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Old 12th February 2007, 11:52 PM   #1
thefish is offline thefish  United States
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Default Sub Rattle Sealed Box

I got an old JBL PS60 powered sub... it sounds good some of the time and bad some of the time I connected it to a tone generator and found the rattle seems to happen around 50-60Hz depending on the volume level. What could cause this? It is a sealed system, and I donít think it is rattling anything in the room... the gain is at about 40% with a input source of only 1Vpp so I donít think it is being over driven. Seems JBL has decided to glue/fix the grill in place so I canít remove it and check the driver... I can remove the plate amp in the back and I donít see anything loose, everything seems pretty well tied up and there is fiberglass damping all around the box. Any ideas?
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Old 13th February 2007, 12:38 AM   #2
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Try a DIY a stethoscope to localize the problem. Make some small cones out of lightweight cardboard and wrap them around a long straw or piece of surgical tube and tape in place. It could be anything, you need to narrow it down.

EDIT: This is assuming you are hearing a buzz and not trying to find a 50 Hz tone.
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Old 13th February 2007, 02:31 AM   #3
thefish is offline thefish  United States
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I am hearing a very clear buzz sound, almost like a paper rattling; it only starts to become audible in the 50-60Hz region.
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Old 13th February 2007, 03:14 AM   #4
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If I were you I would get some ear plugs, crank it up until it rattles and start touching the box with increasing pressure until you hear a change in the sound, test all over the box. Then press around the edge of the speaker frame and listen for a difference. Then gently touch the surround around its perimeter, outside edge first, then middle then inner edge(use balled up sock/towel if excursion is excessive). Then using the sock/towel touch the dust cap and listen for a change in sound.
This obviously requires removing the glued on grill, go for it, there is no sense in protecting a blown/loose driver! If the buzz needs to go away then the grill has to come off.
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Old 13th February 2007, 05:26 PM   #5
thefish is offline thefish  United States
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Quote:
Originally posted by nunayafb
If I were you I would get some ear plugs, crank it up until it rattles and start touching the box with increasing pressure until you hear a change in the sound, test all over the box. Then press around the edge of the speaker frame and listen for a difference. Then gently touch the surround around its perimeter, outside edge first, then middle then inner edge(use balled up sock/towel if excursion is excessive). Then using the sock/towel touch the dust cap and listen for a change in sound.
This obviously requires removing the glued on grill, go for it, there is no sense in protecting a blown/loose driver! If the buzz needs to go away then the grill has to come off.
I got the grill off finally, it took a lot of slow work gently prying and then pop it was off (they had put a bit of glue at the bottom of each of the 4 plastic posts). I set a 45Hz tone with the volume just loud enough to hear the rattle and then begin to play with it to find the noise. I discovered that pushing on the dust cover with the sock ball makes the sound go away almost completely. Does this mean the driver is no good?

Specs (rather small sub):

Sealed Subwoofer
6.5" driver
3/4" fiberboard construction
0.45 CuFt airspace (calculated from a solid model I drew)
50W RMS amp

Just using my ears I verified JBL's statement of 32Hz (-6dB) cutoff.

If the speaker is blown what would make a good replacement? I found these Goldwood Malabu 6.5" drivers but I have no clue about the quality, although with a frequency response of 29-4,800 Hz it would seem to be good for this application.
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Old 13th February 2007, 05:58 PM   #6
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Now get something thin to GENTLY pry under the dust cap around the perimeter, the goal being to find if it is coming off and where. The proper method of repair here is to remove the cap and do a clean install with fresh glue(the right glue), however it is your driver so if you want to just dab a little super glue in the delam area it will fix the problem, it might complicate future repairs though using such a hard/permanent glue. Your choice.

Odds are it is only a loose dust cap, if it is the voice coil applying pressure to the d.c. might stop the noise, so make sure you find the delam before you conclude that is what it is..

Good luck
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Old 13th February 2007, 10:03 PM   #7
thefish is offline thefish  United States
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Quote:
Originally posted by nunayafb
Now get something thin to GENTLY pry under the dust cap around the perimeter, the goal being to find if it is coming off and where. The proper method of repair here is to remove the cap and do a clean install with fresh glue(the right glue), however it is your driver so if you want to just dab a little super glue in the delam area it will fix the problem, it might complicate future repairs though using such a hard/permanent glue. Your choice.

Odds are it is only a loose dust cap, if it is the voice coil applying pressure to the d.c. might stop the noise, so make sure you find the delam before you conclude that is what it is..

Good luck
In the past I have had to do this with some 10" drivers I had, but for this speaker I am unsure how to go about it. The dust cap really does not have any sharp edges to get under. I have attached an image.
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File Type: jpg dustcap.jpg (61.5 KB, 119 views)
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Old 13th February 2007, 10:04 PM   #8
thefish is offline thefish  United States
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Here is a pic of the driver as a whole.
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File Type: jpg driver.jpg (59.8 KB, 112 views)
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Old 13th February 2007, 10:45 PM   #9
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Run the driver while it's out of the box before you do anything with the dust cap. Your cap appears to be mounted directly to the voice coil.
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Old 13th February 2007, 11:10 PM   #10
thefish is offline thefish  United States
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Quote:
Originally posted by Cal Weldon
Run the driver while it's out of the box before you do anything with the dust cap. Your cap appears to be mounted directly to the voice coil.
Well I tried running it out of the box (at low volume) and it has the same problem so I am sure it is the driver now and not the box/amp making the noise.
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