The purpose of this thread is to outline the ideas behind a compound dipole woofer and describe my own version.
The theory, the science and the inspiration.
A dipole speaker (especially the woofer section) produces a figure of eight radiation pattern. This contrasts with a conventional woofer/subwoofer which acts as a monopole and gives a 360 degree radiation pattern at lower frequencies. These differences are critical as they interact with the room in which they operate in quite different ways. In the most simple terms, dipole bass systems fire forwards towards the listener and backwards towards the front wall but have an effective cancellation to the sides, which can result in significantly fewer room modes being excited and little to no sidewall interaction. There are different types of dipole bass systems including open baffle, U-frame, H-frame, W-frame, compound dipole etc.
Why the compound dipole option?
I initially researched ripole systems and have actually bought the SB Audience woofers to build these but I came across the Celestion System 6000 bass system during my research. Although rather brutalist to look at, I liked their simplicity and, being open baffle and therefore not needing a large air volume/enclosure for each driver, such a design would enable to me to have a large cone area in a small form factor.
Image credit: hifido.co.jp
The S6000 was designed to support (literally as well as acoustically) Celestion’s SL600 sealed monitor speakers. The system is a cone-to-cone open baffle system using 12 inch woofers that are electrically out of phase but acoustically in phase - basically one woofer moves forward while the other moves backwards. If they were electrically in phase there would be very little bass at all. This is different from a ripole bass system whereby both woofers are cone to cone, electrically in phase but fire into a shared chamber that loads the woofer, effectively lowering the system FS in the process.
One of the key advantages of a compound dipole over a ripole or H/W frame system is that unlike the framed systems, a compound dipole does not suffer a large resonance peak - these can operate circa 200 hz or above and effectively limit such systems to playing low/sub bass only otherwise the effect is apparently quite audible as well as clearly measurable. I also discovered a recent ‘research paper’ by a chap called Charlie Laub. He built and tested a range of open baffle bass systems, described their merits and pitfalls and concluded that the compound dipole had a similar frequency response (FR) to a conventional open baffle but with increased output/efficiency due to the doubling up of the drivers. This was sufficient for me to have a go at building my own version as an experiment. I just needed some inexpensive drivers that happened to have suitable TS parameters for my the project. Fortunately BK Electronics were clearing their new VERY OLD stock of Challenger PB10 woofers made in the UK by Fane for Warfedale - I ordered 3 pairs.
Once they arrived I quickly built prototypes using furniture cubes from B&Q as proof of concept. Unlike the System 6000, I built them magnet to magnet but they are the same in terms of acoustic and electrical phase. It was very interesting to discover how much bass was lost when wired electrically in phase and also how much is lost when sideways on - there is virtually no output to speak of. However, wired out of phase and pointing towards the LP, the output is quite significant and the quality of the bass is also better than I was expecting, possibly due to the lack of cabinet effects. It certainly is not one-note bass but rather a textured and tuneful reproduction.
Anyway, I was convinced enough by the prototypes to spend more money on some decent materials and more time actually cutting panels and building a second version. These have two intended roles:
-
to act as stand for my Kolster Brandes KS651 omni directional speakers - in this configuration they are wired parallel 2 ohm for use with my Kenwood or Genesis four channel amplifiers that are both 2 ohm stable. The KS651 are high passed circa 140 hz while the woofers are low passed circa 110 hz.
-
as bass modules to support the Naim/Tannoy system. In this configuration they are connected high level to a Paradigm X-300 subwoofer amplifier that produces 300 watts RMS (900 peak). This drives the woofers with considerably more authority than the Kenwood - unfortunately I have not yet used the Genesis as it has developed a fault and I need to send it back to Gordon Taylor (the Amp Doctor) who designed it for a service and repair. In X-300 configuration they run 8 ohm in series and are low passed circa 80 hz.
If you managed to read to the end I hope you found it interesting and thank you for your interest.
