LSi15 woofer sizing

caardvark

Heeding the advice of my elders:

https://forum.polkaudio.com/discussion/89529/db840-sub-in-lsi15s-replace-the-stock-8-driver/p8

I'm starting a new thread with this info and question(s). I purchased some DB842 woofers to do the old DB840 upgrade, as the latter is discontinued. To my surprise, the DB842 is bigger than the stock woofer and cutout, 8-3/8" vs 8-3/16" outside diameter, despite reports that it's a drop-in replacement. Polk's owner's manual and spec sheets only lists it as 8" round.

The DB842's perimeter is the solid metal of the basket, so there's nothing that can be trimmed or removed.

I thought maybe my oddball side wood trims and grills -- the rarer variety that screws in from the back, rather than being pressed in from the outside -- signify an unusual size woofer and cutout.



So, one question is, for those with the press-in style grills, what is the measurement of your stock woofer and/or cutout? Or, alternatively, anyone with the old DB840's, what is their O.D.? (My digital calipers max out at 8", so I had to approximate with a tape measure.) On mine, it's a very snug fit for the stock woofer within the cutout. Do those of you with conventional grills have a bit of wiggle room there?

However... I subsequently found that Soundstream SS8 woofers are the exact same O.D. as the stock ones. Those are the only 8" drivers I had on hand to test, but it seems too coincidental if my stockers are non-standard. I have to think its the new DB842's that are oversized, and that the reports of them fitting are bogus.

FWIW, my stock driver is part # RD2505-1/2 and is dated '01. I don't know if that makes my speakers a very early build, or if they just collected a big stock of drivers early on for assembling much of the production run?

I also have a MM842 to possibly try out, but I have an inquiry in to Polk about its O.D. before opening it. If I had better woodworking tools, I would consider filling the baffle with a trim ring, and just mount the driver further out flush with the cabinet's outer wall.

caardvark

Looking around a bit more this morning, I found a Crutchfield review (1/14/22) of relevance for the MM842:

"I installed a pair of these in a pair of Polk Audio Lsi15s even though Polk engineering team recommended the DB series for a direct drop in. I chosen the MM series do to being a much better woofer, it made huge improvement in deep bass, weight and punch! It made nice improvement to the sound stage do to the cabinet not resonating as much. I did have to use a drimal tool with a sanding bit to widen the opening about a 1/4" for it to sit where the stock woofer was and for the stock woofer grill/cover to fit like stock."

https://www.crutchfield.com/S-8BaJinivZJm/p_107MM842S/Polk-Audio-MM-842-SVC.html

The Dremel work in the pictures accompanying that review makes me cringe, both in terms of a sloppy perimeter and uneven mounting flange, but that's a tough thing to get right by hand. If trying something similar, I would probably trace the additional diameter needed, then use an end mill bit on the drill press to trim that out without digging any deeper.

So, the MM842 probably measures the same [over-]size as the DB842.

Edit: Maybe spoke too soon as far as determining anything regarding standard O.D. sizing... Here are two exceptions to anything resembling consistency:

https://www.parts-express.com/Dayton-Audio-RSS210HO-4-8-Reference-Series-HO-Subwoofer-4-Ohm-295-458

Overall Outside Diameter: 8.39"

https://www.parts-express.com/Pyle-PLBW84-Blue-Wave-8-Woofer-267-093

Overall Outside Diameter: 8.8"

Could it really have been pure luck that my Soundstream drivers were an exact match for the LSi15's stockers? Maybe car audio, where the suitable replacement drivers originate, don't follow baffle cutout dimensions like I'm used to with home audio?

caardvark

I think this thread answers most, if not all of, the questions:

https://forum.polkaudio.com/discussion/171939/lsi15-woofer-mod-if-a-db840-is-good-would-an-mm840-be-better/p1

@ZombieBeaver shows grill removal with the conventional wood trims, and also noted that the MM840 is ~1/4" wider than the stock cutout. So, if there are multiple versions of the LSi15 cabinet -- it's either that or terrible documentation on the part of people calling the non-DB840 upgrade options a drop-in replacement -- then it's either independent of the grill type ... or less likely, his wood trims and grills were swapped with the press-in type at some point between being new and his ownership in '19.

I'll have to look into the mass loaded vinyl solution for filling out the baffle for a single, surface-mount cutout.

skrol

Rather than making the cutout for the woofer larger, would it be possible to fill the recessed area and surface mount the woofer instead. The original grill wouldn't fit though.

caardvark

skrol wrote: »

Rather than making the cutout for the woofer larger, would it be possible to fill the recessed area and surface mount the woofer instead. The original grill wouldn't fit though.

That's actually the direction I settled on, as mentioned in the other thread:

https://forum.polkaudio.com/discussion/comment/2752125/#Comment_2752125

I'm not concerned with the grill fitment, as: a) I think the screw-in style have more clearance in back, and b) I'm likely sticking with the inverted woofer mount which eliminates the grill. Inverted just makes too much sense, the more I think about it and experiment with it. Rather than have a 0.77 cu ft (my calculation, which is a little higher than typical 0.70-0.75 estimates) minus driver volume, which is too little to maximize the capabilities of most options, it's 0.77 plus cone volume -- and a little extra once flush mounting the driver another 1/4" out -- and ~0.9 cu ft "sounds" a lot better to me.