ShinOBIWAN said:
Thats OK. In the end, its only a suggestion.
I assume by looking at the base you'll be doing it similar to the Avalon's then? If thats the case and IF you'll be making two of the things then I'd make your open "slot" airspace effectivly port to the side, NOT the rear.
Slotted to the rear you would have an increase in spl IF near the front wall, however that might not be advantageous given room gain.
Make sure you make the front baffle from cement still..
Additionally you are effectivly showing a "tapper" to the driver's air space (not sure if there is a tube there or not). IF there is then do NOT tapper it - it will mess up driver compliance. (i.e. keep it straight). IN fact despite a resonance build-up from a "tube" in this area - this is one time where I like a "tube" to provide uniform driver excursion, particularly for driver compliance.
Phew, that cement thing is tough to picture. Basically I dont want to be looking at cement though. Can I just make an MDF-cement-MDF sandwich with a driver cutout through the MDF with the driver secured to the cement only?
alexcd said:
Sorry, No.
Again - if your bummed on the cement then go to a steel fabricator and have them pull out a torch and cut the thing for you from 1/2 - 3/4 inch steel plate. It shouldn't cost much at all.
Not unlike what these people have done:
http://www.6moons.com/audioreviews/gemmeaudio/108.html
Its time for me to go off-line now.. sorry.
..as long as the driver frame physically connects to the cement then yes thats OK. (i.e. the frame touches the cement - not the mdf.)
The exterior sheet of mdf (as pictured) isn't required, but if you use one then make it as thin as possible (i.e. 1/2 inch would be better then 3/4 inch mdf).
The cement I spec'ed originally at I believe 3 inches - though even as little as 1 1/2 inches would be OK, it just wouldn't give you quite as much mass.
The exterior sheet of mdf (as pictured) isn't required, but if you use one then make it as thin as possible (i.e. 1/2 inch would be better then 3/4 inch mdf).
The cement I spec'ed originally at I believe 3 inches - though even as little as 1 1/2 inches would be OK, it just wouldn't give you quite as much mass.
ScottG said:
Its just the 18mm MDF bracing shaped around the driver, it also acts as a driver brace, to prevent that heavy driver from rocking around.
I'm not sure if I've explained that very well 2d drawings don't really tell you the whole story. I've peeled away some of the driver walls to reveal the matrix bracing which makes it a little clearer:
An externally hosted image should be here but it was not working when we last tested it.
Is this still a problem?
Not having a "tube" here around the driver will cause greater non-linearity with compliance.. (..i.e. the spider will have more side-to-side motion from internal "open" box pressure.)
-so in this case expect more distortion from say 40 Hz up. (..and a little more below 40 Hz.) Though as I've mentioned b4, I don't find even moderate THD levels to very noticible at these higher freq.s (above 40 Hz).
Again, like the lack of a sand enclosure - it won't "break" the design.
-so in this case expect more distortion from say 40 Hz up. (..and a little more below 40 Hz.) Though as I've mentioned b4, I don't find even moderate THD levels to very noticible at these higher freq.s (above 40 Hz).
Again, like the lack of a sand enclosure - it won't "break" the design.
ScottG said:
The port fires forward in that example. The strange looking things below the cabinet are overhead views of the base - light grey bit is the opening and blue is the port exit.
Is there any advantage to tapering the opening?
Still going with a concrete baffle, not settled on anything yet but the diagrams help me visualise and piece together.
ScottG said:
I was thinking about the tube but wasn't sure if it mattered so much with this being a vented design - I remember you mentioning it was beneficial for sealed/aperiodic.
I'll see if I can work a short one into the cabinet that extends upto the rear of the drivers magnet. One question though, won't enclosing the rear wave of the driver actually cause increased localised pressure build-up right at the cone since it has less options(read: space) to displace air?
ShinOBIWAN said:
to a degree, yes - but not much depending on the length of the tube. Remember of course that the tube isn't really "enclosing" the rear wave, more like "directing" it (..and its still the same volume minus the small amount for the tubes construction).
One thing I've left for (aparently) now - is that this is the one area you don't want to use that nasa paint. Instead you want it as low in air flow resistance as possible. In this case of course you'll likely paint/laquer/polish (probably wax as well) to a "shine" that is very low friction.
IF you go the "tube" route then you don't really need a great deal of length. (..say perhaps a little beyond the magnet assembly.)
ShinOBIWAN said:
AH.. I wasn't looking at the grey vs. black before!
yes there is an advantage tapering the opening - pressure will "release" more with the tappered design. I think the few commercial designs that do this just go for a "U" shaped with the curve in the "U" near the port and the open end of the "U" towards the listening space. I have NO idea if this would be more advantageous or not in comparison to your tappered version.
Yup.. The diagrams are good!
You know with the venting as its currently depicted you will be adding some mass and you might be adding some length - this may effectivly "extend" the port and give you a lower tunning freq..
If you are "wary" of this then have the opening to the side, closest to the port.
I really don't know what the effect would ultimatly be though - you might be able to get someone to model this for you. Maybe someone using Martin's program? Certainly someone with acess to Akabak could model this to see the effect - unfortunetly though I don't know of anyone that uses that program. 🙄
If you are "wary" of this then have the opening to the side, closest to the port.
I really don't know what the effect would ultimatly be though - you might be able to get someone to model this for you. Maybe someone using Martin's program? Certainly someone with acess to Akabak could model this to see the effect - unfortunetly though I don't know of anyone that uses that program. 🙄
Cheers Scott, I plan to build a couple of bases and also with the port just firing at the floor(no base but raised slightly). I'll then be able to check out the differences.
I've decided to go without the tube surrounding the rear of the driver, mainly because I'm lazy and it would take sometime to laminate the MDF into that shape. By the sounds of it, its not giving much extra performance any how.
I've also done the cutting list:
The baffle will be in concrete so that isn't included.
I'll also start a new thread now that this will be going ahead, I'm going to have all the MDF cut by a timber yard as it will save me hours of cutting and allow to put this together in shortish amount of time.
Finally cheers to Scott for taking time out to put most of this together, lets see if it can do the biz.
I've decided to go without the tube surrounding the rear of the driver, mainly because I'm lazy and it would take sometime to laminate the MDF into that shape. By the sounds of it, its not giving much extra performance any how.
I've also done the cutting list:
An externally hosted image should be here but it was not working when we last tested it.
The baffle will be in concrete so that isn't included.
I'll also start a new thread now that this will be going ahead, I'm going to have all the MDF cut by a timber yard as it will save me hours of cutting and allow to put this together in shortish amount of time.
Finally cheers to Scott for taking time out to put most of this together, lets see if it can do the biz.
I wasn't completely happy with the first design Scott, so I went back and re-worked it. The main thing that was annoying me was having to raise my projection screen, its just not comfortable looking up and watching something - imagine getting the naff seats in a cinema.
Instead I've gone for a smallish footprint but made up the volume with a tall design that also meant I could accomodate a 4" port.
The design is exactly the same height as the Perceives, the width is matched to the very top of the bass cabinet and depth is a sort of average that will fit in well when placed at right at the side of the mains.
Plan view from the front:
Side view that I coloured in with my crayons 😀
The angled section near the middle matches up perfectly with the Perceives angled part near the top of the bass cabinet to provide a matched look, basically I wanted these to blend with the mains as if they were one and the same package, subs are an essential augmentation so its natural they should go hand in hand, the other design didn't really do that.
Instead I've gone for a smallish footprint but made up the volume with a tall design that also meant I could accomodate a 4" port.
The design is exactly the same height as the Perceives, the width is matched to the very top of the bass cabinet and depth is a sort of average that will fit in well when placed at right at the side of the mains.
Plan view from the front:
An externally hosted image should be here but it was not working when we last tested it.
Side view that I coloured in with my crayons 😀
An externally hosted image should be here but it was not working when we last tested it.
The angled section near the middle matches up perfectly with the Perceives angled part near the top of the bass cabinet to provide a matched look, basically I wanted these to blend with the mains as if they were one and the same package, subs are an essential augmentation so its natural they should go hand in hand, the other design didn't really do that.
I totally am digging the new deisgn...and IMO will flow the the deisgn of the perceives better....
on another note...you must try veneering again...I just did it for the 1st time today with the wood glue, iron, and t shirt method..and was so damn easy it was scary...
on another note...you must try veneering again...I just did it for the 1st time today with the wood glue, iron, and t shirt method..and was so damn easy it was scary...
I've just finished measuring the drivers.
Here's the manufacturers data:
Qts 0.26
Qes 0.27
Qms 6.9
Vas 37.5ltrs
Fs 42hz
And my drivers after 4 days(~100hrs) running a 10hz cycle at around 200w input:
1st:
Qts 0.21
Qes 0.21
Qms 9.5
Vas 48.6ltrs
Fs 39hz
2nd:
Qts 0.23
Qes 0.24
Qms 9.8
Vas 46.5ltrs
Fs 41hz
Vas probably isn't particularly accurate as I used the added mass method. Apart from that everything seems OK. I've modelled once again with these new figures and it hardly makes a difference.
I was pretty surprised at how close they were to the manufacturers specs. The ATC's were a fair bit out .
Here's the manufacturers data:
Qts 0.26
Qes 0.27
Qms 6.9
Vas 37.5ltrs
Fs 42hz
And my drivers after 4 days(~100hrs) running a 10hz cycle at around 200w input:
1st:
Qts 0.21
Qes 0.21
Qms 9.5
Vas 48.6ltrs
Fs 39hz
2nd:
Qts 0.23
Qes 0.24
Qms 9.8
Vas 46.5ltrs
Fs 41hz
Vas probably isn't particularly accurate as I used the added mass method. Apart from that everything seems OK. I've modelled once again with these new figures and it hardly makes a difference.
I was pretty surprised at how close they were to the manufacturers specs. The ATC's were a fair bit out .
Cutting list finished, timber yard tommorow 🙂
An externally hosted image should be here but it was not working when we last tested it.
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