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Planning the
Design Goals
Before I design
or build any speaker, I like to decide on my design goals. These
would be certain criteria the speaker has to meet when it's
completed. Do I want the lowest 3 dB down point as possible in a
size-no-object enclosure? Or can I sacrifice some space and
get away with a higher f3? Maybe an enclosure that isn't tuned to
give a completely flat anechoic response but will yield a flat in room
response. Are looks important? Can the box be twice as deep as
it is tall? What type of finish will the speaker have?
Will it downfire? What's the smallest diameter port can I get
away with before port noise becomes a problem? Should I go PR
instead? How much money am I willing to throw into this
speaker? And the questions go on and on but you get the idea.
This helps me easily decide on what type of speaker I'll end up with and
how it will sound. For many people who are building their first sub,
they'll simply state, "I want it to go as low and as loud as
possible!" Well I've been there and done that, so my design goals
usually vary a bit more. I am probably more critical about certain
factors than I need to be. But to me it presents more of a challenge
and a greater satisfaction when the final product comes to life after
weeks of making it all come together. So I present you with the
UA3504 15" Subwoofer.
Design Goals
- Ported Enclosure
- Smallest enclosure size
possible (and still be suitable for a 4th order design)
- No active EQ
- Tight and punchy,
powerful and clean bass between 30 and 60 Hz
- 3 dB down point near 35 Hz if
possible
- Not a huge bulky ugly box that
can hit 18 Hz while ignoring WAF
- Must be covered in wood and
stained in something nice
- The front I want to be gloss
black with a huge flared port
- Bracing at a maximum to ensure
there's no panel vibration
That was
the basic idea. Small ported box with strong bass between 30 and 80
Hz using in-room effects to smooth response and help out the really low
end. Here's the driver that I had to work with: 15" Attitude AU1576
Subwoofer *350 Watts RMS 500 Watts Peak *2.5" diameter voice coil *4 ohm
nominal impedance *Frequency response 20 - 200 Hz* Magnet weight 70 oz.
*SPL 92.5 dB 1W/1M *Vas = 9.9 *Qms = 11.25 *Qes = 0.37 *Qts = 0.36
*Xmax = 10.7 mm
Modeling the
Enclosure
The specs above are manufactured specs. I did do my own T/S
tests and got very similar results. I just can't remember where I'd
written them down. The box was designed using the actual T/S
parameters. So the first thing I did was plug and chug some numbers
to see what kind of box this driver would really like to be in.
Using various programs from Brian's ported.xls to Bass Box Pro 2.0 to Juha
Hartikainen's WinISD. I found that the driver would work best in a
rather large ported enclosure of 4.75 cubic feet tuned to 30 Hz with an f3
of 32 Hz. That big of a box was not going to pass especially to
achieve an f3 of only 32 Hz! With that you can easily see that low
f3 is not this speakers strong point. This is due to a high Qms and
Vas which results in a more efficient driver. Which is a definite
plus for low power amplifiers. I wanted a small box at the
expense of having a relatively high F3 but with the bonus being greater
efficiency. This sub sounds awesome with only a few watts
in. So I took the box down to what was an
aesthetically pleasing size for a 15" woofer. 18"W x 17"D x
25"H. Minus all the bracing (which was a lot) and the woofer
and port, I ended up with a net internal volume of only 2.75 cubic
feet. I thought at first that it would be to small for this woofer,
but that was as big as I dared go to still achieve my design goal of a
"small enclosure". So I went ahead and plugged in 2.75 ft^3 and
ended up with an enclosure with an f3 of 39 Hz being tuned to 35 at the
expense of a 3 dB hump in frequency response. The hump I surely did
not want, so by dropping fB to 30 Hz, the hump is reduced to 0.5 dB, which
is much more suitable, especially since the box is so small. I
wanted the box to hit hard in the mid-bass region and this design will
certainly do that very well. You'll notice from the graphs that this
speaker will also work perfectly (with a Q of 0.70) in a sealed enclosure
of the same volume. So if you like the sound of sealed boxes better
than ported, all you need to do is put a PVC end cap on the other end of
the port and your set. Either way this sub sounds great with DVD
movies and most all kinds of music.
Construction
Detail
The
box was constructed of 3/4" particle board. Oh no, I said the 'pb'
word. You're right I didn't use MDF on this box. I went back
to the old school methods and used good old fashioned particle
board. For all you MDF fans (me being one of them) you can get away
with using particle board as long as you understand a few things:
particle board is not as dense as MDF. Therefore bracing technique
and panel width must be doubled to reduce panel vibration. Also
particle board is much more difficult to work with than MDF. Screws
tend to split the wood easier. Bits and pieces can break off corners
easily and be difficult to repair. So taking those factors into
consideration, I tried to make the box as thick as I could and use a lot
of solid bracing. I was extra careful in building the box so as to
not split or break off any corners. I had planned to cover the box
in 1/8" mahogany which would further strengthen the panels and
provide better protection on the corners. I can safely say that I
"got away" with building this sub box out of particle
board. The panels remain motionless during even heavy passages of
pounding bass. By looking at the pictures to the right, you can see
how I have two vertical and two horizontal braces that are interlocked to
each other in the middle. Each brace runs to another brace which
runs parallel to the panel it supports providing 1.5" of thickness
over that particular area. Now the reason I don't have all the
panels 1.5" thick was too maximize the internal volume of this
enclosure. Extra internal bracing would have reduced internal
volume. I had to compromise just a little between maximum internal
volume and maximum internal bracing. I believe I achieved the best
of both worlds by using the technique that I did.
Applying the
Plywood Veneer
After
the box was built and all the bracing in tact, I went ahead and cut out
the 4 sheets of 1/8" plywood veneer. The reason for using
1/8" plywood mahogany in my case, was because I had just enough left
over from a previous project to finish up this one as well. Which is
also why I used the particle board. I already had a bunch of it on
hand. Sometimes that is the deciding factor when building a box -
the stuff you've already got. Okay so I don't like to waste
wood. Anyways, I like to cut my veneer 1/8" too big. This
way I can sand down the edges to ride exactly flush up against the other
piece of veneer. It gives a seamless look. I used Liquid Nails
and a caulk gun and ran a thick 1/4" bead up and down the entire side
panel. I wanted the glue touching every part of both pieces to make
a strong bond between veneer and box. These side panels will never
come off in a million years.
The Finish
After finishing the
mahogany panels, I sanded down everything real smooth - first with 120 and
then 220 sandpaper. I went ahead and stained the sides and the top
with its first coat of stain. Then a second, third and fourth until
it reached the color I liked best. After the last coat of stain
dried, I lightly sanded all the sides with a synthetic steal wool of 800
grit. Now it was time to paint the front and back
baffles. This is done by using painters-type 2" masking tape to
protect the newly stained sides. I then sprayed three coats of gray
primer on the front and back, while sanding with 400 between coats and
after the final coat. I then brushed on the black lacquer with a
fine-bristled, quality paint brush. After one coat, I sanded with
220 and did another coat. After the second and final coat, I sanded
the front and back down again with 220. Now I could peel off the
paint and apply the final two coats of a high gloss polyurethane to the
entire enclosure. This ensures an even shine throughout the whole
enclosure including the black front and the stained top, bottom and
sides. The final look is remarkable. The speaker enclosure
looks absolutely wonderful. The box feels incredibly smooth and just
shines like a light bulb.
The Pictures
To the right are thumbnails of the the
different stages of the building process of this subwoofer. Each one
shows how production progressed until the final product was
complete. This speaker I have to say is the most gorgeous of any the
others I've built recently. I really love the way it looks.
Not to mention the way it sounds. I'd have to say it has a very
robust type of sound to it. It's very heavy and strong. It
doesn't take a whole lot of power to get this sub to start shaking
everything in the house. But when it gets going, it doesn't want to
stop. I really like the unique bass response of this sub - it's
tight, it's fast and it's clean. Most importantly this sub passes
SAF by a landslide. My wife loves this speaker, not only because its
small but because it looks so nice. This sub is also for sale at an
incredible price. I'm practically giving it away. So check out the
For Sale page and see if this sub might not suit your needs. Then
give me a call and we can work out the best way to get this sub into your
own sound system.
Measuring
the Frequency Response
Just
a little bit of info regarding how the frequency response was
measured. The sub was measured in my front yard so as to avoid all
reflections from walls and ceilings. This is about the best
anechoic response you can get. Only the ground is capable of
interfering with the results. I used a precision frequency
generator, frequency counter, amplifier, and my trusty Radio Shack
analog SPL meter. The sub was placed in the grass as far way from
the house and any walls as possible, while the SPL meter was placed at
exactly 1 meter from the front baffle at the same height as the dust
cap. I ran 9.75 volts into the driver which has a nominal
impedance of 4 ohms which roughly equates to 23 watts. Since
impedance varies with frequency, we can't say exactly how many watts the
driver was dissipating, but we can get an idea of how hard the sub is
being pushed. 23 watts is very conservative. I then plotted
the SPL at 5 Hz increments starting at 15 Hz working my way up to 100
Hz. I then accounted for meter correction and plotted the results
in Excel. That is the graph you see to the right. The
response has a small hump of about 2 dB from 50-80. Below 50 Hz
the response begins to drop off where it's 3 dB down point would be
figured around 40 Hz. The response continues to roll off gradually
just as it should dropping to about 93 dB at 15 Hz. In-room
response of the sub would improve the low end response making it sound
flatter than it measured here. For a 15" subwoofer in an enclosure this size, it hits
plenty deep and plays plenty loud.
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The Completed Project
The Driver
Bracing Technique
Stained
Primer
Black Lacquer
Calculated Response
Measured Response
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