Substitutes for Wrap-On 16550 for cabinet lining ?

I am in the midst of a DIY JBL M2 build.

The factory originals are lined on 5 sides with a 1 inch (25mm) blanket (probably fiberglass). The frequency absorption coefficients are unknown and the density (pcf) is also unknown.

So far the recommended substitute has been a 1 inch layer of Wrap-on 16550 (plenum liner used in HVAC applications). This is no longer manufactured and there is no info on what the density or the absorption coefficients may have been. There are other plenum liners on the market but their specs vary all over the place so it is hard to know what a good substitute would be.

The other common recommendation is the "ultra denim" insulation. It's absorption coefficients are great (specified for a 3 inch thickness, so you need to reduce them by about a third). The density appears to be about 3 pcf. As luck would have it, this product is no longer manufactured and Lowes and Home Depot do not have any old stock (at least in my area).

So my thought is to get some Rouxal insulation (sometimes called mineral or Rock wool). I could remove the paper backing and tease apart about a 1-1.5 inch layer. I would apply this to the walls and keep it in place with some loosely stapled lightweight batting used for quilting etc. This would keep the rock wool in place and stop any fibers shedding. The specs on the Rock wool have good absorption coefficients and the density is about 3 pcf. (similar to the "ultra Denim").

My concern (and I am not trying to go off-topic), is what is the liner's "purpose"? Some 1) claim it is mostly to make the volume of the cabinet "appear larger" and you should be mostly concerned about the density not being too great, while 2) others claim it is to absorb the back wave of the woofer (and primarily worry about the absorption coefficients).

Again, remember this is a ported cabinet with sufficient mass and bracing. It is not a sealed cabinet and it is not a transmission line.

Thanks in advance

It does both. Change the quantity in a box simulator. I didn't do a strict clone typically I do cabinets the same way. I was able to find 1" fiberglass uncoated wrap in my local Home Depot. I used a 3" thick piece directly behind the driver. My calculated vs predicted Fb was less that 1/2 Hz off.

Rob :)

In this instance the purpose is to mitigate the sound waves bouncing around the interior cabinets walls and back out through the woofers paper cone. Typically it will add a small amount of 5-10% virtual volume if 1 inch thickness is used. Put the Rouxal on all surfaces except the front baffle.

The gross volume is about 5.2 cu ft of the stock enclosure (147L). The net volume by design simulation released by 4313B is based in a net volume of 130L. It’s a custom electrically augmented over damped system tuning of about 27.5 hertz. The woofer has a QTS of 0.44 and the VAS is about 150L so the enclosure will certainly control the driver. Much more so than a 2235 driver.

What happens is the volume displaced by everything including the ports in the enclosure is deducted from the gross volume. Your right the insulation will add some virtual volume if it’s a viscous lossy fibrous insulation that converts vibrations into heat. The other consideration is what is referred to as box losses QL. Generally these losses are from the enclosure flexing and some other minor losses. JBL generally aim for stiff box QL =10. A diy loudspeaker is typically QL =5 in a large enclosure.
The effect of QL is a loss in low frequency output. QL of 3 is considered a lossy box. A QL of 15 is considered a lossless box.

QL is very difficult to measure accurately so QL = 7 is used at a design starting point. Making the net volume larger than the design 10% larger is a good idea to counter QL losses.

If you like put extra damping on the floor of the enclosure as standing waves were detected in Kippell measurements corresponding to the height of the enclosure on Erin’s Audio Corner review of the M2. It’s one of the best published non JBL system tests l have seen of the M2. If your building a clone l would definitely use those tests for measurement comparisons. Kippell tells you everything about a loudspeaker.