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Design flaw of a Dual 18" Cabinet? [message #321068]

Thu, 08 May 2008 22:23

Josh Billings is currently offline

Josh Billings
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I bought a couple dual 18s off a guy and he used high end drivers (Beyma 18 G50) but the cabinet is built as one big cabinet and most other Dual 18s are like 2 cabinets together. One of the speakers was blown using a PL6 (1 cab per side) and i figured i would be pretty safe with less than 2x RMS at these things, but i guess i was wrong.

I'm thinking it may have been overexcursion because the box was too big. Let me know what you think.

Like I'll do my best to illustrate using crude ASCII

my box
_____
|_____|

What i think it should look like
_____
|__|__|

[Updated on: Thu, 08 May 2008 22:24]

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Re: Design flaw of a Dual 18" Cabinet? [message #321078 is a reply to message #321068 ]

Thu, 08 May 2008 23:06

Art Welter is currently offline

Art Welter
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Josh,

A center isolation baffle makes the cabinet stiffer, and will reduce a little bit of cone modulation distortion from the reflected waves from the opposite side cone. It also will insure the cabinet tuning does not change if one cone fails.

If your speaker failed from over excursion, either the cone would be ripped or the voice coil former would be squashed, in which case there would have been a lot of “clacking” noise before it died.

You should high pass the cabinet just below the box tuning.

Adding a center panel will not change the box tuning.

If you want to add a divider, by using two panel pieces, you could fit them through the speaker hole, then screw them together in the cabinet. Glue and screw, then use caulk to seal any air leaks. If you do that, you can run the single speaker until you get the other reconed.

Divided cabinets are a better from a sound quality perspective, but add weight, material and assembly cost.

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Re: Design flaw of a Dual 18" Cabinet? [message #321079 is a reply to message #321068 ]

Thu, 08 May 2008 23:06

Greg Cameron is currently offline

Greg Cameron
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There are plenty of decent dual 18's out there that use a common chamber. What matters is that the T/S parameters of the box are correct and matched to the driver. If the box tuning is such that it allows the driver to easily unload, you'll have problem with over-excursion. And of course, solid box construction helps with sound quality and reducing wasted energy and distortion.

Greg

Cameron Pro Audio.

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Re: Design flaw of a Dual 18" Cabinet? [message #321130 is a reply to message #321068 ]

Fri, 09 May 2008 06:42

Scott Smith is currently offline

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I would not call it a design flaw. As long as both speakers are wired parallel or see identical signals and have identical excursions, the center baffle has little affect acoustically. The port design is also affected depending on their location.

"Percussive Maintenance" - Bang on it until it works!
Scott Smith, South Florida

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Re: Design flaw of a Dual 18" Cabinet? [message #321133 is a reply to message #321068 ]

Fri, 09 May 2008 07:17

Don Boomer is currently offline

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Josh Billings wrote on Thu, 08 May 2008 22:23

i figured i would be pretty safe with less than 2x RMS |

The power rating is only good if you do exactly the same thing as the bench test. That's tough in the real world to guarantee.

The bench test requires that you shape the signal, that you guarantee the dynamic range, and that you guarantee to NEVER clip the amp. If you don't strictly adhear to these items you won't get the power handling the manufacturer specified.

Also realize that if you clip that "less than 2x RMS" amp" it can put out 4x RMS power (duty cycle wise).

As far as the cabinet ... adding the extra wall might increase power handling a bit but it will come at the expense of losing the low end. No free lunch!

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Re: Design flaw of a Dual 18" Cabinet? [message #321139 is a reply to message #321068 ]

Fri, 09 May 2008 07:59

Paul O'Brien is currently offline

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This is a perfect example of why you can't just throw drivers in any cabinet and get the full potential from them. What are the internal dimensions of the cab and port dimensions(diameter and length)? With this info we can figure out the box tuning and suggest a course of action to prevent further carnage. If the box is way to big for the drivers adding a divider and altering the ports will help quite a bit, but you'll still have to set high pass filters and limit power to prevent over excursion.

Paul O

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Re: Design flaw of a Dual 18" Cabinet? [message #321342 is a reply to message #321139 ]

Fri, 09 May 2008 20:33

Josh Billings is currently offline

Josh Billings
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I believe these are 40x30x24 and they are slot loaded (one long slot along the bottom of the cabinet).

Also I'm using a Pl6 with the clip limiters on and don't even remember clipping them.

It was the double version of this
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=26021 1370398

Bought it from the same guy.

-Josh Billings

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Re: Design flaw of a Dual 18" Cabinet? [message #321375 is a reply to message #321133 ]

Fri, 09 May 2008 23:22

Art Welter is currently offline

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Don,

I usually find your advice to be OK, but this advice:

“As far as the cabinet ... adding the extra wall might increase power handling a bit but it will come at the expense of losing the low end. No free lunch!”

Is simply wrong, if the ports are the same on each side of the divided cabinet.

A) The divider will not make a difference in power handling.
B) The divider will not cause a reduction in low end.
C) The divider will not change the tuning, other than a negligible percent equivalent to the dividers displacement compared to the net volume.
D) No free lunch unless you are a dumpster diver.

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Re: Design flaw of a Dual 18" Cabinet? [message #321377 is a reply to message #321342 ]

Fri, 09 May 2008 23:41

Art Welter is currently offline

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Josh,

You gave the external cabinet size (perhaps?) and said the port shape was that of a slot.

To determine what your the “slot” tunes your cabinet to you need to measure the depth of the port (slot) and the how many square inches it is on the front of the cabinet.

The deeper the duct, the lower the tuning. The bigger the hole the higher the tuning. Small ports are bad when used with lots of cone displacement. Large ports ar good when tuned low, but when a large port is tuned low, it uses up a lot of cabinet space.

Don Boomer said no free lunch, and in this regard I agree.

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Re: Design flaw of a Dual 18" Cabinet? [message #321378 is a reply to message #321377 ]

Fri, 09 May 2008 23:45

Josh Billings is currently offline

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Hmmmm

than can anybody think why i blew up this driver (1 of 4). Seems like 2500 watts @ 4 ohms wouldn't be too much to deal with if it wasn't clipping or anything

-Josh Billings

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Re: Design flaw of a Dual 18" Cabinet? [message #321397 is a reply to message #321378 ]

Sat, 10 May 2008 07:24

Don Boomer is currently offline

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The same reason drivers always blow ... you either provided it too much continuous power and burned it or drove it at too low a frequency and exceeded it's mechanical limitations.

(that's assuming the driver was not defective or that your power amp isn't spitting out DC)

You just don't understand where the power can from .. or ... you don't understand the power rating for the speaker.

If you just stuck drivers in boxes that they were not tuned for the power handling of the driver can become greatly diminished.

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Re: Design flaw of a Dual 18" Cabinet?Here we go again [message #321398 is a reply to message #321378 ]

Sat, 10 May 2008 07:28

Ivan Beaver is currently offline

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Josh Billings wrote on Sat, 10 May 2008 00:45

Hmmmm

than can anybody think why i blew up this driver (1 of 4). Seems like 2500 watts @ 4 ohms wouldn't be too much to deal with if it wasn't clipping or anything

-Josh Billings

Untill we see photos of the failure (that seems to be the hardest thing to get-I have yet to see a single photo of any failure posted here (except mine)) there is no way of know what th epossible failure might be. But people love to guess about things they cannot see. Laughing

Laughing

DON'T GUESS FIND THE ANSWER. Anything else is pure wasted time speculation. I would suspect a defective loudspeaker if only one out of 4 went bad-unless the others are getting ready to fail!

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.

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Re: Design flaw of a Dual 18" Cabinet?Here we go again [message #321399 is a reply to message #321398 ]

Sat, 10 May 2008 07:34

Josh Billings is currently offline

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Well my high pass was set at 37 hz so i can't imagine i was driving them super low. The guy i bought them off of said the box was designed around the speaker. Seems about right though from what ia punched into winisd

-Josh Billings

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Re: Design flaw of a Dual 18" Cabinet? [message #321406 is a reply to message #321375 ]

Sat, 10 May 2008 08:26

Don Boomer is currently offline

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My thoughts here are if you divide the box into two smaller cubic volume boxes the air inside will be stiffer. You'll need to tune the box up (or lose efficiency) and therefore you'll gain a tiny amount of additional power handling ... compared to a larger volume box than can be tuned lower.

Josh ... from what I can quickly find it looks like the drivers are rated 750W RMS (AES) each. Is the VC burnt? Evenly or just at the top? Any creases? Photo?

Ivan ... I missed the 1 of 4 so yes ... DC is probably out. Like you said, the other 3 could be right on the border of going, you could have a case where the frame was tweaked, all kind of things.

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Re: Design flaw of a Dual 18" Cabinet? [message #321411 is a reply to message #321406 ]

Sat, 10 May 2008 08:48

Marjan Milosevic(MarjanM) is currently offline

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PL6 is way to powerful for those speakers. They are 750W (AES) which is around 500W RMS. Even if you dont clip the amp you are pumping 1000W+ RMS to 500W speaker. That is too much considering 4-5 hours of beating per night.

www.totalproduction.com.mk
www.myspace.com/totalproductionmk

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Re: Design flaw of a Dual 18" Cabinet?Here we go again [message #321412 is a reply to message #321399 ]

Sat, 10 May 2008 08:50

Bob Leonard is currently offline

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Josh Billings wrote on Sat, 10 May 2008 08:34

Well my high pass was set at 37 hz so i can't imagine i was driving them super low. The guy i bought them off of said the box was designed around the speaker. Seems about right though from what ia punched into winisd

-Josh Billings

If I look at the .pdf at the Beyma site it appears the speaker drops of pretty fast at 50hz, and 2500 watts is more than that speaker will ever handle. I would be sceptical concerning the box and vent size. You should measure the inside dimensions of the box as well as the length, width and DEPTH of the ports and see if you can either run the numbers yourself or ask some to do it for you.

http://profesional.beyma.com/ENGLISH/producto.php

The roar of the grease paint, the smell of the crowd.

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