass reflex loudspeakers? First, bass reflex systems generally have poorer impulse behavior than closed loudspeakers. In addition: Despite the better deep bass capability, they tend to drop in level quickly in the lower cut-off frequency range.
However, the bass drivers experience a not insignificant workload relief due to the mode of operation shown. As a result, smaller membrane deflections are necessary, leading to fewer sound distortions due to distortion, intermodulation, and the Doppler effect.