ZEN OPEN BAFFLE SPEAKER PROJECT
PART V - THE FINAL CHAPTER
the Zen Open Baffle project I mentioned that I like the
design process to take a long time... The more time that passes
the more confidence I have in the design if it's good. And if
it's bad, with enough time I'll face the facts and just put it out of
It's been a bit over 2 years since my last article and there has been
some real progress towards my ultimate goal with this design. And
perhaps icing on the cake... the enclosure itself has not been modified
in any way since it was first constructed. It had some flaws in
the frequency balance and some minor power handling issues with really
low bass when played at high volumes.
So did this mean the enclosure needs further tweaking... my gut feeling
was no. I wouldn't find out until I was able to try a variety of
drivers and see how each is effected by the cabinet.
We placed the speakers on our web site as a finished product in the
fall of 2010 using a shielded version of our DFR8 full range driver.
You can see in the photo below that this version of our DFR8 started
out life as an FE207 Fostex unit and the reason it worked better than
our normal DFR8 was because it had no shorting plug on the pole piece
and a less aggressive magnet strength giving the ZOB more bass due to
higher excursions. The smaller shape and size of the magnet also
allowed the rear wave of the speaker to launch without obstruction
which helped the imaging to further improve. The downside was a
limitation in low bass control at high playback levels.
The original driver in the ZOB
was the DFR8S
This Summer the US
distributor for Fostex was no longer able to get the FE207's claiming
they were discontinued by the manufacturer. Well this was the
perfect excuse to get serious about finding the ideal driver for the
ZOB so the hunt began and several were tried.
The best driver that was
tested was the Tang Band W8-1808 at $200.00 ea. With a Qts: of
0.44 and an Xmax of 5 mm it lent itself well to this open baffle
application. While not a deal breaker, it was only 93dB
1W/1M. Nevertheless it works rather well and sounds real nice in
the ZOB if your willing to cut
opening in the front baffle larger.
The next best driver was the
SEAS Exotic X1 at $830 ea. With the same Qts: of 0.44 and
slightly lower Xmax of 4.2mm it too works and sounds pretty good.
The mid-range is warmer but wasn't as fast sounding as the Tang
Band. Plus at $1700 a pair it's starting to pinch the wallet a
bit hard and this driver is still only 93dB 1W/1M.
Another possibility would be the PHY 8" Wide Range driver, also
designed for open baffle with a Qts: 0.59 and an unknown Xmax.
The sensitivity is excellent at 98dB although we don't know if that's
at 1W/1M or 2.83V/1M. Besides the price of $1200 ea. the Zmax is
rather high at 155 ohms. I'd like to see that a bit lower.
Obviously at this price I'm not going to try a pair without seeing the
frequency response. I also created a metal frame like the one
used on the PHY that stands out away from the cone by nearly 1/2 inch
making a purely flush mounting into a baffle virtually impossible
and put it on one of my own drivers to see what would happen to the
frequency response. The response was worse with the ring in place.
So here are three off-the-shelf options for your ZOB cabinets if you've
built them yourself from our plans.
A NEW DRIVER GETS DESIGNED
I wanted a driver with a high QTS that had close to 5mm of Xmax and an
efficiency of at least 95dB or more. Preferably I'd like it not
to cost $1200.00 for each driver.
The Tang Band W8-1808 seems like the one to beat and it's frequency
response was unusually good for a full range driver.
TANG BAND W8-1808
While good, it the response
was still indicative of high efficiency full range drivers as evident
by the 10dB rise between 1KHz and 20kHz. This characteristic rise
is something I would desperately like to not have if at all
possible. Without going to a lower efficiency driver that until
now is the only way I know to achieve that result.
About this time Dayton came
out with an inexpensive new full range driver that caught my eye for
$119 ea. The published Qts was too low, but what the hell.
I loved the frame because of the way it was cast to be invisible to the
back wave off the cone. I loved the fact that it had a 1 inch
voice coil instead of 1.5 or 2 inch because smaller voice coils always
have better high frequency response. As I researched this driver
I started liking everything about it. The neo motor assembly is
more than impressive with solid copper shorting rings and a machined
solid copper phase guide that's sure to stop kids from trying to push
in the dust cap. The smaller whizzer cone made possible by the 1
inch neck joint also shows great promise. So I purchased a pair
knowing they would fit perfectly into the existing opening of the ZOB
cabinet unlike the Tang Band. My plan was use them as a donor
driver and make the nessasary modifications required to get them to
sound good, just like I originally did with the Fostex FE207.
When they arrived the first thing I did was put one of them in the ZOB
cabinet on the right side of the room and leave the original DFR8S in
the cabinet on the left to make some comparisons. After throwing
a packing blanket over the speaker for a 24 hour work out I came in the
next evening to hear what it could do...
The two drivers couldn't have sounded MORE different if you had
tried. The Dayton was WAY faster with some promising bass
performance that could easily have been missed due to the insane high
frequency response. I didn't try to measure anything on this
night, just wanted to wrap my head around the sound of this driver by
listening to music and playing with things a bit. It took me
hours to realize the reason the driver sounded 10dB louder than my
DFR8S was because the top end was that peaked.
someone put about ten tweeters on the speaker and
cranked them up so loud it begain to encourage male hair loss.
always said that our full range drivers - actually all full range
drivers - have to warm up before they sound right. The first 5
minutes of playing is far from what they sound like after 20
minutes. The difference is so great that most of us have to leave
the room for the first 20 minutes. I knew this because this is
what I hear happen. It also makes sense that heat would change
the voice coil diameter amoung other things. It wasn't until I
started doing research on a motor for this woofer design that I
stumbled upon some Klippel Non-Linear Test results that show how large
and small signals effect the donor driver when it's cold vs. when it's
warm. The resistance at the rest position due to mechanical
losses goes up almost 600% and the inductance at rest position
representing driver compliance almost doubles. All of the
mechanical aspects of the driver increase. This would be
indicative of all high efficiency full range drivers.
This is pretty much what I
heard... the frequency response has a 15dB rise between 500Hz and
20KHz, but comes on strong hitting that peak as soon as 4KHz. I tried
everything to make this driver sound right encouraged by the few things
it WAS doing right. Regrettably at the end of the night after changing
the complience multiple times, treating the cone and trying dozens of
phase guides, I just had to give up. It was unusable for my
After a few days of wondering what I was going to do about this problem
of having the ZOB on the web site for sale and only one more pair of
the original drivers in stock, I decided to use the Dayton driver as a
parts donor and began work on a modified motor design. l realized
there is no way to beat the tipped up responses of these drivers
without killing the speed and efficiency. The very things that make me love good
single driver speakers. So the work began and my good
intensions were rewarded with a few ephiphonys.
I had a bit of a brain storm during the holidays after pondering these
differences and realized the solution is to create a electromotively
coupled reservoir in the motor assembly for some of this excess energy
to go and then tune the reservoir to damp the rise in frequency
response. To a brilliant engineer this might be simple, but to me
it's clearly getting into the Zen Voodoo zone. During the
development I have to admit I had quite a bit of fun and mystery as I
worked out the kinks. I voiced the driver as close as possible to
the DFR8S originally used in the ZOB. The end result required
some serious machining of T6 aluminum to create the updated motor
assembly that proved to be well worth the expense as it completely
damps the cast speaker frame and offers a nice guide for the rear wave
of the speaker.
The end result (after a good break-in) far exceeded my expectations
which were to simply beat the performance of the Tang Band
driver. I already knew these prototypes were better by the sound
I was hearing but listening to a
speaker in a room is not listening to a speaker, it's listening to the
room with a speaker in it. The response you hear once
you're 8 feet away is FAR different and seriously screwed up compared
to the sound coming off the driver at 1 foot away. So the
measurements which have the ability to show the driver without the room
effects can tell you a lot. I measured the new driver and was
stunned to see it setting the record for best driver I've ever
measured, let alone a high efficiency driver! In fact I haven't
been able to find a driver over 92dB with full range response that can
touch it. I named the driver FRX which stands for Full Range
Your going to like this...
ON AXIS FREQUENCY RESPONSE
This response is insanely
good for any kind of driver and especially impressive in the world of
full range point source drivers.
FOR HIGH RES GRAPH
final product shown on my bench below:
Before we go any further,
these are hand made drivers that I will be building to order as
needed. You can see all
the measurements and read about the new high Qts motor design on the
FRX pages linked to at the bottom of this article. The
measurements found there were all taken with the FRX driver installed
into the ZOB cabinets showing the driver response and waterfall plot of
the speakers and room decays.
PRIOR TO NOW
The ZOB cabinet never measured perfectly. It had a tendency for a
dip in the 60~80Hz region and other small anomalies that proved to
actually be the fault of the drivers that were previously used, not the
cabinet. This was a feel good moment for me but as a result I can
now drive the Zen Open Baffles by themselves at satisfying volumes and
flat response down to 50Hz with usable bass left at 30Hz.
You'll recall the Zen Open Baffle array - which is still viable but
certainly not needed for bass correction like I thought it probably
would be when this project started. I'm thrilled to say these
speakers now stand on their own without augmentation from the broadcast
system approach or a sub woofer. It makes listening to these
speakers on the TORII MKIII a lesson in perfect bass, not to mention
bass that can shake the house if it's asked to by the recording.
Sooo percusive this speaker is.
So now that there is real bass without any holes and you realize that's
it's being created out of thin air, for free... (by the resonant
chamber) it's like watching real magic happen! Personally I think
it's almost a miracle that it works this well. Compared to the
drywall prototype when this all started it seems to have just ignored
physics and granted my wish of a full range point source open baffle in
a narrow tower that disappears and images like no other. Using my
direct drive Acoustat Monitors (full range electrostatics) as a
reference for pretty much everything but especially speed, the ZOB with
FRX driver combination is equally fast beating out everything I've done
to date with moving coil loudspeaker design.
So this finishes the Zen Open Baffle project on a rather high note....
now it's time to listen.
THE DECWARE FRX POINT SOURCE
FULL RANGE DRIVER
Decware is a trademark of High Fidelity
Copyright � 1996 1997
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2002 2003 2004 2005 2006 2007 2008 2010 2011
by Steve Deckert