The Subwoofer DIY Page - Projects
An INF10 subwoofer
26 October 2018
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.

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 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 receiver.

Shown below is the predicted frequency response of both systems:

Predicted frequency response curves

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.

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 (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.

Parameter Measurement
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.

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 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 noise otherwise.

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.

2nd and 3rd HD, driver mounted horizontally 

  2nd and 3rd HD, driver mounted vertically

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.

Free-air driver response

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.

Predicted response with LP filter 12dB/oct @100 Hz

[My thanks to Stephen Tidwell of Layne Audio for providing information about the INF10's past life in a commercial subwoofer design]

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, 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.


Mounting flange

Top panel

Side panels (x2)

Side panels (x2)

horizontal brace

vertical brace

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. 

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Initial Results
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 x-over region).

Port Problems
Click on the image below to see what the output of the port looks like. 

upr_s.gif (4093 bytes)

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.

upr_12_s.gif (3335 bytes)

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 speaker box!). 

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!



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