alexcd said:
No I'm good. In fact this is what I expected - plan, plan, plan - THEN "pull the trigger" and build.
I did it at 1000 (which is closer to the next +3 db multiple of 1024).
I increased volume slightly to still maintain low *driver* excursion to at least the tunning resonance. This allowed for a bit lower *port* air excursion and considerably reduced port length.
I came up with this:
80 Liters (2.825 cubic feet) nominal internal volume.
6 * 2 inch ports = 47.37 inches in length. (..obviously use the "U" bends for bending it here if a lower enclosure height is preferred.)
That lowers port excursion near to what the 5 inch is (in the 3.5 cubic foot enclosure) while keeping driver excursion much lower. No port noise with this design.
Again a rumble filter (LR 4th order at 18 Hz) - should be included.
Will it be able to utilize the full 1000 watts? Probably not, but I think its likely to come fairly close while still maintaining lower overall distortion.
If that sounds acceptable to you then the next thing is to find out what the interior diameter of those 2 inch U bends are really. (i.e. email a supplier or see if its available at a dealer near you.) On the other hand if you are OK with a 5 foot high box then you don't even need to consider this.
Also remember that with virtually all music (other than some organ music and some club music) will *NOT* have large spl's near (or lower than) 20 Hz. Here it will likely be side-band decay and hall reflection decay - and 102 db is *MORE* than enough for it. It isn't enough though for some HT effects material, however.
alexcd said:
Sounds about right, though larger volume and a bit higher. Though not with the v-shape esthetic cut. This would require a downward (or upward) port with no bends. But there are reasons for the downward port and the air loading near the floor that have nothing to do with esthetics or wrapping a port in a box.
Does it matter that the 2" (?) cement baffle + 1.5" MDF and 1" of sand is only slightly less than the mounting depth of the sub (5.3")? Usually I like to give the driver area to breath but in this case that would be difficult to angle out into the inner box.
Seperately, should I brace the rear of the subwoofer (magnet)with any kind of bracketry?
Seperately, should I brace the rear of the subwoofer (magnet)with any kind of bracketry?
alexcd said:
Doesn't matter. 2 inches should be good (..generally the thicker the better, but 2 inches should be good enough). If you wanted to change the cement baffle out for a steel plate (from perhaps a local steel fabricator), then you should prob. consider something approaching 1/2 inch (and of course get them to make the cuts and perhaps weld the bolts or make recessed cuts/drilled for the bolt heads).
The angle cut isn't neccesary here - the passband is so low in operation that any resonance should be much higher and effect it little if any.
Do NOT brace the sub magnet. Here the frame is VERY robust and we specifically want the vibration exiting the frame into the cement baffle and ONLY the cement baffle.
alexcd said:
Thats not too bad - it keeps driver excursion low which will keep distortion low. On the other hand it will also limit spl because of port excursion. Still, if he can get 105 db in-room for music at 20 Hz with low distortion then I don't think he is doing too badly.
(..and if he corner loads the thing it will offer that much more spl's OR lower non-linear distortion.)Note that with this configuration driver excursion below resonance is still pretty low. In this instance then (like Shin's), I wouldn't automatically spec. a "rumble filter", rather I'd try it (with and without) with your friends most demanding music and see if it seems to make a difference.
Will a slot port work as well if I aimed it down? I am thinking of coming straight up the back of the box and along the top. So, one 90* bend but I would use PVC pipe to smooth the corners. Is that okay? I can reduce the height of the enclosure significantly this way which is what my buddy wants.
alexcd said:
I've seen (somewhere) that something closer to a slot was actually better.
IMO its all down to how low friction the sidewalls of the port are and that IF there is any bend, that the bend is "gradual" rather than "abrupt". A "pure" 90 degree bend is horrible. A 90 degree bend that happens over a gradual arc should be OK. Remember, the port represents a "slug" of air mass that is in effect a driver diaphram without a motor (the mechanical compliance is essentially the structure of the pipe and any internal resistance it has). Effectivly that "slug" needs to be able to freely "pump" back and forth.
Bummer about the "sand".. sounds like another compromised design.
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