Monitor 11: Obsessing over foam placement
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motorcityguy
Posts: 62
I am working on my Monitor 11 speakers. These are the original version of the Monitor 11, with the 12" PR and the Sonex acoustic foam under the inboard mid-woofers. Mine are dated 1983.
I am in the process of replacing the old foam, and am obsessing over the correct size and position of the foam. I've searched the forum extensively, and found a surprising amount of pictures and info from other owners. With this post, I'm hoping to get confirmation that I'm on the right track, and (hopefully) document the adventure for future reference to others.
So, I understand that the acoustic foam goes “BELOW” the inboard driver, from the center support to the side of the cabinet. However, based on the size of my original foam pieces (6” x 7”) and location of glue residue, it’s clear that the foam does NOT extend all the way forward to the front baffle, or to the rear of the cabinet. Specifically, it is about 1” shy of the rear panel of cabinet, and ~2” shy of the front baffle. In other words, it doesn’t really extend further forward than the magnet itself.
As evidence, on both of my speakers, there is a magnet imprint on the forward face of the foam confirming it extends no further forward than the magnet. Thus, the foam is not fully “under” the inboard driver. Review of photos from original foam on other M11 speakers, verifies a similar gap at both front and rear of foam. Some examples show the foam LOWER than mine, such that the driver magnet is not contacting the foam. I suspect contacting the magnet is the result of production variation in the foam placement, which was likely done by hand.
In any case, these gaps strike me as odd, because I thought the idea of the foam was to “isolate” the sound waves of the inboard driver. As such, I would have expected foam to extend forward and under the open basket of the driver (i.e. to the front of the cabinet.)
Below is a sketch I made showing the location of my original foam, including the gaps at front and rear.
I am in the process of replacing the old foam, and am obsessing over the correct size and position of the foam. I've searched the forum extensively, and found a surprising amount of pictures and info from other owners. With this post, I'm hoping to get confirmation that I'm on the right track, and (hopefully) document the adventure for future reference to others.
So, I understand that the acoustic foam goes “BELOW” the inboard driver, from the center support to the side of the cabinet. However, based on the size of my original foam pieces (6” x 7”) and location of glue residue, it’s clear that the foam does NOT extend all the way forward to the front baffle, or to the rear of the cabinet. Specifically, it is about 1” shy of the rear panel of cabinet, and ~2” shy of the front baffle. In other words, it doesn’t really extend further forward than the magnet itself.
As evidence, on both of my speakers, there is a magnet imprint on the forward face of the foam confirming it extends no further forward than the magnet. Thus, the foam is not fully “under” the inboard driver. Review of photos from original foam on other M11 speakers, verifies a similar gap at both front and rear of foam. Some examples show the foam LOWER than mine, such that the driver magnet is not contacting the foam. I suspect contacting the magnet is the result of production variation in the foam placement, which was likely done by hand.
In any case, these gaps strike me as odd, because I thought the idea of the foam was to “isolate” the sound waves of the inboard driver. As such, I would have expected foam to extend forward and under the open basket of the driver (i.e. to the front of the cabinet.)
Below is a sketch I made showing the location of my original foam, including the gaps at front and rear.
Comments
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Here are some photos illustrating my observations.
First pic shows front view of the intact original foam on my RH speaker. Notice the gap to the front baffle, as well as a magnet imprint, that shows approximate position of front edge of foam. The magnet imprint identifies the front gap as ~2” back from inside of baffle, which was confirmed by measurement.
Second pic is of same speaker, showing the gap to the rear panel, which I measured at approximately 1”.
Third pic is from another member, showing a view from below, of their original foam. Notice the gap is about twice as large at front of foam, as it is at the back. Note also, that this foam is slightly lower than mine and thus has no interference with the magnet.
Fourth pic is the foam’s glue residue in my LH speaker. Again, notice the apparent end of both the foam and the glue, indicating a gap at the front and rear of the foam. The front gap is about twice the width of the rear gap.
Fifth pic is foam glue residue from another member’s M11. This also shows similar gaps at front and rear of foam. Note that the front edge of the foam is approximately in line with the rear edge of the driver magnet.
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I seem to recall the foam in my father's covered the entire space, but it's been so many years my memory could be off.
Political Correctness'.........defined
"A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."
President of Club Polk -
I would think that there need to be a bit of space between the foam and cabinet to allow enough back pressure for the passive to work correctly. It doesn't take much foam to dampen the driver a bit but not so much to leave the passive dependent on just one of the two drivers in the cabinet.
I'm probably way off and talking out of my rear end, that's my take anyways.
😜 -
Yeah I think the space at the baffle is definitely to not interfere with the passive radiator interaction and the foam is to help with dampening a standing wave on the vertical dimension of the speaker enclosure. Is it pretty much centered on the vertical internal dimension? The standing wave is probably at a frequency that only affects the sound of the midrange driver and therefore no need for foam on the other side.George / NJ
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It's to reduce the sound waves reaching the PR in order to reduce the bass output as they are bass heavy speakers already.Political Correctness'.........defined
"A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."
President of Club Polk
