Question on Adding Batting to Rear of Ported Cabinet

[nkx1]

While I did not build these speakers myself, I may want to modify them slightly, and thought this was an appropriate forum to solicit some help from those with more knowledge than me.

I purchase some JBL Control Monitor speakers. I took them apart like I do with any speakers because I'm curious, and I noticed that the sides and top of the cabinet are covered in batting, but there is no batting on the back wall. I tried adding some polyester fill material, but this seemed to diminish bass response slightly. Does anyone think that adding batting to the back wall might help, or should I just leave the back wall bare as it is stock? The cabinet is ported (of course, I would cut the batting to fit around the base of the port).

Thanks in advance for any helpful advice.

[Inductor]

As a first response to your post, and unless someone has the same speakers and room blah, blah, blah, I before had to change a speaker from BR to closed box that I myself software simulated for lack of bass (/extension) and it maybe has a lot of inside filling/walls covered. Also had to change a brand new pair of Focals (Chorus-826V) and close one of the ports (the one in the front) for excess boom, maybe this one had to do with the room too.

[Loren42]

Originally Posted by nkx1

While I did not build these speakers myself, I may want to modify them slightly, and thought this was an appropriate forum to solicit some help from those with more knowledge than me.

I purchase some JBL Control Monitor speakers. I took them apart like I do with any speakers because I'm curious, and I noticed that the sides and top of the cabinet are covered in batting, but there is no batting on the back wall. I tried adding some polyester fill material, but this seemed to diminish bass response slightly. Does anyone think that adding batting to the back wall might help, or should I just leave the back wall bare as it is stock? The cabinet is ported (of course, I would cut the batting to fit around the base of the port).

Thanks in advance for any helpful advice.

I would ask that question to the forum:

Audio Heritage

And make sure you list the model number. I think there are some former JBL engineers on that site as well.

[woody]

I think you may have 2 things hapening there. I believe stuffing has to be kept away
from the port or the tuning suffers. And when you add fill to an enclosure the effective volumn of the enclosure looks larger. That is a 1 cubic foot box with fill
will look like it's ~1.2 cubic feet in volumn this will also throw your bass tuning off.

[Loren42]

Originally Posted by woody

I think you may have 2 things hapening there. I believe stuffing has to be kept away
from the port or the tuning suffers. And when you add fill to an enclosure the effective volumn of the enclosure looks larger. That is a 1 cubic foot box with fill
will look like it's ~1.2 cubic feet in volumn this will also throw your bass tuning off.

My understanding of the effect of fill (as far as frequency response is concerned) is that when you have a sealed box, what you say is true. With a vented box the effects are a little different, although, it will impact the tuning, but to a somewhat less degree.

What I have seen on simulations in Bass Box Pro is that adding more fill to a vented enclosure tends to suppress the corner or knee of the rolloff slightly and extends the low end a marginal amount, but that extension happens well down on the SPL scale.

Here is a JBL 2235H in a 6.6 cubic foot vented box with a Fb at 35 Hz. The red trace is minimal fill (two opposite sides have 1" of fiberglass fill). F3 = 33.0 Hz. The orange trace has 3" of heavy fill on all sides. F3 rises to 35.5 Hz.



The result is subtle according to the CAD program, but it gives you an idea of what to theoretically expect.

That is a pretty extreme change in fill material, but can you really hear the difference of adding fill to the back of a minimal filled cabinet? Maybe, but I get wary of things that are subjective and actually hearing a difference reliably this close to the "noise" level brings out the skeptic in me. Your actual change in F3 probably will be a little less than the demonstration I showed above.