More DIY cabinet musings.....

Discussion in 'DIY Discussion' started by la toilette, Jul 31, 2006.

  1. la toilette

    la toilette Downright stupid

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    Having settled upon b/bb birch ply for the panels (panels cut to size by my friendly local timber merchants, now lurking in my workshop) I am contemplating further design options.

    Firstly: Mixing materials. I assume that using mdf for some of the internal bracing instead of more expensive ply is OK (?), as I need to keep costs down (my original budget of £100 already exceeded). I have enough ply for the panels and top and bottom, but not for all the bracing. Shouldn't be a problem should it? I'm also using pine handrails to fashion the corner fixings (needed as none of the corners are 90 degrees), not ideal but hardwood poles are gonna be just too expensive. I don't have the tools to cut the complex angles for the corner fixings specified on the original plans.....and I intend to extend the poles slightly above and below the panels for an interesting 'look' :D .

    Secondly: I'm doubling up the panel thickness on the baffle, but is it better to use pva wood glue or a grip adhesive (e.g. gripfill max or equiv.) to bond the two panels together? I'm going to screw them also (from the rear to keep them hidden).

    Thirdly: Port location/size/shape. As I understand it the length of the port can be altered to 'tune' the cabs in some way :)confused: ). But, is the location of the port(s) critical i.e. positioned centrally, or a pair, high on the baffle or down low? I had also contemplated copying the V-shaped ports on the Tannoy Churchill cabs, does the shape alter the sound?

    Fourthly: Where can I buy acoustic foam from, and what type is best? Anyone got a big pile of it going begging? :D
     
    la toilette, Jul 31, 2006
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  2. la toilette

    Tenson Moderator

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    The area of the port is very important. If the area changes, the length of the port will also need to change if you wish to keep it tuned to the same frequency. A larger port area and the port needs to be longer to keep it tuned at the same freq and vice-versa. They can be placed pretty much anywhere though. Putting it nearer the bottom on the cab might be an idea to minimise effects of floor-bounce cancellation.

    For glue I'd go with strong PVA, something like 'no more nails' is utter crap. Evostick wood glue epoxy (green bottle) is also very good though, looks like PVA but is a bit stronger.

    Alternatively you could add the second layer of the baffle with a layer of silicone to isolate it and the driver from the rest of the cab. Maybe use a few small pins to hold the baffle in place nicely.

    The more ridged the internal bracing the better. What sort of bracing method are you going for? Bracing can also double as reflection deflectors to stop standing waves as I did in this standmount.

    [​IMG]
     
    Tenson, Jul 31, 2006
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  3. la toilette

    la toilette Downright stupid

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    The boxes are pretty tall at 1 metre (plus feet), so I was planning to insert three horizontal braces that would extend to all four sides and double-up as places to screw the rear panel to. In addition to those I was thinking of putting in some diagonal bits in to tie the different horizontal braces together. That's about it really, but I'll stick in as much as seems sensible or practical when I come to put them together - I figured that if I start with the horizontal braces in place (slightly rebated into the side panels) when I actually glue the panels together, then I can add further woodwork afterwards.

    As far as port tuning goes, I have to admit I don't really know what I'm doing here :shame:. The plans that I'm approximately following state "There are 2 flared ports 50 mm in diameter and a 100 mm long that provide a port frequency of 48 Hz." However, my cabinets are going to be slightly narrower but 15cm taller (to get the driver at a better height) and I don't know if the tuning is cabinet volume dependant or not, although my cabs will only be 5 litres greater than the plans.
     
    la toilette, Jul 31, 2006
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  4. la toilette

    Tenson Moderator

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    It is volume dependant, its a spring/mass resonance system. This means you don't want the port to end inside the speaker too close to something that might block or considerably damp air movement. The greater the speaker volume the shorter the port has to be for a given port area and desired frequency. What is the volume of the speaker in the plan?
     
    Tenson, Jul 31, 2006
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  5. la toilette

    la toilette Downright stupid

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    Ah, OK. The plan volume is 75 litre, mine will be about 80. Nope, scratch that, quick re-calculation (just drawn out revised x-section on CAD) gives my internal volume as 90 litres, so 20% greater than the original.

    Placing the ports away from obstructions shouldn't be a problem, there's just the 12" driver quite close to the top of the cabinet so it'll really just be the bracing below to avoid, and obviously the exact locations of that is not set yet!
     
    la toilette, Jul 31, 2006
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  6. la toilette

    ShinOBIWAN

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    Agreed, I tried using that stuff called 'Sticks like Sh*t' from Evostick, it didn't seem to work well with MDF and was more 'Stinks like Sh*t' than anything, near impossible to sand too. Best I've used is the Resin W in the blue bottle, same stuff as the green you mention but tougher since its for external use.

    I like the look of those, what's the drivers? Is that BG ribbon and Peerless mid/bass?
     
    ShinOBIWAN, Jul 31, 2006
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  7. la toilette

    Tenson Moderator

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    I'm afraid my modelling in WinISD does not agree with your quote. In a 75L cab, with 2 flared ports 50mm diameter and a frequency of 48Hz the length would need to be 31mm. However, when using the same driver in a bigger cabinet it will act differently anyway, so it isn’t that logical to adjust the port length to keep the tuned frequency the same as the smaller one cab. With a larger cab you can usually get more bass extension. What is the driver?

    Ant, its a B&G Neo8 with a Seas CA18RNX, 12dB/oct electrical Xover. I was planning to use another Seas driver to begin with but found it was now discontinued so thats why the ports are 'extended'. I also added a bit of mass to the driver to extend bass a tad. Only added 1g, but it does the trick. Might have added 1.5g if it were for a larger room.

    If I had to do it again I would use a Hi-Vi ribbon that doesn't have the peak in the high frequencies, but the B&G is what I had and with a 2dB shelf filter at about 3KHz and off-axis listening it sounds pretty damn fine! I got the B&G to start with because everyone said it could cross around 800Hz, but that is crap, it can only cross as low as 1.5KHz (could do a low order if wanted) and even then needs the notch filter or shelf for the HF peak.
     
    Tenson, Jul 31, 2006
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  8. la toilette

    la toilette Downright stupid

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    The driver is a Tannoy 12" Monitor Gold, dual concentric. What would you suggest a sensible port diameter and length would be?
     
    la toilette, Aug 1, 2006
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  9. la toilette

    Tenson Moderator

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    http://www.hilberink.nl/codehans/tannoy38.htm

    Well based on these parameters I found for the Monitor Gold 12” I’d suggest 2 ports each with one flared end to the outside world 5cm dia. and a length of 5.5cm.

    However, some on those parameters on the website don’t fit together. So I took the ones that did not contradict each other and calculated the rest from that. It may not be quite right, but I suspect it will not be far off.

    EDIT: Actually scrap that! Since the length is so short you can go for a much larger port area which will give less distortion. If we stick with 5cm dia. because you can get pipe that size then go for 5 ports with each with one flared end and 18cm long.
     
    Tenson, Aug 1, 2006
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  10. la toilette

    ShinOBIWAN

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    Well, nice work on them Simon. I really like how you've done the ports, very Genelec. Looks like a fair size standmount too and its tough making them look acceptable.

    The CA18RNX measured real well according to Zaph, especially in the lower ranges.

    Any more details on the XO, comments on the sound and maybe a couple more shots?
     
    ShinOBIWAN, Aug 1, 2006
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  11. la toilette

    Tenson Moderator

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    Thanks, the CA18RNX does sound very good in the bass. Pretty good for midrange but not perfect. They sound better than any standmount below £1k. The B&G digs up lots of detail.

    The crossover was 2nd order at 1.5KHz for the B&G and 1.1KHz for the CA18RNX. The B&G then also got a 2dB shelf filter at 2.7KHz. I don’t remember the exact component values, but I have them written down somewhere! Maybe I will publish my plans for then one day. I just have it all in my head at the moment.

    The ports were done by making an 'II' shape. Like a square with the edges overlapping. It acts as bracing quite nicely as well. With the other bits to deflect standing waves it makes for an extremely dead cab, despite being only 1/2" MDF. The front woofer panel is isolated with silicone and then 3 small rods also surrounded by silicone to keep it in place.

    [​IMG]
     
    Tenson, Aug 1, 2006
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  12. la toilette

    la toilette Downright stupid

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    Thanks for your help all, sorted :) .
     
    la toilette, Aug 2, 2006
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  13. la toilette

    KUB3 ciao

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    Looking good Simon. How do they sound so far? I bet that ribbon has some detail. Does it blend in with the midbass ok? What's the low end like?
     
    KUB3, Aug 3, 2006
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  14. la toilette

    ShinOBIWAN

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    Did you get the BG's from Marshall Chong/Design3DW?
     
    ShinOBIWAN, Aug 5, 2006
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  15. la toilette

    Tenson Moderator

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    Sorry guys I didn't notice these replies.

    The ribbon was from parts express! It is an extremely good driver in terms of detail but it has a tricky frequency response to work with. I'd recommend spending a bit more on the Hi-Vi alternatives just for the ease of implementation.

    The two drivers blend very well once you overcome the frequency response of the ribbon. I think the bass driver is a little veiled compared to the ribbon but that is always the way with anything but the very best mid-bass drivers.

    The low end is very good indeed. Quick, tight and tuneful. The only thing I can complain about with these speakers is that they are passive and I can hear it after using the AML1 for so long. They get a little bit muddled and are not quite so decisive about things like heavy rock tracks. I set them up decently the other day and I think they sound as good as any standmount up to about £1.5k-£2k new which is not bad for £300 or so!
     
    Tenson, Aug 10, 2006
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  16. la toilette

    avanzato

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    I know ribbons are probably sexier in the Audiopile world but the B&G is a planar driver, IIRC in the Zaph audio tests the B&G trounced the Ribbon drivers he compared it with in just about every respect. I was thinking about making a line array with them but the costs soon get out of control.
     
    avanzato, Aug 11, 2006
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  17. la toilette

    la toilette Downright stupid

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    la toilette, Aug 23, 2006
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