Help me calculate the size - Home Theater Forum and Systems - HomeTheaterShack.com

Old 04-26-14, 09:41 AM Thread Starter
Shackster

Join Date: Nov 2012
Posts: 10
Help me calculate the size

Hi,

right now I'm finally planning my new HT room and trying to figure out if I can fit an IB in it.

It's gonna be a sealed off, room in room type of HT and around 24x17x9 ft big. No way to cut holes in walls or ceiling.

the only way of doing an IB setup would be to build a huge "box". It seems that some people did it that way and it works, so...

As for the size: It can be as wide as the room and will be placed under the screen.
Something around 17x2 ft. Depth-wise I don't know. It shouldn't reach to far into the room.
I thought maybe 3ft.
That would give a volume of 102 cft. (2.8 cu meter / 2800 liter)

I really would like to build a 4x 18" setup. Let's say the IB318. The specs say 318 L per driver. = 1272 L for all four.
If I calculate this with Vas x 10, thats about 12000 L = ~ 430 cft. If I did the math right

So, according to the formula my enclosure would be too small and essentially just a big sealed subwoofer.
I've seen measurements done with Vas x 4. Some say it is enough. That would mean for 4 IB318 the enclosure should be 180cft. Still a little on the small side.

So... what are your thoughts? Is it possible to fit an IB in my HT and what should I expect from it regarding low frequencies and SPL?.
Right now I have a SVS PB13 and it is "okay", but I want more
And since I'm building a whole new room, I hope I can make my IB-wish come true.

If you need more details just ask. I'm quite a newbie regarding IB's.
relaxo is offline

Old 04-26-14, 11:42 AM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Help me calculate the size

Welcome, your box dimensions would provide a net volume of around 85 cu.ft. assuming a double front baffle which is highly recommended in this case. That would put you in the large sealed category rather than an IB. All 4 drivers would handle 3000 watts, and with room gain factored in, you could probably get 125 - 130 db down to 15 hz.
Mike P. is offline
Old 04-26-14, 12:10 PM Thread Starter
Shackster

Join Date: Nov 2012
Posts: 10
Re: Help me calculate the size

Thanks

I just calculated a little bit more with different driver displacement and sizes and four 15" seem to be the sweetspot for a Vas X 4 in my enclosure.
However... I think they are too small for what I want. Which is low Bass, deep into the single digits and reference level ar at least close to it. Now that I got my dedicated space I would really like to crank it up a little.

If, like you say 4 18" will give me something to only around 15Hz... I don't know... I think there must be something I can do to get more. At least I hope so.
Of coure this is all speculation at this point with no way to predict how the SPL curve will look like in my room and how much SPL I have below 15Hz or even 10Hz.

The other thing you mentioned is Amp power. I need to figure that out as well when I'm at that point. How much I need to have "clean" sound, not blowing the drivers and how to wire the whole thing.
But this is discussion for a later time.

Right now I'm really struggling how to calculate my IB / big sealed sub or rather how to get the most out of my enclosure. Or maybe some alternative build (DBA?!)

//EDIT:
Since I have a little time, I'm looking at those links in your sig. Interesting stuff
relaxo is offline
Old 04-26-14, 01:02 PM Thread Starter
Shackster

Join Date: Nov 2012
Posts: 10
Re: Help me calculate the size

Quote:
Mike P. wrote: View Post
W...assuming a double front baffle which is highly recommended in this case....
How do you mean "double front baffle"? 2x2 manifolds?
I was thinking I arrange them as a line array an the floor. That's almost my only option.
The other one would be to do two separate enclosures left and right of the screen from the floor to the ceiling.
But that would impact my max screen size.
The third option would be a combination of both. A U-shape if you will. Left / Right / Bottom of the screen.
If I do that, the enclosure volume gets pretty much doubled and I would do a 8x 18" setup. But I'm not sure about that and if it is a viable option or would give me a huge improvement compared to the 4x 18" setup.
relaxo is offline
Old 04-26-14, 01:39 PM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Help me calculate the size

A double front baffle means the whole front face is double 3/4" plywood, so 1.5" thick.

Here's the estimated SPL graph of the 4 IB18 drivers with 3000 watts which the drivers can handle without exceeding their rated Xmax, the 15 hz I mentioned was just a specific frequency. The room gain below 15 hz is quite a bit so I'll estimate it the SPL at 10 hz to be 120 db or higher. Not to many movies have any content below 15 hz so wanting high SPL into the single digits is excessive in my opinion.

Mike P. is offline
Old 04-26-14, 02:36 PM Thread Starter
Shackster

Join Date: Nov 2012
Posts: 10
Re: Help me calculate the size

Thank you for taking the time to do all that work for me. I appreciate it very much

My front wall might not be a whole "fake" wall for the IB's, because I have to fit my LCR behind the screen as well.
You mean something like this?

I was talking more about something along those lines, but with for-mentioned 4x 18:

If i understand the graph correctly I should be able to hit reference level without breaking a sweat and heaving enough headroom in case I want to run them a little hotter, without getting thermal issues?
How does it compare to the SVS PB13? Not that I'm expecting too much and get disappointed at the end.

How did you calculate the 3000W amps? Is that much power necessary? I always thought the could do good with less.
I'm not questioning your calculation nor do I have something against the 3000W. I'm just curious why 3000W and what if they get less than that
I would get some Behringer iNuke NU3000 amplifiers. I think they are nice and affordable.

If I'm not already starting to bug you, can you please say a few words about the max excursion point and some damage precautions? Just what to keep an eye on in order to get the most out of the system without the risk of damaging something. I have a little fire paranoia
relaxo is offline
Old 04-26-14, 03:51 PM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Help me calculate the size

I'm going by what you stated:

Quote:
As for the size: It can be as wide as the room and will be placed under the screen.
Something around 17x2 ft. Depth-wise I don't know. It shouldn't reach to far into the room.
I thought maybe 3ft.
4 drivers in a 17 foot wide box that is 24" tall and 36" deep.

3000 watts would take the drivers to thier rated Xmax at 10 hz and won't hurt the voice coils so it's all safe excursion wise and thermal wise. You can use less power for less SPL.
Quote:
If I'm not already starting to bug you,
Not possible, you're here to learn and we're here to help.

As for the SVS PB13, without room gain it was measured as 89 db at 12.5 hz and 105 db at 16 hz. You would be at 115 db at 12.5 hz and 120 db at 16 hz. You won't be disappointed.

Mike P. is offline
Old 04-27-14, 07:32 AM Thread Starter
Shackster

Join Date: Nov 2012
Posts: 10
Re: Help me calculate the size

Quote:
Mike P. wrote: View Post
I'm going by what you stated:

4 drivers in a 17 foot wide box that is 24" tall and 36" deep.
Yep. That's right then.

Quote:
Mike P. wrote: View Post
3000 watts would take the drivers to thier rated Xmax at 10 hz and won't hurt the voice coils so it's all safe excursion wise and thermal wise. You can use less power for less SPL.
I downloaded WinISD and came up with something similar. 2700W to be exact. I had to enable a 1st order Butterworth highpass filter at 2Hz to keep the cone excursion below the rated 30mm line.

Adding more Watts rises the excursion, but the SPL stays untouched. The same thing happens with the box volume. What settings do I have to change in order to modify the SPL curve?
There must be something like lowering the overall SPL and at the same time raising the lower end.

Quote:
Mike P. wrote: View Post
As for the SVS PB13, without room gain it was measured as 89 db at 12.5 hz and 105 db at 16 hz. You would be at 115 db at 12.5 hz and 120 db at 16 hz. You won't be disappointed.
Thanks for the numbers on the SVS.
This sounds really awesome. Can't wait to build this thing

Any advice on the Driver placement regarding room modes? Room size: 24x17x9 / Distance IB to listening position: 11ft.

Would it be better to cluster them next to each other or spread them evenly apart?
This is what I came up with so far. I'm open to suggestions.
Attached Thumbnails

relaxo is offline
Old 04-27-14, 08:11 AM
Elite Shackster

Mike

Join Date: Apr 2007
Posts: 19,397
Re: Help me calculate the size

Spreading them evenly apart across the bottom would be my choice. Being a couple mm over Xmax won't hurt anything. There is no 2 hz HPF, 10 hz is as low as they go as far as I know. As for raisng the bottom end, you can equalize it to raise it up if it is needed once you measure the in room response.
Mike P. is offline
Old 04-27-14, 01:15 PM Thread Starter
Shackster

Join Date: Nov 2012
Posts: 10
Re: Help me calculate the size

Quote:
Mike P. wrote: View Post
There is no 2 hz HPF, 10 hz is as low as they go as far as I know.
Is this a property of the driver itself or do you read this from WinISD somewhere?

You see what I meant with "2Hz LPF" on the attached image.

The top picture is the raw calculation WinISD gave me. The lower one is with the LPF and the shown values.
But maybe I did something wrong there. That was more a trial and error kinda thing and to me it looked good.
Now that I think about it, it may not be that desirable to have the Xmax from 16Hz down to ~5Hz. It may look cool, but is an unneccessary mechanical strain on the driver. I think the Xmax should have a narrow peak just below the redline and the lower the Xmax stays the better. Or am I wrong about that?

And let's take the upper "raw" Xmax curve. It shows it reaches Xmax at 10Hz. Wouldn't lowering amp power mean the curve gets lowered altogether, so that the xmax point is reached at, let's say, 5Hz? (e.g. from 300W to 2500W). Of course that would mean I have less SPL.
The question is what to shoot for. I guess SPL is number one on our list and Xmas is just fine as long as it stays within the specs, right?

Now, If you say the driver only plays down to 10Hz, then I could disregard everything below that and I would be fine. But if it does play lower I need to set a LPF. Correct me if I'm wrong. I'm just starting to get into this right now, so sorry about my stupit questions

I mean it's not that the driver won't play 9Hz at all.. It might drop of and lose SPL, but the excurson is still there or not? At least thats how I read the WinISD graph
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Last edited by relaxo; 04-27-14 at 01:20 PM.
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