DIY Proscenium Subwoofer (1 Viewer)

[Brad E]

Joe, we think alike. I have almost the exact same screen. Only diff is I used 1x3's all the way around and blackout cloth.
For $60 CAD, I can't complain.

 

[Paul Spencer]

Very impressive set up you have there!

Do you have more space behind your screen? My first choice would be IB given such extreme VD. If not IB then a sealed box as big as you can get as this will. reduce the damping effect of the sealed air spring. This gives the best transient response that the driver allows for.

Regarding power handling, it's pointless to talk about amplifier power that is appropriate without considering:

* drivers thermal power rating
* excursion limited power handling (which is different for IB/vented/sealed)

In a vented box with a rumble filter which keeps the max excursion above tuning, the limiting factor may in fact be the thermal rating. In an IB you need less power due to the absense of damping, and this is true to a lesser extent in a sealed box, especially large ones. In this case, the power handling will be determined by excursion.

Also, it is pointless to discuss a recommended vent diameter for a driver as there are too many variables that need to be considered, which will affect whether it is suitable or not. One person may recommend a 6" vent, but in fact another with a 4" vent may in fact achieve a better result if better implemented. In fact, to avoid turbulence, there are two essentials to consider:

1. vent velocity
2. flare radius

If you are able to limit the vent velocity to 34m/s with a decent vent flare (1" min radius) you should get decent results.

Are you able to make your stage higher? It seems a shame to face those beasts down, which will cause cone sag over time, limiting excursion due to the impact on the suspension, as well as altering the parameters. Also you miss out on some volume as well as the wow factor of being able to see such extreme drivers and extreme excursions.

Do you have another space to vent the volume to so that you can get a true IB? (rather than a very large sealed box?)

 

[SteveCallas]

Cone sag due to a down firing mounting? I've never heard of this, and there are numerous subwoofer manufacturers who have down firing subs. Anyone else know if there is any truth to this?

 

[Paul Spencer]

Steve, it is an unavoidable fact. Adire used to have a paper to calculate if the sag was acceptable, however, even in that case there will be some sag over time and compromises in performance. It's up to the individual to consider how much compromise is acceptable. The Adire paper suggested 5% of xmax as acceptable. John from AE speakers posted on this on the cult of the infinitely baffled forum, commenting that you will not normally be covered for warranty in downfiring configuration.

 

[Joe L.]

Paul,

A few answers to your questions...
Unfortunately, no other adjacent space... only adjacent space is up, and if you think my wife would be upset at loosing some storage space behind the screen, I think she would be even more upset at drivers poking up through the floor in the guest bedroom above the theater. :frowning:

I'll probably post another 3d design possibility tonight if nothing else takes priority.

Thanks for the feedback.

Joe L.

 

[Joe L.]

I spent some more time using the 3D graphics program and came up with an alternative for my Proscenium-subwoofer design:


I tried to angle the baffles as much as possible and still fit the drivers within the 15 inch height targeted.

This version has the drivers firing into a plenum and has the advantage that the opposite movement of the cones should cancel out to some degree any vibrations.

With a bit of black grill-cloth across the entire front nobody will suspect a monster subwoofer lurks below the stage.

Joe L.
P.S. This is almost as much fun as making sawdust, but much easier to change my mind mid-design.

My original design had both woofer downfiring

 

[SteveCallas]

Here is something to consider. Is your stage physcially connected to your side or back walls? If it is you might end up losing a lot of energy through transmission. On the other hand, if the floor underneath the stage is concrete, you will probably get some great low frequency gain.

 

[stephanX]

id wager the effect of a concrete vs non concrete floor pretty much negligable, he has enough displacement to get 105 db down to 14hz if he wants it(just pulling a figure out of the air, but it would definately be possable with those things).

Id go for the weird triangular design, as a bonus it will also be stronger. It wont help too much with cone sag, and it will help but not cure mechanichal vibration. If youre really concerned with vibration, maybe build a paralellogram shaped opening, that should be as good as the drivers facing each other at right angles*.

Vibration cancellation wise*

 

[Paul Spencer]

I have the Adire paper (PDF) which has the calculations for how much sag you will get, if anyone is interested shoot me an email.

Regarding storage, I find putting things you don't need easy access to in the roof space (Australian version of attic) is a good way to free up space.

I'm not sure that this "cancelling vibrations" concept really works as you are thinking. I think in fact the vibrations will not be reduced in magniture, but given that the forces of the drivers are symmetrically opposed, it will prevent the whole stage from moving. Everything that is mechanically and acoustically coupled to the drivers will vibrate probably without any reduction on magnitude. I doubt this results in any noticeable difference, however it certainly does no harm and is probably better all other things being equal.

What certainly does make a difference is push pull mounting ie. mount one of the drivers basket forward, the other cone forward, then reverse +/- on one of them. This increases linearity and reduces distortion, as cancellation occurs. Even though XBL2 drivers are very linear even at their xmax, if you ever see the acual measured BL curve and suspension linearity curve, you will notice that the forward and backward motion are different - the driver has more xmax and is more linear in one direction of cone travel. If you have them both in phase but with each moving in different directions relative to their respective magnets, the combined effect is that they are more linear. I believe there is more to it than this, but this is my (fairly basic) understanding of how this works. You may notice Linkwitz mentions this on his site regarding dipole bass driver mounting. I'm a little surprised that this is not more commonplace in IB installations, as it is the one application where there isn't normally a visual penalty for this mounting! M&K use this extensively, but in their box subs they do awkward things to try to make it look acceptable. In your case, you won't even see it.

With your new mounting idea, what you might do is change one of the drivers as I've suggested, then periodically (say every 6 months or year) change the drivers - rotate their mounting 180 degrees, then every 2nd time you change things you might switch the drivers. Your mounting system places some stress on the suspension in two directions, both normal and perpundicular to the direction of cone travel.

 

[Joe L.]

SteveCallas,
The current stage is not connected to the walls of floor. It is currently simply resting on the carpeted floor. I plan on the same for the Proscenium-subwoofer. It will simply sit on the carpet below the screen and not be attached to the room walls.

stephanX,
You are correct in that with enough displacement I will be able to get fairly decent SPLs at most frequencies I'm interested in reproducing. I'm used to a frequency response that goes pretty low. Below is the un-equalized frequency response of my current 15" Tempest based sonosub. I'm not sure it can get too much better down low, but with the two 18" Avalanche drivers at least I know I'll not run into any xmax limits when watching a movie.

My current 15" Tempest sonosub response prior to equalization with BFD equalizer (crossover set to 80Hz)

Paul Spencer,
I'm not trying to eliminate all vibration... that would be nearly impossible. I just don't want the stage "walking across the floor" when things get lively in a movie. I will be bracing the Proscenium-sub enclosure internally so it will not be a light-weight sub... just being cautious as I've read of other folks subs walking across their floors.

I know the driver orientation is not perfect and my sub design skills are that of a beginner compared to many on this forum... with a bit of luck it will sound pretty decent anyway as long as I make the enclosure sturdy and get the internal volume correct. Not sure I'd rotate the subwoofers periodically, but who knows, I do tighten up the bolts on the Tempest sub every once in a while so it is a good suggestion. Biggest problem will be moving the stage-sub to get to the drivers. I expect it to be pretty heavy.

Oh yes, the attic is full with holiday decorations too. My wife likes to decorate the house around most holidays. If we ever move, the house needs to have a 3000 sq-ft basement, a 3000 sq-ft attic, and perhaps 1500 sq-ft to live in..

Joe L.

 

[Joe L.]

Paul Spencer,

The white paper to calculate "sag" was still on the Adire Site. I plugged in the numbers and it resulted in a sag of .97mm (3.59%) and it suggested values under 5% make the driver suitable for downfiring. I think I am fine in that respect especially since angled as I am proposing, the downward stress is even less.

I need to see if I can orient the drivers as you suggested. Might be interesting. The clearances involved might make it difficult.

One advantage I do have with two 18 inch drivers is that it is unlikely I'll ever have to test the BL curve linearity at xmax. I expect that by using two they will be loafing along and hardly moving most of the time. In fact, if they ever need to reproduce anything near xmax it should be one heck of a movie soundtrack.

Chad does not list the SPL/efficiency specs on his web-page for the 18" Avalanche but it is often mentioned they are very efficient down low.
I used the calculations as given in the white paper at the Adire site to calculate the efficiency based on Qes, Fs, and Vas and it calculated out to 89dB SPL @ 1W, 1m. I'll need to run some more simulations in Unibox to see if I have any other excursion limits I need to be aware of.

Joe L.

 

[Paul Spencer]

I think you will find your mains will not keep up. I have a pair of 12" sealed AV12s and my 6.5" MTM mains really can't keep up with them, even though I use a 4th order active xo on them at 80/2.4k. I get about 6db more out of them than the passive version, still the subs go further. While it's nice to have subs that will go a bit further than the mains (I prefer 50-10 db more output capability in the subs), your mains will be the limiting factor I'd say.

I wish getting accuraty & high output were as easy with mains as it is with subs!

 

[SteveCallas]

When you get to the point where your mains can't keep up, you either have a huge room or are losing your hearing. I can see getting large bass peaks every now and then in demanding scenes, but if you're volume is set so high that the mains would be trying to play at that level the whole time, I dunno....maybe look into headphones? =)

 

[Paul Spencer]

Steve, if your goal is to match a comercial cinema at home, ie serious home theatre, you will find most hifi speakers tend to fall short of the goal, unless your room is very small.

Let's consider a typical hifi floorstander with a pair of 6.5" midbass drivers, dome tweeter in a vented box, the midbasses with 4mm xmax, efficiency of 88db 1w1m.

To achieve 105db peaks at a distance of say 3m, you need 114db @1m.

1w >>> 79db @ listening position
10w >> 89
100w > 99
200w > 102
400w > 105
less 3db to allow for power compression >>> 102db

It's pretty obvious the goal won't be achieved here, you aren't likely to use an amp with that much power, and driven that hard they are going to sound harsh so you would end up wanting to turn it down. Also, they will run out of excursion capability, you won't get past 105 db @ 1m in the midbass, however you can overcome this by crossing to the sub at 80 Hz.

If we contrast this with high efficiency speakers with at least 95db 1w1m then you can easily reach 105db with 100w and there will be less power compression and stress on the system.

I'm not talking about ridiculous levels here, but merely achieving what we all want to do - match or even better the experience of a comercial cinema. Of course many of us accept less output because we don't have a room that can contain the sound without disturbing others. But for a purpose-build HT, I'd be aiming for reference levels from 20Hz - 20k. What is the point of having a 120db subwoofer if your mains can't reach 100db?

 

[SteveCallas]

I'd believe your much more likely to hit 105db peaks and beyond with a sub than you are with the mains because LFE channel is usually set higher than the speakers during calibration and a lot of the dynamics are in the LFE channel. That said, if you intend to play movies at 0db or higher, I'd agree that you need mains with a higher sensitivity than 88db.

But I just cringe at the thought of hearing the dragon screech in LOTR (I think part 3) at 105db, lol.

 

[Joe L.]

On LOTR part 3 when the flying dragon's swooped down from the sky the LFE effects/overall sound field were great. You could feel each beat of the wings...
So could my cat... it ran... as fast as it could from the screen, thinking it was being attacked.

 

[Greg Monfort]

Greets!

I only did a quick scan due to super slow loading, but I didn't see any comment WRT to the high tuned bandpass front chamber you've created. Its output may only be partially attenuated by the XO and while the carpet will damp its harmonics some, there may still be enough BW to audibly comb filter with the mains.

GM

 

[Dan Wesnor]

Joe,

I'd be worried that there's not enough area in the holes the woofers are venting into the room through, and that you might end up with some weird quasi-horn loading, or some other weirdness due to front-loading of the woofer. But, hey, you've already got the woofers, for you mays as well put it together and see if it works!

 

[Joe L.]

Dan,

I understand your concern, but two Avalanche drivers feeding a "triangular shaped plenum" is not too much different in my mind than four drivers feeding a square cross-section plenum as commonly used by many folks who have built infinite-baffle subwoofers. I am going forward with the project. Yesterday, I purchased most of the wood needed for the enclosure.

Joe L.

 

[SteveCallas]

Joe, have been following your progress on the CoIB forum, and would just like to ask that you do keep us updated on the construction in this forum, with pictures if possible. I'm very interested in seeing how this one turns out.