Polyfill question

vstarkwell

I recently picked up a pair of Monitor 10's and the cabs were in very rough shape. The played fine and sound great however they were quite the eye sore. I decided to part them out and noticed that they had a lot of polyfill in them. Being the curious type I decided to check my 7C's and noticed they don't have as much polyfill.

My question is too much polyfill a good thing or bad thing? if so why?

zingo

A bad thing.

vstarkwell

zingo wrote: »

A bad thing.

Thanks zingo, might you know why the 10's had so much? I doubt that the previous owner add additional polyfill so I'm curious as to why Polk did.

TennMan

vstarkwell wrote: »

Thanks zingo, might you know why the 10's had so much? I doubt that the previous owner add additional polyfill so I'm curious as to why Polk did.

My 10Bs were the same way and don't think they had ever been opened before. Poly fill was all the way down to the bottom bottom of the speaker behind the passive radiator. I was told it should be stopped above the PR so that is what I did.

skrol

My factory original Monitor 5A's are stuffed top to bottom with polyfil. My 5B's don't have any below the MW's.

westmassguy

vstarkwell wrote: »

I recently picked up a pair of Monitor 10's and the cabs were in very rough shape. The played fine and sound great however they were quite the eye sore. I decided to part them out and noticed that they had a lot of polyfill in them. Being the curious type I decided to check my 7C's and noticed they don't have as much polyfill.

My question is too much polyfill a good thing or bad thing? if so why?

Acoustic damping material, whether Polyfil or Dacron Batting, is normally used for one of two purposes (or both). The open back woofers that Polk uses, produce sound waves in a forward direction emanating from the front, but also from the rear, or back side. The back waves are out of phase, and must be canceled or mitigated. If the back waves are allowed to bounce around and re-reflect inside the cabinet, they will eventually find their way back out through the woofer cone(s), or even the passive radiator. This can smear the image and detract from the overall detail of the sound. When used properly, the damping material absorbs the higher frequency sounds produced by the woofers, but doesn't affect the air pressure being produced.
The other reason damping material is used, is to "tune" the enclosure size. The dead air space created by the fibers of the damping material compress and expand with the internal pressure produced by the woofers. Adding additional damping material beyond that which is needed to absorb the back sound waves, can have the effect of increasing the overall internal volume of the cabinet. If, for example, the speaker had a rather nasty hump in the bass region due to an internal volume that's slightly undersized. Adding additional damping material will increase the apparent internal volume, and can mitigate, or at least reduce the bass hump. If too much damping material is added, the opposite can occur. If the damping material is compressed, leaving little or no dead air space between the fibers, it will decrease the internal volume, and exascebate the problem.
Why some Polk models have more than others is anyones guess. I like to think there was a specific reason for differing amounts, but they were assembled by human beings after all, so mistakes are possible as well.

skrol

Very nice explanation. While generally folks say the batting should not be behind the PR, I left it in place in my 5A's figuring that it was intentional for tuning and likely is. Polk Audio did tweak designs in the middle of a series, so some 5A's may not have the batting behind the PR. Or, they ran out of batting cut for a 5A and stuffed in batting cut for the 7's, who knows.

My 5B does not have the batting behind the PR, however, it also uses a different MW (MW-6502 in the 5B vs the MW-6500 in the 5A) with very different specs and there are corresponding differences in the XO. Ideally, the 5B should be an improved progression from the previous version, though the use of the SL-2000 vs the Peerless is debatable. I do like the bass of the 5B over the 5A. I plan to play with the 5A one day removing the batting behind the PRs and compare again to the 5B.

westmassguy

I like to think the differing amouts were a design decision, but we all know that Polk made in between models, and got creative with different driver/cabinet combinations. I'm sure some on-the-fly decisions were made at the end of production runs. My 5s have the 8" PR, and are fully stuffed with dacron. With my custom 7s, I filled only the top portion with polyfil, and left the bottom half empty. My 10s had dacron from top to bottom, but only a single layer. When I re-assembled them, I positioned all the dacron above the PR.

vstarkwell

@westmassguy,
Thanks for the tutorial I do appreciate.

westmassguy

vstarkwell wrote: »

@westmassguy,
Thanks for the tutorial I do appreciate.

You're welcome, but it's really just speaker building 101.

vstarkwell

My curiosity stemmed from past readings about how some people prefer the 7 as it's less muddy then the 10. I wondered if all that polyfill maybe contributed to that school of thought. Anyway thanks for the replies.

westmassguy

vstarkwell wrote: »

My curiosity stemmed from past readings about how some people prefer the 7 as it's less muddy then the 10. I wondered if all that polyfill maybe contributed to that school of thought. Anyway thanks for the replies.

I own both. The 10s have dual 6.5" drivers, side by side, which blurs precise imaging a bit. The 10s do go a bit lower on the bass side of things. The 7s are the best of all IMHO. They got everything right. Very balanced sound. When I fired my 7s up the first time, I was floored. I love my 5s and 10s, but these were just perfect. Bass has punch, but not sloppy, mids and treble smooth and seamless.

TennMan

westmassguy wrote: »

I own both. The 10s have dual 6.5" drivers, side by side, which blurs precise imaging a bit. The 10s do go a bit lower on the bass side of things. The 7s are the best of all IMHO. They got everything right. Very balanced sound. When I fired my 7s up the first time, I was floored. I love my 5s and 10s, but these were just perfect. Bass has punch, but not sloppy, mids and treble smooth and seamless.

It would be interesting hear a pair of 7s side by side with my 10B CRS+ hybrids to see how them having one stereo MW and one dimensional MW would effect blurring of the imaging as you describe. It would probably be the same but without hearing them side by side...

Mystery

TennMan wrote: »

It would be interesting hear a pair of 7s side by side with my 10B CRS+ hybrids to see how them having one stereo MW and one dimensional MW would effect blurring of the imaging as you describe. It would probably be the same but without hearing them side by side...

I can do that with CRS+ and 7's but CRS+ are considered better than 10's in all ranges including bass so not exactly a comparison of 10 vs 7.

westmassguy

TennMan wrote: »

It would be interesting hear a pair of 7s side by side with my 10B CRS+ hybrids to see how them having one stereo MW and one dimensional MW would effect blurring of the imaging as you describe. It would probably be the same but without hearing them side by side...

Hey TennMan. There's no blurring with your SDA Hybrid, as the woofers are receiving different signals. The outer dimensional woofers are receiving a delayed, out of phase signal from the opposite speaker. In the normal 10, both woofers are receiving the same signal, they're wired in parallel with each other.

TennMan

I guess comparing my hybrids to 10s and 7s would be like comparing apples and oranges... and bananas.

When I compared the bass of my stock 10Bs to my CRS+, my ears find the 10Bs to be a little deeper and have more punch. I prefer the bass of the 10Bs. I've read here that modded CRS+ are better but I've not upgraded my crossovers yet.

westmassguy

TennMan wrote: »

I guess comparing my hybrids to 10s and 7s would be like comparing apples and oranges... and bananas.

When I compared the bass of my stock 10Bs to my CRS+, my ears find the 10Bs to be a little deeper and have more punch. I prefer the bass of the 10Bs. I've read here that modded CRS+ are better but I've not upgraded my crossovers yet.

TennMan, you gotta do the crossovers. Like night and day. If you need help with it, PM me

TennMan

westmassguy wrote: »

TennMan, you gotta do the crossovers. Like night and day. If you need help with it, PM me

Thanks westmassguy. Will do.