Dipole subwoofers
I am very fond of open baffle speakers as you
have noticed. I would never put midrange in the box, it sounds way better on
open baffle. Woofers are a different story. Most of my systems are based
on closed box woofers when it comes to the low frequencies. However, I heard
open baffle bass systems, like Legacy Whisper, and I liked the uncoloured fast
clean bass. So I decided to experiment with open baffle / dipole subwoofers.
Simple 15" woofer on open baffle of reasonable size has quite a
cancellation and lacks low frequencies because of that. Baffle either has to be
enormous in size, or bent. H-frame or U-frame is often used to minimize the
baffle while to prevent cancellation. My experiments with these baffles
did not yield satisfactory bass. Neither slot loaded baffle satisfied. I
constructed big closed box with two 15" woofers on opposite sides. Each
woofer can be powered by separate amplifier. This box can be configured as
bi-pole or di-pole based on the polarity of woofers. I experimented with bi-pole
vs di-pole configuration, and not only by listening, but further by measurement
I found di-pole subwoofer much better sounding. Here are some pictures from the
above mentioned subwoofer and it's frequency responses in bi-pole or di-pole
configuration.
And these are the frequency responses I
obtained by behringer ultracurve. No eq or inductor used, just quick and dirty
measurement. First is di-polar configuration, second is bi-polar. You can see
much better low frequency extension in di-polar configuration, plus much
flatter response. In di-polar configuration the woofers are moving together
(box is isobaric), no pressure changes are inside the box, woofers are not
slowing each other. In bi-polar configuration, frequency response has nasty
peak, I do not know why, plus it lacks real deep bass. Membranes when move,
they are significantly acting on each other, slowing the vibration at low
frequencies. I did not like the sound at this configuration. There are
significant vibrations on the walls of the box too, because there are large
changes in the pressure inside.
di-pole
bi-pole
Later, I constructed two more di-pole subwoofers, one for each side. Each with
two 15" woofers. Below are few pictures from the construction,
which was even simpler, not even the closed box. Just two front and back
baffles connected in the corners. This slides inside the stand, which is not
touching the baffles. This di-pole subwoofer is not even hermetically closed,
no need to. Again, I verified that in di-pole configuration the frequency
response was flatter, went deeper and had no ugly peak. Sounded way better too.
In the final stage I wired those two woofers in series to present easy 8 ohms
for the power amps. You have to be careful and realize that back woofer is
physically in other direction, so its polarity is reversed (on top of its
polarity reversal as di-pole, you get the point).
On top of the di-pole subwoofer sits 2.5 way MTM Betsy WOW with Selenium horn
tweeter. All great sounding speakers. Whole system is bi-amplified by two JVC
SuperA amps.
di-pole
bi-pole
All frequency responses were done without any
eq or crossover for the woofers. In the final application these will be powered
by separate amplifiers through active crossover set at 160Hz. You can see the
bi-pole peak much higher in the second much smaller subwoofer. In di-pole
configuration, however, even compact subwoofer performs well and with little eq
goes quite low.
Just to be absolutely clear about the terminology, what I mean by bi-polar and
di-polar subwoofer, I attached the illustration with arrows. The arrows
indicate the polarity by which membrane moves. Not the orientation of woofer
itself, although that is important too, to reduce the second harmonic
distortion, but that is a different issue I do not want to brink here.
First picture is bi-polar subwoofer, where the membranes move opposite each
other. The polarity of sound pressure created to the front and back is the
same. However, there are big changes in the pressure inside the box, as the
membranes are working against each other. The whole box is shaken, not stirred.
bi-polar subwoofer
In the second case, woofers are wired so, that
their membranes are moving in unison with each other, thus creating no changes
in the pressure inside the box. Membranes are not fighting with each other, box
does not vibrate at all (could be made of flimsy thin material?). However, the
sound generated to the front has opposite polarity to the sound generated to
the back, yielding narrower beam due to the side cancellation. Efficiency may
be lower in the di-pole arrangement, but the sound is not thrown widely against
the walls and above mentioned advantages in flatter frequency response and no
box vibration are additional benefits. Plus it sounds way better to me too.
Enjoy!
di-polar subwoofer