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Discussion Starter #1
You may have seen my subwoofer "build" thread here,
http://www.hometheatershack.com/forums/diy-subwoofers-sealed-ported/15649-ported-ebs-build-pics.html

Well this is a case of a little knowledge being dangerous. I will be starting up in a few days, have an amp coming and all that.
So on the grapevine or somewhere in my forgetful knowledge base, I seem to remember something about watching out for the driver at some resonant frequency? Something about the cone moving way too easy and way too far? Something about having a feedback destroyer and setting it to filter out a narrow frequency range where this could be happening?

Well if you can answer me here, great, or point me to the right place to read up on it. For now, and likely for the next 6 months, I don't want to buy a feedback destroyer. What else can I do? Is a coil / capacitor filter okay? If so what values do I need to get? I have a couple of coils handy and some capacitors, or I can order the right thing if I need to?

Or can I just watch out and take it easy? I built the sub to go down low so I don't want to use the 30Hz filter on the amp. Thanks
Please break it to me gently. I'm a big boy but don't want to cry. :scared::surrender:
 

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I believe you are talking about a high pass x-over to prevent really low frequencies below the tuning of your sub from damaging the driver. A vented subwoofer is great down to the tuning of the vent and very slightly below but below that the driver unloads from the safety of the coupling with the vent and is endanger of bottoming out with a very small amount of power. For example your SW models as hitting it's xmax of 15mm with just 85w at 10hz and even less below that. It can handle 550w above 17hz. You will need a HPF that rolls off all of those really low frequencies in order to run your sub loudly without worrying about damage. Somewhere around 16-20hz is a good bet. Only use one channel of the J2500 for power to the MJ18M also as 500w or 600w is all it needs. With music you should be fine to run your sub without a filter for a bit, since your sub is tuned to 18hz. Anything like that at a high enough level to damage your sub would be very rare in music.

You need to be VERY careful if you run it without a filter on movies though. There is a lot of content below 17hz in movies and the soundtracks are very dynamic, so a big bass transient could come suddenly and you wouldn't have time to react before your driver has been damaged. If you decide to do this I'd suggest putting in the bassiest movie you have (WOTW, or Cloverfield would be perfect) and very carefully watch some of the big bass scenes while monitoring the driver and with the volume at a very low starting level. This will give you an idea of what to expect if you turn the volume up to more normal levels.
 

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How do you determine when such a filter is needed? I have seen contradictory advice on this forum around when one is needed.

I also thought you need to know xmech to know when/where the sub will bottom out.

I would think that this would be important to know when choosing a driver. Given a choice of spending $100 on something to do a filter and spending that same amount on a better driver that may not need a filter I know where I would go.
 

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Any ported sub tuned above 15hz is going to need a HPF for serious movie playback levels. For good sealed designs a HPF is not necessary. The only times that a HPF is not needed is in the case of an IB where you have 4-18's , 8- 15's or something crazy like that and a comparitively smaller amount of power going to each driver. In that case the bass will be really REALLY loud before there is enough of a signal to cause the drivers distress. This is part of the appeal of IB. Most of the time even at loud levels the drivers are just coasting along sharing the load. The other time that you can skip the HPF is if you have a very large, powerful ported sub/s tuned very low, like 10-12hz. I've heard of people getting away with this but I wouldn't recommend it. Most of the time the reality is that the signal chain itself is rolling off below 10hz due to the various electronics in the path and attenuating the <10hz content, which acts like a filter.

The apparent contradictories come from differencies of opinion, design and real world use. When you have a guy with 2 huge ported cabs with powerful 18's tuned to 11hz, lot's of practical experience and he barely ever takes his system past -10db from reference ever, he will most likely be fine just relying on electronic's roll off. Then you have another guy who's relatively new to this stuff, and he only builds half of the other guys system and uses one less expensive and less capable driver, runs the sub +5db hot and likes to crank his music and movies louder than the other guy, you can see where problems could arise. This is without factoring in the effects of different rooms either. With ported builds it's better safe than sorry IMO.

Modeling and real world testing is the best way to determine the correct filter needed. You don't ever want to be modeling something with Xmech as a guideline. Reaching Xmech equals damage and not to mention very bad sound. I'm sure that an MJ18 will go to 25mm or so before it gets in trouble, but you don't want to be flirting with disaster. You could spend another $100 and get an IXL18 which has more displacement, but guess what? it still needs a HPF below tune. I guess you could spend another $125 on top of that and get a Mal-X, but then you won't get the most out of it unless you bridge the amp into it and it will still need a HPF below tuning. Putting a Mal-X into the same box that the OP has it handles a lot more power for sure but it still goes WAY over Xmech with just 500w below tune.
 

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Discussion Starter #6
If (more likely when) the sub bottoms out you can add a external HP filter for about $100.
Please point me in that direction. Is it the Feedback Destroyer?
I also need a "clean box" or other "direct box". Maybe a feedback destroyer is both?
 

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Discussion Starter #7
You will need a HPF that rolls off all of those really low frequencies in order to run your sub loudly without worrying about damage. Somewhere around 16-20hz is a good bet.
What brand, model, do you like? I am not sure I am even looking for the right thing? There is a Behringer Shark. Behringer MiniFBQ?

I would definitely like to filter before I blow it up. Even if I am very careful, my kids might not be, so I need something. Thanks
 

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Here's the cone excursion of your setup. Green shows Xmax exceeded at 16.5 hz with 550 watts input. Pink shows cone excursion within limits with a high pass filter applied at 16 hz.

MJ Excursion.JPG

A very versatile unit would be the Reckhorn B1 sold by Creative Sound Solutions. $109 shipped.

http://www.creativesound.ca/details.php?model=B-1

Click on "More Information" in the above link for a description of the unit.
 

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Discussion Starter #9 (Edited)
A very versatile unit would be the Reckhorn B1 sold by Creative Sound Solutions. $109 shipped.
Thanks Mike
Would this Reckhorn serve as a direct box or signal boost too? I read the manual and it does seem to boost if you want it to, but I am unsure?

I decided to order this item and will let you all know what I find out.
 

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What do you all think of this product for low frequency filtering?
(I can't post links yet - new member. Search for "FMOD Crossover Pair 20 Hz High Pass" on parts-express.)

It seems like a cheap way of avoiding over-excursion, which is why I'm doubtful.
 

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Discussion Starter #11
What do you all think of this product for low frequency filtering?
I don't have that much experience so I can't say for sure. The 20 Hz one looks like it would do the job for me, especially in a pinch. I am tuned at 18 or so. I believe what I need to worry about is 16Hz and under. The 12 db slope would be kicking in pretty good by then I think.

I also needed a clean box or something similar to match my pre amp to amp levels. It sounds like the B1 will do that job as well very nicely. The Fmod part would maybe compound the problem? Possibly not though and could be very good if you have a clean box or similar?

Maybe someone else will weigh in? I don't think it would be ideal for LLT boxes tuned lower than me. :nerd:
 
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