# Monoprice 9723 - cheap subwoofer hack



## Heterosapian (Dec 31, 2011)

I've purchased a few of these over the years and was always pleased with them in small/medium rooms for music, but disappointed with their low-end performance for use in a home theater setting (though I'm spoiled by big-sub mobile installs and four ported 15s in my garage). They recently went on sale for $85 SHIPPED - for a ported 12" in an attractive enclosure with a 150 watt amp, RCA and high-level inputs! SO... I picked up another pair and decided to open them up and hack away until I get what I want! Moved the others I've collected into the living/viewing room for a total of five. Going to brute force some LFE out of these bad boys and, if I fail, it was a <$500 experiment and I have five subwoofers to distribute or resell. 

Anyway, upon disassembly I found some pleasant surprises. 3/5 interior walls are covered in thick batting. The port is a robust 4" job with some room to extend/lower the tuning frequency. I didn't run impedance sweeps but the tuning seems to be in the high 40s, according to some preliminary testing. Paper cone woofer, foam surrounds, stamped basket, soldered terminal connections, vented pole piece. All pretty cheaply constructed but it does the job for very little money. Enclosure is a robust 1/2"-3/4" mdf deal that perfectly matches the appearance of the Pioneer Andrew Jones floor-standers that I'm using for all five channels in my "theater". I mean, it's such a dead-match that they must have precisely targeted this speaker line.

I tried stuffing the enclosures with 1lb of poly fill. That flattened the response a bit (in my room) - less of a droop at 50hz and calmed the peaks at 40 and 60hz. This also netted a small bump at 30hz which made the measurement look all the more flat. I believe there's a subsonic filter kicking in around 30hz as the response falls off a cliff, stuffed or unstuffed, below 30.

I then hacked the existing port in half and installed a few 4" id pvc extensions in place for testing. The pvc slipped right over the short length of existing 4" cardboard tubing (yes, cardboard). I ultimately settled on a 6" long pipe, which extended the port length 3.5" or ~30%. Doing so and returning the stuffing netted a 5db gain at 30hz and 4db at 40! SCORE! A/Bing stuffed v. ported-and-stuffed v. untouched, The difference is noticeable on low-low tracks like 'Bass, I Love You' and should make for a much nicer theater experience when things explode. 

I'm looking forward to hacking the other four and wiring them together this weekend. five of these bad boys may not get me to 110db at 30hz but should make for a very respectable LFE channel, especially for the money! Stay tuned. I'll throw some pictures, sweeps, etc up over the coming days.


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## Heterosapian (Dec 31, 2011)

Got antsy and hacked four of my five subs yesterday. Leaving one untouched for comparison and I may not run it in the living room anyway - it may go to the bedroom  

Stuffed 16oz poly fill and glued the 6” port in all four. No testing yet. Waiting on a batch of cables to come in today. Looking forward to getting it up and running. Fun stuff. All the subwoofers.


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## Heterosapian (Dec 31, 2011)

Got it up and running and it's frankly disappointing. Even with the additional response at 30-40 hz, the subsonic filter in the amplifier must kick on in the 30s because there's not much there. Don't get me wrong - it's loud if I want it to be, but they lack the depth that even a single ported 15" would produce. When they're turned down enough to blend with the towers, they're almost acoustically invisible - which is to say, they're not pumping out that 20-40 hz range that a true SUBwoofer should be able to. Going to eq and redo the crossover and gain setting using REQ, and hope for some improvement. Right now, the crossover on the amp is run wide open and the receiver is sending them <= 50 hz. Turn up the gain and turn down the low-pass on the amps. Ghetto eq <50.

This may have been a fail for my purposes. Put one in a small room and it jams but try four in a large room and it's just kind of "meh".


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