2x18" IB's v's 4x15" IB's

[ThomasW]

Mar 23, 2011 13:30:21 GMT -7 Wull said:

I am considering adding another IB manifold later this year, this will be positioned in line with the original front 4 x 15" FI IB manifold, but at the back of the room.

Why do you want to do this?

[Wull]

Mar 24, 2011 6:05:06 GMT -7 ThomasW said:

Mar 23, 2011 13:30:21 GMT -7 Wull said:

I am considering adding another IB manifold later this year, this will be positioned in line with the original front 4 x 15" FI IB manifold, but at the back of the room.

Why do you want to do this?

Out of curiosity if I'm honest Thomas. My OH doesn't let me crank the volume up when she watches films with me, so I hoped an IB behind would add to the spl. I also have an Audyssey Sub pro, this can EQ two sub's in the same room, so I wouldn't mind seeing how this performs.

[chrisbee]

That's odd, my wife insists we replay all the best bassy scenes after the film with the volume turned up even further. ;D

[Wull]

Why, do you think this a bad idea.?

I have been trying to find a thread I read ages ago on proboards. An Australian chap had decided he wanted an IB behind him after living with one in front for a while and had great results

If anyone knows which thread I am talking about can you please point me to it.

Cheers

[ThomasW]

I didn't say it was a bad idea.

There are numerous installations with front and rear subs. Those were built for specific reasons, usually to increase overall output or to flatten the in-room response..

The wife won't let you turn your existing sub all the way up and your in-room response plots look really good. So I question why you think you want/need a rear sub?

[Wull]

Like I said, purely out of interest, and that itch to try something different. Sometimes when withing films I start to wonder what if, and it's very distracting

I do remember you telling me in an earlier post I could try splitting my IB up, and having a manifold at the front and one at the back to help the response.

So would just having two 15's at the back work, or would I need to make it four like the front?

And I am right In thinking you can not mix 18's with 15's aren't I? 'please don't shout ! '

[ThomasW]

Mar 25, 2011 10:06:06 GMT -7 Wull said:

And I am right In thinking you can not mix 18's with 15's aren't I? 'please don't shout ! '

It's all about displacement (Vd), 4-15 have roughly the same Vd as 2-18"s, so the front and rear output would be a match.

[Wull]

Mar 25, 2011 14:37:08 GMT -7 ThomasW said:

Mar 25, 2011 10:06:06 GMT -7 Wull said:

And I am right In thinking you can not mix 18's with 15's aren't I? 'please don't shout ! '

It's all about displacement (Vd), 4-15 have roughly the same Vd as 2-18"s, so the front and rear output would be a match.

So having 18's and 15's in the same room is OK, this is good news. I was under the impression that you couldn't due to them both having different Q values?

I can fit 2 x 18's at the Rear so I may go with these then if it makes no difference.

[chrisbee]

If you have a crawl space or cellar you could try a manifold right under your chair. Though if a partner sits close beside you then they will also "enjoy" the same output.

Placing a "bass-sensitive" partner in a broad, bass null point would be ideal. Give them a chair right in the centre of the room?

It ought be possible (with REW and a roving microphone technique) to find a spot where there is a minimum of bass. Hopefully near a peak in the bass for you to sit in.

Your partner may even have a particular frequency sensitivity. One which you could exploit with a room mode trough precisely where they sit. (but only if there is a trough in the response to exploit)

Distance from the IB could be exploited if it helps achieve lower levels for the partner. Even just another yard (or metre) away is worth about 3dB in some rooms. Smoothing the response over a larger area of the room, using a rear IB, could actually work against you. Only experimentation in your own room will help to answer this question. You must avoid a bass-negative partner sitting in a room induced peak.

It might be possible to achieve a null point for the partner by using a local subwoofer deliberately made out of phase for her position. But in phase for yours. Good luck with that one! A broad band of very long wavelengths is involved. Plus the close seating locations in most normal sized rooms, I doubt it is even possible to achieve much of a local bass reduction. REW will help you to decide. It would be ironic if you were sitting in a null and your partner in a room induced peak.

[Wull]

I don't have a crawl space below

It really is not that bad Chrisbee, my OH if the bass head in the family, she just doesn't like earth shattering volumes 'she even complains in the local cinema thats its to loud' 'treble' may be I should just nudge the IB's gain up a tad when watching films to gain that extra rumble.

I also fancied having a bit more of a bass wrapped round me affect.

So I thought before carrying on I should ask the experts. I should have said from the start i am going to add another IB behind my seating, as I use 15's should I keep to the same size IB or can I go with 18's taking into account they have slightly different driver parameters.
If putting them behind would be a complete waste of time I would take this no further. Guess I just have to much time on my hands which gets me thinking.

cheers.

Will

[chrisbee]

Dream no small dreams for they have no power to move the hearts of men... Goethe.

I think Thomas should probably put that in the FAQ somewhere.

[Wull]

To be fair I have just watched a film at referefnce level and I started to feel a little nauseous towards the end

plenty of bass

[ThomasW]

Good news.......

I assume this means there's no need for a rear mounted IB?

[Wull]

I guess so.

Out of interest, what would I gain from adding more IB's then. I understand it would give me more headroom, though I never feel like mine are at their limits now.

[ThomasW]

If you don't need more output you don't need anymore drivers

[Wull]

Hi.

Just recently, well the last two films I have watched I have noticed my IB sounding like it's distorting, I have never heard this before.
I have just tried listening to a bass heavy film and it is quite noticeable, the only word I can think of is distortion.

I have checked all my connections and can not see anything wrong.

Any ideas what this could be?

[brady]

With system off gently push on the cones of each driver and listen for scraping/rubbing.

[ThomasW]

To diagnose this is would be handy to see some calibrated measurements of the sub.

For comparison purposes, both nearfield (at the outlet of the manifold) and listening position measurements are needed

Then we need to know what the SPLs are when you hear this distortion.

[Wull]

Thanks Brady and Thomas.

I may have found the problem, I disconnected my sub EQ and all seemed fine.

I will do more checks this week and report back.

[Wull]

Nov 20, 2011 2:21:55 GMT -7 Wull said:

Thanks Brady and Thomas.

I may have found the problem, I disconnected my sub EQ and all seemed fine.

I will do more checks this week and report back.

Right, had some time to run checks today and it appears I had been adding to much EQ.

I am not sure how I never noticed before, I guess recently I have been listening to music more so I've never really pushed it since adding a multi EQ to my system, which is a few months now.

Sorry for panicking/ writting a post before doing basic checks I did't realise EQ could affect output even at low levels.