The Bass Reflex and Passive Radiator loudspeaker enclosures
The Bass Reflex enclosure takes the idea of the infinite baffle one stage further. The enclosure is airtight, like the infinite baffle, except for a small open vent or port at the front through which some of the sound may escape. The basic principal behind this is that the rear wave is delayed and so emerges 180° later at certain frequencies, which makes it in phase with the front wave therefore enforcing it.
The Bass Reflex enclosure
The air in the inlet port is isolated from the rest of the air in the enclosure except at it’s inside end. It has inertia and therefore a resonant frequency of it’s own. The pipe should be proportioned so that this is similar to that of the rest of the system. This may seem contrary to what has been previously said about avoiding similar resonant frequencies, but it is the manner in which these two frequencies interact which each other that gives the desired effect.
At high frequencies the inertia of the air in the pipe is too great for it to respond, so it therefore acts in the same manner as an infinite baffle enclosure. This system therefore is only beneficial for low frequency bass signals. If possible in my design I should like to consider creating this sort of enclosure for the bass driver (woofer) and attaching a second enclosure for the treble (and possibly mid-range) units.
Compared to closed-box infinite baffle systems, the bass reflex vented enclosure possesses several unique characteristics:
- Lower cone excursion near the box’s resonant frequency, resulting in a higher power handling and less distortion. However there is a high potential for cone excursion at frequencies well below the box’s resonant frequency (another argument for getting this frequency as low as possible) for example subsonic noise caused by record warp if using a turntable. Fortunately, many drivers do not respond to such low frequencies, and they could be filtered if necessary u sing a filter.
- In theory a higher efficiency of +3dB compared to the infinite baffle enclosure. This is because of a lower cone mass requirement for the drive unit.
Example of a bass reflex design
Passive Radiator (ABR)
Another variation on the Bass Reflex design is the passive radiator. Also known as an Auxiliary Bass Radiator (ABR), this is a dummy loudspeaker, i.e. a cone suspension without the magnet and coil. It’s mass serves as a substitute for the air in the pipe in a bass reflex enclosure. It avoids the pipe noise and air turbulence and also serves as a barrier to mid-frequencies which may radiate through a pipe due to internal reflections. They provide the same low frequency extension but without the problems of noises and coloration, and work well with smaller enclosures. This method is employed by several manufacturers, including Celestion and many ‘tower’ format loudspeakers.
The passive radiator