sbhattac@rnd.gba.nyu.edu (Shankar Bhattacharyya) (09/14/90)
Some time ago I said on rec.audio that those of us who had built
woofers could probably be persuaded to post designs with budgets in the
$100-200 range. I have since received a few requests to pursue that,
and there has been at least one such follow-up to my article. So, having
been asked to put up, I am putting up. Sorry this has taken some time, but
right now a dissertation proposal comes first.
>From the tone of the questions, I get the impression that something a bit
more comprehensive than just an alignment is in order. So, I propose to
deal with this in bits and pieces. Here I will just list sources of
information. Later, I will try and address choice of drivers, choice of
alignment, etc., etc. If I can dig up a short C program I wrote when I
first learnt C (and woofers, for that matter) I will include that, but that
comes without guarantees. It is a novice's program, and I will have to
trim out fragile elaborations.
If there is any interest, perhaps a hint or two on practical woodworking
for novices can follow.
I'm no expert, just someone with an interest in speaker designs. Woofers in
particular, since I have a strong preference for electrostatics above
100 or 200 hz. So I have almost no real interest in the design of dynamic
systems other than woofers.
If I make a fool of myself on the net, I am sure that someone will point
that out, so I expect I won't mislead anyone for long.
DESIGNS I'M AWARE OF FROM THE NET:
First, Seth Bradley had posted a design for a vented woofer, several months
ago, along with a design for an active crossover, suitable for biamping.
Perhaps we can prevail upon Seth to repost it? I believe he used the
Precision TA305, now no longer available. However, an allegedly equivalent
item is now being sold by Madisound, among others, under the name
"Swan 305" (named for the Swan IV design from Speaker Builder, possibly
their most ambitious dynamic loudspeaker design, incorporating a definitely
solid vented woofer).
Second, I believe that there was a woofer design on the info-high-audio
mailing list, but I don't know anything more about it. Comments, Tom?
CLOSED BOXES AS A DESIGN CHOICE:
I make no claim that closed boxes are best. They are simply the easiest to
build, and perform quite well.
Closed boxes are relatively insensitive to the exact volume. A system Q of
from 0.5 to just over 1.0 seems to work quite well. The cutoff frequency
(F3) does not change dramatically as you cover this range, perhaps about
1/4 octave. This really is not much. So we have a fair bit of room for
error.
Closed boxes are less fussy than vented boxes. Construction is easier.
They are less prone to subsonic excitation than are vented boxes.
There is also a general belief that closed boxes are less efficient and
have higher F3 than vented boxes. That is frequently based on unstated
asumptions, and wrong information. For clearer ideas on the subject, look
at the sources mentioned below, particularly Koonce. I will just state
that it is possible to get quite adequate low bass output in both ways.
GETTING INFORMATION
First, I would urge prospective builders to get some good information.
Plausible sources are:
Vance Dickason: The Loudspeaker Design Cookbook (3rd edition)
I got a copy of this book recently. It is useful for
woofer builders, but does not have enough information to
build complete multi-way systems, because integrating
woofers, tweeters, etc. in a box, with a crossover, is
not trivial, and a little book is not going to teach anyone
how to do that. But, for building woofers, it will do
just fine.
David B. Weems: Designing, Building and Testing Your Own Speaker System -
with Projects
This is better than its earlier version. It now contains
complete projects, with appropriate references to several
Speaker Builder article. (Based on a casual look.)
Since the books each cost only about as much as two CDs, it seems as if one
or more might be a sensible investment. A one year subscription to SB is
still $20, but will go up to $25 later this year.
The vendors of drivers will often have quite good information. Not perfect,
by any means, but first time builders may well be limited to such
information as they can get from the vendor.
The classic papers on closed boxes are those by R.H. Small:
Closed-Box Loudspeaker Systems, Part I: Analysis,
Journal of the Audio Engineering Society, vol 20, # 10, p 798 (1972)
Closed-Box Loudspeaker Systems, Part II: Synthesis,
ibid, vol 21, # 1, p 11 (1973)
Suitability of Low-Frequency Drivers for Horn-Loaded Loudspeaker Systems,
Audio Engineering Society preprint # 1251. (I have seen some of the
results, but not this paper itself. Does someone have a copy accessible?)
The synthesis paper contains some worked examples of various things.
Small's papers are thorough, but they are written for specialists. They are
still well worth a reading, even for us amateurs.
Small's expressions tend to be normalized in ways that make perfectly good
sense, but which make it a bit opaque from an amateur's point of view.
He shows frequency response as F3/Fc, which is fine, except that Fc goes
up as the box gets larger, so his plots can be misinterpreted easily.
They are, too, since a great many people believe that bigger boxes always
produce more bass, which is not always true.
A related article is:
K.P. Zacharia, On the Synthesis of Closed-Box Systems Using Existing Drivers,
Journal of the Audio Engineering Society, vol 21, # 9, p 729 (1973).
It provides a brief, digested account, including a persentation of cutoff
frequency issues in a form much easier to see.
Of course, any half respectable book on speaker design will contain all
this information, if with less thoroughness. The books sometimes have rather
poorly written algebra. Without all of it there to see, it is sometimes
difficult to persuade onself that it is correct.
And, as always, back issues of Speaker Builder, which has carried many
articles on woofers, and many more on complete systems which have woofer
sections of sufficient quality to serve well as subwoofers.
In particular:
G.R. Koonce, Closed v/s Vented Box Efficiency,
Speaker Builder, vol. 2, # 3, p 10, 1981
Trade-offs in Closed Box Alignment,
Speaker Builder, vol. 5, # 2, p 21, 1984
S. Linkwitz (yes, that one), Excursion-Limited SPL Nomographs,
Speaker Builder, vol. 5, # 4, p 24, 1984
F. Ricart, A Modular Three-Way Active Loudspeaker System,
Speaker Builder, vol. 11, # 4, p 36, 1990
Koonce is well arranged for the builder, and, together with Dickason, is
good reading before you read Small, if you are not comfortable with Small
as a first source.
Linkwitz is excellent for figuring out how much driver you need for
specific spl at specific frequency.
Ricart compares a few drivers, including the best ones I know of, in
closed box systems. However, I come up with somewhat different numbers
from those in this article, in at least one case.
SOFTWARE:
Ralph Gonzalez has a program called LMP, available from Old Colony Sound
Labs. He mentioned this program in an article on rec.audio, and I gather,
from articles in Speaker Builder, that it is a very useful program. If your
project is more complex than a woofer, such a program will be very useful.
Bullock's Boxresponse, also available from Old Colony, is another useful
program.
LMP and Box response are dirt cheap.
There are a bunch of programs on Madisound's bbs. Free.
If you are circuit literate, you can use Spice and other such software to
do your modelling, and you certainly have no use for this article. Reading
Small will show you how to model loudspeakers as electric circuits. That is
a mite beyond my capabilities.
More later....
----------------------------------------------------------------------
Shankar Bhattacharyya, Information Systems, New York University
sbhattac@rnd.gba.nyu.edu
----------------------------------------------------------------------bill@vrdxhq.verdix.com (William Spencer) (09/17/90)
in article <6313@uwm.edu>, sbhattac@rnd.gba.nyu.edu (Shankar Bhattacharyya) says: > Some time ago I said on rec.audio that those of us who had built > woofers could probably be persuaded to post designs with budgets in the > $100-200 range. I have one. Sealed box approx. 2.5 cu. ft. 10" woofer (Qts .4 to .5 Free air 19-24 Hz.) Most likely with varivent ("aperiodic loading") & choice of stuffing, wall damping, heavy braced construction. Result is a system Q less than .7 for a less boomy sound. Useful to about 30 Hz. Possible woofers are Carboneau rubber surround models and Audio Concepts AC-10. These drivers have an efficiency of 88-89 dB which speaks volumes on their orientation compared to higher output types: they're oriented towards depth instead of false boom. Design methods include experience and Cookbook calculations. I've tweaked my 12" system as far as it will go and it went from obnoxious bass to acceptable. Cabinet volume is 2.8 cu. ft., response is to 40 Hz. Cone excursion is almost nil. The smaller woofer in a relatively larger enclosure (lower Q) will have much higher excursion and should still stay within acceptable limits. A 12" could be used in a larger enclosure. Crossover frequencies recommended are lower however with a 12. My crossover is 300 Hz, 6 dB/octave. Frequency could be higher with a higher slope. Generally, the lower the better, but that transfers responsibilty to the other drivers. The sealed box design provides better infrasonic control than vented types although the excursion is higher at the lower limits of the passband. > If there is any interest, perhaps a hint or two on practical woodworking > for novices can follow. The simplest box is overlapping (butt) joints using Liquid Nails. Use 2-3 finishing nails per side to increase strength and hold it together until the glue dries. You will have to figure out which sides overlap and the size of the pieces to correct for the overlaps. Fiber or particle board, 3/4" thick or better. finish such as Texturelac for my next project -- tough and fills in imperfections. Don't forget prefab boxes. Parts Express has one less than $30, 1.9 cu. ft. (?), walnut veneer, requires assembly, although it's sure to need reinforcement.McGee Radio carries some unfinished car boxes that can be used as satellites that are very reasonable. bill S.