JBL 4310 fiberglass behind mid-driver

[paulnparis]

Hello,

I'm installing new X-overs (JBL L100 Century) for the 4310 and will be adding bracing and redoing damping in the process.

Question: After opening the speaker and removing the drivers, I found fiberglass behind the mid-driver. Is this normal, what JBL did? Or did someone add it along the way?
If it's a good thing.. do I just leave it there?

For the damping, I'm planning 2mm bitum + 4cm corregated foam..
I have enough foam to replace the fiberglass behind the mid-driver if that's better.

As you can tell this is my first upgrade to speakers. My experience is building a couple guitar amps, and rebuilding a tube hifi amp.

Thanks for the advice, Paul

[DrDyna]

The damping is for the midrange driver that's probably open backed. It probably won't hurt anything to experiment with upgrading it if you're upgrading all of it anyway.

[Cal Weldon]

Yes leave it. The reflected waves can make the mids sound boxy. The fiberglass helps to reduce that. If you notice anything boxy about the mids you can add some more. Maybe use a polyester fill over top of the fiberglass to prevent the old fiberglass dust from getting near the coil.

[erickalan77]

fiberglass is pretty good at damping, but it is better for bass. I would leave the fiberglass since there is not a lot of it and fill the cavity with loosely with long hair wool (madisound) which is going to have the greater effect on the upper frequencies ...foam would change the volume of the chamber and has no real sound absorbtion properties except at extreme high frequency.
Corrogated Foam is used more as a diffuser externally (for the most part foam is marketing hype in regards to absorbing sound)- we line our walls with fiberglass in our recording studio, cover by open weave burlap)...it is never used in speakers. . .You want to dampen the chamber so that the soundwaves do not reflect back into the cone . . .If you are happy with the midrange- don't fix what is not broke. JBL are very good designers. A lot of thought went into that speaker system...check out Lansing Heritage.
I am not saying it sounds good ( I never heard one ).

You should not be using a crossover from a different speaker system, and that is what it is a system. The crossover is interacting with the impedance curves and frequency response of the drivers and the effect the cabinet is having on the impedance curves.
All drivers are very different (some manufactors get similar curves in tweeters as they share voice coils in their lines) . . .You could swap out parts of identical value in the cross over ie: upgrade the caps, mix caps to achieve values/quality and price point (bypassing) and replace iron core inductors with air cores . . .

[erickalan77]

you will realize the greatest benefit with good poly caps (not that Bennic and Dayton junk)

If you brace the cabinet you will change the volume and tuning if it is a "bass reflex" and may lose some intended reveberation (boom) . . .

[paulnparis]

Thanks for the comments. I'll leave the fiberglass as is, and btw, the cylinder enclosure is closed on the bottom.

Erickalan77, The new X-over kit (I got) is specifically designed for the 4310. (It's just on the L100 page.)
After using the speaker a few years, I agree with what's written on this site, JBL L100 Century.
Of course being my first mod, I'm plenty nervous about the results, but going back to original X-over would be pretty easy.
Reversing out foam for fiberglass as well.
Understand the bracing comment, however Troels recommends it - and I would like less boomy bass. That change could be reversible but with considerable more trouble.. Hoping for the best.. as am half way done with it now..

[Cal Weldon]

Hi erickalan77,

I think I need your help with something you said.

If the damping material is less effective as the frequency decreases then I am trying to wrap my head around your statement that fiberglass is better at bass while the long hair wool is better at the highs. To me if it is better at damping the highs then it will be better at damping the lows also. I agree with you that the long hair wool is a very good product but I can't see why the fiberglass is good or better on the low end. To me it's not, it's just cheaper. Correct me if I misread your post.

Is there something I could read on this rather than have you go through it with me? I have done many experiments with cabinet materials, panel bracing, panel damping, and internal damping so I'm not inexperienced but you've got me intrigued and I appreciate any input you have.

[erickalan77]

Hello...Fiberglass is very dense and uniform. It can be used in sheet form. I have been using 1 1/2 myself this week the first few 6 inches from the baffle along the perpendicular walls to reduce reflections back into the cone. but for fill I use wool as I cannot get the density and uniformity of glass. It would not be practical to line a wall with wool as you can not get it in that fashion. Wool is ultimately better than glass in a stuffing scenario. Wool is better for some reasons pertaining to transfering the "soundwaves to heat"....The sound waves lose their energy exciting the wool and glass... Wool has more mass than glass and is softer so it is better...I have raw wool (unprocessed except that it was washed) pretty dense in that form.
I did a test after that first post this morning while was re-stuffing my monitors to experiment with taming midrange irregularities... I put a matted clump off wool to my ears, and 1 1/2 glass (fret not about glass dust and particles I lightly sprayed the sheet pieces with 3M supper 77 - rubber cement to lock the fibers to the sheet and increased mass/damping). . . wool was more effective at higher frequencies especially midrange....

@ Paul...some bracing will reduce boom (reverberation as the panels release their energy) One per panel would have significant effect. It would effectively double the mass by locking the two panels together..3/4 becomes 1 1/2 panels and reduces cabinet panel expansion and contraction...it is so effective I would suggest restrained use so as to not significantly change the volume.

A couple weeks ago I was bored and trying to tweek some 5" Sony's into decent shape for bedroom duty. I put a single 5/8 hardwood dowel from the baffle to the rear panel (the other panels were small and solid). Lined the cabinet with 1 1/2" glass. WoW...the awful boomy midbass, deep bass reverberation and midrange leakage through the port were significantly reduced or gone from perception...Ultimately the speakers are sitting in the closet cause I never could get the midrange resolution out of them that I am use to with the cabinet mods, a new tweeter and good poly caps . . . no great loss, I was just playing around.


I would suggest a little glass ($4 - sold at home depot to wrap HVAC and stuff voids) and 3/4 dowels polyureathane glued...The stuff will expand to fill voids . . I like dowels as they are hardwood and wont pose a surface to create a reflection but what ever floats your boat or wood working skills.
back panel, one side, one top bottom...If it gets over damped for your taste you can take it out . . .have a nice day . . .~erick~

[erickalan77]

Wool is better (if you were stuffing) ...but for restrained, consistent use- such as lining a wall; glass is better. . . I took apart a Cerwin Vega 12" 3 way once . . .they had made a chamber out of 1" wool (or cotton or nylon???) like carpet matting or perhaps say; under-layment . . you might have scene the stuff under your car's carpet. So that the sound waves had to pass through it leaving (and returning to) the cone ... basically a little 1 cubic foot enclosure of damping material inside a 3-4 foot box and a rear port just to be sure of limited coloration of the primary signal . . . an interesting speaker. Don't try this at home folks, that was a factory tweek for that system....just bending your ear . . .~erick~

[erickalan77]

BTW ditching my poly fiil and using glass and wool cleaned up my midrange as hoped for this morning by reducing reflections into the cone (which you can hear ) . . .