PJ Screen Stage Sub Enclosure Design

[elerium]

DrWho wrote:

Just a few random comments....

The "air spring" of a closed box actually allows for more passband efficiency than having an infinitely large box. It stores energy in a way that comes back and helps the driver move (kinda like how you can make big waves in a bathtub by moving your hand back and forth, but waiting for the wave to come back before you push again....same analogy works when moving your feet on a swing set).

Adding a port is like adding yet another spring, which stores even more energy. The tradeoff is that you get a steeper rolloff...the same is actually true between a "sealed box" and an "IB". The Q of the box (which most of these modeling programs provide) will indicate how much gain you're getting from the stored energy.

Btw, keep in mind that the above is true when designing the driver. When you pick a driver off the shelf, the manufacturer has already chosen the passband efficiency....so the DIY approach becomes more of choosing the right cabinet to get the most out of the driver. In the end, it really comes down to choosing how the low frequencies roll off....you can go lower and steeper, or higher and slower. With a true IB subwoofer in too small of a cabinet, you'll notice a peak before the rolloff, which is the result of the cabinet adding some output that the designer of the driver wasn't designing in. The fact that your driver models the way it does indicates that you're close to an ideal sealed box scenario for that driver.

Just to throw some numbers out there, if you decided to go with a different driver that was optimized for porting in your current cabinet volume, then you could gain another 6dB of output for the same amplifier power. With a tapped horn, you should be able to gain another 10dB. If you ported with your current driver, then you could gain another half octave of deeper extension and possibly increase the power handling to allow for slightly more SPL (since you won't be as excursion limited).

For what it's worth, the dimensions you have to work with at first glance look very ideal for a tapped horn on each side. You could probably even fit a single full-sized folded bass horn too, which should be able to go even louder than the tapped horn.

Was there any particular reason you want to go with a sealed box?

As far as the SPL capabilities, you might be able to count on another 3dB of boundary gain, but at those SPL's you might be coming close to 3dB of power compression too. 115dB is not very loud at all when you're talking low frequencies. If you look at the "modern Fletcher Munson curves" (ISO226), you'll notice that 20Hz at 115dB is about the same loudness as 1kHz at 70dB (aka 70 phons):

Amplified speech (like a preacher at church or any kind of similar venue) will be talking at around 70-75dB depending on where you're sitting in the room. You would have to get over 100phons for things to start sounding very loud (which would be about 130dB at 20Hz).

90 phons should be enough to feel the bass, and I think most movie listening is done somewhere between 80 and 90 phons.

So is 70phons loud enough? If you're gonna watch your movies at the same level you might listen to a preacher speak, then that would be the max clean output of your system. You can always crank it up a bit more and rely on the suspension of your driver to behave like a compressor...you can usually get up to several percent distortion before it starts sounding super horrible.

Considering the size of your room, I think your proposed system is right on the hairy edge of being enough. If you ever want to crank it to impress your friends, then I think you might want to look into ways of gaining a few more dB.

Btw, our hearing at low frequencies is nonlinear....so a 10dB increase in bass is actually a 20dB increase in phons, which means it will have the effect of sounding 20dB louder. Another 10dB gets you up to around 90phons, and a tapped horn should be able to get you there....and probably with a cheaper driver too.

If you go the tapped horn or ported box route, then I would highly recommend a Crown XTi amplifier (about $300 used) so that you can take advantage of the built-in DSP.

FWIW, I built a single tapped horn with a pair of $100 12" drivers that can do just under 130dB at 24Hz, so I definitely know it's possible...and you've got more room to work with too.

Btw, what are you using for mains and what are you powering them with?

Thanks for the very informative response. It's going to take me a little while to digest all of the information you've presented here (this is the first time I've ever heard of a phon). Anyway to answer your two specific questions...

1) I chose a sealed box for ease of design / construction. This is my first DIY speaker project so I thought making a sealed box would be simplest.

2) My main speakers are just some Athena bookshelf speakers right now (S.5 model, play down to 100Hz) but it was my intention to do some DIY speakers to replace these in the future depending on how successful I am with the DIY sub. I've left a fairly large cabinet space in my screen wall column design to accomodate bigger mains in the future. These are powered by a Harman/Kardon AVR247.

I'll start digging into a tapped horn designs and see if that is something that I will be able to pull off. Unfortunately, I just bought a Behringer EP2500 for powering my subs but I got it for a good price ($197) so I could probably sell it and come out fairly close to even if required.

[DrWho]

Yikes, I didn't realize this was your first attempt at DIY. I don't think a ported design would be all too much more difficult, but I can't think of any driver off-hand to recommend.

Are these your main speakers right now?
http://www.athenaspeakers.com/products/s-point-5-satellite-specifications/
Looks like they'll do about 99dB, which makes your 115dB from 30Hz to 100Hz more than capable of keeping up. (115dB at 30Hz is about 100phons). The majority of your bass content will be above 30Hz anyway.

Any idea of what you want to do for the mains in the future?

I'm not sure I would go the tapped horn route for a first design, especially with needing to get up to 100Hz. Unless you're Tom Danley, it's real hard to get more than 2 quality octaves out of a tapped horn. Not to say that it can't be done, but it's definitely not something that can be designed without a lot of testing and tweaking. Sealed and ported boxes (when modeled properly) can easily come within a dB or two of the actual final product.

[GPM]

You're welcome!

Hmm, if the drawing is to scale, this puts the sofa very close to the room's fundamental null around 22 Hz, not good. Plus, it's too low for any useful room gain.

OK, most folks don't start with such an ambitious project. I didn't realize how behind the times I've become. The last time I auditioned PJs, decent ones began at $10 k, so $3 k for a complete big screen, multi-channel HT plus building materials no less! has me wondering if it's time to roll the 65" RPTV that cost considerably more years ago into the computer room since it has no decent resale value and begin anew..........

Hmm, I thought you already had the drivers........ Unfortunately, I'm about as out of date WRT high output sub woofer offerings as I am about PJs, so Mike and others will have to field that Q. Historically I've used multiple low Qts drivers in large EBS or TL cabs. The only IB I've built used four 20 Hz Fs Altec theater horn drivers in a 300 ft^3 false wall chamber, then drove it an adjustable high output impedance amp to adjust its Qtc to best blend in-room.

Regardless, to get the most bang-buck output in the 15-30 Hz BW out of this much cab Vb would probably be with a tapped horn, but the trade-off will be spending some time figuring out how to fold it up in the given layout, then of course physically building a pair of labyrinths.

Anyway, browsing some conic TH designs I've done for others, the Dayton RSS315HO shows promise if its published specs are reasonably accurate with a 10 Hz dual driver TH hitting DD/DTS/THX reference from 15 Hz - up in your 1 pi space layout, so with two you'll have ~6 dB of dynamic headroom which will probably negate the need for a high pass filter. Four drivers aren't much more than your $500 budget and all you'll need is a used 100 W stereo amp to drive them, so should be well under budget overall to more than cover any additional material costs.

GM

[elerium]

DrWho wrote:

Yikes, I didn't realize this was your first attempt at DIY. I don't think a ported design would be all too much more difficult, but I can't think of any driver off-hand to recommend.

Are these your main speakers right now?
http://www.athenaspeakers.com/products/s-point-5-satellite-specifications/
Looks like they'll do about 99dB, which makes your 115dB from 30Hz to 100Hz more than capable of keeping up. (115dB at 30Hz is about 100phons). The majority of your bass content will be above 30Hz anyway.

Yes, those are the Athena speakers I picked up just to get things going ($200 for the entire set). I'm assuming that the rest of your comment means that the two Exodus DPL-15s would be fairly well matched to these speakers in terms of output capabilities. I have watched a movie with just the Athena surrounds and was satisfied with the output level they were delivering. Does that mean that I should be happy with the Exodus DPL-15s performance as well?

DrWho wrote:

Any idea of what you want to do for the mains in the future?

I haven't started looking into what DIY build I would do for the mains but I do have about 2'x3'x2' (WxHxD) space reserved for new mains in my current design for when that time comes. Right now I'm just trying to get the sub / stage design finished up so I can get some LFE going on. Big screen with no LFE is pretty embarassing.

DrWho wrote:

I'm not sure I would go the tapped horn route for a first design, especially with needing to get up to 100Hz. Unless you're Tom Danley, it's real hard to get more than 2 quality octaves out of a tapped horn. Not to say that it can't be done, but it's definitely not something that can be designed without a lot of testing and tweaking. Sealed and ported boxes (when modeled properly) can easily come within a dB or two of the actual final product.

To be honest, I'm somewhat relieved to hear that the tapped horn probably isn't a good idea for a first DIY speaker build. It's nice to have some restrictions so I don't get in too deep. Ported might be something that I could pull off. I'll have to look into some ported designs and drivers and see if I can come up with something.

[elerium]

GPM wrote:

You're welcome!

Hmm, if the drawing is to scale, this puts the sofa very close to the room's fundamental null around 22 Hz, not good. Plus, it's too low for any useful room gain.

The room is too scale within 0.5' measurements. I pretty much rounded all the measurements to the closest 0.5' mark. In regards to your comment, does that mean that there will be no 22 Hz audio present where the couch is?!? How can I fix this?

GPM wrote:

OK, most folks don't start with such an ambitious project. I didn't realize how behind the times I've become. The last time I auditioned PJs, decent ones began at $10 k, so $3 k for a complete big screen, multi-channel HT plus building materials no less! has me wondering if it's time to roll the 65" RPTV that cost considerably more years ago into the computer room since it has no decent resale value and begin anew..........

I should be clear in that I would consider my project to be a very budget build. I'm not sure what I'm putting together would be quite up to the standards of the people on these and other forums. This is what my costing looks like at present:

Projector - Optoma HD65 (720p DLP projector - refurbished) - $650
Screen - 5'x12' Sheet of Wilsonart Designer White Laminate - $200
Speakers - Athena Point 5 MkII Speaker System (center channel and four surrounds - used) - $200
AVR - Harman/Kardon AVR247 (used) - $155
Blu-Ray Player - Samsung BDP2500 (refurbished) - $200
Sub Amp - Behringer EP2500 (used) - $200
Sub Drivers - ??? - $300 (est)
Cabling - Speaker wire, HDMI cable to PJ - $150 (est)
Building Supplies - MDF Sheeting, 2"x4"s, drywall, paint, etc. - $1000 (est)
=================================
Total - $3055 (est)

Items in blue are still just estimates at this point and may end up being more (or less) than anticipated. Everything else has already been purchased for the prices indicated (close enough anyway).

GPM wrote:

Hmm, I thought you already had the drivers........ Unfortunately, I'm about as out of date WRT high output sub woofer offerings as I am about PJs, so Mike and others will have to field that Q. Historically I've used multiple low Qts drivers in large EBS or TL cabs. The only IB I've built used four 20 Hz Fs Altec theater horn drivers in a 300 ft^3 false wall chamber, then drove it an adjustable high output impedance amp to adjust its Qtc to best blend in-room.

Regardless, to get the most bang-buck output in the 15-30 Hz BW out of this much cab Vb would probably be with a tapped horn, but the trade-off will be spending some time figuring out how to fold it up in the given layout, then of course physically building a pair of labyrinths.

Anyway, browsing some conic TH designs I've done for others, the Dayton RSS315HO shows promise if its published specs are reasonably accurate with a 10 Hz dual driver TH hitting DD/DTS/THX reference from 15 Hz - up in your 1 pi space layout, so with two you'll have ~6 dB of dynamic headroom which will probably negate the need for a high pass filter. Four drivers aren't much more than your $500 budget and all you'll need is a used 100 W stereo amp to drive them, so should be well under budget overall to more than cover any additional material costs.

The drivers are about the only component that I haven't purchase yet. As I mentioned in my previous post, I think a tapped horn design might be a bit ambitious at this point in time. I think I would like to try to stick with a sealed box or ported box at best for my first DIY speaker project. I was pretty close to buying the Exodus DPL-15s (almost did yesterday). But I figured I should take a step back and make sure that this was the right way to go. From a budget point of view, it still looks like it is the most economical. However, after getting feedback from you guys, I will look at some ported options before making that decision.

[Mike P.]

I think I would like to try to stick with a sealed box or ported box at best for my first DIY speaker project.

Agreed. If ported is an option I would suggest a pair of 18" subs in the $250 range if it would be possible for you to make them fit in your stage design, or 15" subs if it's not possible. Let us know and we can figure something out for you.

[GPM]

DrWho wrote:

Adding a port............

....so the DIY approach becomes more of choosing the right cabinet..........

If you're gonna watch your movies at the same level you might listen to a preacher speak.........

.......I would highly recommend a Crown XTi amplifier..........

Greets!

A nifty animation for graphically explaining reflex vent action.

Actually, the serious DIYer should begin the design process the same as a pro, i.e. choose the 'close enough' right driver specs to meet the needs of the app, not vice versa. Unfortunately, this requires learning the so-called 'art' of speaker design at a level few have interest in even if their lifestyles afford them the luxury of time, tool expenditures, etc..

FWIW, watching movies at a preacher's sermon level doesn't even meet the reduced reference levels for compressed broadcast TV movie presentations (79-82 dB), so it seems reasonable to me that for a dedicated HT the de facto minimum/maximum recommended peak SPL goals should be 99/109 dB - 105/115 dB/viewing position if for no other reason than those interested in lower SPLs might just as well save themselves the effort and just buy the best HT-in-a-box system based on their budget now that there's a good enough selection available.

Interesting, didn't know they were adding DSP to low end amps now, Good to know, thanks!

GM

[GPM]

DrWho wrote:

I'm not sure I would go the tapped horn route for a first design.........

While I agree that TP/TTQWT/TH design is no trivial pursuit for the newbie and/or casual DIYer without a decent grasp of TL/compression horn design theory and how to choose the 'close enough' right driver for the app, we'll have to agree to disagree that successful wide BW designs be left to the relatively few that do and that they require more testing/tweaking than a sealed or vented alignment if used in a HIFI/HT app now that Hornresp has such a powerful/user friendly TH Wizard to find an acceptable driver for the intended BW and data export capability to AkAbak for fine tuning the basic design.

Since this is the OP's first attempt at DIY audio though, I hadn't planned to leave him 'swinging in the breeze' if he wants to pursue a TH solution and why I made the effort to find an appropriate driver/alignment for the usable 160 Hz BW required for an 80 Hz LFE XO, but I see he's had an overnight change of heart, so no sense in me wasting any more time on it.

GM

[Owen Bartley]

Chris, I'm sure you have quickly realized the dangers of getting more involved with HT. The more you learn, the more you need to know, and the more you know, the bigger you want to go! No matter what project you settle on, you will get great advice here, as I'm pretty sure we have some serious experts on every type of enclosure you could want.

If I'm going to add my 0.02c, I would suggest going for a ported design, along the lines of an LLT. Ported really isn't much harder than sealed, and if you're constrained by height to using 15" drivers, you can squeeze more low end out of them this way. Two high excursion 15's should easily outpace your bookshelf speakers, and keep up if you decide to upgrade eventually. I think we sometimes forget around here that especially when you're just starting out, even a single 15" is going to sound great to you! I remember adding my first sealed 10" sub (borrowed from a car) to my setup and really loving the extra low end extension. For a first time project on a budget, don't worry about shooting the lights out. I'm pretty sure that you will be happy with something fairly basic... at least until the upgrade bug kicks in, but that happens no matter what you build.

[Ricci]

I agree with a lot of what has been posted so far. I think that you should skip the sealed option unless you can afford more drivers and more power. If you did want to go sealed definitely jump up to a pair of the FI IB18's. They'll move a lot more air than the DPL15's.

With your size of a room and the airspace that you have available it makes more sense to go for a pair of huge 18" ported subs. Again the FI IB18's, or better yet the Q18's if you can afford a pair would be your friend. Either of those drivers in about 700L each with a 10" port tuning it to the 15-16hz area and about 1000w should do pretty well paired up.

A TH or other horn type could be very sick as well, but I don't know about designing and building a pair of huge ones into a stage as a first DIY build is a good idea. They are quite a bit more complicated to both design and build over a normal ported or sealed cab.