Digging deep, 4th order bandpass subwoofer build

[Welder]

Looks like an interesting Home Cinema project Mark

A bit of warmth and comfort is essential so having some room to dedicate to such projects is great.

[Reid Malenfant]

These will be the left & right bass speakers for the new enclosures John 2 drivers per enclosure, probably mounted on the side to keep the front baffle as narrow as possible. I was going to build subs but decided against it, might as well have the whole lot in a couple of boxes rather than 4 boxes.

If you know anyone that wants some cheap 15" high excursion woofers send them my way. No point in keeping the old ones, just more clutter in the house

[Puffin]

I have built a coupled cavity sub in the past to very good effect and I still use a B&W acoustitune Coupled cavity sub. I think they sound as good as any other. The Acoustitune was originally a passive design with different length ports to tune the lower frequencies. I converted it to active operation.

[Reid Malenfant]

Hi Rob, yes i'm in 100% agreement if they are done right Done wrong like a lot of other speakers they can sound "one note" in nature.

Blimey, you just reminded me of that acoustitune thing! I has been years since i have heard that mentioned & if i remember right the only time i heard it mentioned was a test in a magazine?


I'm not sure at all if these new drivers will be used in a coupled cavity, the others had to be or a sealed box simply because of the high driver Q. These new ones are spot on for anything, be it reflex, coupled cavity (4th order bandpass) or whatever. Some fiddling realises i can now squeeze 115Db 1M @ 10Hz with the SDX15 woofers

That's bloody crazy Well i guess earthquakes will feel more realistic!

[Puffin]

Nice thing about the B&W is the proper cast frame drive units. Unusual for a budget sub.

10 Hz, enough to mske you feel very queasy at the least and result in death at the worst

[Reid Malenfant]

Nothing like pushing the boundaries I'm just about dead sure i'll never hear that kind of SPL at 10Hz as it'd be too much, but it also means that whatever volume level i listen at the speakers will take it & at a reduced level distortion will be negligible

It's not about making it go loud, but about getting the least distortion at any level i care to listen at..

I could happily listen at 100Db & know that absolutely nothing is stressed no matter what i throw at it, the resulting sound will have far less distortion than just about any speakers available unless you happen to have £50K+ to throw at some

[Reid Malenfant]

Well i have been messing about & got some results. If i happen to make a pair of 300L enclosures which is te same internal volume as ATC300A they'll certainly be blown to bits

These graphs are for a 600L enclosure with 4 drivers, though i'll possibly be building 2 x 300L enclosures, i really haven't made my mind up yet I don't have to please anyone except myself so a pair of coffins in here isn't a problem. All i have to decide is would i prefer to build a sealed box with fairly low Qtc (really clean bass), a reflex but very low tuning (should be damn clean still due to the low tuning frequency) or a 4th order bandpass

I think i'm going to go the reflex route

First up maximum SPL @ 1M, 129Db is crazy I much prefer looking at the 115Db @ 10Hz Looks like well over 122Db @ 20Hz The blue trace is the 4 x 9" carbon bass mids i'll use for upper bass/lower mid duties. Green is in this pic combined with a red trace but you can't see the red trace, this is the 600L reflex tuned to 11.5Hz. The yellow trace (looks white ) is the same 4 SDX15 drivers in a 600L sealed box.



After a bit of fiddling with various filters that won't be difficult to make you can see how the SDX15s will combine with the bass/mids & cross over at 68Hz. As you can see from the box on the right system input power is 320W which results in 118.9Db @ 29Hz but with a -3Db frequency of 11.88Hz, -6Db @ 10Hz There would be 320W RMS with no boosting, so assuming there is no silly frequencies (below 20Hz - which i love) the whole system can be driven to 129Db, however if there is silly stuff i can only go to a paltry 119Db



Here is what the drivers would be doing being fed 3KW RMS (forget the bass mids [blue trace]). As you can see they have a 30mm linear excursion but a 40mm mechanical limit. Now most people say that due to capacitors in equipment that a high pass filter isn't needed in an LLT sub. I guess that the fact that i'll never be able to handle the volume level that these things could kick out would make it even safer. But as you can see, with a high pass filter i can restrict the excursion to 39mm even if a full power signal at 1 - 5Hz came along (red trace again).



I guess if i try a single one of these subs in a 150L enclosure & feed it every bass heavy CD & blu ray i'll find out if i need that filter or not...


ATC SCM 300 AT eat your heart out Totally out monstered

[MartinT]

Anyone heard of Graham Holliman?

[Reid Malenfant]

Sounds like it's a bit room fussy

These won't be They'll generate the pressure just like the response shows, well, as long as i make the enclosures well & you can be assured of that. Something tells me that they'll end up at about 200Kg each loaded with the drivers. Concrete floor, nothing to worry about They'll sit on spikes to

Just don't get hand trapped under spike

[Reid Malenfant]

Oh well, as usual these things end up a pain in the proverbials

If i port this thing the largest port i can get away with is 6" diameter for one speaker with two drivers & 300L internal volume. The port ends up 129cm long which is pushing things & the first port resonance is only 130Hz which is a bit close to 68Hz which will be the crossover frequency. Also the port air velocity is just a tad high at nearly 40M per second = noisy

If i use two passive radiators per speaker the ABRs will run out of excursion. I can't fit more ABRs even though i can buy them from the Netherlands simply because I'd need more mass than the PRs can handle to tune them low enough in the enclosure

I have considered using both passive radiators & a port, though i have no idea how they'd get along with each other? If anyone happens to know then please let me know!

So it looks like i'm going to have to build my own passive radiators It's not as difficult as it first appears, i have done this before though i can't honestly say i'm relishing the idea of doing it again! What would be used to do two large PRs would be a 1M^2 sheet of 5mm acrylic, 4 x 21" motorcycle innertubes, 2 new bolt up schrader valves, lots of evostik glue, MDF, woodscrews & M4 bolts, nuts & locking washers

In true Blue Peter style i'll show you one i made earlier I'll get my camera & fight my way into the garage...