Subwoofer Placement

Systems

I have one polk momo 10" sub, and i was wondering the difference in sound if i change the possition of my sub, if i turn it a round and stick the magnet outside the box. Its a ported box. Thanks alot

neomagus00

very little... it's all about looks; but, the volume the sub takes up matters - there will be more air inside the box if you invert the sub, so this will change the tuning of the box. for reference, the average sub takes up about 0.1 cubic feet of airspace in the box.

exalted512

porting MOMOs is a no no
unless its the baby momos, then i dunno
-Cody

AustinKP

Actually, one of the Polk techs said that Momo's could also be ported with no problem. I think it was Josh that was just talking about it last week...

neomagus00

well... 'no problem' vs. 'there will be no problems'? 'no problem' i take to mean that it will act just like any other driver in a ported enclosure, in that it has sufficient suspension to handle that. 'there will be no problems' implies to me that, if you're careful, you won't break it, but it may not sound the same (even accounting for the difference b/w ported and sealed).

Systems

Yea, i was reading the forums, and something about the suspension of the sub couldn't hadle it? What did Josh say about it?

neomagus00

i just ran a search, and he hasn't said anything recently that i could find... i don't remember anything like that, but i'm looking right now.

Systems

Ok, thanks. I was just wandering, because i just recently purchased a mm2104, and an alpine MRV-T320 amp in a ported box. So that would really suck if it broke!

neomagus00

you wander too? try talking to MTXMAN...

it should work - just make damn sure you use a subsonic filter, or tune it damn well. if you need help to do either of those, just ask. till then - break it in properly (play it at 1/4-1/3 volume for 2 weeks) and keep it down till you're sure you won't break it. again, ask if you don't know when you'll know you won't break it .

Systems

I've been using the sub, probly to the max i can get it, because i didn't know it would hurt it. I've had it for about 2 weeks. It hasn't broken yet, how could i tune it better?

exalted512

I'd DEF. put in a call to Polk and ask them if the sub can handle being ported
I know for a fact that the bigger momos are not supposed to be ported b/c of their suspension
-Cody

neomagus00

well, theoretically, any sub could be ported as long as you keep the volume down. whether they're supposed to be ported or not is a different matter.

i would not turn it up for another two weeks, but if it hasn't broken yet you're probably one of the lucky ones - but better safe than sorry. there is only one issue you have to worry about with powering a sub in the vast majority of cases, and that is excursion.

okey, this part gets a little detailed. a ported box has a specific tuning frequency, say 40 Hz. if you were to play a 100 Hz tone through the sub, and measure how far out the cone moves, you'd get a measurement of say 10 mm. now, if you play a 60 Hz tone, which makes the cone move slower, at the same power level, the cone moves, say, 15 mm. your cone can only move so far, namely 1 inch, or 25 mm (i think this is right for your sub). keep in mind the tuning frequency is 40 Hz. if you play a 45 Hz tone, even lower than the 60 Hz tone, it'll move even further, say 23 mm. drop the tone to 40, the tuning frequency, and the excursion becomes 25 mm. note that we've moved 5 Hz here, from 45 to 40. now, we move another 5 Hz, down to 35. only now, the cone tries to move 29 mm - a big jump. look at the graph below. (you're halfway done!!!)

this example is a long way of saying that, below the tuning frequency, there's not enough support behind the sub to hold it back at those low frequencies. again, look at the graph. the red line represents, in this hypothetical subwoofer, its maximum excursion. the sub here is being overdriven a bit - the notch in the blue line should coincide with the red line for optimum performance. note that the tuning frequency is about 40 Hz. see how the excursion rapidly rises below that? when you turn the volume down, that has the equivalent effect of moving the blue line down on the graph. if you turn it down far enough, the whole blue curve is under the red line and you won't over-extend your sub. however, look at how far down the stuff on the right is once you move the blue curve down that far - very little volume here.

so, the solution is to take away the blue line to the left of your tuning frequency. one uses a device called a subsonic filter to do this - it's just a highpass filter set at a very low frequency, namely 40 Hz in this example. what this does is hack off all the blue line (roughly) thats to the left of 40 Hz, or whatever the filter's set at. this means that you can turn it up much louder and not over-extend and thus destroy your sub. you may hear this phenomenon referred to as 'unloading'. the reason the momos are not supposed to be ported is because they depend quite heavily upon the air behind the cone to control its motion - their powerhandling will be quite low in a ported box.

the next step is to adjust the gains properly, explained here

Systems

thanx a lot man, like i said i really don't want to mess it up. also i bought a momo grille for it about a week ago and its on freakin backorder, how long does that take usually?

AustinKP

D'OH! It wasn't ported, they were talking about putting the MM124 in a Bandpass. My Bad! And it wasn't even Josh, it was Thom :rolleyes: Here

Systems

Ok thats cool

neomagus00

jdm - if you read that whole essay (i'd be surprised if you did), are there any questions i didn't answer or anything that's unclear? and if you didn't read the whole thing, go back and read it you lazy bum .

austin - no prob; i actually found that thread as i was searching for the other one, it just didn't register that it was the one you had been talking about.

Systems

naww....dude, i really did read it, the only question i can think of, is how to ajust the hz?

neomagus00

on which part? the tuning of the box? or the subsonic filter?

if it's tuning the box, you can change the port length. to change the subsonic filter, a very high end amp may allow you to do so, or you can make your own for like five bucks.

Systems

naww...dude, i read the whole thing, i guess the only question i have is, how do you change the Hz? And i guess that filter your talking about is the switch on my amp, if it is i already have it on, well theres two of the, i have both on

Systems

well, how could i change the port? I didnt' make the box. What exactly is a subsonic filter?

neomagus00

to change the port length, you'd have to unglue it (or just tear/cut it out, but gently) and replace it with one of a different length. you can't change the tuning too dramatically in this way, but you can do some.

a subsonic filter is a highpass filter set really low. a highpass filter is a filter that passes frequencies higher than it's setting (hence high...pass). so, if you set it to 40 Hz, it cuts off all the sub-40-Hz music (not a whole lot) before sending the signal to the amp. the one in your amp is probably for your other speakers - normally, one crosses over the tweeters at like 3500-4500 Hz, again with a highpass, so they only play the real high frequencies. the midranges take the rest, so they get both a highpass and a lowpass filter. the highpass is set to wherever the sub kicks in (usually around 80-100 Hz) and the lowpass is set to the same point as the tweeters, so where the tweets start to fade is where the mids pick up. same happens with the subs. you may have a switch that will enable/disable the subsonic filter on your amp. if you told me what amp you have, i'd probably be able to provide more specific advice.

Systems

I have an alpine MRV-T320, its a 2 channel amp, i have it bridged, so its pushing about 220 watts RMS