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  1. #1

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    Default LSi25 Subwoofer Upgrade

    I replaced my LSi25 subwoofer with Polk MM1040UM and listened to it for the first time last night. I can say that I am very happy with the results. I listened to The Battle of Kerak and I felt vibration from the floor and sofa where I sit. Then I listen to some pop beats and it really filled where the upper drivers left off with solid bass.

    This is just my first impression and it is only from one speaker as I've not done the other one. In my plan I have prepared that I might have to modify the stock port to 4" x 26" (more data about this at a later time) but it seems I may just be satisfied without that extra work.

    I need to do more listening as it requires 20 to 30 hours of break-in according to Polk and re-adjust my settings when I have my other speaker done. There are other detail information I will provide but just wanted to share this quick news for now.

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  2. #2

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    Interested in your findings.
    I was thinking of doing the DB1040 swap as the 15s take the DB840. The DB requires little port modification from what I have read.

    Anyway, hope to read more as you finish the second one and port mods.
    "Unwad those panties and have a good time man. We're all here to help each other, no matter how it might appear." DSkip
    I may have an addiction... RTA-15TL, SDA 2, LSi25, LSi15, LSi9, LSi7, LSiCx2, LSiFX, LS/FX, RT/FX, DSW MP2000...and that's just the Polks...

  3. #3

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    Hi zltful,
    I am away from home for about 2 weeks. Once I am back I can provide the driver and enclosure measurements so you can check if a DB1040 can physically fit or not. In my case, MM1040UM can fit but not without enclosure mod. I also simulated driver performance to make sure it would work sonically. Again, I can run a simulation for DB1040 if you want me to when I am back home.
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  4. #4

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    That would be great. They seem to run around the same price so that's not a concern.
    Thanks.
    "Unwad those panties and have a good time man. We're all here to help each other, no matter how it might appear." DSkip
    I may have an addiction... RTA-15TL, SDA 2, LSi25, LSi15, LSi9, LSi7, LSiCx2, LSiFX, LS/FX, RT/FX, DSW MP2000...and that's just the Polks...

  5. #5

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    Zltful, the interior Lsi25 width is 5.5" but DB1040 has a 5 7/16" top mount depth which is not going to fit well.
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  6. #6

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    I finished the sub upgrade on my 2nd speaker. This upgrade takes about 2 hours for the work but I can say the difference is significant and totally worthwhile. The bass becomes much tighter, stronger and integrates well sonically with the other drivers. I chose not to run additional subs in the past but for some type of music I wished I did. With this driver upgrade, I don't think I'll need additional sub anymore.

    Old vs. New
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    Stock subwoofer
    diameter 10 1/16"
    cutout hole diameter 9 3/16"
    depth (bottom mount) 5 3/16"
    4 screws

    MM1040UM diameter 10 7/16"
    cutout hole diameter 9"
    depth (bottom mount) 5 3/16"
    8 screws

    You can see that the two sub have the same mounting depth which is very important due to the way the sub needs to be mounted on the side of the speaker. Any other sub with longer depth would either not fit or have issues of not allowing adequate air flow. The existing cutout hole is a tiny bigger than what's required by MM1040UM which works well without any change. However, the diameter of MM1040UM is a little bigger than the stock - 3/8" to be exact. For this small difference, the recess ring where the sub would sit on needs to be widened. To do this, you'll have to use a router, and a plunge router would work better than a fixed router in this case. You will also need a circular router jig to make a circular cut.

    I made my own circular jig with a piece of acrylic. I can drill a hole anywhere on the jig for the radius I need. You can also buy one with many pre-drilled holes. To widen the recess ring for 3/8" more in diameter, I used a 1/4" router bit with a radius of 5 3/32"
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    To use the jig, you must have a center hole where you can pin down the router. With the hole already cut on the speaker, this is a problem. So, what I did was to glue a temporary board on the back of the hole to close the hole and waited for it to dry. I used silicone adhesive so that the board and the adhesive can be removed easily and cleanly afterwards. You don't want to use Gorila Glue or alike for this. Then I made a 3/4" thick and 9 1/4" diameter round MDF (using router) to sit in the hole. Now I have a center point. Make sure it sits tightly without room to move (the center point). If it does, you can tape around the edge to make it bigger until it sits tightly. I then glue another layer of 3/4" MDF square to sit on the top. It's very important to ensure the center point is maintained on the square. Now this 2nd layer is flush with the speaker cabinet and ready for the router work.
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    To prevent the cabinet getting scratched by the jig, I taped the surrounding areas. But, I still made some scratches. Oh, well, I am not selling this speaker so - so be it. It's better to cover up all the surrounding area.

    Now that the recess is done, the temporary board can be removed and start to mound the new sub. Surprisingly or not, the 4 screw holes from the stock sub fits precisely to the new sub, so you'll just drill 4 new holes. It's that simple. I'd suggest to tighten all 8 screws because this new sub is powerful and really can move.
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  7. #7

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    About this sub replacement, without any port modification, the frequency response is tuned to 42.5Hz. This is a enjoyable frequency that most speakers do for home theater and general music listening which has more thump. There is no data I can find for the original stock subwoofer, but I wouldn't be surprised if it is tuned to the same result by knowing how the rest of the LSi drivers sound.

    However, if a true flat bass response is desired for critical listening, then the port would need to be modified. Changing the port diameter would be very difficult due to the existing Power Port at the bottom of the speaker and that the interior space is only 5.5" wide. My recommendation would be just to extend the length of the port tube and keeping the same diameter. According to WinISD simulation, extend the tube to 20inches would result a tuned 30Hz frequency for a fairly flat response above that. Although MM1040 has a frequency response down to 24Hz based on the spec, 30Hz would be a good realistic frequency. Rarely music really go down to that low anyway. There is a bit of concern in port air velocity as normally it should be kept under 55ft/sec, and making the port wider (such as 4" diameter) would decrease that concern. However, it should be fine as long as you are not listening at a very loud volume (who can hear that "chuffing" when the overall volume is at that level). If anyone tries to modify the port length or diameter, please share your result with us.

    Initially, I didn't think it is possible to extend the tube as the sub section of the cabinet is only 26" tall. Then totally by accident I looked up inside the cabinet, I found that there is a long and narrow chamber that goes from the sub section all the way up to the top of the cabinet behind the MTW section (see picture). So it can be extended very long as long as you want to make the effort. I am sure there is somewhere that sells 3.5" PVC pipes and fittings. I've not done this myself because I am satisfied without modifying the port. I can only imagine that it can be done by cutting down the existing tube somewhat and then use different angle elbows to extend the tube around the subwoofer (because the tube is right underneath it) and then goes up straight. One must also check into that there is enough space between the amp and the subwoofer.

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    Although it may appear to be a concern that MM1040 is capable of a 350W continuous rating and the LSi25 amp output is only 150W (300W peak). In reality, it's only a 3dB difference. For most ears, 3dB is the threshold of human hearing, so I don't think there is going to be a noticeable difference in volume if you want to play it loud. We must also consider the coherence to the other drivers, so I don't see a need nor recommend to change anything to the amp section if that's even possible.

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    LSi25 subwoofer enclosure
    lower chamber 13.5" D x 5.5" W x 26" H (volume 1.12 ft3 including port)
    upper chamber 4.75" D x 5.5" W x 18,5" H (volume 0.28 ft3)
    round port 3.5" diameter x 8" length (volume 0.045 ft3)
    Amp 150W RMS, 300W Dynamic, 4 Ohm

    Polk MM1040UM SVC
    Fs 35Hz
    Xmas 1"
    LE 3.0 mH
    RE 3.25 ohm
    SD 54.25 ft2
    Vas 0.65 ft3
    Qts 0.45
    Qes 0.529
    Qms 3.0
    Power Handling 350W Continuous, 700W Peak
    Nominal Impedance 4 ohms
    Efficiency 91 dB

    In the pictures, green lines are w/o the tube extension (8"), white lines are with tube extension (20").

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