twin 18 design

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ok, this is a take on a twin 18 reflex im planning on prototyping this week.

id welcome any comments at this time.

the net internal volume is 400l, and its tuned at 39hz. seems to simm quite well in winisd, but i dont trust sims much

im hoping to get a very low amount of port noise from the large central slot port.

drivers will be b&c 18nw100 neo units, wood will be 18mm birch.

these will need to be run from 38 - 130hz, and i will be using these as a 1 box bass solution.
whilst im aware that a lot of people will think ill need a 15" kick bin, thats not an option. these will play below community sls920 cabs, which ive been playing around with, running them down to 104 hz which they are happy at. to give me plenty of kick im happy to mess around with  dsp eq / phase / timing settings to get the whole system flat and sounding good. 

so, if you have any thoughts, im all ears!

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csg Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 17 September 2007 Location: bedford Status: Offline Points: 6085	Post Options Post Reply Quote csg Report Post Thanks(0) Quote Reply Topic: twin 18 design Posted: 03 August 2008 at 1:22pm
	ok, this is a take on a twin 18 reflex im planning on prototyping this week. id welcome any comments at this time. the net internal volume is 400l, and its tuned at 39hz. seems to simm quite well in winisd, but i dont trust sims much im hoping to get a very low amount of port noise from the large central slot port. drivers will be b&c 18nw100 neo units, wood will be 18mm birch. these will need to be run from 38 - 130hz, and i will be using these as a 1 box bass solution. whilst im aware that a lot of people will think ill need a 15" kick bin, thats not an option. these will play below community sls920 cabs, which ive been playing around with, running them down to 104 hz which they are happy at. to give me plenty of kick im happy to mess around with dsp eq / phase / timing settings to get the whole system flat and sounding good. so, if you have any thoughts, im all ears!

darkmatter Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 26 February 2005 Location: LDN Status: Offline Points: 2419	Post Options Post Reply Quote darkmatter Report Post Thanks(0) Quote Reply Posted: 03 August 2008 at 4:27pm
	i've got no experience so im not sure whether this is right, but you might want to take a little more off the port length to account for the natural extension the port will see from the cab walls at either end. guess you may have thought of this though, as you've already shaved off 8mm or so... don't know whether it's worth making bracing less evenly spaced to split up the number of similarly sized vibrating panels. Edited by darkmatter - 03 August 2008 at 4:28pm

_djk_ Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 5992	Post Options Post Reply Quote _djk_ Report Post Thanks(0) Quote Reply Posted: 03 August 2008 at 8:59pm
	Port needs to be braced too.
	djk

csg Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 17 September 2007 Location: bedford Status: Offline Points: 6085	Post Options Post Reply Quote csg Report Post Thanks(0) Quote Reply Posted: 03 August 2008 at 9:16pm
	yes, bracing the port given its dimensions would be a good idea. ive always made my bracing symetrical in the past, and from the insides of commercial designs ive seen, most bracing seems to be reasonably symetrical. whilst im awate that breaking up resonances / modes is a good idea, how much of an advantage will this be at these frequencies? cheers for your input so far guys

ceharden Members Profile Send Private Message Find Members Posts Add to Buddy List The 10,000 Points Club Joined: 05 June 2005 Location: Southampton Status: Offline Points: 11851	Post Options Post Reply Quote ceharden Report Post Thanks(0) Quote Reply Posted: 04 August 2008 at 1:09am
	The higher you want the cab to be useable, obviously the more you need to think about resonances. I think the more you can avoid having two panels which will resonate at the same frequency the better. I definitely avoid square unbraced areas of panel. One of the reasons I'm really loving the triangular ports is the inherent stiffness it provides. Slightly veering off but interesting is something I noticed on a mid top I did recently. The 15" bass/low mid drivers had a sharp notch at about 300Hz around where I wanted to cross to the midrange. At first I thought it was a measurement artifact but after a while realised that the distance between the driver and the back of the box was a quarter wave at that frequency and what I was seeing was the cancellation by the reflected wave. This was a 300mm-ish deep cabinet. Scaling that up to your bass bins, that puts the cancellation at about 110Hz. So.... is having a very deep box a good idea for something you want to use quite high? Maybe consider not having the baffles parallel to the front of the box. Actually completely changes things from a panel and internal resonances point of view.

csg Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 17 September 2007 Location: bedford Status: Offline Points: 6085	Post Options Post Reply Quote csg Report Post Thanks(0) Quote Reply Posted: 04 August 2008 at 8:26am
	mmm, good point. one option would be to move the front pannel as you suggest, maybee another woud be to introduce larger pannels of bracing, placed at various angles to the external pannels. this would also have the benefit of increasing the stiffness of the box, and i can easily allow for the extra volume absorbed by the bracing ill draw something up later with my high tech pencil cad!