Film-Tech Forum ARCHIVE: Sub Woofer Cracking Noise

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Posted by Don Furr (Member # 1469) on 11-27-2010, 05:39 PM:

The sub in my screening room is a JBL 4645B. I'm feeding it with a 400 watt amp and as of late I'm hearing a cracking sound coming from the speaker. Of course it's in the louder passages in action movies such as Iron Man 2. (Blu Ray) I've never heard the noise while running Dolby Digital on film..just from the Blu Ray player. Questions: How old is the 4645B speaker? Could it need replacing (dry rot)? The processor is a CP500.
Thanks...

Posted by Mark Gulbrandsen (Member # 72) on 11-27-2010, 05:58 PM:

Check the surround edge of the cone and gently press straight in and see if the voice coil is rubbing. Usually one sub isn't enough.

Posted by Don Furr (Member # 1469) on 11-27-2010, 06:03 PM:

I'll try that Mark...thanks. BTW the room is only 15X20 feet.

Don

Posted by Marco Giustini (Member # 4544) on 11-27-2010, 06:22 PM:

Well if I'm not mistaken my excel sheet says 770W for a 4645B. You're powering it using a 400W amp: do you see it clipping during loudest passages?

One of my 4645B was a bit noisy during louder scenes, despite the amp was not clipping nor overpowering it. The cone didn't have any apparent defect and the other 4645B I have was not doing it. Eventually I had to recone it.

Posted by Don Furr (Member # 1469) on 11-27-2010, 06:27 PM:

Marco I haven't noticed any clipping either. I can't see any defect in the cone so I'm wondering if it's just time for a replacement speaker? Does anyone know the first year JBL made the 4645B's?
This might be a really old speaker.

Don

Posted by Steve Guttag (Member # 268) on 11-27-2010, 06:30 PM:

Presuming it is 20-feet long and you have one 4645B...then you need 656-watts. The speaker can handle 800-watts so it is your amplifier that is deficient. Note, with two 4645Bs (4645Cs or a mix of the two) your amp needs drop to 330-watts total and you have a 4-ohm load which likely your amp can handle.

The 4645B uses the 2242 driver which has a pleated damped cloth surround...as such, unless it has cracked/ripped it shouldn't have failed due to age (it is a good idea to periodically rotate a driver to prevent sagging).

Checking the cone for rubbing is a good idea, as Mark suggested. It is also possible that the tensile leads might be rubbing the basket of the frame...including when the driver is moving at extremes...if they touch the frame it will give a staticy sound. It can also be as simple as a loose connection at the cabinet terminals and at the driver terminals.

Lastly, since it is a 4645B...I would check the port tubes for tightness...I've seen many just fall into the cabinet. They are built rather flimsy and are angled at about 45-degrees in the cabinet. They are not well supported.

-Steve

Posted by Don Furr (Member # 1469) on 11-27-2010, 06:36 PM:

Thanks Steve. I'll get busy checking things.

Don

Posted by Marco Giustini (Member # 4544) on 11-27-2010, 06:48 PM:

Don

I second Steve. My two 4645B were BOTH squeaking when I got them second hand. I found that the reflex tubes and some woodden parts were loose. A good quantity of glue solved the issue. Anyway, my noisy cone was perfectly fine, couldn't see any issue with it. I couldn't feel any resistance pressing it with my hand.

Steve
why we have that slightly difference in result? I used 97dB sensitivity and 113dB SPL, 2/3 of 20 feet. The result I have is 770.7W for a single speaker.

Posted by Steve Guttag (Member # 268) on 11-27-2010, 07:09 PM:

I dunno...perhaps having grown up with Lucas Electric in cars, you are used to over compensating to minimize inherent problems?

I've use the same spread sheet for many years now to calculate my power requirements and it has never failed me yet. I did a quick glance at my formula in the cell and I didn't see anything amiss.

Perhaps you mis-keyed the feet/meter conversion? Mine is only to four sig-figs (.3048 feet/meter).

-Steve

Posted by Louis Bornwasser (Member # 3063) on 11-27-2010, 09:53 PM:

I'm voting for the "bad surround" 4645b. Foam fails after a number of years. Can be reconed or replaced with another design, with slightly different results. Louis

Posted by Gordon McLeod (Member # 33) on 11-27-2010, 10:18 PM:

i think the issue off he foam has been discussed it is a non isue

jbl does not use that one the subs

Posted by Don Furr (Member # 1469) on 11-27-2010, 11:27 PM:

Update:
Running my finger around the outer ring of the speaker, there is a soft spot about 6-7 inches long. When you run your finger around that area there is a noticeable cracking sound. The cone is flatted out in that area. I've got to assume the speaker is "shot".
Where can I purchase a replacement speaker for this 4645B?

Thanks for all the help.

Don

Posted by Jeremy Weigel (Member # 4208) on 11-28-2010, 02:29 AM:

It will be cheaper to just have it re-coned (it's about 1/2 the cost of new). You can find authorized JBL centers here or Steve can also do them. I recently found this out the hard way with subs I inadvertently over drove.

Posted by Marco Giustini (Member # 4544) on 11-28-2010, 08:34 AM:

Steve

Yes, feet/meters conversion mistake thanks!

Posted by Steve Guttag (Member # 268) on 11-28-2010, 11:26 AM:

Damn...I was thinking it was the Lucas "prince of darkness" electric thing. [Wink]

Posted by Marco Giustini (Member # 4544) on 11-28-2010, 12:20 PM:

Sorry Steve, didn't get that, I thought you were talking about yourself or someone else!

Posted by Mark Gulbrandsen (Member # 72) on 11-28-2010, 01:06 PM:

I knew you were going to say my room is only XX by XX however my thoughts about using two or more subs are always in terms of achieving the lowest possible THD from that channel. Very few if any every day theaters have LF harmonics of less than about 10% and are usually much higher. A single 4645B when driven by the proper 656 watt amp as Steve says will be needed will net you incredibly high amounts of harmonic distortion. It is also a prooven fact that the bigger the woofer the higher the harmonic distortion it will produce. The laws of physics come into play on this and there is no way around that except to go with multiples... but if you like distortion than this is all great!

It takes lots and lots of power and multiple speakers to create clean realistic low frequencies. I'd go so far as to say say the 656 watt amp wouldn't even be enough for a single 4645B woofer for it too would also be straining. More liike a kilowatt of power might be adaquate as then at least you have an ounce of head room!! Multiples are always better then singles to both lower the distortion to reasonable levels and to keep the power levels and operational excursion of the woofers with in reasonable limits. Unfairly, all this in the end is dictated by budget constraints and price cutting on jobs and installs [Frown]

.

RE: The 4645 box... What a piece of crap! I too had to repair several of them very recently... they literally fall apart while in operation. It's amazing that very many of them make it to a job site intact...

Don... Just send the woofer to Steve...

Mark

Posted by Marco Giustini (Member # 4544) on 11-28-2010, 01:17 PM:

speaking of rooms: small listening rooms are more prone to room modes. My listening room had a huge dip of -25dB in the middle of the room at 40Hz. Dreadful.

Being a small room, it is likely to have some room modes to some degree. This may affect your low frequencies and you may need more power to compensate them. Usually the 2242 do not break, after reconing it please try to assess why it broke or you may find another hole in the cone very soon!

Posted by Steve Guttag (Member # 268) on 11-28-2010, 03:51 PM:

Both lower distortion and distortion from power-compression are reasons to go with multiples.

That said...I think you would be amazed at how little distortion from that driver would be reduced. The 800-watt rating of it is for continuos pink noise over several hours. It has an incredible amount of power handling and cooling built into its design. It is one of the most impressive drivers I've ever dealt with. The only knock some may have with it is that it does not go as low as some drivers. Most drivers that go lower have foam or, better yet, rubber surrounds (compliance). However, with those you typically also loose out on power handling as the suspension becomes much more sloppy.

-Steve