This is a speaker we've wanted to create for some time. Perhaps our most versatile ever. We wanted to create a compact point source speaker. It would cater to some unique situations:
- nearfield monitors
- a small room where a deeply involving sound stage is desired
- valve and lower powered amplifiers
- a centre channel speaker that can be placed on its side
- a surround speaker with equal horizontal and vertical dispersion to serve more than one row of seats
- a premium Atmos ceiling surround speaker (which doesn't measure so badly the response plot has to be hidden)
These are quite diverse situations but all of them benefit greatly from a point source speaker. We decided to commence working on this project and the first step was finding the right driver.
One promising driver was the Eminence Beta8CX. The parameters were quite useful and initially it seemed quite appealing that any compression driver with a thread could be tested. We're quite fussy on which compression driver we use as they are very often make or break with HE designs. Price was also an attractive feature. We know that people don't tend to spend big on surround speakers, so this was a key issue.
The Eminence driver has useful sensitivity and works well in a reasonably compact enclosure. However, when we tested various compression drivers, we noticed a serious problem.
There is a broad 15 dB valley from 4 - 11k. We tried various compression drivers but all exhibited a similar problem. Even with the benefit of a DSP active crossover, the sonic result was poor. One of our requirements was the ability to use a passive crossover, where we don't have the option to simply add EQ to a valley this large.
A further challenge with coaxial drivers is that they tend to be expensive. Money does not necessarily buy you exceptional performance. The reality is the coaxial drivers are difficult to get right.
Here is an example of one of the better units:
This woofer is mounted in a different enclosure so it's not directly comparable in all respects, however, the far superior response in the treble region is clear. Despite being one of the best, we can still see some obvious problems. Numerous dips in the woofer response above 1k where we are likely to want to cross to the tweeter. A peak just above 4k which is very audible. We can also see that the compression driver has very limited bottom end response.
The Game changer
Here we have a driver that changes the game. Without this driver, this project would not get off the ground.
In contrast to the cheaper stamped steel frame of the Eminence, we have a very solid cast frame. There is no dust cap here. An accordion cloth surround ensures very long life and very control of the cone edges.
The compression driver is integral to the design. Shown above is the back of the phase plug and you may also notice the copper shorting ring. This arrangement provides much better alignment of the acoustic centres.
Here we can see the best of both previous drivers. The woofer has a well behaved response, free from the problemmatic dips and breakup of the pro coax. We also have a very nicely extended top end without a falling response that needs to be corrected - this makes for a simpler crossover. Overall, a much more usable driver. Our first trial with a very crude crossover revealed a musical result - this is a very good sign.