We've had several discussions about this over on AudioRoundTable.com. There's lots of good information there, so hopefully some of it is useful for you.
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GM said:
do you mean noises from the pole piece vent of the inverted driver ? - if so, yes, I will be just as curious ! I've heard of the risk but have not seen any reports from users of push-pull that this has been an issue, even from those who could readily change their configuration to push-push (i.e. they were not overly vested in their particular solution to ignore the problem).
Noise from "pole-vent" was suggested an issue if one woofer was faced with magnet out
Possible "port-noise" is another different issue
Possible "port-noise" is another different issue
No, I mean the recess the 'rear' driver is mounted in. It acts as a very high tuned low pass filter. WRT pole vent 'wheezing', I've read where some folks could hear it when the tuning was low and the driver was basically just moving air.
GM
GM
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My box design does allow the inverted driver a recess as I don't like to see the gizzards of my drivers hanging out. But the recess is fairly shallow and is wider than the driver. Part of the magnet will extend outside of the box. At these frequencies (80Hz and below) I really can't see this 'cavity' causing any problems - but then I'm a novice in these waters.
The driver of choice is the CSS TRIO8. There are so many choices out there I decided that a rational approach was too much work. I figured that most modern drivers are darn good so it's hard to go wrong. I noticed that the Trio8 had a good price/performance ratio (on paper).
I liked that it was a relatively shallow cage, the back of the magnet-to-surround distance is only 9cm, which makes the inverted driver easier to partially hide in a shallow recess. Also, with an inverted driver it's harder to provide physical support to the magnet. When the magnet isn't so far away from the surround the force of gravity on the magnet doesn't have as much leverage. The driver is very solidly constructed (to my eye).
The driver of choice is the CSS TRIO8. There are so many choices out there I decided that a rational approach was too much work. I figured that most modern drivers are darn good so it's hard to go wrong. I noticed that the Trio8 had a good price/performance ratio (on paper).
I liked that it was a relatively shallow cage, the back of the magnet-to-surround distance is only 9cm, which makes the inverted driver easier to partially hide in a shallow recess. Also, with an inverted driver it's harder to provide physical support to the magnet. When the magnet isn't so far away from the surround the force of gravity on the magnet doesn't have as much leverage. The driver is very solidly constructed (to my eye).
Bigun said:
The cavity is a 1/4 WL TL and its harmonics are excited by the driver's pumping action, so is a musical instrument in a BW dictated by its length and its HF cutoff by its CSA, ergo will 'sing' over some BW well above the sub's. The question is, will it be audible? Don't know. Some have claimed that it's audible in that while they don't notice it being loud enough to comb filter with the mains output, it can be loud enough to draw attention to its location in the room, something you normally don't want.
Even if it does, some stuffing will solve the 'problem'. Ditto a felt disc suspended behind the VC vent. Really, making the tunnel deep enough to completely cover the driver and lining it with acoustic fiberglass insulation or similar would be my choice and awhile back there was a tubular consumer sub done this way with grills on both ends IIRC.
Anyway, yeah, the TRIO8 does look like a 'keeper' and its specs looks good for TH alignments in general and a 'couch potato' sub in particular, so looking forward to your review.
GM
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Well it looks like it will be an interesting thing to finish and test out. Lots of trade-offs in the end. A quarter wave TL only 9cm long is up above the 10th harmonic of my planned XO frequency but I guess it could get excited somehow.
Another opportunity for some science is the steel rod connecting the mounting holes between the two drivers. The rods will be in tension and could have a piano-wire oscillation in which the box would be a wonderful sound board. The lowest frequency mode is a half-wave at around 600 Hz.
So my box is a wind instrument (qtr wave TL), a string instrument (metal rods), a percussion instrument (any idiot can bong on the box) and a sub-woofer. I feel a patent application coming on 😀
Anyhow, I'll continue to report progress and eventually results, but it really needs one of you guys to listen to it.
Another opportunity for some science is the steel rod connecting the mounting holes between the two drivers. The rods will be in tension and could have a piano-wire oscillation in which the box would be a wonderful sound board. The lowest frequency mode is a half-wave at around 600 Hz.
So my box is a wind instrument (qtr wave TL), a string instrument (metal rods), a percussion instrument (any idiot can bong on the box) and a sub-woofer. I feel a patent application coming on 😀
Anyhow, I'll continue to report progress and eventually results, but it really needs one of you guys to listen to it.
Hi Gareth, just a quick comment on your box ( Nice bracing BTW ) perhaps some square 25*25 in the corners to couple the sides to each other, I've always found it advantageous and cost effective.
Regards
Ted
Regards
Ted
Bigun said:Right now I have only a couple of little clamps which aren't long enough to get across the side of the cube. There are two sides of the box gluing in the basement, two bags of cement on top and my tool box hanging off some string on one corner to close a gap and some rubber mats on the floor in case the string breaks.
Yes, I've been there too. The funkier it looks, the better it clamps - bucket o' screws, paint cans, 18" woofer... it's all good.
🙂
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Hi,
When I mentioned rods earlier I did think of solid rods thick enouth to be threaded in the ends. Thus allowing use of ordinary machine screws securing the driver units.
Thoughts of this idea?
When I mentioned rods earlier I did think of solid rods thick enouth to be threaded in the ends. Thus allowing use of ordinary machine screws securing the driver units.
Thoughts of this idea?
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MJL - looks great, I think we're going to need a dedicated thread for everyone to show off their jury-rigged gluing operations !
4fun/Dave - I've gone with the no.10 threaded rod and I'll put something on them to damp vibrations. I don't have heat-shrink but something else will be found. I don't fancy starting with plain rod and cutting my own threads - don't own the tools and it's not worth it for this one project.
Moondog - definitely need some bracing in the corners between sides. I just happen to have some very nice quality thin-ply (packaging from an Italian football table). Thin means quick and easy to shape and drill holes in. Thin doesn't mean it's not strong. The floor joists in my house are made from engineered wooden I-beams with very thin verticals. The trick is that they are deep - a beam's stiffness depends very much upon it's depth. So I've cut up the thin ply and glued it all over the inside of the box, edge on, to create a very stiff structure with little penalty in weight or volume. A bang on the side makes a nice satisfying 'thunk'. Not sure if this photo explains it any better...
4fun/Dave - I've gone with the no.10 threaded rod and I'll put something on them to damp vibrations. I don't have heat-shrink but something else will be found. I don't fancy starting with plain rod and cutting my own threads - don't own the tools and it's not worth it for this one project.
Moondog - definitely need some bracing in the corners between sides. I just happen to have some very nice quality thin-ply (packaging from an Italian football table). Thin means quick and easy to shape and drill holes in. Thin doesn't mean it's not strong. The floor joists in my house are made from engineered wooden I-beams with very thin verticals. The trick is that they are deep - a beam's stiffness depends very much upon it's depth. So I've cut up the thin ply and glued it all over the inside of the box, edge on, to create a very stiff structure with little penalty in weight or volume. A bang on the side makes a nice satisfying 'thunk'. Not sure if this photo explains it any better...
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Photo explains every thing very well, what damping if any are you using inside the cabinet??
That tapped rod idea did sound interesting, easy to do if you have a lathe
That tapped rod idea did sound interesting, easy to do if you have a lathe
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Moondog55 said:
I have some man-made poly-bat left over from the Moon-Onkens that I bought locally. It's white, fairly stiff, doesn't shed fibres and is in a large sheet about 3/4" thick. Since I have no other use for it I planned to cut it up and use it in the sub. I don't really need much in the way of 'stuffing' to increase apparent box volume so it's more in the way of damping.
I'm still thinking about the BASH500 plate amp. It has some internal heat sinks so I have decided not to put it into it's own box within the enclosure. This allows some air movement around those heatsinks. But, it's a Class-D amp and they produce EMI. So I'm now thinking about adding some conducting mesh and foil. I have some local expertise where I work and will seek some advice on that today.
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end of this discussion about driver quantity and time to show results
continued at: http://www.diyaudio.com/forums/showthread.php?s=&threadid=140069
continued at: http://www.diyaudio.com/forums/showthread.php?s=&threadid=140069
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