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By JULIAN EDGAR
Little Dynamite
Subwoofer
Build your own compact subwoofer – ideal
for use in a car or home unit
S
UBWOOFERS ARE NOW almost universal in any car system that attempts to produce
quality music. They’re also prevalent
in home theatre systems and also make
a great upgrade for a compact sound
system that needs a bit more punch.
The subwoofer described here is
dead-easy to make and gives excellent
performance, considering its compact
size and low cost.
And if you think that it’s really a
68 Silicon Chip
bit too limited in power handling
and low-down frequency response,
stay tuned – we’ve got another more
complex, higher-power design coming
soon. But honestly, unless you want
to make a social statement to sidewalk
pedestrians or your neighbours, this
subwoofer will be more than adequate
for most music!
The design
We’d been considering doing a sub-
woofer project for some time – and in
fact went as far as designing a bandpass model using a 10-inch driver.
However, by the time we’d priced the
particle board and added in carpet,
glue, terminals and a grille, the cost
was around $130 for the enclosure
alone. What’s more, it would have
taken quite some effort to build.
It was about this time that Jaycar Electronics released a range of
sub-woofer enclosures, all pre-built
www.siliconchip.com.au
Main Features
•
•
•
•
•
Small enclosure
125 watts power handling
Excellent in-car response
Competent in-room response
Easy to build & low cost
and finished with carpet and loudspeaker terminals. Their boxes start
at just $59.50, so we made a radical
change to our plans. The bandpass
design was scrapped and we set about
designing a traditional ported design
instead.
A ported enclosure is one of the
oldest box types around (see “Ports
Enclosure Size & Other Design Considerations”). However, the design
of this type of enclosure has been
revolutionised in recent times with
the development of loudspeaker design computer software. Rather than
building a box and then laboriously
testing it in many different configurations, computer software now allows
the virtual construction of dozens of
different enclosures.
In addition, good subwoofer design
software (like the Bass
Box package
that we used) makes various suggestions during the design process. However, if you want to aim for an optimal
trade-off in terms of size, response,
sensitivity and power handling, such
a design process can still take many
hours to complete.
The first step in the design can be
either the selection of the driver or
the enclosure. In our case, we did a
The driver used in the Little Dynamite subwoofer is 10 inches (25.4cm) in
diameter, is rated at 125W RMS and uses a voice coil 50mm in diameter. It
costs $99 and is available from Jaycar.
bit of both. First, the enclosure – the
new pre-built subwoofer boxes come
in 25, 35 and 45-litre nominal interior
volumes (we’ve written “nominal” for
good reasons which we’ll come back
to later) and the hole for the driver is
already cut. This means that if you
want to use a 10-inch driver, then the
25-litre box is the only one to go for
(the 35-litre box is precut for a 12-inch
driver, while the 45-litre box is cut for
a 15-inch driver).
Note that the boxes are available in
both sealed and ported designs.
We leaned towards a 10-inch driver
as in general terms they’re cheaper
than larger sizes and can usually be
fitted into a smaller box, which again
has a cost advantage.
Choosing the subwoofer
So with the 25-litre box the chosen
BassBox: Loudspeaker Design Software
The BassBox software package is
available in two forms – BassBox Lite
and the BassBox Pro professional version. The Lite version is quite sufficient
for any speaker design work that most
people will want to carry out and is
the package that was used to design
this subwoofer.
In addition to being able to model the performance of bass-reflex
(ported) speaker enclosure designs,
this software can also model sealed
and passive radiator boxes and a
range of bandpass designs. The Lite
version comprises just a single CD
www.siliconchip.com.au
and the Help and background explanations are all available on-screen.
In fact, this information is very good
indeed – with some study, there’s
enough information for even a beginner to start designing sophisticated
speaker enclosures.
And if you have a background in
audio, the flexibility and detail of the
program will be even more useful.
BassBox Lite is available in Australia for $165 from ME Technologies
at www.me-au.com and also from
Harris Technologies at http://www.
ht-audio.com
March 2003 69
The “step” in the inner surface of the
port that would otherwise be formed
between the flared ports and the
plastic pipe is smoothed away using a
half-round file and sandpaper.
The port is formed from two flared speaker vents (available from Jaycar) joined
by a length of 65mm-diameter plastic pipe.
we’d like is sensitivity – with a sound
pressure level (SPL) of 88.2dB at 1
Watt 1 metre, it needs to have both
an enclosure design which is efficient
and a power amplifier with at least 100
watts RMS behind it.
Port size
The completed port – the two flared ends reduce turbulence so that there are
no problems with port noise. Make sure that you don’t glue the ends on at this
stage, though – otherwise you’ll never get the assembly into the box!
enclosure, which speaker would be
suitable for it? Keeping it in the Jaycar
family, we opted for the CS-2274, a
10-inch “titanium” finish driver with
125W RMS power handling, an X-max
(maximum cone movement) of 9mm,
a voice coil diameter of 50mm and a
70 Silicon Chip
resonant frequency of 33Hz.
What these specs add up to is a
competent driver that should be able
to produce good bass in the right
enclosure without breaking the bank
(the driver retails for just $99). The
only spec that isn’t quite as good as
While free speaker design software
is available on the web, we’ve not
seen any program that comes close to
BassBox – so it makes sense to pay the
necessary dollars if you want this kind
of package. One area that we were very
conscious of when software modelling
the design was the behaviour of the
air within the port. Typically, ports
are made too small in diameter – it’s
easy that way because then they can be
made shorter for the same box tuned
frequency.
However, small ports give rise to
high air velocities, which in turn causes port noise or “chuffing”. Bass-Box
can predict maximum port velocities
and recommend minimum port diameters to eliminate this problem.
However, after looking at the modelled
behaviour of a system using the prebuilt ported enclosures (the 25-litre
box comes with one 50mm ID port),
we decided to buy a sealed box and
then cut a hole for a port of our own
making.
So what does the final design look
like? First, the Jaycar “25-litre” box is
actually 23 litres – an important difference when it comes to the design.
Second, the volume taken up by both
the driver and the port need to be
subtracted from the available internal
volume (BassBox does both of these
things automatically if the right data
is entered). In addition, the effect of
www.siliconchip.com.au
Ports, Enclosure Size & Other Design Considerations
A speaker’s role in life is to create
sound waves and it does this by moving its cone back and forth in response
to electrical signals. When it pushes
forward, higher air pressure is created
in front of the cone; conversely, when
it moves back, lower air pressure is
the result.
If a bare woofer is sat on the bench
and driven by a bass signal, the pressure waves find their way around the
edge of the frame and partially cancel
each other out. A fundamental task of
a loudspeaker enclosure is to prevent
this wave cancellation and its adverse
effect on bass response. A sealed
enclosure, for example, dissipates the
energy from the back of the cone into
the box (and/or the box filling). However, because these pressure waves
are being wasted, the efficiency of a
sealed enclosure is not as high as for
a ported enclosure.
In a ported enclosure, radiation
from the back of the cone is used to
reinforce the pressure waves being
generated by the front of the cone.
This is achieved by using a tunedlength port or vent, which has the
effect of altering the phase of the
waves emanating from the rear of
the speaker. Put your hand near the
port used in this type of design and
you’ll notice that as the woofer cone
moves forward, so does the air in the
port – the pressure waves from the
back of the cone are now reinforcing
those coming from the front.
However, if the port isn’t just the
right length and dia
meter, the response of the speaker can be all
“wrong”. For example, in this design,
we used a port 63mm in diameter and
360mm long. But what if the port had
been only 150mm long?
the acrylic speaker fill also needs to
be taken into account (confus
ingly,
the fill expands the volume that the
driver sees).
In the end, we had 19.5 litres available and chose to use a port 63mm in
diameter and 360mm long. This gives
a box tuned frequency of 33Hz. Note
that the tuned frequency in a ported
design is often close to the speaker’s
resonant frequency (in this case, it is
identical).
www.siliconchip.com.au
Fig.1: the red curve show the in-car response of the subwoofer using a
360mm-long port, while the yellow curve plots the predicted response if the
port is reduced to just 150mm long.
Fig.2: the green curve here shows the effect of dropping the enclosure volume
to 12 litres and using a 75mm-diameter port that’s just 100mm long.
Fig.1 shows the changed response
– the red line is the in-car performance
of the subwoofer as described in this
article, while the yellow line shows the
predicted response with the shorter
port. As shown, with the shorter port,
the bass falls away more quickly and
is also “peakier”. (Actually, the selected driver is quite tolerant of design
changes – with some speakers, the
results of such a port mismatch would
be much worse!)
The speaker enclosure volume is
Modelled in a car environment, the
response is effectively strong down to
15Hz, while in a room the -3dB point is
at 38Hz. The modelled maximum port
velocity is only 23 metres/second at
125W input power and 22Hz which –
not coincidentally – are also the exact
conditions where the cone reaches its
maximum excursion.
To avoid any possibility of port noise
in this long vent, both ends of the vent
are flared. This is achieved by using
also a vital part of the design. In the
case of the response curve indicated by the green line in Fig.2, we’ve
dropped the enclosure volume to
12 litres and used a short, fat port –
75mm in diameter and 100mm long.
The resulting in-car response is very
peaky at 100Hz.
As a result, this type of enclosure
design that would give “one-note”
bass, a problem often found in ported
loudspeakers enclosures that are not
well designed.
two Jaycar CX-2688 flared speaker
vents, joined with 65mm-dia
meter
plastic pipe. The two vents slip tightly
into the pipe, allowing the easy construction of a double-ended flared port.
Building It
If you have available an electric
jigsaw, some hand tools and a tube
of general purpose building adhesive
(eg, water clean-up Liquid Nails),
building the complete subwoofer will
March 2003 71
Parts List
1 10-inch Response Subwoofer;
Jaycar Cat. CS-2274
1 25-litre sealed subwoofer enclosure; Jaycar Cat. CS-2520
1 acrylic speaker damping material; Jaycar Cat. AX-3690
1 10-inch protective grille; Jaycar
Cat.AX-3522
2 flared speaker ports; Jaycar
Cat. CX-2688
1 500mm (approx.) length
65mm-dia. plastic pipe
1 short length heavy-duty speaker wire
1 tube building adhesive (eg,
Liquid Nails)
8 speaker attachment screws
The hole for the port is marked on one side of the enclosure, towards the back
of the woofer. This 85mm-diameter food tin was an ideal size for marking out
the hole which was the cut out using a jigsaw.
take you nearly no time at all – in fact
an hour, tops!
The first step is to remove the sharp
inside edge of the flared speaker vents;
ie, at the end opposite the flare. This
is done to eliminate any sharp steps
between the flared vents and the plastic pipe when they are joined together.
You can use a round or half-round file
for this job, finishing off with some
fine sandpaper.
If you don’t have any sandpaper
handy, a stainless steel kitchen scourer
works quite well on the soft plastic.
With these edges smoothed, cut the
plastic pipe to the right length so that
when both flared ends are pushed
firmly into it, the total port length is
360mm. Don’t be tempted to glue both
flared ports to the plastic pipe at this
stage – you still need to fit the port
into the enclosure!
Once the port has been assembled,
spray some black paint inside it to cov-
Drill a hole just inside the marked area for the port cutout to allow the insertion
of the electric jigsaw blade.
72 Silicon Chip
er any scratches that you have made
and to hide the white plastic.
The next step is to cut the hole for
it in the side of the speaker box. An
85mm hole is ideal – we drew the
cutting line with the help of a can of
food that conveniently had the right
diameter. The port must be placed at
the magnet end of the speaker to give
sufficient clearance, with the hole cut
in an end wall. Before marking the
hole, consider the placement of the
terminal strip and how you intend
positioning the box in its final home,
as these might have a bearing on where
you want the port to be.
The hole should be cut so that the
edge of the flare ends up about 20mm
in from the edges of the box. This
causes the other end of the port to sit
with its flared outer edge against the
inner walls of the box, allowing it to
be further held in place with some
dobs of adhesive. For environments
where the subwoofer will be subjected
to lots of shaking, use an additional
internal fastening to hold the port
tube in place – for example a bracket
made from aluminium strip wrapped
around the port and then attached to
the inner panel of the box with short
woodscrews.
The flared port can now be assembled within the box. If you push the
ends on after you’ve applied a smear
of adhesive to the pipe, they will be
held in place firmly. The flared end
that sits flush on the surface of the
box needs to be thoroughly sealed
from behind using Liquid Nails or a
www.siliconchip.com.au
silicone sealant. Make sure that you
give the sealant time to set!
Next, the acrylic speaker damping
material can be cut to size and stuck to
the inner walls of the box. We suggest
350 grams/square metre material (Jaycar AX-3690) but any similar material
is fine – eg, acrylic quilt wadding. Be
careful that you don’t block the port
– in fact it is wise to be quite sparing
in your use of the material around the
port entrance.
Next solder some heavy duty speaker cable to the box terminals and attach
the other ends to the screw terminals
on the loudspeaker. Make sure that
the positive terminal on the box is
con
nected to the positive terminal
of the loudspeaker and similarly for
the negative terminals! The woofer
can then be slipped into its precut
hole and the positions marked for its
mounting screws.
That done, remove the speaker and
drill small diameter pilot holes for the
screws. Clean away any shavings, then
reinstall the driver and fasten it into
place using eight coarse-thread MDF
screws. Here’s an important note: the
carpet will compress as you tighten the
screws, so go right around the speaker
at least three times, tightening them
each time.
You’ll note that we’ve left off the
protective grille at this stage. Instead,
it’s time to do some testing.
A jigsaw will make quick work of the port hole. The rim of the flared port
covers the edge of the hole so don’t worry if you don’t cut a perfect circle.
Testing
two reasons for this buzz. First, at high
output levels, the interior of the car
was getting excited – ie, bits of trim
inside the car were resonating. And
second, we were driving the 150 watts/
channel amplifier so hard that it was
going into distortion.
The latter is pretty important to
avoid – while this is a rugged speaker
with cone movement well controlled
down to about 22Hz, start pumping
up the SPL and you might find that
The first step is easy – apply a 1.5V
battery across the terminals and make
sure that when the positive and negative battery terminals are applied to
the positive and negative subwoofer
terminals respectively, the woofer
cone moves forward. If it moves
backwards, open up the enclosure
and swap the wiring connections at
the speaker!
The next step is to connect the
subwoofer to an amplifier. Begin by
driving the unit quite gently. Moisten
a finger and move it around the edge
of the driver, to check for any air
leaks past the frame. Now do the same
around the edge of the port – there will
be air movement within the port itself
but there shouldn’t be any around the
edge of the flare.
Next, listen carefully for buzzes,
rattles and whistles. During the
prototype’s development, we had a
buzz that we chased and chased and
chased. It turned out that there were
The flared speaker port inserted through the hole. The striped background is
the rug on which the box is sitting while the work is being done – unlike an
untrimmed box, this one needs to have its finish protected during the build
process.
www.siliconchip.com.au
you’re working the amplifier harder
than you thought. And remember:
more speakers get destroyed by pushing the amplifier into distortion than
ever get damaged by exceeding the
speaker’s limits!
If everything seems fine at low
volume, wind up the wick a bit more.
Naturally, during this test procedure,
you should disconnect all the other
speakers, so that you’re just listening
to the subwoofer. It won’t sound won-
March 2003 73
This is the inside view – the plastic pipe is slid over the external flared port after which the second flared port
is added at the other end.
derful (no subwoofer does on its own)
but it will allow you to easily identify
any problems.
As stated earlier, because of its relatively low sensitivity, this subwoofer
isn’t going to deliver ear-splitting SPLs
from modest amplifiers. However,
if you have 100-125W available, it
should be capable of delivering lots
of low-end bass.
If all is well, you can now reconnect
the other speakers and then balance
the system for the bass response that
you want.
Finishing
Acrylic filling is glued to the walls of the enclosure, making sure that the port
entrance remains clear. The port is held in place by sealant/glue applied at each
end. If the subwoofer is going to be subjected to lots of movement, add an extra
bracket to secure the port tube in place.
74 Silicon Chip
The final step is to attach the protective speaker grille. To do this, you’ll
have to take the driver out again so that
you can place the “T” nuts on the back
of the baffle. While the speaker is out,
check that the port is still rigidly held
in place by its glue and/or brackets
and that none of the acrylic speaker
filling has moved around. You should
especially check that the port entrance
has remained clear.
It’s then just a matter of completing
the assembly and enjoying the fruits
SC
of your labours!
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