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View Full Version : Swap foam in the back cap of 4" drivers to felt?



sebackman
11-14-2015, 03:11 AM
Hi,

I have read somewhere here that people swap the foam in the back of their 4" compression drivers for felt and that JBL uses thicker stuffing in the back cap of the K2 drivers (476 and others). I even think the back cap is deeper on these models.

I tried to search but did not find the thread.

Can someone please help me with info what type of felt to use (and where to get it) and what thickness would be appropriate?

Is there any advantage of using thicker felt that the original foam pad?

I am going to change in some 2451’s and a couple of 2447’s.

Kind regards
//RoB

sebackman
11-28-2015, 02:21 AM
Hi again,

Surely there must be members here that have exchanged the foam in the back cover of their drivers for something else and potentially thicker?

If so was there any measurable advantage?

–And if so what thickness would fit in the back of a 2447 / 2450SL / 2451 safely behind the diaphragm.

Kind regards
//RoB

pos
11-28-2015, 02:38 AM
I have never measured the difference myself, but I know TAD uses felt in the über expensive and over engineered TD2002 and TD4003 (precisely cut to fit, with angles and all, covering the whole surface), so that must be good...

gibber
11-28-2015, 04:33 AM
I tried to search but did not find the thread.

Can someone please help me with info what type of felt to use (and where to get it) and what thickness would be appropriate?



Hi Rob,

http://www.audioheritage.org/vbulletin/showthread.php?26757-foam-or-felt-in-the-cover

That seems a kind of soft material.

I opened my 476Be and it has fairly stiff felt inside (single driver, still looking for a mate, guys!)
br,
Ralph

sebackman
11-29-2015, 02:58 AM
Hi,

Thank you for input. Sounds like 1/2 - 1/4" felt with high wool content is the way to go.


kind regards
//RoB

pos
11-29-2015, 03:44 AM
Please let us know your findings if you measure any significant difference :)

gibber
11-29-2015, 06:22 AM
Please let us know your findings if you measure any significant difference :)

Hi Thomas,

it will probably end up like it did here when changing to a different back cap in post#28/29:

http://www.audioheritage.org/vbulletin/showthread.php?33269-Upgrade-SAM1HF-or-2452H-SL-in-ScreenArray-horn&p=339320&highlight=#post339320


While i do believe these things are indeed audible, i'm not sure they would deliver a significant difference in a measurement.

Ralph

pos
11-29-2015, 07:10 AM
Yes Ralph, I remember those measurements!
That was a very interesting experiment.

I think the difference with felt vs foam, if any, should occur in the >kHz range.
To be able to see this with good resolution and without comb filtering from reflection, it will be better to use a good gating (and as long as the measurement setup allows) rather than frequency domain smoothing.

By the way, regarding the extension/snout (2432 vs 2453, 2447 vs 2451, etc.), I should be able to try something: one of the D2430K I have here (from a German friend who got them off ebay) has a snout that can be detached (no glue), so that I can easily adapt it on any 1.5" driver and measure what happens...
I could even try the D2430K without this snout, with the phasing plug protruding inside the throat of the horn :)

gibber
11-29-2015, 08:28 AM
By the way, regarding the extension/snoot (2432 vs 2453, 2447 vs 2451, etc.), I should be able to try something: one of the D2430K I have here (from a German friend who got them off ebay) has a snoot that can be detached (no glue), so that I can easily adapt it on any 1.5" driver and measure what happens...
I could even try the D2430K without this snoot, with the phasing plug protruding inside the throat of the horn :)


Hi Thomas, that would be great and perhaps worth an own thread. I once talked to Mark Ureda on one of the AES converntions and he said the 2431/35 drivers had shown a certain unexpected rugged respose. Given the shallow Neodymium structure, the team's design goal had been to eliminate any snout, but on the 2352/53 horns this proved an advance in distortion (and less apparent apex shift) not without a compromise elsewhere.

Your measurements will be much more meaningful than what i did here with that rough particle wood surface in the extended throat :
http://www.audioheritage.org/vbulletin/showthread.php?33972-JBL-Master-Reference-Monitor&p=346086&highlight=#post346086

I even bought a 2432 later but then was surprised to find you can't fit the 2431/35 domes into it.
Looking forward to seeing your measurements !

Ralph

pos
11-29-2015, 10:12 AM
Well, I am afraid it wont happen: I just measured the snout and it appears it is not a tube, the inner side is diameter is 1.7 inch... :crying:

sebackman
11-07-2016, 03:08 AM
I had two 4" drivers with bad foam and went looking for some kind of felt to replace the foam as per above.

The original foam in my drivers was 8mm thick and had a self-adhesive back side. I took it out using a shop vacuum cleaner. It just fell apart. The self-adhesive tape was rather easy to peel off by hand. Then I cleaned them with ethanol.

The inner diameter of the rear lid in the JBL 4” neo drivers (2450SL&2451) is 122,5mm at the bottom and 150mm towards the driver, when laying with the terminals down.They are slightly tapered at the back end and hence the dual diameters. The original foam only covers the 122,5mm circle at the back.

Depth of the phasing plug backwards is 18mm over driver deck where the lid attaches and the inner depth of the back cap is 32mm.

The clearance for any foam or felt is maximum 14mm and I believe that a max of 12,7mm =½” or possibly more conservative 10mm should be used. There is also the rubber gasket that adds roughly 2 millimetres.

I found some 8mm compressed natural wool felt that was done for sound proofing boats. It was sold by the meter and I bought 3 meters (1,5 meter wide). That should be enough for all drivers near me for the rest of my life J.

I then laid the back cap on the felt (terminals facing up) and used a pen to mark the outer rim of the back cap. With a pair of scissors I cut out a round piece slightly smaller (10mm) than the circle. That should give me a diameter of about 140mm. I had to take out a piece for the connectors but it fit nicely including covering the sides.

I then used contact glue and painted the inside of the back cap and just pushed the felt in. To get it well attached I filled a balloon with warm water and dropped into the back cap. The weight of the water/balloon will push the felt evenly towards the glue in the back cap and it will assume the form of the back cap to apply even pressure over the entire area. It worked well.

The day after I threw in some SL diaphragms to measure and while I have not done AB testing but comparing to my older curves it look like there are less pronounced shifts in the curve now.

Maybe just placebo but I will from now on swap to felt in all my drivers and make certain that the felt covers the entire back cap, also the sides.

Kindregards
//RoB

744227442374424

Earl K
11-07-2016, 03:50 AM
Good info RoB . Thanks for sharing!

http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=74424

audiomagnate
11-07-2016, 04:36 AM
I have never measured the difference myself, but I know TAD uses felt in the über expensive and over engineered TD2002 and TD4003 (precisely cut to fit, with angles and all, covering the whole surface), so that must be good...

I'd love to see a picture of that.

srm51555
11-07-2016, 05:54 AM
Thanks for sharing Rob!


I'd love to see a picture of that.

From a TD2002

74425

sebackman
11-07-2016, 11:11 AM
Here are some additional pictures from TD2002's. The felt they are using seem really thick. These are not mine, I found them on the internet for sale.

Kind regards
//RoB

744307443174432

Flodstroem
11-07-2016, 02:45 PM
I found this on eBay:
http://www.ebay.com/itm/JBL-375-2440-2441-2480-2482-REPLACEMENT-FELT-LINER-FOR-BACK-CAP-PAIR-/282233644862?hash=item41b66fa73e:g:WnkAAOSwIgNXwY-G
He also sells other formats too.

My self purchased felt (self adhesive) from the McMASTER-CARR (1/4" x 12" x 12") a brownish type of felt that works excellent Only drawback, I had to cut the pattern my self :(, but its OK with a sharp scissor......

toddalin
11-08-2016, 10:58 AM
When I redid my three 2420s, I used the carpet cushion padding to replace the deteriorated foams. It's the stuff that looks like blue head cheese.

sebackman
11-21-2016, 11:35 AM
Dear all,

Attached is a measure of a 2450SL with a brand new 475Nd diaphragm and both foam and felt back cap. They show very little (if any) change when swapping from foam to felt as per above.

Soundwise I do think the felt version sounds tighter in the mid but I'm sure it is just placebo saying that I should hear some payback for the work I put in. :skeptical:

74637

Kind regards
//RoB

pos
11-21-2016, 12:39 PM
Very interesting, thank you!
Main changes appear to be just under 3kHz.
That is were I always measure accidents, and also where the 2452 and 2450SL cores differ the most in my measurements.
Could you use a longer window to get more precision in that range, with a close measurement?

sebackman
11-21-2016, 01:15 PM
Hi POS,

Please tell me what you want me to do. The M2 will give odd results numbers close up but the VTX could be fine. -Or do you mean without horn? Please advise.

Indoor and longer than 6ms gives to much artifacts on 60cm. I can probably do out door measurement in about 2 weeks, won't be home until then.

Kind regards
//RoB

pos
11-21-2016, 01:25 PM
Yes a VTX/PT waveguide will probably be more consistent up close.
A very close measurement, something like 10cm, would let you use a long window without issue in door.
Of course it would not be representative of the driver/waveguide behavior, but perfect for comparison purpose.
A very long window (ie no added window and something like 1/24 frequency-domain smoothing would probably give accurate results.

sebackman
11-22-2016, 11:40 AM
I will give it a try and post here. I have a few different horns/wg so I can try them.

Kind regards
//RoB