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View Full Version : Replacing cabinet's fiberglass lining, pardon the stupid question



Kreshna
12-28-2014, 12:15 PM
Well, the fiberglass lining in my speakers has thinned out, so I think I need to replace them, either with new fiberglass sheet or with something like Monacor MDM-3. The speaker in question is a pair of JBL 120Ti, by the way.

The question is: should I better disassemble the speaker enclosure (taking off the back panel, for example), or can I do it fine through the driver holes? The woofer is 12", so for the lower part of the speaker, I guess I could insert my hand fine to insert the new fiberglass sheet, as well as lining it properly to the inside wall.

The upper part is another story. The only hole available is the tweeter hole. My hand can fit, but naturally the hand's movement is not as free compared to inserting it from the woofer hole below, not to mention obstructions like the bass port tube. I imagine I could have more freedom to press the fiberglass against the inside wall if the port tube is removed, but the tube is very hard to remove (I haven't been able to remove it).

I noticed some members of the forum has replaced/added fiberglass lining to their speakers (like on this thread (http://www.audioheritage.org/vbulletin/showthread.php?32169-loudspeaker-fibreglass) and this thread (http://www.audioheritage.org/vbulletin/showthread.php?19059-Best-Way-to-Add-Fiberglass-to-an-Existing-Cabinet).)

So here is the stupid question: do you have to disassemble your speaker cabinet to replace its dampening material? Or can you do fine through the driver holes?

I guess the easiest side to disassemble the 120Ti enclosure (if I have to, but I hope I don't) is taking apart the backside panel. If the back panel is opened, then it would be easier to re-line the new fiberglass sheet (or Monacor MDM-3) to the inside wall of the cabinet. But I just don't have the heart to pry it open. In fact, I believe the proper way is to use wood glue dissolver to the joints, then carefully open it. But I'm totally inexperienced in this matter.

How did you guys replace the fiberglass lining inside the cabinet? Did you do just fine through the driver holes?

And while we're at it, how do you take off the bass port tube?

Many thanks.

Odd
12-28-2014, 12:41 PM
Why do you think the fiberglass lining in the speakers has thinned out?
If you add more it will change the bass response.

Horn Fanatic
12-28-2014, 09:19 PM
Well, the fiberglass lining in my speakers has thinned out, so I think I need to replace them, either with new fiberglass sheet or with something like Monacor MDM-3. The speaker in question is a pair of JBL 120Ti, by the way.

The question is: should I better disassemble the speaker enclosure (taking off the back panel, for example), or can I do it fine through the driver holes? The woofer is 12", so for the lower part of the speaker, I guess I could insert my hand fine to insert the new fiberglass sheet, as well as lining it properly to the inside wall.

The upper part is another story. The only hole available is the tweeter hole. My hand can fit, but naturally the hand's movement is not as free compared to inserting it from the woofer hole below, not to mention obstructions like the bass port tube. I imagine I could have more freedom to press the fiberglass against the inside wall if the port tube is removed, but the tube is very hard to remove (I haven't been able to remove it).

I noticed some members of the forum has replaced/added fiberglass lining to their speakers (like on this thread (http://www.audioheritage.org/vbulletin/showthread.php?32169-loudspeaker-fibreglass) and this thread (http://www.audioheritage.org/vbulletin/showthread.php?19059-Best-Way-to-Add-Fiberglass-to-an-Existing-Cabinet).)

So here is the stupid question: do you have to disassemble your speaker cabinet to replace its dampening material? Or can you do fine through the driver holes?

I guess the easiest side to disassemble the 120Ti enclosure (if I have to, but I hope I don't) is taking apart the backside panel. If the back panel is opened, then it would be easier to re-line the new fiberglass sheet (or Monacor MDM-3) to the inside wall of the cabinet. But I just don't have the heart to pry it open. In fact, I believe the proper way is to use wood glue dissolver to the joints, then carefully open it. But I'm totally inexperienced in this matter.

How did you guys replace the fiberglass lining inside the cabinet? Did you do just fine through the driver holes?

And while we're at it, how do you take off the bass port tube?

Many thanks.

Fiberglass insulation has an impedance no different than the other components in the electrical, acoustical, and mechanical circuit of a speaker system. The impedance of course, is based on the density and thickness of the material. I typically shy away from using fiberglass insulation in vented enclosures, and opt for using the chewed up cotton denim material manufactured by Bonded Logic. I do have a customer however, who prefers me to use one inch thick duct board in the enclosures I build for him. Duct board would not be an option for you. I apply the duct board in the enclosures I build for my customer as I'm assembling the box, as there is no other way to install it after the box is completed.

What damping material does is decrease the amplitude of high frequencies, and reduce the velocity of sound at low frequencies. For example, the acoustic suspension enclosures developed in the fifties, and re-popularized in the seventies were completely stuffed with damping material. The material changes the air density in the enclosure, thereby fooling the woofer into thinking it's in a much larger enclosure. The more dense the material, the longer it takes for the initial wave to return to it's origin. Stuffing a cabinet in that manner provides a great deal of bass from a small enclosure, but by doing so it also results in producing mushy, inarticulate, ill-defined bass, flawlessly.

ALTEC Lansing for example, used four inch thick fiberglass insulation in many of the large HiFi cabinets manufactured in the fifties, although lining only three sides. The result was the heavy bass the company was known for. Using to much loosely packed damping material can literally suck the life out of a full range woofer, leaving it sounding muffled. I prefer to be stingy with damping material by not covering every internal surface with it. Although I line every internal surface, duct board on the other hand is rigid, and offers a slightly reflective surface that allows the woofer to sound more lively.

As for your "thinning" problem, I can't imagine how something like that could occur. It would mean the material is literally disappearing, or falling apart for some strange reason. I can't say that I have noticed the fiberglass JBL used succumb to disintegration. The material JBL used in cabinets such as yours, the L65 Jubal, and L100 Century, was not thick, but more like 1/2" thick. Perhaps that is why you think the material is thinning out. My advise, if the the rear panel is not removable, as I suspect is common JBL practice, I urge you not to try and remove it. The rear panels are secured in place by using a 1/4" x 1/4" dado. Prying it out will destroy pretty much all of the glue joints. Just remove the woofer and secure the speaker wires inside so as not to be in the way should you wish to replace the damping material. I will also not recommend removing the tuning duct either, as you can also damage the veneer face. Personally, if the material is still intact, I would leave it alone. Just sayin'

Good luck,

H.F.

Kreshna
12-29-2014, 08:36 PM
Fiberglass insulation has an impedance no different than the other components in the electrical, acoustical, and mechanical circuit of a speaker system. The impedance of course, is based on the density and thickness of the material. I typically shy away from using fiberglass insulation in vented enclosures, and opt for using the chewed up cotton denim material manufactured by Bonded Logic. I do have a customer however, who prefers me to use one inch thick duct board in the enclosures I build for him. Duct board would not be an option for you. I apply the duct board in the enclosures I build for my customer as I'm assembling the box, as there is no other way to install it after the box is completed.

What damping material does is decrease the amplitude of high frequencies, and reduce the velocity of sound at low frequencies. For example, the acoustic suspension enclosures developed in the fifties, and re-popularized in the seventies were completely stuffed with damping material. The material changes the air density in the enclosure, thereby fooling the woofer into thinking it's in a much larger enclosure. The more dense the material, the longer it takes for the initial wave to return to it's origin. Stuffing a cabinet in that manner provides a great deal of bass from a small enclosure, but by doing so it also results in producing mushy, inarticulate, ill-defined bass, flawlessly.
I see, thanks. Just wonder: how much would stuffing/lining will help the bass in an already large cabinet? Like JBL's typical "bookshelf" speakers.



ALTEC Lansing for example, used four inch thick fiberglass insulation in many of the large HiFi cabinets manufactured in the fifties, although lining only three sides. The result was the heavy bass the company was known for. Using to much loosely packed damping material can literally suck the life out of a full range woofer, leaving it sounding muffled. I prefer to be stingy with damping material by not covering every internal surface with it. Although I line every internal surface, duct board on the other hand is rigid, and offers a slightly reflective surface that allows the woofer to sound more lively.
If I can safely open the back panel of the cabinet (not prying it or hammering it, of course), I guess I'll be very interested in duct board.

I'm also interested in the modification of this particular JBL L20T (http://www.ebay.com/itm/JBL-L20T-Restaurated-upgraded-with-stands-/331401885674) (unfortunately the buyer wasn't me). The seller added more insulation on the area behind the tweeter, and he claimed it improves crispiness and sound stage. Of course, adding the isolation is not the only thing he did; he also improved the crossover components as well as phase-aligned the drivers. But I became curious whether my 120Tis will be better with (allegedly) better insulation materials like Monacor MDM-3 (http://www.ebay.com/itm/Monacor-MDM-3-aus-Schafwolle-High-End-LS-Dammung-090133-/151444227591?pt=Lautsprecher_Selbstbau&hash=item2342c78607) or Sonic Barrier acoustic foam (http://www.ebay.com/itm/Sonic-Barrier-1-1-4-3-Layer-Damping-Material-w-PSA-18-x-24-260-535-/321510819170). A friend of mine is skeptical about using acoustic foam inside the speakers, because such foam is supposed to be used as room treatment instead of speaker insulation. But what would you think?

I knew changing the insulation would alter the sound, but if it's for the better, I really don't mind. For example, I love the 120Tis detailed imaging (especially from nearfield listening), as well as its excellent HF cleanliness (cymbals and the likes), but I wonder if I could still squeeze out better sound from the already excellent speakers by using different insulation materials (as well as using better crossover components and phase-aligning the drivers, of course :bouncy: ).




As for your "thinning" problem, I can't imagine how something like that could occur. It would mean the material is literally disappearing, or falling apart for some strange reason. I can't say that I have noticed the fiberglass JBL used succumb to disintegration. The material JBL used in cabinets such as yours, the L65 Jubal, and L100 Century, was not thick, but more like 1/2" thick. Perhaps that is why you think the material is thinning out.
I see, thanks again. Yes, I think you're right; the fiberglass is not very thick to begin with.

Nonetheless, the fiberglass insulation is dirty like hell (probably because the speakers are old). I'm thinking of cleaning the inside of the speakers (particularly the fiberglass surface) with vacuum cleaner, but I'm afraid the vacuum would yank and suck the fiberglass strands as well, making the fiberglass insulation thinner than it originally was.

Worse, the fiberglass insulation already sagged, probably because of age as well. The sagging is not as bad as this illustration (http://img1.basementsite.com/uploads/photo_gallery/600_800/488-3-years-from-total-replacement.jpg), of course, but it has already sagged nonetheless, so I think I'm going to replace it anyway, either with fiberglass or with more "exotic" material like I've mentioned above.




My advise, if the the rear panel is not removable, as I suspect is common JBL practice, I urge you not to try and remove it. The rear panels are secured in place by using a 1/4" x 1/4" dado. Prying it out will destroy pretty much all of the glue joints. Just remove the woofer and secure the speaker wires inside so as not to be in the way should you wish to replace the damping material. I will also not recommend removing the tuning duct either, as you can also damage the veneer face. Personally, if the material is still intact, I would leave it alone. Just sayin'

Good luck,

H.F.
I see, thanks. :)

Well, I guess I can safely replace the fiberglass insulation from the driver holes. 12" woofer hole is pretty big anyway, and the tweeter hole isn't exactly small - my hand can still fit.

However, I think the bass port tube still has to go away while the fiberglass insulation is replaced, because it really gets in the way. You see, the bass port tube is very close to the right side (for the right speaker), which makes it hard to press the new fiberglass insulation evenly against the right inside wall. And vice versa for the left speaker.

As for the veneer, it has to be restored anyway (http://www.audioheritage.org/vbulletin/showthread.php?34733-How-much-restoration-do-these-JBL-120Tis-need). Recently I found a guitar craftsman who is willing to sand and restore the veneer for a reasonable fee :bouncy:. Nonetheless, I think everything sticking to the front baffle must go away while the veneer is sanded, to expose every bit of veneer surface for sanding and restoration. Taking off the speaker drivers are trivial, while the JBL emblem can be taken off safely using dental floss (http://www.audioheritage.org/vbulletin/showthread.php?34733-How-much-restoration-do-these-JBL-120Tis-need&p=351669&viewfull=1#post351669) (as SEAWOLF has mentioned). Still, I wonder about the bass port tube and the grille retainers. Can they be removed safely? I'm not talking about brute force methods like prying or yanking, but probably subtler, safer methods, like using 90% alcohol to dissolve the glue then using dental floss to pull them safely.

As for the back panel, well, this is pure curiosity, but I wonder if the back panel could be safely removed as well. No prying, no hammering, but maybe using glue dissolver like 90% alcohol at certain joints or intersections, followed by gentle push from the inside. Is it possible? Have you ever done such thing? Well I don't really need to remove the back panel if I can remove the bass port tube, but I just wonder anyway.

PS: I am of course inexperienced in such thing, not to mention my hands aren't dextrous anyway, but the guitar craftsman would do it. I would feel safer if I could provide him with tips and instructions from you and other folks here. :)

audiomagnate
12-30-2014, 09:59 PM
Fiberglass insulation has an impedance no different than the other components in the electrical, acoustical, and mechanical circuit of a speaker system. The impedance of course, is based on the density and thickness of the material. I typically shy away from using fiberglass insulation in vented enclosures, and opt for using the chewed up cotton denim material manufactured by Bonded Logic. I do have a customer however, who prefers me to use one inch thick duct board in the enclosures I build for him. Duct board would not be an option for you. I apply the duct board in the enclosures I build for my customer as I'm assembling the box, as there is no other way to install it after the box is completed.

What damping material does is decrease the amplitude of high frequencies, and reduce the velocity of sound at low frequencies. For example, the acoustic suspension enclosures developed in the fifties, and re-popularized in the seventies were completely stuffed with damping material. The material changes the air density in the enclosure, thereby fooling the woofer into thinking it's in a much larger enclosure. The more dense the material, the longer it takes for the initial wave to return to it's origin. Stuffing a cabinet in that manner provides a great deal of bass from a small enclosure, but by doing so it also results in producing mushy, inarticulate, ill-defined bass, flawlessly.

ALTEC Lansing for example, used four inch thick fiberglass insulation in many of the large HiFi cabinets manufactured in the fifties, although lining only three sides. The result was the heavy bass the company was known for. Using to much loosely packed damping material can literally suck the life out of a full range woofer, leaving it sounding muffled. I prefer to be stingy with damping material by not covering every internal surface with it. Although I line every internal surface, duct board on the other hand is rigid, and offers a slightly reflective surface that allows the woofer to sound more lively.

As for your "thinning" problem, I can't imagine how something like that could occur. It would mean the material is literally disappearing, or falling apart for some strange reason. I can't say that I have noticed the fiberglass JBL used succumb to disintegration. The material JBL used in cabinets such as yours, the L65 Jubal, and L100 Century, was not thick, but more like 1/2" thick. Perhaps that is why you think the material is thinning out. My advise, if the the rear panel is not removable, as I suspect is common JBL practice, I urge you not to try and remove it. The rear panels are secured in place by using a 1/4" x 1/4" dado. Prying it out will destroy pretty much all of the glue joints. Just remove the woofer and secure the speaker wires inside so as not to be in the way should you wish to replace the damping material. I will also not recommend removing the tuning duct either, as you can also damage the veneer face. Personally, if the material is still intact, I would leave it alone. Just sayin'

Good luck,

H.F.

Good stuff, but L100 cabinets are the standard 3/4, not 1/2 inch.