Peter's Loudspeaker Designs
Subwoofers based on the Dayton 15" Mark II Driver
Building a subwoofer system to match my electrostat satellites was my first
experience in speaker design. Since electrostats image so incredibly
well, and I wasn't sure of exactly how I was doing to low-pass the subwoofers
(I'm still not sure), I decided to build a pair of subwoofers instead of
single unit. In this way, even without a crossover, I can play room
placement games to protect imaging and even affect a room-based low-pass.
The choice of the Dayton driver was mostly due to my being enamored
by the specifications - in particular the < 20Hz fb and the small Vas.
The driver is available from Parts
Express. I have no affiliation with them. You may
want to check out the specification sheet
(not scanned yet) for the driver, which also includes the beginnings
of a vented box design.
I developed three designs: sealed box, vented box, and an insane transmission
line. Ultimately I built the sealed box design because I wanted the
subwoofers to have some remote chance of being as "fast" (i.e., have a
good transient response) as the electrostats.
Sealed Box
Sealed boxes with Qtc=0.707 do well at preserving
transients. A 0.707 box for the Dayton 15" driver has Vb of about
2.8 cubic feet. F3 for this design is about 40 Hz, but since sealed
boxes fall off at -12 dB/octave, there is still plenty of oomph left at
20 Hz. If you really want very heavy, very deep bass, consider either
Dayton's or my own vented box design. If you also care about transient
response, you might want to check out my transmission line design, although
it is really anyone's guess how it will actually sound.
The following scans from my notebook show the calculations and the dimensions
for a golden ratio box. Please note that although these scans are
only 50-100K, they are physically large, 300 dpi scans from a notebook,
and are probably best printed rather than viewed directly.
The "t" in the description of the box components is the thickness of the
material used to construct it.
This is the design I actually implemented. The wall material I used
was 3/4 inch Luann hardwood plywood. MDF may be a better construction
material, but I found it very difficult to find. Home Depot carries
Luann for about $30 per 4x8 sheet, and they will cut it for you to the
necessary dimensions so the only cutting you'll need to do is for the holes
for the driver and the terminal block. The speakers require 1 1/2
sheets. If you buy 2 sheets, replicate the cuts on the extra 1/2
sheet and you'll be ready to build a pair of these spiffy
little transmission lines, too. The above drawings show the driver
offset to one side of the front panel, but I ended up putting it dead center.
The front of the box was constructed from two panels laminated together
with wood glue using finishing nails to hold it together until the glue
dried. For the other panels, I laminated smaller pieces of scrap
to the insides in order to increase stiffness. To put it together,
I used wood glue with finishing nails to hold it together until the glue
dried. You may prefer to use wood screws instead, but the nails do
work fine. I sealed the box using the kind of silicone caulking ("Kwik
Seal") used in bathroom and kitchen tile work. The inside of the
box was lined with 1.5-2 inches of dense fiberglass insulation, the kind
used to insulate pipes and the like. Internal wiring is 12 gauge
"Sound King" speaker cable. There is no internal crossover.
The driver and terminal block were sealed to the box using that black silicone
goo that Parts Express provides with drivers.
In the future, there will be pointers to photographs of the construction
process and the finished speakers here.
Vented Box
Although Dayton includes a vented box design with
the driver, it seems to be very biased to produce the "one note samba"
that some bass aficionados prefer. I did a design with a middle of
the road alignment which puts f3 around 26 Hz. Keep in mind that
with a vented box, the fall-off is -24 dB/octave. The following
scans describe the vented box design. No vent calculations are provided
since I lost interest in a vented box design before I did them.
If you build either my design or Parts Express's, I would like to know
your impressions.
Insane Transmission Line
Having developed a taste for transmission line speakers
after hearing (and buying) a friend's Voigt Pipes, I decided to do a TL
subwoofer design too. I didn't fold the line since that makes construction
far more difficult. There is only one change in direction for the
sound wave and the end of the line is intended to fire at the ceiling to
get as much dispersion as possible. Unfortunately, a 1/4 wavelength
of 20 Hz in free air is around 20 feet, so the line needs a lot of the
right kind of fill. According to Dickason's figures for the speed
of sound in wool, 6-8 feet is long enough for a 1/4 wavelength at 20 Hz
transmission line. Unfortunately, that amounts to hundreds of dollars
of long hair wool for my design, which is why I didn't build it.
If you are interested, the following scan describes the design:
Please let me know what it sounds like if you build it.
Spiffy One Way Full Range Transmission Lines
I find crossover-less speakers quite beguiling.
It's amazing how good a simple, cheap, one-way driver can sound in the
right enclosure, like a Voigt Pipe or a transmission line.
If you build the sealed box subwoofers described
above using two 4x8 sheets of material, you'll have exactly the pieces
you need to build this fun little set of full range transmission lines.
While the design is totally ad hoc, when fitted with the Panasonic 8" full
range drivers ($20 each) it has that certain foot-tapping quality.
It also has various defects, but the overall effect makes it worth implementing,
especially since it's so cheap. Several designs are in the following
scan. The design I implemented is the last one.
The construction was the same as for the sealed box
subwoofers described above, except that these speakers must be DENSELY
filled with a SAFE material. I used two to three pounds of pillow
stuffing material per speaker. Note that the line can be lengthened
by increasing the length of the front piece. If you use the cuts
described in the sealed subwoofer section, you'll have lots of scrap of
exactly the right diameter. I ended up adding about 3.5 inches to
the design.
If you build these speakers, let me know what you think.
The Importance of Soup in Audio
Are you an audiophile? Do you think soup cans matter in audio?
Paul Placeway and
I think
so! Check out this letter to the
editors of Stereophile that we wrote on the subject.
Peter
A. Dinda
pdinda@cs.cmu.edu
