A few years ago we owned two pairs of
Wharfedale Valdus loudspeakers. Not a high end
system, just a Pioneer Dolby 5.1 receiver coupled
with these speakers. They were fairly decent
speakers for the price, I'm not going to launch into
a review here but after some listening I started to
feel that the bass units were much better than the
tweeters and that they'd sound a lot better if they
had more space to push their volume of air around in
- one thing I'll still say about these was that the
low end bass was impressively tight, if quite thin,
harsh and boxy overall, a weakness of many cheaper
speakers - the tightness I later found out was down
to the bass drivers themselves. These were the days
long before I got a digital camera, unfortunately no
trace of the speakers remain now.
Loudspeakers, even low end 'High Street
grade' ones, are designed to a close tolerance (Thiele-small
parameters) and this includes the volume of box they're
fitted into. So taking those drivers out and messing with
the size of box they fire into is judged to be a definite
no-no in audio circles. However, I craved some more bass and
had a ton of 25mm MDF left over from fitting a kitchen, a
cunning plan ensued.
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Wharfedale Valdus 500 |
We had two pairs of Valdus, a pair of
500's which were the largest of the range with 2 bass
units and a midrange driver, and a pair of 400's, one
bass and one mid driver per box. The mid and bass units
were the same type and the same for both models of
speakers which meant a total of ten matching 8"
bass drivers to play around with, all far too much
temptation for me.
The cabinets I built weren't actually
designed to any theorem - I'd assume that Wharfedale, being
a large company, knew what they were doing and the drivers
were actually pretty well matched to the boxes they were
fitted to.
Also I didn't have any means to do quality tests
on the drivers at the time to calculate any of their
acoustic parameters, so the mathematical approach was
discounted for the artistic one with huge boxes of 38" high
x 13" wide x 19" deep (trying to stay reasonably
near to the fabled 'magic triangle' of loudspeaker design
and away from cubic dimensions) with 4 woofers
in each box firing through a 4" diameter front facing port,
designed on graph paper and tuned by nothing but ear.
However, these were only to be sub woofer units, anything
above 150hz would be channelled to the regular pair of
speakers I had sitting on top. So, disobeying the
mathematics was, to a point, excusable in the circumstances.
I did toy with the idea of adding mid and
high drivers too to make a full range loudspeaker later on
down the line, these would have been added in a separate
matching cabinet stood above on isolated feet.
The total load for the four 8 ohm woofers
wired parallel in each box was 2 ohms, I knew for sure the
(modern) Pioneer receiver which was powering that system
would've suffered a hernia just from being shown a large
pair of speakers so that task got handed to my trusty
Technics A900, which, on paper, couldn't cope either but
which in reality drove them with aplomb.
After first writing this article, I came
across some pages of plans and calculations which I did at
the time and some of the original plans - one of the scale
drawings is shown below. From the scribbled notes I made, I
had found the specs of a similarly priced/sized 8" bass
driver which was made to work in a similar sized box to the
Wharfedales and made a theoretical set of specifications
from that:
Size = 95cm x 33cm x 46cm (HxWxD)
Box volume = 113,129 cm³
Frequency response (-3dB) = 37 - 3,500Hz
(upper limit lower, depending on crossover fitted)
Sensitivity = 94 dB @ 1m
Power handling = 200 watts RMS (50 watts
RMS
per woofer)
Impedance = 2 Ω
Required bass port length @ 10cm
diameter = 19cm
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Original hand-drawn plan |
The boxes were cut from the MDF sheet, all
the edges rebated with a router for an airtight fit,
everything being fixed together with 50mm single thread wood
screws. The holes for the drivers were cut with the hole
cutting attachment on the router, the bass port and
connection plate being cut with hole saws.
For something
which was only an experiment for temporary use the build
quality was quite high, all the edges were nice and flush
and everything fitted together well, I even cut rebates for
the woofer mounting flanges so that they would fit flush in
the assembled boxes. I didn't plan on veneering them at that
point so they were all bare MDF and japanned black wood
screws, with the gloss Wharfedale woofers they actually
looked quite striking, even if it was obvious that I'd spent
a weekend murdering two sheets of MDF and a pair of
speakers..
The crossovers were basic 2nd order with
150Hz cut-off. Final touch
was to literally stuff the boxes to the brim with 50mm loft
insulation to act as acoustic padding. Everything was wired
up and when it came to the first test I wasn't sure what to
expect from the bass response - I had put them into a far
larger box than they were designed for, there was bound to
be some loss of sound somewhere in the bass region. Both box
volume and bass port length/diameter are critical in vented
loudspeaker designs to get the correct response, air is
basically a giant spring and needs to be tuned to the
correct resonance to enable the best performance from the
drivers installed in the box.
First test was positive, the bass was still
very there, the resonance peak from the original speakers
was gone completely - probably partly because the larger box
was losing some of the bass response and a portion was
probably now lacking - but what was there was impressive in
comparison with the stock Wharfedales, besides, my regular
Celestion bookshelves were picking up on anything above the
150hz low bass. The bass port was made from a length of
thick cardboard tube, 4" (10cm) diameter. I gradually
adjusted the length of this to 'tune' the resonant response
of the bass, in the ideal world the ideal size for this
would have been calculated along with everything else.
This wasn't mudded bass like the boot sub in
a car, this was purely eight 8" bass woofers working in sync
from a regular audio signal, all firing into a large, solid
box - the sheer amount of air being moved in the collective
effort of all the cones gave them a hell of a kick without
the need for any bass boost or loudness, probably the reason
my Technics coped so well. Not audiophile by any stretch of
the imagination but still an impressive, room filling sound,
soft bass with plenty of deep low end without being
overpowering and wheezy like the original speakers tended to
be, enough bass to literally rattle the door furniture.
I ran this system for about 6 weeks until
the fun was over and I craved a 'normal' system with
soundstage and imaging once again, also my Technics was
being pushed hard even though it was coping well and I knew
I was pushing my luck...this was before the days when we had
the internet and the ample supply of vintage audio
powerhouses which came with it, the Technics at the time was
the biggest amp in my system. Eventually the drivers were
refitted to the Wharfedales and the empty boxes stored in
the shed until I managed to source some more high end subs
which would work in the boxes - that never happened and that
gap in my system is filled by a pair of Celestion A3's
instead.
And then there were the donor speakers, I
eventually put both pairs back together and eventually sold
both pairs as I gradually replaced them with better
speakers.
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