Dipole

Almost all manufacturers place the midrange/woofer driver in a box. This is done for one primary reason, to produce bass.

Keep in mind that any driver, whether it is a woofer, a midrange or a tweeter, radiates equal sound pressure to the front and to the rear. This means that during even normal listening, a large amount of sound energy leaving the rear of any driver has to go somewhere. Imagine, 90dB into your 3000 cubic foot listening room is also radiating into the 1 cubic foot enclosure behind the midrange. Under ideal circumstances, it is entirely absorbed by the 1" thick, heavy enclosure, and damping material, but what about the 1/64" thick driver membrane? In real life, some of this energy escapes out of the cabinet walls, but much more comes back through the membrane of the driver.

A beneficial side effect of not enclosing the midrange is to take advantage of the natural dipole radiation pattern. Because of the fact that the sound waves radiate towards the front of the baffle are exact opposites of the sound waves that radiate out of the rear of the enclosure, they sum to zero at 90 degrees off axis. This means that through the entire range that the midrange is operating, much less sound will be reflecting off of nearby walls, floors, ceilings and other objects, only increasing the clarity of the source.

Coaxial Midrange and Tweeter

You probably either assumed that this design used one large full range driver, or that it was a coaxial midrange-tweeter arrangement. The latter is correct. It uses a 28mm silk dome tweeter mounted inside of a 22cm magnesium midrange driver.

Choosing a driver arrangement like this solves many of the problems that speaker designers have struggled with for years. No matter what steps are taken, when one driver is high-passed (the tweeter) and one driver is low-passed (the midrange), there is a small band of frequencies where both drivers must play to keep an even frequency response. Unfortunately the midrange and tweeter are located at different locations, and at all angles, the band of frequencies that they both play in is not reproduced properly at all frequencies and at all angles. While a speaker will sum to a flat frequency response directly on-axis, because of these path-length differences there will be severe dips and peaks in the frequency response at various angles off axis. This plays a large part in the overall sound that you will hear. Not only are you listening to the direct on-axis sound emanating from the speaker, but you are listening to literally hundreds of reflections bouncing off every surface in your room, and most of these reflections are off-axis of the speaker, so you do indeed hear these peaks and dips off axis. This can be general summed up as ‘power response’, the measure of a speaker taking into account it’s entire radiation pattern.

Locating the tweeter at the center of the midrange mitigates all of these problems. The sound radiated at all angles travels the same distance to both drivers, thus eliminating any of the off axis peaks and dips, ensuring a more natural reproduction in your room.

This leads directly into the next design optimization….

Limited Tweeter Dispersion

Consider most speaker where the tweeter is placed alone on the front baffle and the midrange is crossing over to the tweeter. At most typical crossover points, the radiation pattern of the midrange has begun to narrow due to the size of the driver (larger drivers ‘beam’ more at high frequencies than smaller drivers do) Then at a frequency slightly above this point, the tweeter has fully taken over. Due to it’s small size, it has very wide dispersion just above the crossover region. This causes an overall imbalance in the power response.

Our design places the tweeter inside of the midrange. This gives the tweeter a ‘wave guide” that is approximately 155 degrees wide. This controls the dispersion at the low end of the tweeters frequency spectrum, and provides for a better balance when the off-axis energy is considered from the midrange and tweeter.

This has none of the side-effects that typical horn-loading does, horns typically use a compression chamber in front of the tweeter diaphragm and also use much steeper angles to further increase sensitivity. Many horns use 90 degree angles at the throat. At higher frequencies due to the tweeter itself starting to beam as all tweeters do, it has an off-axis response like any conventional 28mm dome tweeter.

Bamboo Construction

The entire speaker is constructed out of bamboo. This provides several key improvements over most standard construction methods.

Due to the cross-ply construction, the fibers of bamboo run vertically on the faces of the speaker, and horizontally in the core of the wood. There is an extreme number of glue joins, providing incredible strength and stiffness.

The Teragren brand bamboo is almost completely formaldehyde free and non-toxic unlike other enclosure materials like MDF. It complies with US Green Building Concil’s LEED (Leadership in Energy and Environmental Design) and classifies as both a rapidly renewable material as well as a low emitting material.

Bamboo is a very quickly renewable resource, it is actually a grass that can be harvested after only 5 years.

Basics

Of course all of this is backed up by solid design fundamentals such as proper integration of drivers, quality crossover components, internal wiring, and accessory parts such as binding posts and spikes.  Almost all of the crossover capacitors are ClarityCap SA series.  These were evaluated to be the best suited to this design through careful listening tests.  All critical inductors, on the midrange and tweeter circuts are composed of copper foil coils.  The midrange series inductors are 12 AWG pure copper foil for low resistance, the main coil weighs in at 4.5 lbs each.