[rick57]

Hi

A year ago I built an extra room for music & HT on my house, with a concrete floor. But I miss the feeling of bass in the timber floor of my old main ‘original’ house, which I’ve also enjoyed in timber floors at concerts.

A sonotube would be easier, but more obvious (ie a big cylinder), and not give the desired under floor bass:

I plan to build in situ a timber ‘platform’ in the room’s center, under the seating area. Say 6 inches ” * 7’ * 10’. On top sheet flooring, say 15 mm (here) 0.6” thick (with a rug on top).

The platform will sit on some commercial grade carpet I have. Around the perimeter I think the weight of the platform, people and the sofas on top should give a good enough seal.

Inside the platform will be enough space for the EBS, to be checked for excursion etc.

As the driver is wider than the platform is tall, I’ll have a ‘driver box’ at one end, about externally 20 inches cubed. So the clearance behind the driver will be 20 inches, less the baffle thickness (say 1 inch), less the driver’s depth.
 
A little thing to even out the bass within the room, at the very low end where room gain is highest: below about 18 Hz more output comes from the EBS tuned port(s) than the driver. So other than the driver’s side, I’ll put one port in each of the other three sides, so bottom octave output is better spread.


Does this sound a good idea? Any suggestions?


Thanks 

[dyohn]

Hmm... I have installed systems using a raised seating platform as an enclosure and it can work great... I am not confident your idea of using the commercial carpet as a gasket between the platform and the concrete floor will provide a sufficient seal.  You may need to actually construct a "box" using something cheap like 1/4" plywood as the bottom and literally seal it up.  In any case, you don't need to try and port that large an "enclosure" just seal it up and install a woofer or two IB.  Also, rather than try to use the platform as the enclosure is there a closet or adjacent room in this space to use as an IB?  You may be happier with conventional subwoofers and then use "bass shakers" under your raised platform if what you are trying to do is create a physical sensation under your feet...

[Kevin Haskins]

It can work with the right construction.   The disadvantages of going that route are that you have to concentrate on building the riser the same way you would any other subwoofer, well braced.    You also don't have the advantage of moving the sub to optimize location.    If you get any kind of port chuffing or out of bandwidth noise, you will hear it much easier than you would with the subwoofer at a more traditional location.     If you can live with all those things and manage to design it into your space.   Go for it.   

[rick57]

Hi dyohn,

no, there’s no closet, adjacent room or roof space to use as an IB.

> In any case, you don't need to try and port that large an "enclosure" just seal it up and install a woofer or two IB. 

If its not ported, its not an EBS (no that it has to be), but why would you not port it?

> You may be happier with conventional subwoofers and then use "bass shakers" under your raised platform if what you are trying to do is create a physical sensation under your feet...

I considered "bass shakers", but only an EBS goes really deep, ie f3 is 11.8 Hz!


When I increase Ql (leakage) in Unibox from the default 15 to (throwing a number up) 200, ie about 12*, f3 moves up 2.6 Hz to 15.4, though peak ouput at about 14 Hz increases by 1.6 dB. Fairly modest chages.
This is with 700 litres, 25 cubic feet.

[rick57]

Hi Kevin

> The disadvantages of going that route are that you have to concentrate on building the riser the same way you would any other subwoofer, well braced. 

Yes, harder than a sonsosub

> You also don't have the advantage of moving the sub to optimize location.   

True, though with the driver and ports on 3 other sides I hope will partly mitigate this

> If you get any kind of port chuffing or out of bandwidth noise, you will hear it much easier than you would with the subwoofer at a more traditional location

That’s mitigated by sizing ports so that port speed at high volume is less than 25 m/sec (and at moderate volumes, less)?

[Kevin Haskins]

Hi Kevin

> The disadvantages of going that route are that you have to concentrate on building the riser the same way you would any other subwoofer, well braced. 

Yes, harder than a sonsosub

> You also don't have the advantage of moving the sub to optimize location.   

True, though with the driver and ports on 3 other sides I hope will partly mitigate this

> If you get any kind of port chuffing or out of bandwidth noise, you will hear it much easier than you would with the subwoofer at a more traditional location

That’s mitigated by sizing ports so that port speed at high volume is less than 25 m/sec (and at moderate volumes, less)?

For me, I don't prioritize the < 18Hz that high on my list.     I'm much more interested in what the subwoofer does from 18-60Hz.   The port contribution for a sub is typically in a fairly narrow bandwidth around the tuning frequency.    You are forced to use VERY long ports with either a Maelstrom or a Tempest due to their swept volume.   Do the modeling and you will see that without MASSIVE ports you will never keep a Maelstrom/Tempest build <25ms.     Also, large long ports develop resonances that are out of the bandwidth of the subwoofer design.   Those resonances can be very audible, especially when placed close to the listening position.   

In terms of the ports being in a different location than the driver and hence smoothing out the room response, it won't.   The port output is in a fairly narrow bandwidth.   The driver is covering all of the bandwidth up above 20Hz in the EBS design and up above 20Hz is where you have all your room problems.   

As David has said, a large sealed cabinet will give you close to the same effect, without the problems of having to deal with the port.   If you need more output, add more drivers.    It will also be much easier to build.    The KISS principle works.   

[dyohn]

Thanks Kevin, my thoughts exactly. 

[rick57]

I appreciate most of your points, eg that 18-60 Hz is most important.

Though as I see it, an EBS compared to sealed, moves the f3 down a little more than am octave. So for sealed KISS, even if of a lesser importance, the trade-off is quite some extension is lost.

I hadn’t checked group delay, but thought that below 50 Hz, it wasn’t really audible(?)

The long ports can be at the far end from the listening position, about 10 feet away, facing the opposite direction.

If I’m using Unibox correctly, the Tempest in 25 cu feet tuned to 13.5 Hz: with 4 * 3.6 inch (stormwater pipe) ports need ports lengths of 21 inches, and has its 1st resonance at 294 hz, well outside the passband.

So it appears that done this way, chuffing and resonance would not be a problem. . ?

[Kevin Haskins]

I appreciate most of your points, eg that 18-60 Hz is most important.

Though as I see it, an EBS compared to sealed, moves the f3 down a little more than am octave. So for sealed KISS, even if of a lesser importance, the trade-off is quite some extension is lost.

I hadn’t checked group delay, but thought that below 50 Hz, it wasn’t really audible(?)

The long ports can be at the far end from the listening position, about 10 feet away, facing the opposite direction.

If I’m using Unibox correctly, the Tempest in 25 cu feet tuned to 13.5 Hz: with 4 * 3.6 inch (stormwater pipe) ports need ports lengths of 21 inches, and has its 1st resonance at 294 hz, well outside the passband.

So it appears that done this way, chuffing and resonance would not be a problem. . ?

You are right, the group delay is not an issue and with those port sizes, you probably won't have an audible problem for a single Tempest.  They might chuff at really high output, depending on how much power you have on them but shouldn't under most circumstances.   Make sure you flare the ends of the both sides of the port opening.       

I don't worry about the simulated f3 overly much.   I try to keep in mind the in-room response, which will be pretty ugly compared to anything you model.    In most rooms you get some pressure vessel gain so the low-end of the response is a lot higher than you model. 

You can try the port, and plug them and try it sealed.   That way you can make up your own mind.   Nothing ventured nothing gained. 

[rick57]

“Nothing ventured nothing gained”

Always. Though this venture is not a trifling project . . .

("depending on how much power you have on them" - 100 watts, ie about 107 db: ample)

”You can try the port, and plug them and try it sealed.
That way you can make up your own mind”

Yes, and with three ports, that gives scope for tuning, over that last octave

“In most rooms you get some pressure vessel gain so the low-end of the response is a lot higher than you model.”

In your experience, say for a room 20 foot long – from what Hz and at what rate does room gain typically operate?

Cheers

[dyohn]

Post the dimensions of your room and I'll predict your room mode frequencies for ya.

As you know, we cannot "hear" below 20Hz, although we can detect and "feel" frequencies as low as 2Hz.  If your port noise is below 20Hz, it can be neglected.  Also, designing a system for a predicted relatively small SPL increase in subsonic bands (like pushing f3 down from 20Hz to 12Hz) may or may not be detectable and may or may not be worth the efforts, especially once the system is installed in a space where those effects can be masked or completely cancelled by room modes.

Now don't get me wrong, I think if sub-sonics interest you then you should go for it and build the system that models best to you and see what happens.  Only the meters can tell you for sure what you are getting, and that requires building something to take the measurements.    My only interest is making sure you understand the trade-offs, especially when attempting such a large scale construction.

As an aside, I once built a theater system where we consumed over 100 cuft under the stage in an EBS alignment using eight 18" Harbinger woofers in push-pull alignments.  It modeled out to 8Hz at over 120db.  However, after all the careful planning, the custom ports made from machined sewer pipe, and many hours of hard work building the thing, it measured and sounded better with the ports sealed and the woofers operating IB.  Who knew? 

[Kevin Haskins]

Post the dimensions of your room and I'll predict your room mode frequencies for ya.

As you know, we cannot "hear" below 20Hz, although we can detect and "feel" frequencies as low as 2Hz.  If your port noise is below 20Hz, it can be neglected.  Also, designing a system for a predicted relatively small SPL increase in subsonic bands (like pushing f3 down from 20Hz to 12Hz) may or may not be detectable and may or may not be worth the efforts, especially once the system is installed in a space where those effects can be masked or completely cancelled by room modes.

Now don't get me wrong, I think if sub-sonics interest you then you should go for it and build the system that models best to you and see what happens.  Only the meters can tell you for sure what you are getting, and that requires building something to take the measurements.    My only interest is making sure you understand the trade-offs, especially when attempting such a large scale construction.

As an aside, I once built a theater system where we consumed over 100 cuft under the stage in an EBS alignment using eight 18" Harbinger woofers in push-pull alignments.  It modeled out to 8Hz at over 120db.  However, after all the careful planning, the custom ports made from machined sewer pipe, and many hours of hard work building the thing, it measured and sounded better with the ports sealed and the woofers operating IB.  Who knew? 

Ha!   I have had the same experience but not on that scale.   All of the big EBS ported designs I've done always sound better after you plug the port.   Why?   I don't have a clue.   I could speculate but the bottom line is that if you have the space, big sealed subs and lots of them are a fantastic solution that matches up great with room gain.   It is simple too, just mount your drivers, no need to worry about port noise or subsonic filters.   If you need more output, just add more drivers.     It is a simple solution and it works.   

[rick57]

Hi dyohn ,

Thank you for helping me better understand the trade-offs, much appreciated.

I'm seeing what I can do to get 100 dB with one Tempest at about 12-15 Hz, to feel (not hear o course) what "is there" . .

The room is 17 feet wide, 22 feet long * 12 feet high.
Per Linkwitz, room gain occurs below the lowest room mode frequency.

http://www.linkwitzlab.com/thor-intro.htm
“Room modes cannot exist when 1/2 of a sound wavelength exceeds the longest room dimension. If this is 7.5 m (24.6 ft), then a wavelength will be 15 m and the lowest mode frequency is 343 m/s / 15 m = 23 Hz.
Below this frequency bass response may increase due to room gain . . “

http://www.linkwitzlab.com/thor-estim.htm
“actual room behaviour is almost impossible to predict”

I think that in a “perfect” room with stiff walls and floor and no leakage due to openings, in my size room, by 12 Hz, gain would theoretically add about 6 dB.
Mine is pretty stiff with few openings, so by 12 Hz I’d expect say 5 dB.


Does that sound right?

> pushing f3 down from 20Hz to 12Hz) may or may not be detectable and may or may not be worth the efforts, especially once the system is installed in a space where those effects can be masked or completely cancelled by room modes

That’s true. For the Tempest-X, a maximally flat sealed box with Qtc = 0.707 is still a big 480 litres (17 cubic feet), about 9 db short of flat at 12 Hz, but with room gain likely only about 4 db short of flat.

As the ports only add output below about 18 Hz, with 3 ports, to tune the bottom octave,  1- 2 ports could be  plugged. Maybe EBS will add little of value, or be worse. But if built that way, as you say, it can always be sealed.

But if built sealed, it can't be turned into EBS quite as effectively, if I understand it all correctly.

Cheers

[dyohn]

Siegfried is of course correct that it is impossible to completely predict room modes since there are just too many variables such as furniture, speaker placement, etc.  But it is entirely possible to calculate what room modes exist due to the geometry of the room, and these tend to be the ones most present no matter what else might be going on.

HERE's a good on-line calculator to try.  If you enter your measurements you'll see a likely huge room mode around 60Hz since there are instances of all three types in a +/- 6Hz band around 60Hz.  This means your room is likely to sound boomy and you may need to make efforts to reduce this resonance.  Also note that this calculator is not showing any subsonic modes although due to the length of the room there is a likely axial 1/4 wave mode at 12.8Hz.

I still am not sure why you feel you need 12Hz subsonics, but to each his own.

I say go for it.  If you don't like the effect you can always seal the ports.  The only thing you might lose is time. 

[rick57]

Thanks again dyohn,

That on-line calculator is good and does highlight a problem. When I input the room dimensions more accurately, I have a big problem at 51 – 56 Hz: one axial and two tangential modes in that small range.

Maybe I should diy (is it?) a Helmholtz resonator tuned to 55 Hz?

> still am not sure why you feel you need 12Hz subsonics

I do enjoy the odd bit of organ music
What Hz might a T Rex’s foot stomp and similar effects be at?

Btw, the word from an EBS evangelist
http://www.hometheatershack.com/forums/diy-subwoofers/print-1820-llt-explained.html?pp=50

[Kevin Haskins]

Thanks again dyohn,

That on-line calculator is good and does highlight a problem. When I input the room dimensions more accurately, I have a big problem at 51 – 56 Hz: one axial and two tangential modes in that small range.

Maybe I should diy (is it?) a Helmholtz resonator tuned to 55 Hz?

> still am not sure why you feel you need 12Hz subsonics

I do enjoy the odd bit of organ music
What Hz might a T Rex’s foot stomp and similar effects be at?

Btw, the word from an EBS evangelist
http://www.hometheatershack.com/forums/diy-subwoofers/print-1820-llt-explained.html?pp=50

Yes... I've seen the arguments.   Much of the problem that people have, even technically astute ones, is that people fall in love with a philosophy.   The one that I think you see made in this circumstance is the chase for "accuracy".    In that quest I often see people chasing an arbitrary goal, based upon a philosophical underpinning and we sometimes get blinded and loose sight of the bigger picture.

For one, there isn't anyone who knows the research that gives any real weight to a simple THD measurement as a sound quality indicator.    At the range of operation of a subwoofer, our ears are terrible at discerning gross levels of distortion.    We become more capable at doing so the closer you get to the midrange, but at 20-30Hz our ear/brains cover a multitude of sins.     Our perception of distortion is strongly affected by the overall spectrum, and it's content.   It is a complex subject but you can have 10% THD in one circumstance and be totally unbothered by it.   In another situation it can chase you from the room.    Obviously, we need more data than simple THD to make engineering choices by.   

Also, once the distortion levels (and spectrum) are such that they are inaudible, it doesn't pay to make them TWICE as inaudible.   Inaudible is inaudible and you don't get extra brownie points for making it twice as inaudible as another solution.   

My experience is that sealed systems will have higher distortion, especially as you push lower in frequency but it isn't a problem until you reach gross levels.   In that circumstance, you just add more drivers until it is no-longer an audible problem.     That is a simple solution, it gives you a tremendous amount of flexibility in building enclosures, and you can equalize until your heart is content.    I don't worry about the extra distortion as long as it isn't audible.    When it becomes so, just keep adding drivers until that is no longer the case.   

Where ported systems make the most sense to me, is when you have a finite amount of space and a tight budget.   For commercial products companies are fighting to make the smallest sub, with the most output, at the lowest price and in those situations it makes sense to pull out all the tricks to maximize the output of a small driver.     

For most people building theaters, our situation is different.   The goal isn't necessarily to maximize the output of a given driver in a given amount of space.   It is to achieve a goal, within a budget, that is easy to build and design into the room.    If you are building a room you have tremendous flexibility in placement and construction.   Those are things that are not available to designers of commercial subs.    One thing that DIYers DON'T have is access to tools and testing to determine how to fool-proof the design.   Commercial subs have to be built so that the customer won't blow the sub.   That often means hundreds of hours of testing and complex electronic limiters, filters etc.. that allow the engineers to run a driver right up to it's limits.     DIYers don't have that kind of research budget.   The answer to that is you just overengineer the project so that you have a lot more wiggle room in operation.    You will find that the hard components, the drivers/amps are not the most expensive part of the project.   It is the time and resources you have wrapped up in building the damn thing.    It makes no sense to me to build an entire room and try to squeak by saving every penny on the driver & amp.   Design it and overbuild it enough so that you don't operate the devices anywhere near their limits.    You won't regret it after it is done.

[dyohn]

That was one of the best posts I have ever read on any forum or paper about audio Kevin.  You should cut and paste it everywhere.  I especially like "Inaudible is inaudible and you don't get extra brownie points for making it twice as inaudible as another solution."  Brilliant.

The "true believers" out there will argue with you, of course, but they will anyway no matter what you post unless you are simply reinforcing their preconceived notions.  But anyone with any engineering background - and especially acoustics engineers - will applaud you, as I do.  Thanks for writing that!

[rick57]

Hi guys

Thank you for an interesting and wise post. (I didn’t think the pro arguments fo EBS would be new to you, but possibly to some other readers).

I agree that THD is not a good measure of sound quality. Though your primary points seemed directed more at the idea that minimizing THD is my #1 aim.

My #1 aim is output at a Hz, *comfortably within XMax. About 105 dB at 12 Hz.

If below 20 Hz turns out to be only of occasional benefit, nothing ventured, nothing gained, and nothing lost.

Finite space or budget isn’t overly relevant. I’m not building a room, but fitting something extra in. By doing a sub as a “false floor”, no floor space is lost, which is a small plus.

Maximizing the output of a driver also just gives a modest sense of efficient dsign.

* Good point about the most precious inputs: time and resources.

In terms of different inputs to achieve my aim:
To achieve a similar output at a low Hz, is a sealed box + say a Linkwitz Transform or one or two extra drivers any simpler?
Less box to build, but a circuit to measure and or model and build; or more to mount & provide an amp to.

To me, time and resources seems about the same in total, either way. What do you think?
 

[Kevin Haskins]

Build it ported and you can always plug it and try it sealed.    Nothing ventured nothing gained. 

[Kevin Haskins]

That was one of the best posts I have ever read on any forum or paper about audio Kevin.  You should cut and paste it everywhere.  I especially like "Inaudible is inaudible and you don't get extra brownie points for making it twice as inaudible as another solution."  Brilliant.

The "true believers" out there will argue with you, of course, but they will anyway no matter what you post unless you are simply reinforcing their preconceived notions.  But anyone with any engineering background - and especially acoustics engineers - will applaud you, as I do.  Thanks for writing that!

Oh come on now David, you are making me blush.        

Anyway, I don't take praise from most people too seriously.   It doesn't mean much unless the person giving it is someone I know and respect.   You are one of the few whose praise is meaningful.   Thanks for the kind words.