NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/rec/audio/18978

Internet Message Format | 1993-01-21 | 5.6 KB

Path: sparky!uunet!spool.mu.edu!agate!ames!ncar!noao!amethyst!organpipe.uug.arizona.edu!news
From: [e-mail] (Thomas J. Trebisky)
Newsgroups: rec.audio
Subject: Speaker Building -- a progess report on my project.
Message-ID: <[e-mail]>
Date: 21 Jan 93 05:59:42 GMT
Sender: [e-mail]
Organization: University of Arizona, Tucson, AZ
Lines: 98

In a bold attempt to actually build myself a set of speakers (encouraged
by a friend who built a pair and loaned them to me for a while to try out
-- very nice, it definitely can be done), I have begun my quest as follows:

    Comments are invited, consider this a design review....

Step 1:    Gather information -- at first this was reading a copy of the
book by David Weems.  Used to be sold by Radio Shack (a mark against it,
I know), but now out in a 3rd edition by McGraw Hill.  Then fired off
requests to Madisound, A and S speakers, and others for catalogs and info.
Then ordered Dickason's cookbook.  These catalogs are gold-mines of info.
Finally got bold enough for step 2. (Oh, and our library had a copy of
the AES anthology).

Step 2: order drivers.  My strategy is to first build a small set of
satellites with pretty decent bass response, then in the future build
a subwoofer and enhance my system.  But for now the satellites:
What I ordered was a pair of Peerless TP165R/8 -- a 6.5 inch woofer
actually part number 833599 with Fs=36.2 Hz, Qts = 0.28, Vas = 32.2 Liter.
Also ordered a pair of Vifa D25AG35-06 aluminum dome tweeters.
The cost is $35.00 for the Peerless and $24.00 for the Vifa,
so for a pair of speakers, I have invested $118.00 in drivers.
My target was $100.00 for the pair (for drivers), but the extra should
be money well spent.  If I did it again, I would probably get the
Peerless CC series driver for $41.50, but this one should be fine.

Step 3: design the box.  The Peerless is a low Q driver intended for
a bass reflex design.  I ignored the advice that a beginner should start
with a closed box design -- we will see about that.  Choosing a QB3
alignment and using Dickason's tables for Ql = 7, I get a box volume
of 573 cubic inches (9.39 liters) for this driver.  I am gonna overvolume
my box by 25 percent -- just for safety -- I can always throw bricks inside
later to consume the extra volume --  My final box will be 8.5 inch deep,
10 inches wide, and 15 inches tall.  Purchased my sheet of 3/4 inch MDF
for $16.00 and have been getting along pretty will with my skill saw with
a carbide blade and a guide to bang the box together.
It is using less than half the 4x8 sheet, so if I really screw up, I
am only out $8.00 bucks and I can try again!

Step 4: design the vent.  I did buy a piece of 2 inch PVC pipe for this
purpose -- calculations indicate a piece of this about 6 inches long
would be about right (and be a bit too close to the rear of the box).
BUT, I decided to ignore a bit of advice and try using a "shelf port".
Dickason blows this off and refers you to Thieles paper.  Thiele says this
is his prefered method.  Something about it just appeals to me -- it
allows the widest and longest vent for a given box.  Fiddling with the
equations, I am gonna have a 1/2 inch tall slot the full width of the box
for a port.  A shelf port is a scheme like this in cross-section, BTW:

    ________________
    |              |
    |              |
    |           Tw |
    |              |
    |              | <- may put a brace (sheet with big hole) here.
    |              |
    |              |
    |              |
    |         Woofe|
    |              |
    | _____________|
    |_______________ <- here is the shelf port (see why the name?)


We will see how this all turns out.  Like I say, I have the drivers in
hand and am working on the box, I will update you all as things progress.
Anyone with comments (like, "You idiot, why are you doing that", is welcome
to email me or post or whatever (that is why I am posting this actually)).

Ultimately I intend to use an active crossover and biamp (well tri-amp
when I have a subwoofer, some fine day), but for now (lacking the "iron"
for such ventures), I am gonna hack together a "get me by for now" crossover.
The plan is to use a 2nd order job for the Tweeter (a 7 uF cap in series
and a 0.5 mH coil across the tweeter).  This should be a 12dB per octave
roll-off at 3 Khz.  The woofer will get a 0.45 mH coil in series with it,
just a simple 6dB per octave first order treatment.  Oh yes, and I have one
L-pad, I am gonna adjust it to balance the two drivers, then replace it
with a fixed array of power resistors (I have a big junk box full of them).

Other bizarre schemes await a future post to be revealed.

Oh, and a footnote -- there has been some ridiculous harangue in this
group about whether it is better to build speakers or build them.  I would
say, if you just want speakers, buy them!  If you enjoy building things and
want speakers, build them (well, maybe evaluate your woodworking skills
first).  I doubt that speaker building is for everyone, but there are those
to whom such things has a certain appeal -- and I do fully expect that my
speakers will compare favorably with commercial designs that cost more
(whether this hope will be realized or not remains to be seen, but I do
expect it at this stage. :-) ).  Particle board is cheap -- as long as I
do not burn up my drivers, I can build boxes till I am satisfied, the
duds can feed the fireplace if necessary, but I am hoping for better things,
even on the first round.
--
    Tom Trebisky    [e-mail]

This program posts news to thousands of machines throughout the entire
civilized world.  Your message will cost the net hundreds if not thousands of