Cheap and easy compact
speaker enclosures
by Julian
Edgar
Want to build speaker enclosures but put off by the woodwork involved?
Julian Edgar describes a novel technique using clay-fibre flowerpots. They
look stunning – and sound excellent, too.
S
peaker building is great
fun, but one of the greatest challenges in home Hi-Fi speaker
construction is making the enclosures. Typically, speaker enclosures
are constructed from veneered or
plastic-coated particle board, and to
develop good-looking and stiff enclosures usually requires access to some
pretty serious woodworking machinery. A different approach is to base the
enclosure design around flat-pack furniture – but when taking that route,
the expense quickly adds up.
However, the enclosure design covered here takes a completely different
approach – you could say, a radically
different approach! Rather than make
a rectangular enclosure from composite board, this approach uses clay-fibre
flowerpots (planters) that are glued together. Constructing the compact enclosures takes hours rather than days,
and the cost can also be quite low. Fascinatingly, this method of enclosure
design also has major benefits to the
sound that’s developed.
First, let’s go back to the beginning
– how did such an unusual approach
come about?
Beginnings
Near where I live, there is a smallscale manufacturer of very expensive,
esoteric speaker enclosures. The enclosures are spherical and are handmade from concrete. The outer surfaces are polished or painted, and the
enclosures sit on avant-garde sheet
metal stands.
I wandered in one day to experience the sound. On the one hand I
was impressed – the music sounded
incredibly open and spacious – and on
the other, underwhelmed. What I was
most underwhelmed by was the price.
The cost of the speakers was way out
of my budget, and so I started thinking about building my own.
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The constructor of the concrete enclosures uses custom moulds in which
the concrete is cast. On my visit, he
told me of the years of development
that had gone into the technique, and
how he’d had plenty of failures along
the way. For a one-off pair of enclosures, the time and effort in my making moulds would be horrendous, so I
wondered – was there another way of
achieving a similar outcome?
That’s when I discovered the joys
of flowerpots!
The enclosures
The enclosures shown here use ordinary flowerpots, also called ‘planters’,
but crucially they are made from clayfibre (also often listed as ‘fibre-clay,
‘fibreclay’ or similar). This material
uses a mixture of traditional clay and
fibre – the latter is apparently a waste
product from pulp mills. Acoustically, it is relatively dead (the pots don’t
ring when tapped) and can be worked
and cut with normal tools – more on
this in a moment.
Pot shape – not just spheres
The pots I use are hemi-spherical –
that is, when two are glued together,
they form a sphere. However, depending on the shape of the pots that
you start with, you can have all sorts
Practical Electronics | September | 2020
of shapes. For example, if you glue
together two pots having extended
sides, you will end up with more
of a prolate spheroid. (Yes, I had to
look that one up – it means an elongated sphere.)
The curved walls of the enclosures
provide excellent stiffness, reduce internal standing-wave reflection, and
potentially have some advantages in
reducing diffraction effects (there are
no front enclosure edges).
Although the precise shape is not
critical, do use circular ones, not the
box-shaped ones, which are weaker,
heavier and lack the acoustical advantages of pots with continuous rotational symmetry.
While I haven’t done so, it may
also be possible to glue a flat piece
of particle board within the mouth
of an elongated pot and mount the
speaker driver on that. (This would
still massively reduce the amount of
work needed to be done over building a conventional enclosure.)
Flowerpot selection
When selecting appropriate pots, you
first need to consider the internal volume required for your design. For example, does your final enclosure design need to have a volume of 15 litres
– or 30 litres? Using off-the-shelf flowerpots, you obviously won’t be able
to have an infinite variety of internal
volumes available, but if the sound
is to be good, you’ll need an internal
volume that is close to the required
design spec.
Second, ensure that the flat surface
(the one on which the pot normally
sits) is large enough in diameter to take
the main bass driver (the woofer). In a
two-way system, I mount the tweeter
externally, but if you want the tweeter
to be in the same panel as the woofer,
the flat part of the pot will need to be
larger. In ported designs, I use the rear
flat area of the enclosure for the port.
Finally, the quality of clay-fibre
flowerpots varies considerably, so
ensure that you have solid examples.
(And where to source the pots? Just
wander along to your nearest large
garden supplies store. Looking online,
Homebase has a wide variety of clayfibre pots.)
In addition to the clay-fibre pots,
you will also need:
Water clean-up industrial adhesive
Polyester fibre internal damping material (I use low-cost quilt wadding)
Acoustic components (drivers,
crossovers, terminal blocks, ports)
Basic tools, including an electric
drill and an electric jigsaw. An angle grinder is useful and hastens
the work, but you could use a hand
file instead.
How to construct your enclosures
The accompanying photo panels and
captions give you the main construction details. There is room for considerable innovation and personalisation,
so stick to the basic techniques, but
don’t be afraid to add your own design flourishes.
Fig.1. (above) The first step is to buy a pair of the bowls. At
the time of writing, these bowls were available in 28cm, 34cm,
40cm and 51cm diameters, and in black or white finishes. The
one used here is 34cm (external) diameter with a black finish.
Two glued together give a 16-litre internal volume. (The ear
defenders give a sense of scale and are a reminder to wear PPE
when using power tools or working on a dust-generating job!)
Fig.2. (above right) Here is the as-bought bowl turned upsidedown. While it isn’t super clear in this pic, the bowl rests on
‘feet’ which are formed into the bowl. To gain a flat surface,
these feet need to be ground off.
Fig.3. (opposite) A normal metal grinding disc mounted in an
angle grinder does a good job of removing the ‘feet’. Again,
it’s a dusty job and so it’s best done outside with the operator
wearing goggles, hearing protection and a dust mask.
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Fig.4. Here’s the view with the base smoothed. To get it as flat
as possible, follow-up the grinder with the use of a belt sander,
or moderately coarse sandpaper and a sanding block.
Fig.7. Here is the hole immediately after the jigsawing had been
done. It’s difficult in the clay-fibre mix to follow the line accurately
(there are small pebbles in the material too), so it’s better to err
on the side of undersize rather than oversize. The hole can then
be ground back to the line by carefully using the angle grinder or
a half-round file.
Fig.5. The next step is to mark the cut-out required for the driver.
Be careful in sizing this hole – too small and there will be the
need for lots of further grinding; too large and you will not get
the driver to seal in the hole. Note the pot drainage hole.
Fig.8. It’s hard to see, but the arrow points to one of the speaker
mounting holes that has been drilled. If you have a masonry drill
bit, you could use that (not with a hammer drill!) but these holes
were drilled with a normal HSS (high-speed steel) bit. Drill the
holes sufficiently large that machine screws can be inserted –
nyloc nuts and washers will go on the inside of the bowl.
Fig.6. The hole for the driver can be cut out with an electric
jigsaw – this pic shows the piece that’s been removed. Use a
coarse wood-cutting blade – and expect to get only a few holes
out of the blade before it is blunt. Don’t go like a bull in a china
shop – you can crack the bowl if you push too hard.
Fig.9. Use silicone sealant under the edge of the speaker frame.
This is important because despite smoothing with the grinder
and sandpaper, the mounting surface is likely to not be perfectly
flat. You don’t want air leaks around the edge of the driver.
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Practical Electronics | September | 2020
– Flowerpot speaker acoustic design –
The complete acoustic design of
loudspeaker systems is well beyond this article – however, here
are some tips.
The most critical component to
get right in terms of enclosure sizing
is the woofer. Woofers are available
with Thiele-Small specifications,
and there are free online calculators into which these specs can be
plugged. The software will quickly
show you the required enclosure volume, and whether it is better that it
be sealed or ported. (And in ported
designs, the software will also show
you the required diameter and length
of the port.)
Note that the easiest way of measuring the internal volume of the pot
is to fill it with sand, measuring the
required volume of the sand with a
measuring jug. (If using two pots, obviously double this volume.)
Thiele parameters
If you would like to measure the
Thiele-Small specs of the driver yourself (perhaps you have an unknown
driver for which the specs are not
available) I recommend Woofer Tester
2, a combination hardware/software
package that connects to a PC and allows quick and easy driver testing
and enclosure modelling. I reviewed
this package in the May 2017 issue
of EPE/PE.
However, probably the easiest
(and potentially cheapest) way of
sourcing decent drivers, crossovers
and an enclosure design, is to buy
a good quality second-hand pair of
speakers close to the enclosure volume you are chasing. If the particle
board cabinets are a bit knocked
around, you can pay very little for
such speakers. You then just transfer
everything (woofer, tweeter, crossover, port and terminal block) into
the flowerpot enclosure. With the
added wadding and usually acoustically better enclosure, the sound is
very likely to be improved over the
original – and of course, the speakers will look stunning.
In the enclosure design you can see
here, I used Alpine SPR-17S 6.5-inch
Type R Component Splits – these are
sold for use in cars. This set comprises
nominal 6.5-inch woofers with cast alloy frames, 1-inch dome tweeters, and
full crossovers (not just capacitors).
The bass drivers lend themselves
to compact ported enclosures, and
the crossovers have variable tweeter attenuation so you can tweak the
high-end response as you wish. The
drivers also have plenty of power
handling and, since in ported enclosures their efficiency is fairly high,
lots of sound volume is available.
At the opposite end of the spectrum,
they also still sound good with only
a few watts powering them.
The enclosures have an internal
volume of 16 litres and are tuned to
49Hz, with a port 50mm in diameter and 118mm long. The tweeter
is mounted below the main enclosure, and the crossover and port are
mounted on the rear.
The sound of the finished speakers
is outstanding – especially in their
transparency and their ability for the
vocalist to be ‘with you in the room’.
Fig.12. The port tube
needs to be the correct
diameter and length to
match the enclosure
design. PVC pipe is
cheap and easy. This
port is 50mm in diameter
and 118mm long.
Fig.10. The driver installed in the hole. Note the use of washers
and nyloc nuts on the speaker mounting screws. Tighten these
when the sealant is yet to set – but be careful not to over-tighten
or you could crack the bowl.
Fig.11. With the driver
installed, it’s time to tackle the
other bowl. The other bowl,
its ‘feet’ already ground off,
needs a hole made for the
port tube. I found the easiest
way to do this was to use
the jigsaw to cut a series of
radial slots like this, with the
pieces then broken off by
the careful use of pliers. The
hole was then filed to final shape. The hole doesn’t have to be
perfect – the industrial glue used to secure the port in place will
fill any small gaps. (Don’t use an expensive holesaw to make this
opening – the teeth will soon be blunt and the holesaw ruined.)
Practical Electronics | September | 2020
Fig.13. The port glued
into place. For this task,
and also gluing the
bowl halves together,
I used water cleanup Liquid Nails. Note
that this glue was also
used to seal the ‘water
drainage hole’ provided
in the original bowl.
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Fig.14. Now is also
the time to feed
the cable through
a hole drilled in the
bowl, and seal it
with further glue.
Either solder the
wires to the woofer’s
terminals or equip
the cable with pushon terminals to make
this connection.
I chose to mount
the crossover on
the rear of the
enclosure, but
it could also be
mounted inside.
Fig.17. The tweeter is mounted below the main enclosure. I
could have run the tweeter cable within the square tube, but in
normal use the cable is not as visible as it is here. The physical
position of the tweeter makes a substantial difference to the
sound – it must be as far forward as the main driver.
Fig.15. This photo shows two steps. The first is that polyester
quilt wadding has been placed around the inside of the lower
bowl. Ensure that there is enough projecting upwards from the
lower bowl so that when the two bowls are glued together, the
wadding covers the inside surfaces of the upper bowl as well.
Also ensure the rear of the driver and the port are not blocked
by the wadding. The second step is to glue the two halves
together. Use a generous amount of glue around the rim of the
lower bowl. The upper bowl is then carefully lowered over the
lower bowl, ensuring the wadding goes inside the upper bowl as
it is lowered. (This job is best done by two people.)
Fig.16. The bowls
are pushed together
and then a wet finger
used to smear the
glue around the join,
ensuring that the
gap is filled. A damp
cloth is then used to
carefully remove the
surplus glue. (The glue
must be of the water
clean-up type!) Note
the chalk witness line
(arrowed) that shows
the correct alignment
of the bowls as they are joined. This line is made earlier when the
best rotational fit is found – because their lips aren’t dead-flat, the
bowls fit together better in some orientations than others.
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Fig.18. (above) The next step
is to mount the crossovers on
the rear ‘flat’. The crossover
was glued into position just
below the port.
Fig.19. (right) The stand
shown here was made from
square tube. You can make
any type of stand you wish
– but the speaker must be
off the ground to work at its
best. (Also, when you are
buying the pots, check out the
available pot stands!)
Practical Electronics | September | 2020
