Bass reflex loud speakers design have an uncertain past. Most authorities would agree that the bass reflex loudspeaker was conceived of in the late 1940s, but there is some data from old audiology textbooks that this idea was used in hearing aids long before it appeared in loudspeaker design.
The bass reflex loudspeaker has a simple design; it is a conventional loud speaker but with a port or tube running from behind the loud speaker to the outside of the box. Depending on several parameters, such as the diameter of the port, the length of the tubing, and the volume of air trapped in the loud speaker enclosure, this port would have a resonant frequency. If the tube was long and wide enough, this resonant frequency would be quite low and could be used to extend the low frequency end of the bandwidth down to quite a low note.
Graphic design of the bass reflex loud speaker. Figure courtesy of www.en.wikipedia.org
For the audiologists reading this blog, this is precisely what a hearing aid vent does. The vent decreases the amount of low frequency gain that the hard of hearing person may receive from a hearing aid, but also increases (near its resonant frequency) that same gain. The net effect for many vented hearing aid systems is that the hard of hearing person receives more low frequency gain than is sometimes specified.
This low frequency resonance (sometimes called an inertance) is the result of the mass of air in the vent, oscillating as a single lumped element according to the equation:
F = 5500 Hz (cross sectional area/LVe)1/2
In this formula that I lifted from the work of Dr. Robyn Cox, the wider the vent (top of the equation), or equivalently, the shorter (L) the vent (bottom of the equation), the higher will be the resonant frequency. In contrast, a narrow, long vent will have a very low resonant frequency. The Ve term is the equivalent volume between the end of the earmold and the eardrum, not unlike the volume of the speaker enclosure.
Bass reflex loud speaker systems would be able to generate significantly more low frequency bass notes than conventional loud speaker designs of the same era. Of course, they were not without their drawbacks. The bass reflex loud speakers have a poor transient response near the low frequency end of their range with an associated lack of a flat frequency response. And depending on where the port was, there could be other reflective surfaces in the room such as a wall that made these loud speakers sound quite poor.
Nevertheless, with proper modelling and control of many simultaneously occurring parameters, these loud speakers could generate a wide range of sounds with minimal distortion. This was one of the inventions that the Bose Corporation took advantage of in order to manufacture a broadband sound system in a small box.
And now that we know all about air suspended loud speakers and bass reflex loudspeakers, part 3 will discuss why with six loudspeakers, the system is still called 5.1 and not 6.
