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Old 18th December 2001, 06:42 PM   #1
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Join Date: Dec 2001
Location: California
Question Focal 4k Kevlar mids

Hello everyone... I am a new speakerbuilder and new to this forum. I have read the following books from the following authors: two from David Weems, and one from Vance Dickenson. Therefore I understand a little... (comprehend is a another matter.)

Here's my delemna, I got myself a little exited at building a new home theatre system using a sub/sat combo. And subsequently really didn't do much homework before just forging ahead... primarily because I located some focal 4 in. mids for a great price and wanted to use them for my sats all around.

I have built the sub (two actually) and I am VERY pleased with thier performance. I have also built the sats (front two anyway). And I am disatisified with the crossover point between the subs and sats.

I am using a B&K AVR202 reciever w/ bass mangement that has a fixed xover point of 80hz. (I don't know the slope). My sats are down 3db at around 120 hz. (sealed enclosure). Therefore without any additional processing (ie, graphic equalizer) I have a hole in the response that makes the combination sound thin in the upper bass.

I am currently tweaking the passive crossovers for the sats, the subs use thier own plate amps with LFE input. My main question is this.... Does anyone have any ideas how I might help the low-end response of the small sats in a sealed enclosure? Would a zobel help? I have seen reference to baffle step response additions to the woofer circuit. Is there a a possible passive network component that may boost this specific frequency (opposite of a notch filter)?

Would "super stuffing" the box help? Maybe something with the subs (although I can't see how)? I am open to all suggestions. I am not in the mood after two months of building this system to toss it out yet!

Here are the specs (focal kevlar cone mids):
fs 76
qes .47
qms 8.23
qts .445
vas 3.58l
re 5.97
p 50w
lvc .22
xmax 1.25
sd 50.27

I have them in a .08cf sealed enclosure in an MTM arraingement with a focal 120dx tweeter. I used a separate enclosure per mid in each cabinet (hoping to stop any inter-driver backwave problems). I currently have the crossover (between the 4s and tweeter) at 3k using a second order on the tweeter and a first on the mids (no impedence equalizer) and a lpad on the tweeter. I do have a peak around 1k that i think is the fs of the tweeter getting through the second order filter, therfore I may try a zobel or separating the crossover point between the tweeter and mids.

I have access to a Rane RA-27 spectrum analyzer to help with locating reponse problems.

Thanks in advance....
mike

ps. Any sources for reasonably priced crossover components? This is getting PRICEY!!!

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Old 19th December 2001, 02:05 AM   #2
CHRIS8 is offline CHRIS8  United States
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Default well...

I do not think that you are going to ever find a satisfactory way to make the transition from the satellites to the subwoofer. The 4" speaker simply does not resolve frequencies under 140-150 or so Hz realisticly. You may be able to get it to respond decently down to 100 hz or evena little lower with good 'measurements' in a sealed box but this does not nescarrily translate to 'good' sound. Unfortunately, even female voices have significant information in the lower 100 hz range so this is going to be forever be a problem. Your sub is of course extremely directional over 100 hz, and IMO even 80 Hz is still easily localized. Now, the focal polykevlar mids are amazing, but this size mid(4") would be ideally suited to a 3 way speaker IMO. As a rule, I never use anything smaller than a 6.5" bass driver in a satellite. That is a personal preference, not a set rule.

As for solutions....you can achieve slightly improved lower frequency response by using a resistor in line with the woofers to increase overall qts of the woofers. However, there is no free meal ticket and this will decrease the overall sensitivity of the woofers and will still probably only result in a f3 no more than 8-10 hz lower, and also require you to redesign the passive filters to the woofers since the network will now see higher impedances. You could just bite the bullet and port the enclosures. Can you live with the midbass transient degradation as compared to a huge hole in response? I think that I would choose the imperfect midbass as compared to no midbass.

Since you have two subwoofers, you could use them front firing and get a stereo sub amp and a component crossover(just make your own (http://sound.westhost.com)), and place the subs under the satellites, or very near the satellites and cross at 100-120 hz. This is likely not practical though.

Good luck.

-Chris
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Old 19th December 2001, 03:47 AM   #3
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Location: California
Thanks for your resonse Chris.

Ooooo, I was afraid of this...

I AM amazed at the resolution offered by these focal 4s, but I am disappointed in the midbass. I was really hoping to hear someone comeup with a magic bullet to solve this problem... good thing I've only built the two sats at this time. (I did however build the center cabinet to match the two front sats.)

As far as the subs go, I've built and completed these (as downfiring) therefore I'd rather modify the sats. It IS a possibility to modify the sats to a ported design by removing some mdf braces inside, but off the top of my head I think they would be too small for an "optimum" alignment. I'll check the dimensions and let you know.

But first, I'd like to try your idea to raise the qts.... can you recommend a resistor value to start?

Anyone else have any thoughts, ideas?

Thanks,
mike
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Old 19th December 2001, 03:55 AM   #4
ucla88 is offline ucla88  Tahiti
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i agree that you will have great difficulty getting these to 100 or even 80. the problem isn't so much getting down to a frequency of 100, the problem is that you're just not going to get much in the way of useable spl. at 80hz, two of these will be displacement limited to around 80 db. i don't think you can get around this in a sealed box. active eq will just get you to the displacement limit. i don't think fiddling with any passive circuit will get around this fact.
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Old 19th December 2001, 06:46 AM   #5
CHRIS8 is offline CHRIS8  United States
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Yes, regardless...your sealed enclosure SPL will be severely limited. If you still wish to try and lower Qts, just start with around 1 ohm and increase until you find a maximum acceptble result. I am not aware of any math to predict the outcome. However . I have heard that the degree of manipulation possible is relative o the qes:qms ratio or the woofer. Maybe someone knows a forumula? Try asking at www.audioasylum.com or the www.madisound.com board. Keep in mind that a 4" driver is already very inefficient, even when used in parallel in a MTM to gain efficiency, decreasing sensitivity is just going to complicate things further.

I would try porting first. A vent will also make up for some of the driver's displacement capability issues as well as lowering F3 considerably(pending you have enough internal volume). You mentioned removing some bracing? I am strongly against this, but if you do be sure to put in plenty of damping material such as vinyl or bitomous sheets so that you hear more speaker than board.

-Chris
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Old 19th December 2001, 09:21 AM   #6
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You could make the speaker a "2.5 way" system by filtering one of the mid-bass units so it only operates from 120Hz and down. This would have the effect that below 120Hz both drivers are pumping away but only one is driven above 120Hz.

This would add +3dB (I think) to the bass but overall efficiency will drop by 3dB (again, I think). The tweeter would also have to be attenuated to compensate.

Might be worth trying before you hack the boxes.....

Nice one,
Dave
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Old 19th December 2001, 02:16 PM   #7
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Default The lower end

I have used some of the Focal Kevlar midrange drivers in the past and found that some of the Scanspeak carbon fiber 6.5 or 7 inch drivers work quite well in filling the void on the lower end.

Two of these carbon fiber drivers working together can quite easily obtain a almost flat response down to 32 Hz in a properly designed enclosure.

The Scanspeak carbon fiber drivers are not pretty to look at but their performance is excellent.

I did not particularly like the Focal Kevlar midrange drivers I used. I was not pleased with the construction of the diffusers in the middle of the cone and also the lack of a cover over the front voice coil opening. I would suspect that the magnetic fluid keeps most of the dirt out but being old fashioned I like things like this sealed.

John Fassotte
Alaskan Audio
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Old 21st December 2001, 02:29 AM   #8
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I went back and checked the internal dimensions of the box, and using winISD I AM able to port the box the turn these into vented sats. (And I *think* I can do it without tearing-up too much of the internal bracing).

So, that is one possibility.

I then wondered if I could save these puppies by removing the lower MTM 4, routing out the hole, and replacing it with a Focal 6.5" midbass. A quick check in winISD suggested it might work. The box dims are good and I have room on the front baffle to just squeeze one in.

Since I have'nt purchased the center channel speakers I could take the two leftover fours and put them in the center cabinet that's done (and port it) without spending too much *more* money.

Anyone have any thoughts on this?

And then there is the COSTLIER solution (toss the baby with the bath water) and make these expensive rear surrounds. If I go this route (I might) I'd be looking for some good advice from all of you on a GREAT MTM design.

My main focus is music, say 90% and 10% HT. This system has to fit into the old SAF, and a relatively small space.

BTW: to CHRIS8. I saw a post you left another newbie (such as myself) about speakerworks. I downloaded it and I am very impressed. Kinda makes me want to start all over again and try it! Thanks.
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Old 21st December 2001, 07:29 AM   #9
CHRIS8 is offline CHRIS8  United States
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Sounds like a good plan. Might I suggest however, just make a couple of matching cabinets(same width and finsh)to contain the 7" or 8" midbasses so that you can stack thte mtm on to it and it look like a single loudspeaker. This way you can have enough volume for 7", etc. to use an adequate ported alignment(decent transients). You will now also have a large enough cabinet to house the new 3 way crossover you are going to be building.

You will likely not even want to use a subwoofer if you use those scan speak drivers. They should provide superb lower midrange and bass.

This will be a perfect chance to try out that new software you just downloaded.


-Chris
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Old 22nd December 2001, 09:47 PM   #10
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I really like the idea of building a new lower cabinet for the 6 1/2 to 7" midbass.

I think I can make this work and still look good.

Can you guys recommend a Scanspeak driver by model number and a source with a good price? I know it sounds stupid, but... since I am using the 4" kevlar focal drivers, and they are YELLOW... (umph). I think it might *look* better to stick with another kevlar driver (Scanspeak, Focal, etc) primarily because I will not be putting grills on these speakers. And it might look a little funny to go with a yellow 4, then a black tweeter, then a yellow 4... then a black woofer.

(I know, I know... I am not building them to look at, but my fiance' can be like Martha Stewart at times. I mean hell, I think my motorcycle would look great parked in the living room when its raining outside... but she has a different opinion. Needless to say the bike stays outside.)

Thanks,

mike
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