2121H Dust cap issue fixed: improvement

Hi all,

Just thought I'd share this discovery:

The other day I noticed the dust caps on the midbass drivers (2121h) of my 4343 had started to seperate from the cone and were no longer sealed to the cone. There was a gap of 3mm in some place. So I glued them back in place and made sure the cap was sealed to the cone all the way around.

Well i may be going crazy but after the fix i believe I noticed a bit of an improvement in mid bass punch midbass tightness and overall imaging.

The only thing I can imagine is that the dust cap forms part of the acoustic seal of the driver to the dogbox. I'm not sure if this is the case, but if it is it may explain the perceived improvement. Or perhaps improved the rigidity of the cone.

Anyway check those inverted dust caps on your drivers.

Thanks.

Hi Goldjazz,

RE "were no longer sealed to the cone."

Going crazy? Probably not with regards to improvement in mid bass punch. Speakers should be an air tight system, except for the vent when the design calls for one.

Air leaks rob you of some of a driver's low end. The effect of losses is nicely illustrated in Vance Dickason's Loudspeaker Design Cookbook, 5th ed., P. 51.

Bullock on boxes (e.g. P. 20) has repeatedly insisted on having air tight cabinets and that includes the driver(s) sealing the front baffle holes (excludes vent when there's one). He describes this as the single most important requirement, and adds " I cannot emphasize this point enough". He did have to seal porous dustcaps to settle some losses issues.

An air leaking dust cap may well explain some of your sound issue. Can't say about better imaging though. Regards,

Richard

Same. My 4343 was making air noises and turned out to be my 2121 mid driver's dustcap wasn't sealed (air was leaking). I removed the dustcap and I'm gonna put a new one with a new glue line.

Should make a big difference, you really want a good seal on your 2121.

Quote:

---

Originally Posted by Goldjazz

Hi all,

Just thought I'd share this discovery:

The other day I noticed the dust caps on the midbass drivers (2121h) of my 4343 had started to seperate from the cone and were no longer sealed to the cone. There was a gap of 3mm in some place. So I glued them back in place and made sure the cap was sealed to the cone all the way around.

Well i may be going crazy but after the fix i believe I noticed a bit of an improvement in mid bass punch midbass tightness and overall imaging.

The only thing I can imagine is that the dust cap forms part of the acoustic seal of the driver to the dogbox. I'm not sure if this is the case, but if it is it may explain the perceived improvement. Or perhaps improved the rigidity of the cone.

Anyway check those inverted dust caps on your drivers.

Thanks.

---

Thanks. I have the cookbook which is what drove me to fix them up, quite surprised by the result. Yeah the image seems to be more sharply focused I the centre. I can explain why they would be either. In the cookbook they also mention the stiffening provided by some inverted dust caps to the cone, but I agree yh a t it's probably the acoustic seal. There was still the original JBL swing tag on my speakers cautioning against opening the cabinets and disrupting the seal. The 2121h dog box is such a small volume, I imagine a gap of 3mm around much of the dust cap would get In the intended sealed box total Q which must have an effect.

Cheers.

Quote:

---

Originally Posted by RMC

Hi Goldjazz,

RE "were no longer sealed to the cone."

Going crazy? Probably not with regards to improvement in mid bass punch. Speakers should be an air tight system, except for the vent when the design calls for one.

Air leaks rob you of some of a driver's low end. The effect of losses is nicely illustrated in Vance Dickason's Loudspeaker Design Cookbook, 5th ed., P. 51.

Bullock on boxes (e.g. P. 20) has repeatedly insisted on having air tight cabinets and that includes the driver(s) sealing the front baffle holes (excludes vent when there's one). He describes this as the single most important requirement, and adds " I cannot emphasize this point enough". He did have to seal porous dustcaps to settle some losses issues.

An air leaking dust cap may well explain some of your sound issue. Can't say about better imaging though. Regards,

Richard

---

Yeah seal them up, I'd be keen to know your listening impression before and after. The 4343 is a great speaker and this just adds to the many things (along with getting rid of biamp switch etc) that pull more and more magic out of these older beasts. These monitors have so much to give but a lot of tweaking is needed to get them to theor best, particularly after all these years.

Quote:

---

Originally Posted by jbljr

Same. My 4343 was making air noises and turned out to be my 2121 mid driver's dustcap wasn't sealed (air was leaking). I removed the dustcap and I'm gonna put a new one with a new glue line.

Should make a big difference, you really want a good seal on your 2121.

---

...also i'm now suspicious of the gasket ring on the basket. Mine has gone pretty hard, I bet getting a fresh one of those would help. Are there any tricks for softening them up, or just get a new pair of gaskets?

Hi Goldjazz,

I would simply replace the gaskets with similar new ones if you can find some. Trying to salvage gaskets at the end of their life may turn out to be a waste of time... Regards,

Richard

Hi Goldjazz,

if these drivers are front mount and you don't mind the somewhat compromised look, even removing these old, useless gaskets might be an option.

Best regards!

Edit: Or did you mean the O-shaped gasket behind the frame? Then the above is nonsense, of course!

Hi yeah I meant the O ring on the back. Though you raise a point I've never really thought about the foam gaskets that were attached to the front on my 2231H. It never occured to me they are for when you rear mount the driver? But Mine for my 2231H are sitting im a drawer after i refoamed years ago, I guess not needed due to the front mounting of the 2231H in the 4343.

Quote:

---

Originally Posted by Kay Pirinha

Hi Goldjazz,

if these drivers are front mount and you don't mind the somewhat compromised look, even removing these old, useless gaskets might be an option.

Best regards!

Edit: Or did you mean the O-shaped gasket behind the frame? Then the above is nonsense, of course!

---