I have a soft spot in my heart for that fringe group in audio who swear single full-range drivers are the only way to go. I admit there is a compelling argument for that belief. No crossovers to rob amplifier power, introduce potential signal degradation or cause phasing problems and a true single point source, the holy grail of audio reproduction. These gains, they argue, far outweigh the problems of beaming and excursion created distortion. Personally, I believe there are many roads to audio bliss and individually we all have our preferences, which is why I added Aria to my product line.
When the driver used in the Aria came along I was pretty excited because it’s one of the very few affordable full range drivers that has a minimal amount of anomalies in its frequency response and can even produce reasonable bass response. It is based on Adire Audio’s patented XBL2 motor design, which is capable of long excursion rates while simultaneously maintaining excellent control for upper frequencies. The front driver used in Aria has a free air frequency response of 68Hz to 20kHz +\- 3db. The rear driver has a natural high frequency roll off beginning around 12kHz.
One of the challenges to using single full range drivers is an anomaly known as “baffle diffraction step”. Briefly; when a frequency equal to or longer than the width of the front baffle is reproduced it doesn’t have the benefit of the front baffle to “support it” resulting in a perceived lack of bass response from the speaker. As an example, a 2 foot wide baffle can “support” a frequency of 190Hz. Below 190Hz the drop occurs at 3 6db per octave depending on room placement. Not too many people will tolerate a 2 foot or larger front baffle in their living room. Aria’s very narrow 7.5″ cabinet would begin suffering from baffle step around 1900 Hz. The typical solution is to employ a passive circuit that “pushes down” the treble response in opposite and equal proportion to the drop of bass response. Other solutions involve line-level filters that essentially do the same thing.
For purists, using any type of circuitry is sacrilege.
Using a second driver on the back, directly opposite the front driver creates an acoustical solution to baffle step rather than an electrical solution. The reason has to do with the frequencies of concern operating in 4pi space rather than 2pi space. For more details type “baffle diffraction step” in your favorite Internet search engine. Other benefits gained are increased sensitivity and cancellation of the mechanical motion of the driver(s) on the cabinet. “Bipole” also means that the drivers are “in phase”; both drivers move in and out at the same time.
What is a restricted terminus quarter wave resonator?
This is the evolution of the “transmission line” speaker cabinet experimented with by Voigt and many others. Quarter wave resonators attempt to increase bass response by having a “pipe” of sufficient length and cross sectional area effectively “stack up” the frequencies produced by the back wave of the driver inside the cabinet. The lower frequencies are allowed to exit the cabinet at the “terminus”. Early TL designs had an open end and a closed end. The driver was mounted at or near the closed end and the opposite, open end was the terminus. Through much experimentation the design began incorporating a restricted terminus, such as a port or port tube, and stuffing (fiberglass or polyester wool) to help minimize higher frequencies exiting the terminus. The driver was moved from the end of the pipe to a point around 1/3 to 1/2 the length to minimize odd order harmonics resonating inside the pipe. For these reasons a restricted terminus quarter wave resonator cannot accurately be called a transmission line. The Aria design is another step in the evolution of this design because, in addition to a restricted terminus and stuffing, it utilizes a series of internal chambers. The combination of these three components act like a quarter wave resonator, minimize harmonics and provide driver damping closer to that of a sealed cabinet than a typical bass reflex enclosure. Aria’s demure 40″ tall cabinet behaves as something much longer and allows the driver to be moved back to the “closed end of the “pipe”