The Redesigned 1956
by Steve Deckert
The original Jensen Imperial started back in
the days before Stereo when people listened only in mono. Speakers were
sold as raw components and consumers built their own enclosures or
had them built.
Manufactures like Jensen offered plans for various speaker cabinets
that would accommodate their new fixed magnet moving coil speakers.
You might think "...mono, gee what's the point?" but trust me,
one of these speakers would fill a room with live sounding music very
well. I've tried this myself and am still amazed at how only a
single speaker can be so enjoyable to hear! In many ways it's
actually better, or should I say more believable that stereo!
My introduction to it came about 20 years ago
when a fellow carpenter and I were finishing a house. He showed
me an old book with speaker plans and had the page marked for the one
he wanted ME to build. Why me? In his own words... "it's so
big my wife would see me building it so you're going to have to do it."
He also informed me he wanted TWO of them. I studied the
page and tried to imagine how big it would actually be... about the
size of a commercial refrigerator! Then I tried to imagine what
it would sound like and that's all it took.
I built our first pair, and the fun began.
It lasted about 2 years as we tried every possible combination of
drivers we could think of. We discovered right away that A) the
box was very particular about the type of woofer you used. B) the
engineers who designed it in 1956 didn't bother to see what happens
when you dump 800 watts into a pair!
Along the journey we quickly found out that
nothing man made that you could buy in any stereo shop could ever come
remotely close to this cabinet as a sub-woofer. It would make bass come
out of almost anything. A 25 watt 10 inch pioneer woofer shook
items off a neighbors shelf in the house across the street.
Distance: 220 feet. It did that with one channel of a 22
watt class A receiver. The same woofer in the box it came out of
with the same signal wouldn't shake a pencil, in fact, by comparison it
was inaudible. It was in that moment I saw the light. It's
been almost 20 years since and the passion it ignited actually created
DECWARE. Since then, I've had the opportunity to hear most of the best
speakers in the world. I bring that up because even today with
all I've heard nothing in the consumer marketplace that's not custom
built seems to be able to kick its ass.
In the years that followed we built several
more of these cabinets and used them as sub-woofers in large night
clubs. This continued R&D gave us the opportunity to try a
variety of things that otherwise would have been too costly.
You're probably thinking who needs a bunch of thick heavy bass -
night clubs always have sucky bass. Well, ironically the bass
from the Imperial was just the opposite. Fast and effortless it
was the benchmark of clean, flat, low bass
On one occasion a club owner wanted to
hear what one would do in his own place so he called us up. On
the spur of the moment we had to take what we had so we packed up a
single Imperial, my frequency generator, and an old Harmon Kardon
Receiver with one channel blown. Just before we got this call, we
had been experimenting with a unique driver arrangement. We
removed the 15 inch woofer, and replaced it with a 12 inch woofer that
was already mounted in a 1 cubic foot sealed cube. The response
of the woofer in the cube was 3 dB down at 120 cycles. Basically
ZERO bass. We installed this cube into the Imperial with the
woofer facing the back of the enclosure. The original 15 inch
speaker opening was sealed. Installed the response was a bit
different. Starting at the same 120 cycles and measuring the SPL
at 90 dB in the room we slowly started to sweep the frequencies down
until at 28.5 cycles the SPL had risen to 118 dB. That's a 28 dB
of gain at 28.5 cycles! I really can't begin to describe
what happens at that frequency when you hit that SPL, but it's serious.
We saw a mouse stagger out of a crack in the concrete floor and
We gave the same demonstration in the night
club for its owner, using only a single 12 inch woofer and 80 watts we
were able to move the ash trays on 51 tables throughout the facility.
It was an effective demonstration. He bought it, and we
placed it into service with the 12 inch woofer that you couldn't move
for the sealed box behind it - just to see how long it would last
with 800 watts driving it all night long. The woofer lasted about
1 week and we got a service call. Upon inspection I was shocked
to see 1/2 of the pulp speaker cone missing. It was all still
inside the cube in the form of a powder. The largest fragment was
less than 10 millimeters across. We saved that woofer for over 10
years just to look at it.
And frankly the stories go on and on and on
and on. The speaker casts a spell on people - even before it's
After building several pairs of these
speakers and flexing their muscles some very obvious design flaws
reared their ugly head and we found ourselves having to stiffen up the
cabinets on every one. Many years passed and as always we sold
our personal pairs along the way and hadn't had a good Imperial buzz
for a long time so we decided this Christmas to make one more pair for
ourselves. We've had ideas about how to modify the design to
elevate the resonance issues and make it less finicky about woofer
types. Also learned a lot about how to handle the mids and highs
on full range applications so it was time to pull it all together and
redesign the cabinet.
If you've ever built this imperial from the
original Jensen plans you've probably had a few ill thoughts about the
guy that drew the plans as you work out the discrepancies between
angles and panel sizes.
Fortunately we've built
enough of them now to get a good feel for what works best. We
made some fundamental changes to the cabinet construction that make
layout and assembly much easier. This modified design has many
advantages. It is stronger, it has less distortion, faster
transient response, higher efficiency, flatter frequency response and
can accommodate a larger selection of 15 inch drivers. The only
thing that stayed the same is the final horn flare itself which bends
waves up to 600 cycles.
As you can see the new design uses two 15
inch drivers rather than one. There is also a high frequency horn
on top of the cabinet. Of course there are several ways to
configure this cabinet. Two drivers are optional, the high
frequency horn is optional.
Before you build one or a pair of these
speakers you really should know the intended application of which there
- Subwoofer - this is the simplest
implementation of the cabinet. All you have to do accomplish
decent response across 2 octaves between 25Hz and 100Hz. You can
also use a very wide variety of drivers for this application.
- P.A. Speakers/theater - As shown above.
- High Fidelity Playback - As shown above with
special considerations or with a single 15 inch coax, using the 2nd
woofer for 50 Hz and below.
In all applications there is one consistency
- the room must be large. 24 x 24 feet is really about the
minimum and frankly rooms 2 to 4 times this size would be less
Shown set up as sub-woofers.
These horns launch a very large wave front
below 600 Hz. That makes bass response difficult to handle when
compared to a smaller speaker. A smaller speaker having a tiny
wave front creates dozens of nodes or pockets in your listening space
where cancellation has created quiet or soft spots in the low frequency
response. This is common with all speakers used in smaller
residential listening rooms. It also makes it possible to arrange
your speakers and listening chair so that you are not sitting in one of
these nodes, the result being a frequency balance that has solid bass
response. If you were to increase the wave front launching from
the speaker to say 30 times it's original size, it would fill the room
with greatly simplified cancellation, ie. less nodes. That might
sound good, but it is actually bad because rather than resulting in
dozens of complex standing waves that create the pockets where you will
hear some bass, a large simple standing wave will have near
perfect cancellation. The result is less pockets, and each pocket
becoming very large. That makes it difficult to hear any low bass
response when you are in the center area of the room. In fact you
will probably have to sit against the back wall to hear any bass.
In a perfectly square room it is possible to create an almost
perfect standing wave. To listen to one is a puzzlement, because
it is no different than having a 24dB active crossover set at 80 Hz.
If you go outside your house or to the other end of your house it
becomes quiet a different story. There you will hear the bass.
To prevent not hearing the bass from a
speaker like this with a reputation for being a benchmark in low
frequency response, you MUST take your room into account. Having lots
of clutter in a small room, combined with plenty of bass traps is the
best approach. In a large space the urgency for room preparation
is less of an issue.
CABINET SIZE = 28 cubic feet each.
Aprox Dimensions 62" x 38" x 28"
Materials need are 3/4" plywood, we recommend
mahogany, and lots of carpenter's wood glue. An air nailer with
2" finish nails makes life a little easier during construction.
Each unit takes between 5 and 6 4 x 8 sheets of plywood.
PLANS CAN BE
DOWNLOADED FOR $19.95
or CHECK OUT
We also have the same plans in a full
size blueprint (1:4 Scale)
There is also a support
forum for this loudspeaker design.
Decware is a trademark of High Fidelity
Copyright © 1996 1997 1998 1999 2000
2001 2002 2003 2004 2005 2006 2007 2008 2009 2010
by Steve Deckert