AVS Forum banner

Will This Space Work For An IB

1K views 23 replies 10 participants last post by  Thomas-W 
#1 ·
I'm still in the planning stages of my HT, and one of the big decisions I'll need to make is whether or not I'll be able to use an IB sub. My room is huge, but once I pack in a false wall, two rows of seats, and bar seating at the back, I end up a little limited for space for an IB. There is a place in the back that looks promising, but I have a few questions about it.


1) I've read that it's not good to put your IB at the back of the theater. Is this true, and why? Also, I thought I could put another sealed sub at the front to help with room modes. Is it OK to put the IB at the back if used with multiple subs?


2) Is my space big enough? I understand it needs to be around 7xVas (IIRC), but I'm not sure what the units are on the subs I've looked at (mm, s.f., square inches, etc). My space looks like it will be a little less than 247 c.f. once you account for drywall (2.5 x 11 x 9). Is that big enough for a dual IB setup?


3) Could I use the space under the riser to add volume (rather than a bass trap), or will I end up with a T-line?


Here's a pic of the space to help. The red line indicates the space I would like to block off for the IB.



Thanks!
 
See less See more
1
#4 ·
Vas is 13.735cu ft for that driver. If you go x10 Vas then you need about 550cu ft according to their guidelines. Here is a model of 4 fi drivers in 240cu ft with 1600watts.

Attachment 240274


If you use a 1000w amp, spl drops about 2db.

http://home.comcast.net/~infinitelyb...0timesVas.html

Quote:
The Adire Shiva White paper talks about an IB sub being 4 times Vas, and says that's where Qts = Qtc.


If we run the numbers to calculate the closest reasonable box volume where Qts = Qtc, we need to increase the box size to 25 times Vas. With a box volume 25 times the Vas, then the Qts and Fs will be raised only by a factor of 1.02. One can safely say at this point Qts = Qtc.


Now a volume of 25 times Vas is impractical for most people's installations, as a compromise we use 10 times Vas as our baseline.....

Note that the 10 times Vas although recommended isn't etched in stone. Why? Well even the smaller 4 times Vas volume IB, will have better SQ as compared to a standard small sealed box. This is a function of there being less 'box' induced colorations.

 
#5 ·
I built an IB with 4 FI 18's in the rear of my room. My thinking was that the 2 sealed atlas 15s on LT plate amps in the front of the room would work well with the IB in the rear. Well, it sounds great but I have to keep the gain really low on the IB to not overwhelm the subs in the front. The IB drivers are not using hardly any of their potential. Consequently I'm planning on selling the FI's and adding a few more sealed subs to the room.

Hope that helps,

Brad
 
#6 ·
I agree with gracey - I would be a bit hesitant to have the IB's in the rear of the theater. I think you'd have a hard time balancing them with other subs in the front. I think you would be better off using 2 1/2 feet by the screenwall and putting them up front and possibly using the rear chamber for some sealed subs or a smaller IB.


My room was about 30 feet in length. I built a 4 foot chamber in the front of my room leaving 26 feet which was enough to have 2 rows of seating with 3-4 feet of space behind the rear row. I also have almost 3 feet of space behind my AT Screen. I think with your room length you could do a 2-3 foot IB chamber at one end particularly if you are not doing a false wall for the screen. I think you could possibly make the 2 rows and bar work in that size room. If I wanted to make it work I could have easily made a bar fit in my room.
 
#7 ·

Quote:
Originally Posted by Mrkazador /forum/post/21769128


Vas is 13.735cu ft for that driver. If you go x10 Vas then you need about 550cu ft according to their guidelines. Here is a model of 4 fi drivers in 240cu ft with 1600watts.

Thanks for modeling this for me! I need to study a bit more to get a comparison of how other subs would compare to this. Specifically, I'm looking for that sub 20 Hz response. I was under the impression that an IB is a good way to get there.

Quote:
Originally Posted by gracey /forum/post/21769287


.......Well, it sounds great but I have to keep the gain really low on the IB to not overwhelm the subs in the front. The IB drivers are not using hardly any of their potential. Consequently I'm planning on selling the FI's and adding a few more sealed subs to the room.....

Good info! Can you elaborate on how the front subs are causing problems? From what I've read, if the crossover is set below 80 Hz it shouldn't be possible to localize the sub. With that in mind, I thought the other subs in the room would be there only to fill in the response at higher frequencies where room modes cause problems (say around 40 Hz or so).

Quote:
Originally Posted by stockmonkey2000 /forum/post/21769383


...... I think with your room length you could do a 2-3 foot IB chamber at one end particularly if you are not doing a false wall for the screen. I think you could possibly make the 2 rows and bar work in that size room. If I wanted to make it work I could have easily made a bar fit in my room.

I'm planning a false wall with an AT screen (my drawing is a bit out of date
) I may have to rethink my layout to gain some room for an IB chamber in the front, but I'm really hesitant to give up that space. I'd probably be more inclined to go with some sealed subs or even something like a tapped horn if I decided to put the big subs at the front. It's certainly something to think about, though.


Could you offer anymore insight on what makes it difficult to balance the subs when they're in the rear?


Thanks again for the feedback, I really appreciate it!
 
#8 ·

Quote:
Originally Posted by J_P_A /forum/post/21770142




Good info! Can you elaborate on how the front subs are causing problems? From what I've read, if the crossover is set below 80 Hz it shouldn't be possible to localize the sub. With that in mind, I thought the other subs in the room would be there only to fill in the response at higher frequencies where room modes cause problems (say around 40 Hz or so).

At this point my system is pretty basic. I'm relying on nothing but setting this room up by ear. No mini dsp, no measurements (yet). I've treated the room with 4 bass traps, and first and second reflection absorption. I would guess that if you had more elaborate processing and measurements you could probably set up a rear IB and make it work. In my room, you can hear huge bass behind you even at 80hz x-over.


I'm going to change my sub arrangement because I want bass to hit me in the face!
 
#10 ·
Localisation is the problem. You can get the sense that some of the bass is coming from the sub. You end up having to cross the sub over lower, limiting its range and also giving up some of the potential to use a sub to smooth out the bass. On the working I was measuring this system:

http://redspade-audio.blogspot.com.a...asurement.html


It had a good quality JL audio sub. We found it measured best on the right rear corner, but it took a 4th order low pass at about 45 Hz to avoid problems with the image. This meant giving up some of the reduction in a midbass broad null that the sub could otherwise have offered.


I'd be wary to build in a sub in an untested location that has any chance to have this problem.


There are all kinds of ways to do bass, but in a very large HT room I'd be inclined to start with a big bass horn, be it tapped horn or front loaded, placed where it gives the most oomph up to the midbass. Then add extra subs or woofers to cover mostly 40 Hz on up, placed to optimise a smooth response. Those might be sealed, they might be IB. With a beefy pro amp one might use one channel for the bass horn, the other for the other subs.


You can combine IB with a multisub setup. In that case I'd be inclined to mount them all dual opposed to reduce vibrations, and place them for the best combined in-room response. IB potentially lets you use ceiling locations as well. If your room is built then you can put a sub in the listening position, then move the mic to intended sub placements. You can then check for locations that will give you the least amount of dips.
 
#11 ·
i have a 4 18 ib useing xbl^2 drivers and a crown xti 4000. and it is insane i mean will shake the whole house even olny ataching it to the concrete and isolateing fromm anything that touches the celing still shakes things up.


buld the thing and you can allways buld bass traps to fix some problems.
 
#12 ·

Quote:
Originally Posted by Thomas-W /forum/post/21770887


Generally speaking 80 Hz is too high for a rear IB crossover point. Typically 40Hz-60Hz are used

Could you elaborate on what makes IB's different in this regard? Is there something mechanically different about IB's that require a lower crossover? I'm new to this (well, subs in general), but I've read just enough at this point to be dangerous and very confused



Quote:
Originally Posted by paulspencer /forum/post/21771026


Localisation is the problem. You can get the sense that some of the bass is coming from the sub. You end up having to cross the sub over lower, limiting its range and also giving up some of the potential to use a sub to smooth out the bass. On the working I was measuring this system:

http://redspade-audio.blogspot.com.a...asurement.html


It had a good quality JL audio sub. We found it measured best on the right rear corner, but it took a 4th order low pass at about 45 Hz to avoid problems with the image. This meant giving up some of the reduction in a midbass broad null that the sub could otherwise have offered.


I'd be wary to build in a sub in an untested location that has any chance to have this problem.


There are all kinds of ways to do bass, but in a very large HT room I'd be inclined to start with a big bass horn, be it tapped horn or front loaded, placed where it gives the most oomph up to the midbass. Then add extra subs or woofers to cover mostly 40 Hz on up, placed to optimise a smooth response. Those might be sealed, they might be IB. With a beefy pro amp one might use one channel for the bass horn, the other for the other subs.


You can combine IB with a multisub setup. In that case I'd be inclined to mount them all dual opposed to reduce vibrations, and place them for the best combined in-room response. IB potentially lets you use ceiling locations as well. If your room is built then you can put a sub in the listening position, then move the mic to intended sub placements. You can then check for locations that will give you the least amount of dips.

Thanks for the link! I can see there will be a lot of reading in my future.


Assuming that localization has some lower cutoff frequency near 80 Hz (it sounds like it may be lower, but let's just assume 80 Hz for the sake of argument). It seems like applying the crossover at that frequency should eliminate the localization issue. I've read that harmonics can be the cause, but if we're filtering out the harmonics above 80 Hz, how can they contribute. Is it a mechanical resonance independent of an electrical input?


I've also been considering some horn subs as well. Any suggestions for something that can play down in the sub 20 Hz region? I'd really like to get down to 10-15 Hz if possible?

Quote:
Originally Posted by theirishgonzo /forum/post/21771121


...............buld the thing and you can allways buld bass traps to fix some problems.

My only concern with that is I'll have to make some significant changes to my riser design for the IB. If I change my mind later, I'd have to correct that, and even then fixing the carpet would be tough.
 
#13 ·
View enclosure size as a continuum, with "small sealed" the lower extreme and "true infinite" the upper extreme. Anything in between will require less power for a given LF output...and, if adequately braced, it will likely sound better than several small boxes.


Moving from a true IB (over 25x VAS) to a new home, my choices were about 3x VAS on the front wall vs true IB on the rear wall. For overall SQ I chose the front wall...and am pleased as punch with the new install.


I recommend the front wall even if the enclosure must be shallow. Since it will be undersized, be sure to build it very stiff (overkill bracing)...you don't want large panels vibrating. If needed, use small filler subs around the room to smooth FR.



PS: Part of the localization issue is that sub crossovers are not "brick wall" filters. Even if the sub drivers are well behaved, there can be significant output 2,3,4 octaves above the crossover frequency...and I have measured some sub drivers with a 10-12 db cone breakup spike at 300Hz...very ugly!
 
#14 ·

Quote:
Originally Posted by J_P_A /forum/post/21771942


Could you elaborate on what makes IB's different in this regard? Is there something mechanically different about IB's that require a lower crossover? I'm new to this (well, subs in general), but I've read just enough at this point to be dangerous and very confused

This is not an IB specific situation. Placing any type of sub at the rear of the listening room creates a different set of problems compared to having the sub on or near the screen wall.
 
#15 ·

Quote:
Originally Posted by pnw /forum/post/21771969


PS: Part of the localization issue is that sub crossovers are not "brick wall" filters. Even if the sub drivers are well behaved, there can be significant output 2,3,4 octaves above the crossover frequency...and I have measured some sub drivers with a 10-12 db cone breakup spike at 300Hz...very ugly!

Absolutely, not just breakup but any harmonic distortions will still make their way through as the electronic crossover does nothing to stop them. IMO, this is very common in undersized subwoofer systems where a 50-60hz tone is producing significant harmonic distortion at usual listening levels thus causing significant output at the 100-120hz and 200-240hz ranges. Those are both very easily localized.


I'd follow the advice of these guys and skip the rear IB due to localization issues. Yes, you could run a steep crossover at 30-40hz but IMO that is a waste. You'd be better off using that displacement in subs up front that cover a wider range.


I'd only suggest using rear subs with the Geddes method where the rear subs only contribute a very small portion of the bass output relative to the front subs. I'd also suggest only using low distortion small sealed or 4th order bandpass enclosures for these rear subs.
 
#16 ·
Great advice offered.


In my opinion, the essence of what's been stated is if you're after what an IB approach has to offer, ie., highly efficient bottom octave performance*, then it would serve you well to implement this into a front of room location. Whether that's in the front wall, front side walls, ceiling, or even the front floor, what's important is the acoustic output must be psycho-acoustically tied appropriately to the LCR mains. Response filling/balancing individual small sealed subs could be employed elsewhere to mitigate the commonplace SBIR issues associated with a sub system.


What's been suggested is even if you don't hit your VAS goals, the hit in bottom octave efficiency can be accommodated by frequency EQ'ing of the proper driver.


Consistent and smooth response over as wide an area as possible is a very desirable trait. PNW's method of addressing height and width modes is very effective by spreading the driver across th e front end of the space. Here is a decent tutorial you may have already read, however Toole describes width and height spacing of subwoofer drivers to mitigate modal excitation.



Good luck

*a true IB alignment in a typical ~3k cube space, config'd of four co-located FiIB3-18's, modestly powered via 200w/each, can achieve ~125dB@1m@20hz.
 
#17 ·
Thanks for the feedback, guys. It sounds like I need to shelve the IB for now. I'm not going to dismiss using one at the front completely, but as it stands now, I'm not sure I'm willing to give up the space that's necessary to build the chamber. The space in the back may turn out to be a good place for a supporting sub.


FOH, thanks for the link. I've read Toole's book, and I think most of what's in that link is covered there, but I'm going to re-read the link and see if I've missed anything. It's always good to get a refresher from time-to-time.


Well, off to find a sub that will reach down into the teens without requiring a fortune in amps!
 
#19 ·
Good thought, but unfortunately no. The entrance is at riser height and is at a stair landing just outside the theater. The only other possible entry location is in a mechanical closet with limited head room. That closet was the only area we were all willing to compromise headroom to allow the HVAC ductwork to drop below some of the beams in the basement.
 
#21 ·
The primary reason is sound isolation. I need to be able to build a build a sound isolated room for the IB. I could potentially move that front wall back another foot with only minor changes to some plumbing. Or I could possibly build below the plumbing, but I also need to maintain a walking path for things going, well, into storage



I could possibly build a 4x7x8 box at the front where the drawing below shows, but it would not be centered in the space. My IB would be offset towards one wall. That would still give me room to move things in and out of that lower area. Would the offset location still affect imaging?

 
#23 ·
Are you saying I could isolate just the ceiling with clips+channel+DD+GG and get good results? I suppose I would need to box in the rim joists as well.


If that would work, it could be an option. I'd have to look at the costs of the additional drywall etc.
 
#24 ·

Quote:
Originally Posted by J_P_A /forum/post/21773234


Are you saying I could isolate just the ceiling with clips+channel+DD+GG and get good results? I suppose I would need to box in the rim joists as well.

The rearwave off the IB is just as loud as the front wave. Whatever you use to isolate the ceiling of the main listening room will work for the ceiling in the storage area.


Personally I'd wait on the storage ceiling until you get the HT operational. Fire up the IB and listen on the first floor. That will tell you if more heroic measures are needed to isolate the rearwave/storage room.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top