Need ballpark TL for a bastardized B&W

koja

Member
2009-10-04 12:10 pm
Ontario
Here's the scoop:

I have a bunch of commercial speaker designs and parts stored away "for when I find the time to ...", which is more or less never. Well, I am thinking of just grabbing a few of these and cobbling up something which, if it turns out decent sounding, could find its way to some family.

I do not have the time to do it "by the book" so I need help just ballparking the design and then I can tweek it by filling the box, plugging the ports and changing miniDSP settings (oh yes, it will be bi-amped with the luxury of an active Xover ;)).

The top will be the excellent B&W DM302 monitors. And the bottom should be the mid/bass unit from its bigger brother DM305, sidemounted in an open ended TL (see my sketch). The factory DM305 are bass-reflex units targeting about 55Hz (-3db) so I figured the TL should probably target about the same?

Unfortunately I have no T/S parameters for the bass drivers and have no time to pursue measuring those, thus my plea for a little wisdom from the more experienced.

Can you please help guesstimate some dimensions? I will stay with the same width of the speakers (narrow baffle of 217mm outside width). The depth could be 300+ mm as needed, the height and the taper TBD.

I attached some info as referenced.
 

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Make its net volume [Vb] the same size cab as the DM305, do the taper, say 3:1 and ~60" long, mount the driver down between 1/3rd - 2/5th from top. Make the terminus ['vent' area] = driver effective piston area [Sd]. Damp to 'taste'.

If you can 'afford' a larger cab, then make [Vb] 1.5-2x more to allow more tuning flexibility.

Before 'burning' wood though, ask B&W for its specs unless you know they don't honor such requests.

GM
 
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koja

Member
2009-10-04 12:10 pm
Ontario
re: the T/S parameters. I tried passing the question through their sales front end people, even through a university researcher who at some point worked with B&W guys in the U.K., but no luck. it looks like the B&W engineers made it clear to everybody that they were not to be bothered :mad:.

Thanks for the pointers. exactly what I am looking for. :)
 
You like the silly putty too huh ? :)

DATS v2 is indispensable tool. Always ready to be used within seconds and data within seconds of pressing "go". I now measure all drivers that arrive the moment they are unboxed. Save the info and am ready to build to measurment vs build to factory wishful thinking specs. Many brands are close (big companies like SS, Vifa, Peerless, Eminence, B&C, PRV, Faital Pro, and Dayton are generally close). Others not so much. Not low enough Qts (weak motor) and high fs are two of the biggest culprits.

GM's guide above is excellent. Although I personally have had great luck with adding moderate stuffing all the way down the line until last third (leave space around driver) to get an excellent low group delay but still have pretty deep bass extension. Adjust atuffing to taste - but no stuffing sounds boomy like a bass reflex.
 
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eriksquires

Member
2013-05-10 4:11 pm
Actually I don't use silly putty at all, just know some like to. I usually use coins and tape.

ScanSpeak close? What are you buying? :) I've never had a SS driver be close to spec. Raal had the same issues. Maybe I just don't know how lucky I've been.

Best,


Erik

You like the silly putty too huh ? :)

DATS v2 is indispensable tool. Always ready to be used within seconds and data within seconds of pressing "go". I now measure all drivers that arrive the moment they are unboxed. Save the info and am ready to build to measurment vs build to factory wishful thinking specs. Many brands are close (big companies like SS, Vifa, Peerless, Eminence, B&C, PRV, Faital Pro, and Dayton are generally close). Others not so much. Not low enough Qts (weak motor) and high fs are two of the biggest culprits.

GM's guide above is excellent. Although I personally have had great luck with adding moderate stuffing all the way down the line until last third (leave space around driver) to get an excellent low group delay but still have pretty deep bass extension. Adjust atuffing to taste - but no stuffing sounds boomy like a bass reflex.
 

koja

Member
2009-10-04 12:10 pm
Ontario
I am in the process of making this TL based on the recommendations I received here (B&W did not provide me with driver T/S parameters :mad:).

I only calculated the added vented volume in their DM305 speaker from which the driver came and it was 30L, so my TL is going to have the same volume. I have 45 inches overall length (31 +another 14 after fold) and the length is 5 x the depth (9); slope ratio (3.2):1; driver offset by 0.22xlength from top.

As I understand the offset length (above the driver) is to be heavily damped?? I am planning on a couple of inches (~3-4) of open cell foam. does that make sense?

My other question has to do with the terminus (a.k.a open vent) area being equal to driver Sd (~120cm^2 for a 6-1/2 inch driver). That opening is pretty big and I read that the stuffing might reduce the sound pressure wave speed by ~20% so I was thinking maybe i should extend it by another couple of inches even if I am to get slightly under Sd? does that sound reasonable?

btw for stuffing below the driver in the fold I ordered AcoustaStuff from Madisound.

the design is based on this picture (my dimensions are smaller though as the overall height is only 33 inches; DM302 monitor will be sitting on top of it).
 

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