Marek has made these stylish and very nicely crafted cabinets.
The plan for the speakers is
basically the transmission line approach
taken by Bailey in his Wireless World article of the 1980s, but
extending the approach by heavily bracing the cabinet and using bitumen
panels to deaden vibration internally. As per the article, the total
pipe length is about 8ft and the area of the pipe is kept equal to that
of the 15" HPD driver.
The driver is front mounted behind a very thin recess, which when
covered up gives the impression it is mounted internally. See photos
with and without front cover. To avoid children's fingers smashing into
the dustcap, the wooden cover panel has a plain funeral veil on the
inner surface. This makes excellent speaker grille as it is totally
transparent in every way - you can only tell it is present from very
close up. The cover panel is mounted in the conventional manner using
velcro.
As far as I can see, the transmission line approach is basically similar
to a tuned horn-loaded Tannoy cabinet design, except that the
construction is far simpler, due to the smaller number of panels and
corners the sound wave has to go round before being lost/attenuated. The
complexity of the Tannoy designs probably mean they act a as a cross
between transmission line and infinite baffle design. The TL approach is
designed to attenuate all except the lowest frequency signals which are
ported to the outside world to give an extended bass response.
This particular implementation uses solid oak for the frame and braced
MDF oak verneered panels externally. The legs are turned oak and are
part of the frame. Internally, the cabinet is sectioned into three,
i.e. when looking into the speaker through where the driver sits, it
divides into three equal boxes, each as per the plan view so the front,
rear and internal panels are braced at one-third and two thirds of the
cabinet width.
The whole of the interior is covered with 5mm thick bitumen panels,
which are available in bulk to the automotive industry to stop car panel
vibration. Internal wadding consists of long haired wool. This is placed
into "sleeves" constructed from plain funeral/wedding veil so they can
be filled with wool to the required density and sewn shut . Using
sleeves/pockets like this means that the wool cannot settle or move
around. Maintenance can be carried out on any part of the unit easily -
there is no loose wool inside the speaker cabinet. These internal
additions make a massive difference to the sound of the cabinets,
attenuating almost all of the higher frequencies heard at the external port.
15 inch HPD driver on the floor in its wool pocket prior to being replaced plus triangular top long haired wool pocket and one of three long pockets in the signal path.
cabinet is braced by dividing into four - all four pockets in place.
view from top - entire speaker is split into four about vertical axis
speaker showing the out of four channels populated with long haired wool pockets
fully stuffed cabinet
one of three vertical pockets and top pocket - there are four sets per speaker plus the main poket which sits snugly on the back of the driver.
three types of pocket - each is made from wedding veil and has long haired wool inside to the correct density - 17 pockets in total.
close up looking into speaker - the internals are completely covered in bitumen panels to dampen vibration
thinnest possible speaker grille is made from funeral veil and secured using velcro
view from below looking straight upwards - the last set of pockets in the signal path are held in place by wooden dowel
The crossover (the standard Tannoy unit) is mounted externally and is
only visible from the underside, between the legs. The controls are
easily accessible.
Internal volume is about 180 litres.
Currently, the units are being driven by a
DIY system comprising Quad
405 Mk2 boards, made up as monoblocks using a regulated 400VA power
supply per channel.
© PE1MMK Hans Hilberink & Marek. 2001/2003.
