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Sonotube
FAQ
By
Gordon McGill
Updated
22-June-2002
Index
Q: What
is Sonotube ?
Sonotube
is the commercial name for a paper tube used for forming concrete.
It is used when builders want to make a concrete pillar. They
set up this paper tube (comes in diameters from 8 inches to
56 inches and in lengths to about 18 feet or so) and pour
concrete into it and let it dry. When the concrete is dry,
they strip the paper tube off, and voila, instant pillar.
Q: Why
is Sonotube good for Speaker enclosures ?
Strong.
A tube is a very strong shape for resisting internal pressures.
Coincidentally, this makes it ideal for use as a speaker enclosure.
When building speakers, wall strength is an important consideration,
especially in subwoofer cabinets where internal pressures
are relatively high.
Cheap.
A Sonotube is very inexpensive when compared to the cost of
the wood required for a conventional speaker cabinet. A sonotube
enclosure will probably be at least 1/4 the cost of a conventional
wood speaker cabinet.
Easy
to Build. A cabinet using Sonotube needs a top plate
and a bottom plate (to seal either end of the tube) and that's
it. A conventional cabinet requires much more exacting woodwork
and much more time, wood, and effort (not to mention weight!)
to get a cabinet as strong as a Sonotube enclosure.
Q: What
are its disadvantages ?
They are
ugly. If you put up something resembling a missile silo in
your livingroom, your spouse or significant other may object.
You can cover them with paint or with a big black cloth sock
slid over the top. On the other hand, you could veneer them,
or just paint them red with a big "radioactive"
sign on the front and let friends draw their own conclusions.
So, this is the drawback, they are hard to hide or make decent
looking. Some builders have disguised Sonotube enclosures
as plant stands or as Greek columns. If you can overcome the
nasty look, you are all set.
Q:
How thick does the tube have to be ? It looks too thin to
be effective !
The tube
walls don't have to be very thick at all. Sonotube walls are
usually no more 1/4 inch thick. Their shape provide amazing
shape - more than enough to hold the weight of concrete without
deforming.
Doug Purl
posted the following on the DIY
Loudspeaker list on 12-January-2001 :
"Once
again a thread on tubes as speaker enclosures has appeared.
And once again all the posters have been laboring under a
misapprehension of how the thing works. Folks, the walls don't
flex. You don't have to lard them up with damping. It is precisely
the reason that pipes conducting fluids and gasses under pressure
are round and not square.
If
you stood inside a garbage can and pushed your arms outwards
the walls would flex. But if you had an infinite number of
arms and pushed outwards equally at every point, the can would
not flex. The net effect would be to try to stretch the walls
rather than bend them.
Boxes
flex because their panels have different resistance to flexure
at different points. Tubes have the same resistance to flexure
at every point, and receive identical pressure at every point."
Q: How
is a Sonotube used as a speaker enclosure ?
In constructing
a Sonotube enclosure, the first decision you have to make,
is the location of the drivers in the tube. Will the driver(s)
be mounted in the ends firing up or down? Or will they be
mounted in the side of the tube firing out (perpendicular
to axis of tube) ?
Mounting
in End of Tube. This method is the most common for
subwoofer cabinets. Building a driver into the side is much
more difficult, and the frequencies below 80 Hz are omnidirectional,
(mounting a subwoofer driver facing down or up, does not have
much effect on the sound) so most people building with Sonotube
just mount them in the end caps.
To mount
a driver in the ends of Sonotube, it is first necessary to
fabricate the end plugs. The plugs each generally consist
of two circular layers of MDF. One of the circles is cut to
the inside diameter of the tube so that it will just barely
fit and the top layer is cut to the outside diameter of the
tube so that it sits on top and is flush all the way around
the tube. The layers are glued together and glued into the
tube, perhaps with epoxy. Make sure to fill any cracks or
gaps with epoxy or silicone caulk. Now that you have one endcap,
repeat and make another one.
Next, measure
out the hole(s) for the driver in one of the end caps. Try
to leave space between the drivers so that the endcap does
not become too weak. Now that you have fabricated the endcaps
and cut out the driver holes, you are just about done. The
next part of construction is the addition of threaded rod(s)
to the enclosure to maintain tension between the endcaps.
Here, you want to use some reasonably stiff rod, perhaps 1/2"
or so and run 1 (for aSonotube to 12" dia.), 2 (to 16-18"
dia.) 3-4 (to 24-30" dia.) or more if your Sonotube is
even larger. The idea here is to put holes in the endcaps
in roughly the same place and then drop the rod in and tighten
at each end with nuts and washers. Some people have used 3-4
rods in their enclosures, and cut the threaded rod long enough
to act as feet on the bottom!
Well, other
than adding some stuffing (pillow stuffing (Dacron) or fibreglass
insulation) to damp out standing waves, you are pretty much
done. You can drill the holes for wiring the driver, bolt
it into the endcaps and fire it up! Of course, your Sonotube
looks terrible, it still has its waxy coating and is still
that nasty brown colour with the construction company's lettering
on it. Ah well, we'll take care of the finishing a little
bit later.
Q: What
about Standing Waves ?
Stuffing
to Get rid of Standing Waves. We will use either the
fluffy pillow stuffing (sold around here, (Toronto, Ont.)
as Polyester Fiberfill) or the standard pink fiberglass insulation.
How
to calculate Freq of Standing waves. Any enclosure
will have standing waves associated with it's shape and size.
These standing waves will exist between to parallel surfaces.
For instance, a sphere will have one standing wave, because
all points are perpendicular to a line drawn through the centre.
A tube enclosure will have two, one between the walls of the
tube and one between the endwalls. A rectangular enclosure
will have three, top to bottom, right to left, and back to
front. To caculate the frequency of the standing wave that
will exist between two points use this equation:
Wavelength
= speed of sound/ frequency of sound
W = v /
f
v = 340
meters/second
How
to Reduce the effect of These Standing Waves. These
standing waves are harmful to the sound because they influence
the cone making it vibrate in the absence of a signal. That's
the short explanation. The long explanation involves resonant
frequencies.
Resonant
Frequencies. A standing wave is actually a type of
resonant frequency. A resonant frequency will exist for any
panel you may build. Each material has it's own resonant frequency
as well. So, if a signal of all frequencies is put into the
space or panel or material, and we measure the loudest frequencies
coming out of it, those will be the resonant frequencies.
The general
strategy used when dealing with resonances is:
break the
resonant frequencies into many resonant frequencies that are
much smaller. You see, when two resonant frequencies are the
same, they add to each other, doubling the power at that frequency.
When the freq. are different, the power cannot sum and influences
the response we hear less. In a speaker box, this can be done
by using non-parallel interior walls and bracing the panels
so that no two unbraced areas of the panels have the same
area 2) Reduce the energy of the standing wave. This is usually
done with stuffing. The stuffing may line the walls, but is
more effective if suspended few inches off the walls. This
is the primary method to be used in our Sonotube enclosures.
3) Make the enclosure walls lossy.
How
to nullify Standing waves.
<Not
written yet>
Q: Can
I use them for other speaker enclosures? (not just subs?)
<Not
written yet>
Q: How
do I contact the author of this FAQ ?
Send all
questions, comments, and corrections to this site - see the
About page.
(permission
to post this granted by original author)
From: Szewczyk,
Robert
Date: Thu, 3 Dec 1998 13:53:46 -0600
I read your
instructions on building a subwoofer using SonoTube. I have
been building subs with this medium for a bit over a year.
But my method to assembly is a bit stronger and perhaps simpler.
I have only
used the 12" and 20" tubes. I prefer the 20"
tubes because their sides are a bit over 1/4" thick.
I use 3/4" MDF throughout.
For the
bottom (this is where I mount the 12" driver, the 4"
flared port, and the 5-way binding post cup):
- I cut
two 20" diameter disks (I use 2 because this is the
drivers mounting flange and should be strong). I glue them
together and cut out the required holes when the glue has
dried.
- I use
whatever color fabric I want (usually black, but I have
done a Hunter green, and white) and sew a seem on the inside.
I usually use polyester because when stretched over the
sonotube, you still can not see the lettering behind it.
I fold about an inch of the fabric over the bottom, then
insert the bottom plate until it sits about 1/8th"
to 1/16th" below the bottom lip. Once the disk is in
place, I use drywall screws in a criss-cross pattern (,',',',',',',',',',)
to secure it in place. But using several screws, (and taking
into account that I took my time to make the outside edge
of the disk as round as I could) I can seal up the bottom
without going back with silicon or caulk. (I paint the bottom
and top the same color as the fabric)
For the
Top:
- I cut
a 20" ring that is about 4" to 6" thick (or
you could say the ring has an 8" to 12" hole in
the center). I place this in the top and mount it flush.
I secure it again with drywall screws. I make the ring fairly
thick to give the sub some weight.
- Next
I pull the fabric up and fold it over the ring. I staple
the fabric to the inside of the ring.
- I cut
a 20 1/2" disk and router the upper edge. I then prep
it for a glassy smooth finish. I place this over the flush
mounted ring and, entering from the bottom through the 12"
driver cutout, I use drywall screws going through the ring
and into the top plate. I usually apply a bit of caulk from
the inside.
As for legs,
I use 1" dowels with little rubber pads on the bottom.
This process works very well. I found that when using the
messy epoxy, it mad my skin break out! So I stopped using
it and figured out an alternate method.
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