Best way to handle 25hz & down 2 Tempests

[johninCR]

I have 2 of the original Tempests to use, a 200sq ft room (20 sq M), and space isn't an issue due to some dead space adjacent to my room. I need to handle only 25hz and down with this sub as 25hz and up is dipole. The room is well sealed, so I can count on room gain. I need that bottom end extension just for HT. What is the best way to accomplish this ?

Driver specs-
Fs 18.8
Qts .38
Vas 317
Sd 779
Xmax 16
Power max 750

 

[paulspencer]

Dipoles down to 25 Hz?! Why so low?

Sounds like a job for a pair of big EBS subs designed to get down to 15 Hz. I have a RTA permanently in my system and I notice some movies have more output in the bottom octave than the rest - this occurs in quieter but tense moments, often in thrillers, horror movies, sci fi - movies like Blade, Resident Evil. This is where you will really appreciate the depth, as it gives a dark feel and a sense of weight and depth - dark and ominous. My RTA doesn't show below 20 Hz, but I suspect that quite often the SPL in these moments actually continues down well below 20 Hz, however you have to determine the point at which you just don't have enough displacement to get an impact.

 

[zobsky]

for that extra sense of "impact" and a free bass shaker effect , you could try positioning the subs nearfield.

for instance, i place a single shiva / ported EBS / 17Hz sonotube in the empty space behind my couch in a 150 sq. ft room and believe me, it does more than i need, even with only 100W.


your amplifier power requirements are also reduced to an extent, with the sub closer to the listener.

 

[simon5]

If you don't care about space taken, I would use something like two 15 cu.ft internal boxes, then you tune around 12 Hz. You would get around 123 dB from 11 Hz to 25 Hz or 129 dB corner loaded (but you will probably have some room loss at such a high SPL level... hehehe). You would need only a single 400W amplifier for both subwoofers, or dual 200W if you prefer.

 

[DanWiggins]

OK, how about getting crazy...

Do a 6th order bandpass box (drivers in the box, two chambers, each ported). Isobarically mount the two drivers, wire voice coils of each driver in series, then parallel the two drivers...

Make the rear chamber 340 liters (12 cubic feet), and tune to ~11.5 Hz with a 4" diameter by 17" long flared port.

Make the front chamber 56.6 liters (2 cubic feet) and tune to 28 Hz with a 4" diameter by 17" long flared port (same as the rear chamber).

All output will be via the two ports. Goosed with 200W into the 8 Ohm nominal load, you should have more than 110 dB SPL in-room, and a bandwidth that is +0/-3 dB from ~11 Hz to 40 Hz.

How's that strike ya?

Dan Wiggins
Adire Audio®

 

[GM]

Greets!

'In for a penny, in for a pound', so I'll 'up the ante' :

For such a low, narrow BW, only a BP makes sense to me also, especially since you have a spare room available. With a ~IB rear Vb (preferably at least 224 ft^3), a ~20 ft^3 filter chamber and a ~10.125" dia. throat (to keep the CR reasonable) placed as close to the floor/wall corner junction as practical. Plug the info into Hornresp to fine tune/get an idea how much gain there will be, with the understanding that at some point the corner typically becomes increasingly transparent if not built like a bomb shelter, so will probably begin to roll off quicker than predicted.

GM

 

[simon5]

Hi, I want to know why you guys prefer bandpass since the group delay is worse and the output is lower?

Why not go even more crazy? Two dual chamber bass reflex boxes. 10 cu.ft main chamber with 5 cu.ft small chamber, using 6 four inches diameter ports 28 inches in length. One chamber will be tuned around 10.5 Hz and the other around 14.5 Hz. I guess this would beat bandpass IMHO.

 

[johninCR]

I like all those ideas, but Dan's will prove the easiest and very little work because I already have a sealed cube for a 15. I think it's 90L, so I'll need to shorten the port. I also have an EBS box for a Maelstrom which I think is 360 liters (why aren't those box designs on the Adire site any more?). I was wondering what to do with it anyway since my Maelstrom is part of my dipole bass. It already has dual 17" by 4" flared ports, so I can play with resistance in the port(s) to tune it however I want. I also already have the other flared port.

It won't look pretty, but will be hidden by a false stage. The only work to to is fix one baffle to fit a Tempest, add a port to the cube (the hardest part), and join the boxes.

I'm leaning toward making a sizeable airspace between the 2 drivers which really smooths out the response of the Decware Housewrecker, a 6th order BP design. It I do that, then I can add a removeable plexy window for viewing and driver access. It's a no pressure zone, so plexy is OK. Then just stand this monster up in the one unused corner which just happens to have concrete walls.

The big question is, should I go all the way and intentionally make it look like a cyclops robot? It will be such a monster that it deserves a name, maybe Iron Man, and initiate him with some Sabbath cranked up on the HT system.

This is going to be fun. I'll have to warn the neighbors so they don't think we're having an earthquake because no way 10hz is staying in the room.

 

[simon5]

Just checked your box volumes in WinISD Pro, you could leave the volumes like they are, if it's exactly 360 liters and 90 liters internal, it will work well at the frequency Dan recommended you to tune to.

You would need to use a 17 inches by 4 inches port for the 360 liters part and a 10.5 inches by 4 inches for the 90 liters part.

 

[johninCR]

Hi, I want to know why you guys prefer bandpass since the group delay is worse and the output is lower?

Why not go even more crazy? Two dual chamber bass reflex boxes. 10 cu.ft main chamber with 5 cu.ft small chamber, using 6 four inches diameter ports 28 inches in length. One chamber will be tuned around 10.5 Hz and the other around 14.5 Hz. I guess this would beat bandpass IMHO.

If I understand group delay correctly, it won't be an issue because I have a variable distance setting for the sub on the receiver, so I can adjust timing that way, plus it is only going to be used for HT. Music will be 100% OB.

 

[johninCR]

Simon,

Thanks for that port length on the cube.

If I really can reach my goal of 25hz with OB, then I'll try an easy 4th order BP first. Clamshell the Tempests on the cube and put it face down directly over 18" driver cutout on the big box. With the 2 drivers the cube will weigh close to 200lbs, so just some foam rubber will make a good enough seal for testing. Do you think iso loading a 90L sealed rear chamber will get me a wide enough pass band from 25hz down with the 360L front chamber? If I can avoid cutting my cube, I'd like to because it has a beautiful layer of 1" hardwood laminated over a 3/4" well braced plywood box.

 

[GM]

Hi, I want to know why you guys prefer bandpass since the group delay is worse and the output is lower?

Why not go even more crazy? Two dual chamber bass reflex boxes. 10 cu.ft main chamber with 5 cu.ft small chamber, using 6 four inches diameter ports 28 inches in length. One chamber will be tuned around 10.5 Hz and the other around 14.5 Hz. I guess this would beat bandpass IMHO.

Greets!

First off, in the intended BW our hearing acuity isn't good enough to notice the high GD and will be decayed enough higher up so that it's not an issue. Second, my 'BP' is a conical compression loaded horn that morphs into a catenoidal one once the expansion hits the ceiling, so GD isn't an issue.

WRT to peak SPL capability, mine has a great enough compression ratio and much lower vent losses to achieve considerably more efficiency than any of the other designs posted so far, so in theory will 'blow their doors off', including yours.

Like I said previously though, the room will probably 'bleed off' a considerable amount of all this craziness, so the differences between the various designs probably won't be all that much. Indeed, had I known about the other boxes JCR has lying around, I'd of just designed around them.

GM

 

[johninCR]

Indeed, had I known about the other boxes JCR has lying around, I'd of just designed around them.

GM

I wasn't thinking about using those boxes until I saw the dimensions Dan recommended. That made me realize very little construction work would allow me to utilize the only significant boxes I have laying around gathering dust, which makes it pretty much a no brainer on the way to go.

 

[simon5]

WRT to peak SPL capability, mine has a great enough compression ratio and much lower vent losses to achieve considerably more efficiency than any of the other designs posted so far, so in theory will 'blow their doors off', including yours.

Oh, I agree with you on this, a horn is way better than anything else in the SPL department. My question was only about bandpass. Dan said bandpass and you said that's the only thing making sense to you too.

Then I check and see exactly zero advantages for a 6th order bandpass box over a ported box, that's why I asked my question. Why prefer something who looks inferior to me? There's probably a reason that I don't know? Especially since you are both good subwoofer designers with alot more experience than me.

If I understand group delay correctly, it won't be an issue because I have a variable distance setting for the sub on the receiver, so I can adjust timing that way, plus it is only going to be used for HT.

Group delay is not constant delay over the whole subwoofer bandpass. You can't compensate for it, unless you can set a different delay at each frequency, something like a 10 band delay EQ, which I think isn't in use alot yet! The group delay is usually a big peak of delay just a few Hz under the tuning frequency.

Yes, you don't need to cut your boxes, WinISD Pro simulations are really close to Dan Wiggins recommendation. Since the boxes are bigger, you'll have a bit more efficiency.

 

[johninCR]

I've come up with a new idea. I've had good results with the Decware housewrecker subwoofer, a 6th order bandpass with a sizeable sealed chamber between the two iso loaded drivers. There is no pressure in the chamber and if you opened it up there should be no sound.

My idea is this, to mount the small box on top of the big one with the driver facing directly at each other, but raise the small box 8-10" above the larger box on 3 legs and leave that space open. That way the drivers are visible, I can easily get to them if necessary and even up close I believe there would be zero sound coming from that open area even at high output which would be a cool effect.

The only side effect I can see is that the loose iso coupling wouldn't give me the halving of Vas, but I have a feeling that has no effect on response with a 6th order bandpass OR am I all washed up and the different chamber sizes and tuning will make the 2 drivers behave differently resulting in output from the center if I leave it open.

 

[Mikey p]

"" There is no pressure in the chamber and if you opened it up there should be no sound.""

Sealed or not a woofer produces soundwaves on both sides of the cone.

"" I believe there would be zero sound coming from that open area even at high output which would be a cool effect.""

Wrong. See above.

""The only side effect I can see is that the loose iso coupling wouldn't give me the halving of Vas""

There's no such thing as "loose iso coupling". The drivers are either coupled together or they're not. Unless there was a flaw during construction causing an air leak.

""....resulting in output from the center if I leave it open.""

Now your on the right track.

 

[johninCR]

"" There is no pressure in the chamber and if you opened it up there should be no sound.""

Sealed or not a woofer produces soundwaves on both sides of the cone.

"" I believe there would be zero sound coming from that open area even at high output which would be a cool effect.""

Wrong. See above.

""The only side effect I can see is that the loose iso coupling wouldn't give me the halving of Vas""

There's no such thing as "loose iso coupling". The drivers are either coupled together or they're not. Unless there was a flaw during construction causing an air leak.

""....resulting in output from the center if I leave it open.""

Now your on the right track.

Mikey,

Yes the cone moves, but the other cone is moving too, but in the same direction because it's fed an identical but out of phase signal. The distance between these 2 equal but opposite sound sources is insignificant in relation to the wavelengths produced so they will net to zero except to the extent that the chamber differences affect cone movement, which I believe is the hole in the idea.

Just take 2 speakers and face them together say 1 foot apart. Switch the wiring polarity on 1 and play a mono low frequency signal. Magic, no sound. Haven't you ever used this approach to break in drivers quickly using high volumes without disturbing anyone? In fact, with the kind of frequencies we're talking about, the speakers wouldn't even have to face each other for silence.

That would make an interesting test. Identical drivers + 2 differently tuned ported cabs side by side with one out of phase + a mono signal. See how closely they imitate a 6th order bandpass enclosure.

 

[Mikey p]

johninCR , I'm really looking forward to your results. Let us know how you make out.

 

[SpeakerBob]

Yep - I'm interested in what you ended up building too.


Rob.

 

[johninCR]

I did a test run since I already had the boxes laying around to build Dan's suggested isobarik 6th order BP. It worked fine and as predicted.

I decided that I really didn't need that much potential output and have opted for a dipole arrangement consisting of 2 sealed shiva's in a coffee table immediately in front of my seating position and 2 sealed tempests under my screen firing out of phase.

The initial tests of the dipole arrangement worked perfectly and the room is taken almost totally out of the equation, leaving much cleaner bass. Due to the 3.5M separation distance I don't lose output going dipole. In fact, the out of phase wave reinforces the primary wave down to 17hz, so it's just a matter of flattening the sloped response which I have to do anyway with the sealed alignment.

To me, I give up room gain plus about 10db of max potential vs the BP alignment to achieve flat down to 13hz and I'll need to use the 2 additional drivers and more power. However, I realized I'll never use 120db at 15hz anyway, plus the dipole really keeps the sound in room drastically more so I'll be able to listen louder without disturbing others and/or causing structural damage to the house, all with better sounding bass.