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The published T/S parameters are as follows:

Vas	109 litres (3.85 cu.ft.)
Qts	0.34
Fs	22 Hz
Xmax	0.31
Pe	100 Watts

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 Modelling
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 receiver.

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. @30 Hz. Power-handling is better for the Madisound alignment, but is less important to me than the extended frequency response of the large system.

Personal Observations
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 (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.

Parameter Measurement
Measuring the T/S parameters of the INF10 driver using my usual methods have proven 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 posible to correct any frequency response anomalies that may arise by retuning the box to a higher or lower frequency.

Motor Noise
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 runnng 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 noise otherwise.

Construction Plans
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.

Construction Steps
Box construction should be relatively straightforward. First of all, the two braces are fitted together in the shape of a cross. Then the mounting flange is attached to the rear of the baffle such that the two circular cutouts are concentric (this "flush-mounts" the INF10 driver). The other panels are then attached (glue and screws) to the braces, starting with the larger side panels, then the top and the baffle, then the smaller side panels. If done correctly, one edge of the brace should be resting on the mounting flange. Once the glue has dried, cut a hole for the terminal cup and install the terminal cup, the driver, the leads from the driver to the terminal cup, and finally the port. The walls of the enclosure can also be lined, to tame any internal resonances. Finish the exterior walls of the box according to taste (I plan to use one layer of fiberglass mat and resin).

Note: I haven't built this box yet! These plans are DRAFT plans, and may be changed in the future, to incorporate further improvements, or correct any errors in the design.

Distortion Measurements
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 design]

Brian Steele
brian@caribsurf.com