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Path: utzoo!linus!decvax!harpo!eagle!mhuxi!houxm!ihnp4!we13!mgweed!rjr
From: rjr@mgweed.UUCP
Newsgroups: net.audio
Subject: building speakers
Message-ID: <4118@mgweed.UUCP>
Date: Thu, 13-Oct-83 16:56:47 EDT
Article-I.D.: mgweed.4118
Posted: Thu Oct 13 16:56:47 1983
Date-Received: Fri, 14-Oct-83 20:22:24 EDT
Organization: Western Electric - Montgomery Illinois
Lines: 65


After reading ihldt!dbg's thought on building speakers, I thought
I would throw out mine:

I have experimented with many speaker box designs. There were quite
a few articles in "Popular Electronics" (I think the author's name
was Webb) about building cabinets, mostly dealing with bass reflex types.

I never had good luck with ported, or bass reflex designs.

After consulting with a friend of mine who is an acoustical engineer
at a well known acoustical lab affiliated with IIT, it seemed the
best bet was to go for the infinite baffle design. It is a good one
since, for one thing, the enclosure size is not critical, as it is
with vented enclosures. One can try all kinds of speakers in the box
without worrying about cone resonances etc.

The main rules for constructing a cabinet (which apply to many types)
are to make it heavy. Don't use 1/2 inch plywood. Use 3/4 inch stuff
at least. If the cabinet is much larger than a bookshelf size, fasten
ribs inside to stiffen the sides and back. It is a matter of choice
whether you want to use screws or nails, but definetly use glue as well!
The only exception to this is the front or back, if you want to be able
to remove it. Put at least 4 or 5 inches of fiberglass padding on all
inside surfaces, except the front. The idea is to make the box look as
large, acoustically, as possible. 

I mounted a screw terminal strip on the back and brought out all
speaker wires individually. The crossovers are mounted on the outside
of the cabinets.

I prefer a 3-way crossover system. My cabinets have a 12 inch woofer,
a 8 inch cone type mid-range unit, and a horn tweeter. I selected
crossover frequencies of 500 and 2500 Hz. My crossover is a 12dB/octave
design taken from an old audio handbook. I wound the inductors with
22 Ga wire using laminations from a couple of old power supply filter
chokes. The capacitors are all mylar or paper types. Do not use
electrolytics! Even the non-polarized speaker caps that are sold
(Radio Shack and others) are back-to-back electrolytics and the
tolerance is lousy and they deteriorate after some years. If you have
room, you can even use oil types. 

My measured crossover characteristics came out very close to the
design figures. The low frequency crossover turned out at 535 Hz
rather than the intended 500, but otherwise there were no other
departures. 

For serious listening to classical music, I also use a pair of
"Ionovac" tweeters. These have a high impedance input and I connect
them across the existing horns. Today, I would recommend one of the
dome tweeters. They are clean and efficient. One should connect
the tweeter to the crossover through a "L" pad. Many speaker systems
used to incorporate one as a "brilliance" control. It helps balance
out the system. 

Single driver wide range "coaxial" speakers are OK for background
use only, as far as I am concerned. I don't think one can expect
a 10 or 12 inch cone, being pounded by bass, to simultaneously
work well on the mid and high frequencies.  

Be sure the high and mid range speakers have solid backs so the
back-wave from the woofer does not reach them.

Bob   mgweed!rjr