Here is another in our series of basic speaker projects, designed
to provide a weekend of learning and fun.
By Lex Luthor
All too often we speaker builders undertake some huge project, only to be overcome by it, especially if things don't go the way we intend. Maybe this occurs because we haven't warmed up enough, like a pianist running the scales to limber up before a special performance. Or sometimes we just procrastinate and never begin. But here's a fun project just to tackle and enjoy.
This project is a simple bookshelf sized speaker. I designed it to be about the size of a regular hard-cover book; in fact, it can fit on virtually any bookshelf. It uses a single full-range 5” speaker, without a crossover or any other special construction techniques.
A FEW OLD PARTS
I desired a speaker that would fit right on the shelf next to my receiver-nothing pretentious, for I listen to the receiver infrequently. I had some simple 5 ohm speakers on hand-just basic stamped frames with treated paper cones. I had picked them up with the idea of using them as midranges in another project, so I thought I might as well see how they would sound in a small closed box.
I cut some scrap pieces of ¾” MDF into enough sections to make almost all the sides of the box, but there wasn't enough for the two back panels. That didn't stop me; I just glued together everything I had at the moment, figuring that I'd pick up a small piece of MDF and cut some back-panel sections another time.
The external measurements are 8.75” tall × 6.5” wide × 6.75” deep (without the back on). Since I have a ¾” round-over bit for my router, I rounded over every thing but the incomplete back. It looked a little bit like a Chicklet (candy-gum), so I stained it with some whitewash.
An old tweeter-baffle template, which is nothing but a masonite board with an appropriately sized hole for guiding a router around the inside to make a perfect circle, was just about right for the front-speaker opening. You can use a saber saw as well; just draw a penciled circle, drill a "start-hole" big enough for the blade to slip through, and carefully cut out the circle.
Check to see that the speaker frame drops down flush with the front surface. I used a half-round file to produce the exact fit. The speaker must be tightly seated against the wood so that no air can leak past the frame. Any unintentional leakage will degrade the sound quality.
SPEAKER MOUNTING
Solder some wire onto the terminals of the speaker, red to the "plus," and black to the other. Mount the speaker and mark the face with a pencil for drilling the pilot-guide holes for the retaining screws. Remove the speaker again and drill the four guide holes, then brush everything clean. Now mount the speaker and carefully install four fine-thread sheetrock screws to hold it firmly. Do not over-tighten, or the MDF will break out, and you'll need to turn the speaker and start again after cleaning and repairing the MDF.
A speaker box needs something in side to damp the sound that's bouncing around, and there are many materials being used today. If you have some polyester batting, hobby-store quilting, or pillow stuffing, use two big handfuls and loosely fill each box.
The project was close to completion, but I wasn't willing to wait. Since I did not have a back, but did have some Masonite on hand, I quickly cut two pieces to size using this material. It would just be a temporary fix, right ? So, to seal the backs, I used some silicon tub and tile sealer I had already opened. Some small brads held the back flat while the sealer set up.
The very next day, I hooked up the new speakers to the receiver, and they stayed there for a good while-all summer, in fact. The sound was satisfying, especially the voices of newscasters and Garrison Kiellor. But when recorded music came on, I would turn the set off, go into my listening room, and get my music fix on the big system.
After "Pairie Home Companion" one evening, a local jazz show started. I was feeling lazy just sitting there listening to a piano and bass duet, when it stuck me that the sound was just not right. It was compressed, if you know what I mean; it just didn't have any life. I played with the remote, trying to adjust the tonal balance and anything else the receiver offered, but after minutes of fussing, I turned it off and headed for the listening room.
DAWNS THE LIGHT
The next morning I awoke with an idea as to how to fix those little speakers. I took them over to the bench and ripped off the Masonite backs. Then I cut some wire screen, the half-inch-square stuff that you use around plants, to fit over the back. I found a small sheet of fiber glass padding and, cutting out one piece for the back of each box, I placed it behind the existing polyester stuffing and stapled the wire screen over that.
Several days later, I bought two small pieces of white felt to cover the wire screen, and attached them with spray adhesive. I used scissors to cut the felt to size and shape while it was right on the speaker.
I took the units back to the book shelf and reconnected everything. Wow! The compression was gone, and the little babies were singing music. The polyester fill plus the fiberglass was damping the back-wave, but not completely. If I stood close, a couple of feet away, I could pick up some of the reflected wall sound. But seven or eight feet away in the easy chair, it was just great.
The male voices lost any chestiness that was there, and the sound of the small speakers filled the house as never before. They were now much louder, by almost ten clicks of the re mote's volume control--a transformation! Now I can really pick out the recorded promos from the live news, and poor taping techniques are very discernible. But the music was alive.
THE OPEN-BACK SOLUTION
There are many old radios still in use that utilize an open-backed cabinet as the speaker enclosure. They all sound pretty much the same, and not bad at that, considering what's there. Usually it's a little stamped-frame paper unit that comes in different sizes--3, 4, 5, or even up to 6 × 9 inches. You can usually find these at a discount electronics shop for a few bucks.
The principle is simplicity itself--the sealed enclosure was restricting the in expensive unit's ability to move. And it's not that this speaker didn't have a de cent magnet; it does--almost as large as the frame--but it wasn't designed for what I was attempting to do with it. Trying to turn a cheap speaker into an acoustic-suspension design (another name for a closed box) didn't work out.
FIGURE 1: Speaker-enclosure dimensions.
But that was initially; you should hear them now! As for you purists, remember that the open-back cabinet was John Dalquist's claim to speaker fame.
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