In August 1998, my attention was caught by a new subwoofer driver offered by Madisound under the What's New section on their web
site. The INF10, a 10" driver, appeared to have Thiele/Small
parameters that suggested very good performance in a ported enclosure. The INF10 appears
to be the same driver used by Infinity in their $600 SSW10 servo subwoofer, which received
quite favorable reviews. The SSW10 was a 1.1 cu.ft. sealed enclosure housing a
10" driver that was powered by an 80W amplifier.
T/S parameters are as follows:
||109 litres (3.85 cu.ft.)
These parameters suggest that a port box about 2 cu.ft. or greater, tuned to
frequency at or below 27 Hz would provide very good low bass performance, with cutoff
frequencies below 30 Hz, for a 10" driver that cost $32? Of course I'm interested!
Note that the INF10's Sd is about 330 cm^2, which is quite low for a 10"
driver (effective diameter works out to 20.5 cm (8.07 in.)), but it's not as low as some
other popular 10" drivers, including a few from Peerless.
Investigation and Modeling
Madisound suggests the use of a 2.3 cu.ft. box tuned to 27 Hz, which is a simple
"maximally flat" alignment that should produce decent results. However, I think
I'm going to try something different - a 2.5 cu.ft. box tuned to 22 Hz. Theoretically, my
system should provide and extra 3dB at 20 Hz, at the expense of some midband efficiency. I
think that this alignment would be a better match for my listening room, as well as the
100 Hz 12dB/oct LP filter built in to the subwoofer amplifier circuit of my Technics
Shown below is the predicted frequency response of both systems:
Both systems can be expected to produce a 30 Hz tone at just under 100dB at 1
metre at a 25 Watt input level. In-band power-handling is slightly better for the
Madisound alignment, but is less important to me than the extended frequency response of
the large system.
The INF10's cone is reasonably stiff, though not at stiff as some car audio
10-inchers I've come across. Physical cone throw is just about 1 inch. The INF10 has
unmatched spade terminals, which suggests that this was a driver destined for use in a
commercial full-range speaker system. Note that there's no cardboard spacer ring on the
surround, so isobaric mounting might be a bit difficult (basically, you'll need to make
your own spacer ring). The INF10 has a vented pole piece, a stamped frame that rings a
little when I tap it a 2" voice coil former, and a 5" flat spider (good news for
ported systems). Mechanical damping is quite low (matching the high Qms is indicated on
the LEAP specs). There are no pinging sounds when I tap around the sides of the cone
(good), and it's almost impossible to "rock" the cone from side to side (very
good). Altogether very good value for a 10" woofer that costs $32.
Measuring the T/S parameters of the INF10 driver using my usual methods proved to be
extremely difficult, because of the low resonance frequency and high Qms (low mechanical
damping), the latter which produced a high and difficult to measure impedance peak.
However, the results from several measurement runs suggest to me that, if a large enough
ported box is chosen, it should be possible to correct any frequency response anomalies
that may arise by re-tuning the box to a higher or lower frequency.
Most of the drivers I've come across usually exhibit some kind of motor noise, but usually
when they're nearing their maximum excursion limits. However, the INF10's characteristics
were a bit different. When I subjected it to my usual "motor noise" tests
(basically running a high-level 20Hz tone for the driver and listening for any out of the
ordinary sounds), at about 1/8" excursion (20 Hz), the INF10 started to exhibit a
ticking noise, which grew louder, then softer as the volume was increased. I traced the
source of the noise to the point where the ribbon leads are bonded to the former. Just
above this bond, the leads are bonded to the cone by a small bead of glue, but it seems
that this bead came loose and was tapping against the cone, hence the "ticking".
A small dab of silicone in the right place cleared up this problem immediately. Those of
you who want to use INF10s might want to look out for this ticking "problem"
with your driver - it's quite easy to solve, and the driver exhibits very little motor
Shown in the graphs below are distortion measurements I took of the raw driver
driven with a 20 Hz signal at varying voltages. The graphs were generated using my PC as a
signal generator/frequency analyzer, and a Radio Shack SPL meter as the recording device
(reverse-C weighting was applied to the results). The results indicate that 2nd harmonic
distortion increases significantly when the driver is mounted horizontally, and 3rd
harmonic distortion decreases slightly. This suggests that the best results might be
obtained with the driver oriented forwards instead of facing down.
Free-Air Frequency Response Measurements
The graph below shows the free-air response of the driver (measured via CoolEdit
and my SPL meter). The lower end of the graph is probably inaccurate and should be
ignored. Of concern here is the twin peaks at 900 Hz and 2.1 kHz.
The graph below shows the predicted response of this driver, when driven by my
Technics receiver's subwoofer output (12dB/oct @ 100 Hz). I suspect the 900 Hz peak,
though reduced in size, might still be audible under certain conditions. A 12dB/oct filter
centered at 450 Hz should take care of this problem, if it occurs.
[My thanks to Stephen Tidwell of Layne
Audio for providing information about the INF10's past life in a commercial subwoofer
Coming up with a 2.5 cu.ft. ported box for this driver proved to be a bit of a
task. At first I wanted a box that could fit below my couch, but that proved to be
impossible. Shown below are the latest drawings that I've come up with. Once constructed,
the net internal volume of the box (with speaker installed) should turn out to be 2.54
cu.ft. Note that my measurement of the volume displaced by the INF10 via the usual bucket
and water technique turned out to be 183 cu.in., and this accounted for (along with the
volume occupied by the braces) in the design. Note also that the 5.25" cutout is for
the flared 3" port (also purchased from Madisound). The box is designed so that the
driver and port can be faced downwards or forwards.
Box construction was relatively straightforward. First of all, the two braces
were fitted together in the shape of a cross. Then the mounting flange was attached to the
rear of the baffle such that the two circular cutouts were concentric (this
"flush-mounts" the INF10 driver). The other panels were then attached (using
glue and screws) to the braces, starting with the larger side panels, then the top and the
baffle, then the smaller side panels. Once the glue was dry, a hole was cut for the
terminal cup and then the terminal cup, the driver, the leads from the driver to the
terminal cup, and finally the port were installed. The following pictures were taken
while I was constructing the subwoofer.
For a subwoofer based on a driver that cost $32, the initial results I'm getting
seem to be quite nice, except for the driver's tendency to "bottom" hard with a
loud cracking noise when it receives a large transient signal. If you're looking for a sub
to play the 1812's cannons, then I recommend that you look elsewhere. OTOH, it seems
to be quite a good match for any system where you don't have a maniac at the bass
controls. Preliminary listening tests tell me that, compared to my "El Uglito", the INF10 subwoofer delivers more low bass
(expected), but a but less "punch" (which could be the signs of problems in the
Click on the image below to see what the output of the port looks like.
With this type of response, this system definitely can't be used full-range! In
my opinion, this "organ-pipe" effect is one of the main reasons why ported
systems should probably be considered for subwoofers only, not for full-range systems,
unless in the full-range system, the driver is crossed over well before the
"organ-pipe" resonance frequency. Note also that this is a measurement
taken with the enclosure undamped. With a lining of fiberglass damping on the sides
of the enclosure, I expect that the resonance peaks will be significantly reduced.
The image below displays what the port response will look like if the subwoofer
is fed with a signal from a 12dB/octave @100 Hz filter.
Note that, while the "organ-pipe" resonance effect is
still present, it's much lower in level compared to the port's peak output. With
additional damping, I expect the effect will be even further diminished.
Improvements (01/2000 - 04/2000):
The first thing that need to be improved in this particular subwoofer was the problem of
cabinet vibration. It seems that, even with the cross-brace inside of the enclosure,
the top and the bottom of the cabinet still suffered from noticeable vibration at higher
volume levels. In an attempt to fix this, I inserted hardwood dowels lengthwise in
the enclosure, and also include a few plywood cross-braces on the most troublesome panels
(top and bottom). This fixes solved some of the vibration problem.
I was starting to get a bit tired of this rough plywood box with its visible 50+
deck screws sitting in my living room, so I decided to veneer the box to give it a more
attractive natural wood finish (with no visible screws). The problem is, no-one in
Grenada seems to stock wood veneer (the stores' ideas of what constitutes
"veneer" was Formica and Arborite - not the stuff that I want to put on any
What to do?
I decided to kill two birds with one stone. I purchased a sheet of
1/8" ply, and cans of MINWAX Pre-Stain Wood Conditioner, Red Oak Wood Finish and
Clear Satin Polyurethane Varnish, and set to work creating my own veneer and strengthening
the box (the 1/8 ply adds additional stiffness) at the same time. The results seem
pretty impressive, if I say so myself!