Hi All,
I've been planning my new (and first ever) DIY HT/music setup for a little while, and have been playing around modelling a few subwoofers. The constraints are all too familiar: can't be expensive, must be small, can't be ugly (WAF). An agreement has been struck: I can build a subwoofer, as long as it can serve dual duty as a side table!
Size constraints are (more or less) no more than 30cm wide, about 45 long (max) and 45cm tall (max).
I was almost set on going down the Peerless XLS 10" route (830452), but that really doesn't seem to model too nicely without using a PR, which I'm trying to avoid on cost grounds as much as anything.
One design I knocked up (apologies, can't post a picture from work) was a dual JBL GTO 804 setup. In brief: Two 8" subs, in about 40-45 litres, port tuned to mid-late 20's Hz, it looks pretty darn flat down to 25Hz, and with two 120w RMS amps (which I already have) specs to about 105dB at 25Hz (within excursion limits) ignoring any room gain. Group delay looks reasonable, XMax isn't reached until ~120w at under 20Hz. I know I'm not going to get any real 20Hz and under authority out of two low-powered 8s, but it should have some authority at 30Hz which will beat anything I've had before. Being a simple box I can knock it up fairly easily, veneer it with iron-on oak(!) and put a real oak top on it = instant side table! The JBL 8s are also very cheap in the UK - a pair can be had for about £70...
Any thoughts? Sound like a good plan? Is 105dB at 25Hz a reasonable goal for this sort of thing (no demands of 120dB @ 20Hz in 20 litre box with a £100 budget or anything!)
Thanks,
Seb
I've been planning my new (and first ever) DIY HT/music setup for a little while, and have been playing around modelling a few subwoofers. The constraints are all too familiar: can't be expensive, must be small, can't be ugly (WAF). An agreement has been struck: I can build a subwoofer, as long as it can serve dual duty as a side table!
Size constraints are (more or less) no more than 30cm wide, about 45 long (max) and 45cm tall (max).
I was almost set on going down the Peerless XLS 10" route (830452), but that really doesn't seem to model too nicely without using a PR, which I'm trying to avoid on cost grounds as much as anything.
One design I knocked up (apologies, can't post a picture from work) was a dual JBL GTO 804 setup. In brief: Two 8" subs, in about 40-45 litres, port tuned to mid-late 20's Hz, it looks pretty darn flat down to 25Hz, and with two 120w RMS amps (which I already have) specs to about 105dB at 25Hz (within excursion limits) ignoring any room gain. Group delay looks reasonable, XMax isn't reached until ~120w at under 20Hz. I know I'm not going to get any real 20Hz and under authority out of two low-powered 8s, but it should have some authority at 30Hz which will beat anything I've had before. Being a simple box I can knock it up fairly easily, veneer it with iron-on oak(!) and put a real oak top on it = instant side table! The JBL 8s are also very cheap in the UK - a pair can be had for about £70...
Any thoughts? Sound like a good plan? Is 105dB at 25Hz a reasonable goal for this sort of thing (no demands of 120dB @ 20Hz in 20 litre box with a £100 budget or anything!)
Thanks,
Seb
I have a pair of those drivers and they are good. Your plan is OK, but you may find it difficult to keep port noise down, i.e. you need a lot of mouth area which means a long port. I looked at a slot port that ran along the bottom, up the rear and folded back down, almost like a transmission line.
Exactly what I was thinking. I was planning on using the floor as the bottom face of the port, running under the enclosure, then turning the corner. So three sides of the box would sit on the floor, the fourth cut short to reveal the mouth. I radius on that edge could help port noise too, I guess.
Okay, I did a little more WinISD modeling on this last night. I came to a point where I wasn't sure which way to go in the design.
I can get flat to 26Hz, then pretty steep drop-off below that (F10 maybe 23Hz - really steep!) or a slightly falling response to about 26Hz - not flat- but then a more gradual fall off below that. What would be the preferred option?
I can get flat to 26Hz, then pretty steep drop-off below that (F10 maybe 23Hz - really steep!) or a slightly falling response to about 26Hz - not flat- but then a more gradual fall off below that. What would be the preferred option?
You are building pretty similar what I'm doing at the moment. The result should be a subwoofer with a TangBand W8-670 element with a plate amp. The enclosure will be 23 liters with a tuning frequency of 30 Hz, the 3 dB roll-off at 25 Hz. The size will be 40 cm * 32 cm * 38 cm (H * W * D) and the port will be flat on the bottom.
I already started a new thread with a title: Subwoofer with Postal Money.
I already started a new thread with a title: Subwoofer with Postal Money.
Do you have a link to the paper, Brian?
I've been sketching (on Post-It notes!) some ideas, and the current favourite includes a nice large radius to the slot port at both ends. It'll need to be folded to fit the length in a reasonable size/shape box, but with a 180 degree bend the port open end radius can also form part of the port entry radius. I'll have to do a sketchup later to illustrate what I mean.
I've been sketching (on Post-It notes!) some ideas, and the current favourite includes a nice large radius to the slot port at both ends. It'll need to be folded to fit the length in a reasonable size/shape box, but with a 180 degree bend the port open end radius can also form part of the port entry radius. I'll have to do a sketchup later to illustrate what I mean.
Okay, taking this a step further.
I'm now splitting the two 8s into two subs. Lots of benefits to distirbuted subs, and more amplifier headroom as I have two monoblocks to use. All good.
I've started teaching myself hornresp for modelling slightly more complex vented enclosures, as I've been toying with the idea of using a downfiring port, then using the subwoofer's height off the floor as a tuning variable in-room. That way the bottom of the sub enclosure and the floor will act as a port extension with a large flare - hopefully reducing port noise, and simple threaded spikes should allow the hieght adjustment.
My floor is currently carpeted - so I guess this is similar, in a way, to partially lining a port or transimission line? My hope is that'll absorb some of the hf port noise and result in a cleaner sub - but we'll see how that works out.
I'm a little unsure on moddeling the enclosure, though. This thread http://www.diyaudio.com/forums/multi-way/239691-gap-down-firing-ports-subs-more-less-2.html has lots of helpful pointers - but assuming I build a cuboid enclosure, with a round port that exits the centre of the bottom of the enclosure, the path length isn't fixed as per a cylindrical enclosure.
For example, a 10cm port emerging at the centre of a 30cm x 40cm panel, has a minimum path length from the edge of the port to the edge of the enclosure of 10cm, or from the centre of the port to the same edge is 15cm. Extending a line from the centre of the port out on a radius to a corner of the subwoofer gives a path length of sqrt (15^2 + 20^2) = 25cm. Any thoughts on how best to model this, or how it will behave in reality?
I'll try modelling it as a cylinder with expansion lengths to represent both the short and long paths and see how much difference it makes. My guess is that in the real world it'll behave on the whole like a slightly longer than the shortest path port, with a slightly broader but lower effiicency peak around the tuning frequency - and hopefully with a slightly gentler roll-off too.
I'm now splitting the two 8s into two subs. Lots of benefits to distirbuted subs, and more amplifier headroom as I have two monoblocks to use. All good.
I've started teaching myself hornresp for modelling slightly more complex vented enclosures, as I've been toying with the idea of using a downfiring port, then using the subwoofer's height off the floor as a tuning variable in-room. That way the bottom of the sub enclosure and the floor will act as a port extension with a large flare - hopefully reducing port noise, and simple threaded spikes should allow the hieght adjustment.
My floor is currently carpeted - so I guess this is similar, in a way, to partially lining a port or transimission line? My hope is that'll absorb some of the hf port noise and result in a cleaner sub - but we'll see how that works out.
I'm a little unsure on moddeling the enclosure, though. This thread http://www.diyaudio.com/forums/multi-way/239691-gap-down-firing-ports-subs-more-less-2.html has lots of helpful pointers - but assuming I build a cuboid enclosure, with a round port that exits the centre of the bottom of the enclosure, the path length isn't fixed as per a cylindrical enclosure.
For example, a 10cm port emerging at the centre of a 30cm x 40cm panel, has a minimum path length from the edge of the port to the edge of the enclosure of 10cm, or from the centre of the port to the same edge is 15cm. Extending a line from the centre of the port out on a radius to a corner of the subwoofer gives a path length of sqrt (15^2 + 20^2) = 25cm. Any thoughts on how best to model this, or how it will behave in reality?
I'll try modelling it as a cylinder with expansion lengths to represent both the short and long paths and see how much difference it makes. My guess is that in the real world it'll behave on the whole like a slightly longer than the shortest path port, with a slightly broader but lower effiicency peak around the tuning frequency - and hopefully with a slightly gentler roll-off too.
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