Sun-Sational Savings At Parts Express (Prices Good Thru 8//31/2014) - page 16

Speaker Building
Design Goals
As an electrician, I’ve always wanted nice sound on job sites. I’ve noticed recently the impressive (not great, but quite surprising) audio
quality of some of the little Bluetooth speakers being sold. This led me to look at some of the newer high excursion small drivers being
made. Glancing at the specs of some 4", 5", and 6" drivers, I realized that using lots of power would enable me to build something
impressive. The Pipe is my take on the portable boombox.
Component Selection
First, I needed power and plenty of it. The Lepai LP7498E amplifier fit the bill nicely. About 80 watts per channel, Bluetooth, and just two
switches and a knob... nice and simple. Just so you know, this is not battery powered. It could have been, but I wanted the boombox to be
light and run all day.
I chose 6" diameter, solid core PVC for the enclosure. This light and rigid material also lets me show off my molding and design skills. Considering
the woofers, I thought about using a couple of 5" or 6" drivers located in the ends, but then the whole thing would just look like a bunch of PVC,
and that’s no fun! I was quite surprised with the specs that I found on a couple of 3-1/2" drivers (I’ve never really looked into small woofers,
I’m a bigger-is-better kind of guy when it comes to low end), and the price was great on the Dayton Audio ND91-8 drivers.
I’ve always wanted to try out the Vifa XT ring tweeters; I just never got around to it. I wanted something small, but with a low Fs for a crossover
point that would minimize intermodulation. The Vifa XT25SC90 fit the bill perfectly. 2-1/2" overall diameter and an Fs of 830—wow, that’s low!
To begin, I molded a place to mount a steel plate for the amp. I used a
heating blanket to do this, which made the larger PVC sections pliable
enough to flatten with a piece of 1/2" particleboard. A piece of bisected 4"
diameter PVC was incorporated to accommodate the amp’s depth, and it
added 0.05 cubic feet of air space.
To hold the amplifier in place, I mounted it (and a stereo 3.5 mm jack) to a
Penn-Elcom D2101K 5" x 7" blank dish. I sandwiched the reversed plate
between the amplifier’s body and its faceplate.
I molded spots for two Dayton Audio ND91-8 3-1/2" woofers on each side
of the amp, using a heat gun and a 1/2" piece of particleboard. These fit
together perfectly, with the speaker frames 1 mm apart. Rear-mounting
these onto an internal plate really made them look nice, while distributing
the weight evenly.
I molded a couple of spots for the tweeters centered above and about 50°
off-axis from the woofers. 50° seems a lot, but when this is on the floor and
standing about 10 feet away, the woofers and tweeters will both be about
25° off-axis. My choice of angle should help overall dispersion quite a bit.
I cut 1/2" plywood end pieces for the pipe and used them to cap the
enclosure. The ends are also the legs of The Pipe and support some
weight. I ran a few 1-1/4" dowels through the ends into the pipe to make
the joint stronger.
The PVC cabinet is designed to be a ported enclosure for all of the
woofers, and works out to about 0.40 cubic feet. To tune the enclosure and
save as much space as possible, I took a section of 1-1/2" PVC, heated it
up in the oven, and squashed it down to a smiley face shaped port tube.
As it stands, there is about 3/4" of surface area on the port. I tried different
tuning frequencies, and settled on 35 Hz as being the best sounding while
allowing the woofers to hit an f10 of 28 Hz. The final port length is 9-1/2".
For transferring the power supply’s output into the cabinet and the
amplifier—pro audio guys look away—I used a rear-mounted Neutrik
speakON connector. Quick and easy to connect, and heavy-duty enough
for the job sites I go to.
Cabinet Design and Construction
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