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Discussion Starter · #1 ·
Hi there,

I have been thinking over the last few days about how a single 21" maesltrom slot ported or with PR's would compare to both my FI Q18 subs for output and SQ. Would I gain alot more output. I could afford to sell both my drivers to get one of the 21"'s.

Here's a pic to show them together. The green one is one of my Q18 subs. They are co located so I guess its roughly an extra 3db in gain. I still would want to tune low to avoid a hpf.

I have based the single on 15 cu ft as I could get away with one big one to two smaller ones.

I have based the 21" maelstrom on 2300 watts of input but am not sure if I can bridge the maelstrom from the EP4000 or if I would have to buy a second and power the single maelstrom with two EP4000's. I have to say I love the maelstrom driver because of their huge output and excursion capabilities.

One thing that would be an issue is port velocity as it is at 40 m/s at 10hz.

If I am not going to gain much then I will stay as I am but it would be good to know if I am missing out on something. I would love to have a bigger driver to look at.

If this is a better way forward then I am open to suggestions.

cheers

Graham
 

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2 drivers can get 6db more in the right configuration, but it's best to even out the room modes instead. In a contest 2 subs usually beat 1 sub everytime due to evening out room response.
 

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2 Fi Q18's should have the advantage due to 2 motors to share the power, more SD, more displacement, and the ability to have 2 subs to even out room modes.As long as your subs are within 1/4 wave length of each other they should get the full 6db. That's about 9.4ft for 30hz.
 

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Discussion Starter · #7 ·
Ricci,

The subs are approximately 15 feet apart facing each other.

Dylan,

Yep that's correct 11 cu ft and 13.5hz tune.


I still wonder if I am getting an input problem though as even watching transformers 2 on -6db I was only getting 5 green lights and that was on the really heavy bass scenes. There did not appear to be much cone movement or air coming from the drivers even at that volume. I am not sure what the frequencies are in that film but they didn't seem to be alot of sub 20hz stuff.

cheers

Graham
 

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Ricci,

The subs are approximately 15 feet apart facing each other.

Dylan,

Yep that's correct 11 cu ft and 13.5hz tune.


I still wonder if I am getting an input problem though as even watching transformers 2 on -6db I was only getting 5 green lights and that was on the really heavy bass scenes. There did not appear to be much cone movement or air coming from the drivers even at that volume. I am not sure what the frequencies are in that film but they didn't seem to be alot of sub 20hz stuff.

cheers

Graham
Graham, The part where a few of the enemy transformers go to rescue megatron from the depths of the ocean has some pretty heavy 18-20hz stuff in it. That happens at Chapter 7, 40:08 into the movie.


Ricci, how would you calculate what 1/4 of the wavelength is? Just multiply the frequency by .25? The reason I ask is because 1/4 of 30 is 7.5. I'm not trying to be nit-pick or anything, I just want to know how you got that number (~9.4ft.)
 

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Discussion Starter · #9 ·
Dylan

I will check that out. Don't get me wrong there was alot of bass but my subs barely even struggled. I would expect more cone movement for 18-20hz stuff than I am getting right now. I think on the spl meter the most I was reading with all speakers and sub was 102db from the listening position which is about 2.5 metres. It was very loud. The subs never drowned out the speakers at all but then the onkyo 876 has great output.

cheers

Graham
 

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Graham, The part where a few of the enemy transformers go to rescue megatron from the depths of the ocean has some pretty heavy 18-20hz stuff in it. That happens at Chapter 7, 40:08 into the movie.


Ricci, how would you calculate what 1/4 of the wavelength is? Just multiply the frequency by .25? The reason I ask is because 1/4 of 30 is 7.5. I'm not trying to be nit-pick or anything, I just want to know how you got that number (~9.4ft.)
You take the velocity of sound and divide it by the frequency to get the wavelength

343 m/s = 20 x w

w = wavelength


343/20 = 17.5 m =w

divide w/4 = 4.625m
 

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Discussion Starter · #11 ·
As my subs are only 15 feet apart can someone tell me if I would be getting the full 6db gain from the extra subs.

cheers

Graham
 

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Discussion Starter · #13 ·
Hi there,

Sorry but what is the function real time analyzer. I have done measurements for both subs but it only gives me the same but with different curves.

cheers

Graham
 

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The RTA gives you a frequency response but it continually changes in real time instead of the snap shot of a sweep you would normally take. for instance if you have variable phase adjustment on your sub you can measure your sub and mains at the same time and adjust the phase. The spl will go up and down as you move the sub in and out of phase will the full range speakers, at the frequencies where they cross over. I assume the spl would also increase when you add a second sub into the picture, although I've never used it that way.
 

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You should have the full 6db below about 20hz based on your 15ft estimate. In practice in may be better than this. Isiberian provided the math for how to calculate this. Divide the distance travelled in 1 sec at the speed of sound (about 1125ft) by the frequency 20hz (hertz is cycles per second). The answer to that tells you how long one full 20hz wavelength is. Now divide that by 4 and that is your 1/4 20hz distance.

The easiest way to do this is, set your crossover to the mains as high as it will go momentarily, disconnect one sub and run a 0-200hz sweep at a decent level. Reconnect the other sub and run the same sweep again. Do not touch any settings between. Superimpose the measurements on each other in REW with the measured tab and see for yourself. Post up the results here for fun.
 

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Discussion Starter · #16 ·
Hi Ricci

So just to recap, turn the crossover to my mains as high as poss. What would ideally be the minimum. Disconnect 1 sub and then do I run a sub only or sub and speaker measurement. Then add the second sub and run again. I takes I still need to calibrate to 75db as normal and then turn it up to do the measurements.

cheers graham
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Sorry about the delay...Time is short to come by as usual.

You don't necessarily have to mess with the crossover at all. It just would've made things easier to see. The level used does not matter either. 75db, 100db whatever. Basically 1. unplug one of the subs. 2. run a sweep measurement. 3. plug the one sub back in and touch nothing else. 4. run the same sweep measurement again. 5. go to the measurements tab in REW and display both graphs overlayed and it'll show you the difference that adding the second sub made. It'll vary with frequency.
 

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Discussion Starter · #18 ·
Ricci,

Here you go.

Blue is one sub and the green is two subs. When I do the first the EQing goes out the window but then is ok with the second measurement.

What do you think. Have I done it right.

cheers

Graham
 

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That's it. You are getting the full 6db 30hz down. You also fill in the big dip by using the second and gain more than 6db there because the placement of the second sub does not result in the same null. At other frequencies you loose a little bit of output or gain a little. Don't worry about EQing one sub you will be listening to both.
 

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Discussion Starter · #20 ·
Ricci,

I have to say I am really impressed with them. I am in the process of working out how to stain them. I want them to match my cherry Monitor Audio RS6's and have the same gloss effect but I am not sure how you go about doing it. I have some american white oak veneer and have tried about 6 different varnishes but with not much success. These are big and I dont want to get it wrong. The veneer for each box is £120 so you can see why.

cheers

Graham
 
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