About my Dual Woofer Design

The primary goal with the design was to eliminate woofer induced vibrations in the loudspeaker cabinet. Instead of using mass/double wall cabinet and similar, I wanted something that worked with a more normal cabinet design. And yes, it works. The woofers face each other, and so to say squeeze the air trapped between them out, to form a sound wave. Membrane movement/vibration from one woofer is balanced by the other woofer, since they move in opposite directions. Furthermore, vibration direction is vertical, as opposed to movement from midrange and tweeter, that move their membranes in the horizontal plane.
The effect is simple, no vibration from woofers affect midrange and tweeter sound reproduction. No frequency modulation of midrange and tweeter sound reproduction. Result: cleaner sound, especially when volume settings are higher (read: more realistic!!) and woofer membrane movement is more pronounced. There is one design concern that is crucial: the mounting between the two woofers has to be absolutely rigid, they should not be allowed to move or flex.

The distance between the woofers is also important, depending on the desired frequency range and choice of crossover frequency.
There is damping material between/behind the woofers, this is an absolute requirement. The damping material should absorb all sound moving backwards, it should work as a real cabinet, preventing any kind of omni-directional behavior. If you remove the damping material, the frequency response will be bad, with pronounced peaks and dips, known as a comb filter behavior. Look at the shape and size of the damping material on Junior, the image says more than many words.
There is another interesting damping material property, one can, to a certain extent, control woofer dispersion! With a conventional design, woofer dispersion is only controlled by the woofer itself, and the width (and general dimensions)of the front board. As you can see on the Junior picture, it is partly open towards the sides of the loudspeaker. With the damping material, you can control the size and shape of this opening. Damping material sound absorbing properties is also decided by its thickness, this means that a thinner material absorbs less low frequencies. All in all: to some extent you can now control both dispersion and it's frequency content – this is unique when it comes to a woofer in a loudspeaker cabinet.

Drawbacks – yes and no...
Number one: cabinet construction is more complicated – absolutely! Number two: a bit more complicated, and can be turned to something useful?? The special woofer arrangement with the units pointing up and down affects the frequency response. At some point in the frequency domain, depending on woofer size, the sound wave emerging from the rear edge of the membrane gets behind the sound from the front edge. It gets out of phase. Result: a dip in the frequency response. Bigger woofer, lower frequency, smaller woofer – higher frequency. This is the reason for my 4 inch woofers in myBaby design – since it is a 2-way speaker, I have to go higher in frequency with the woofer than in a 3-way design.
This could look as a problem, but in most cases it does not matter, because the normal crossover frequency is far below the previously mentioned dip. It could also be turned into an advantage. If the desired crossover is close to the dip, you get additional acoustical roll-off! This could mean a simpler crossover filter, and better impulse response, since high order crossovers have worse impulse response than lower order filters.

To sum it all up from my own point of view: This loudspeaker construction gives some unique possibilities to (very, very close to) eliminate woofer induced vibration in a loudspeaker cabinet, and an equally unique ability to control dispersion properties. Many (music and hifi) enthusiasts have listened to my prototypes, and there has not been any negative response whatsoever about bass reproduction. You can also check some measurements in the Proof of Concept section, (see below) this shows quite clearly the difference between a standard loudspeaker and my design.
In short – it works.




Thoughts about Loudspeakers and DIY

Proof of Concept