Subwoofer help?

Discö Rice · March 01, 2012, 07:36:27 PM

So a couple of weeks ago my bass player blew out her 18" sub. It had a fairly low wattage rating, and it was an 8 ohm speaker, so we decided to boost her output by getting a more powerful 500w continuous, 2000w peak speaker. This one's 4 ohms, and we're powering it with a GK 700RB MK. II, which is 480w at 4 ohms. Less than ten minutes after install, smoke, then silence. Sub no longer works. We're most definitely returning the fucker, bit what can we do to make sure this doesn't happen again with the replacement speaker?

Mr. Foxen · March 01, 2012, 07:45:35 PM

You just learnt about how thermal wattage ratings mean dick in practice.

Speakers an cabs need to be matched. Using this is the standard way: http://www.linearteam.dk/default.aspx?pageid=winisdpro

You are probably pretty far off figuring how to use that right, so best off telling me the box dimensions, inside, and the porting dimensions, I need to know how much air is int he box, and cross section and length of any ports, slot/shelf or tube.

Pretty unlikely that there was no audible warning that the speaker was unhappy, so main ting to do is listen. Also midrange content is much more important to bass than you might think.

Discö Rice · March 01, 2012, 08:12:42 PM

Vented sub cabinet. Measures 26" wide, 19" deep, 36" tall. Vent faces forward 5" below the speaker, and is 14.5" wide and 3" high.

Mr. Foxen · March 01, 2012, 08:43:11 PM

How thick are the walls? Need to know inside dimensions.

What speakers did you have in mind?

Do you have a separate midrange device? If you haven't, your main problem is that. If you do, what is it, where does it need to be crossed over?

How deep is the vent?

Edit: Ran quick and dirty calculations with a box that big and an Eminence Omega pro 18, since I have one of those, if the port is about a foot long, then its tuned to about 30hz an goes plenty far into sub low territory flat enough for sub function. Its maximum clean power handling at its poorest frequency is about 135w at 40hz, probably take close to double that for death and be making horrible noise at that point. That is with a fairly serious, although obsolete (as pretty much any 18s are) pro audio sub driver. Cheaper stuff will take bunches less.

jibberish · March 01, 2012, 08:47:33 PM

what exactly smoked?
humor me here, maybe we can find something more out.

many possibilities like crap like a poor alignemt can cause rubbing of the voice coil and that wattage foiled by the resistance to motion gets let off as heat on top of the normal resistive heat when it is moving correctly

how hard do you drive that amp? SS clips nasty. if you are pushing a busy waveform into hard clipping, you are putting nearer-and-nearer to DC@ power rail value into the speakers. hitting a 2x volume spike at near max volume is asking 10x the power. by tweaking the max distortion spec to huge, you may be able to get a burst of that power at the 4500watts your 2x volume spike asked for until the power supply is tapped. the problem is the huge distortion.
maybe check for a dc offset too, but that's not likely, but ez2 check.

edit: was thinking about dc offset. a shorted filter cap could do it since ss isn't safely isolated by an output transformer. anything that pulls a node up to + or down to gnd could put dc on the speaker outs since they are usually somehow commoned into the whole works also. just a thought as to what could burn up a speaker without circumstances really looking like that speaker should have burned up lol.

Mr. Foxen · March 01, 2012, 08:54:11 PM

Likely failure scenario is exceeding Xlim an smacking the voice coil against the magnet, distorting it and causing it to fail. There is pretty much no failure scenario that won't be accompanied by lots of distorted noises.

Lumpy · March 01, 2012, 10:31:05 PM

I would call Peavey tech support and get them to recommend a speaker.

Lumpy · March 02, 2012, 08:42:19 AM

The Peavey basket replacement instructions says to use sticky tape to pick up any leftover bits of debris from the previous voice coil...

Other than that, weird because old speaker had worse specs than new speaker, I think.

Discö Rice · March 02, 2012, 11:39:23 AM

We didn't replace the basket, Lumpy. We got a whole new speaker. Foxen - the walls are about an inch thick, including the front wall where the vent is cut.

Jibberish, I have to admit (and forgive my ignorance) I don't know how the hell you'd get 4500 watts of power from a 480 watt amp. It's making my head hurt. The speaker we had in there before (a much lower wattage 8 ohm version) lasted for months under the same kind of strain, and was pushed to even more ridiculous levels by a bass player in a previous band (years ago). Head hurts.

Mr. Foxen · March 02, 2012, 12:10:01 PM

Is the vent hole only the thickness of the wood, or is there a shelf inside? It needs to be close to a foot long for a sensible tuning for a cab that size, if it is shorter than about 6 inches, then that will be your problem, the speaker is unloaded at low frequency and is acting like it isn't in a box at all.

Watts aren't very relevant, unless the amp is totally broken and shorted and it won't be doing much like music then. The amount the speaker is moving is going to be causing you problems, the idea of a ported box is that it is a tuned resonator and the port loads the speaker at a certain frequency and stops it moving so much whilst producing more output. If the speakers own resonance isn't suited to the box one they'll break very easily, it is part of the Thiele/Smlal specs that decent speakers are provided with, but people like the utterly useless 'wattage, voice coil diameter, magnet weight' specs that don't tell you anything but are less hard to understand.

Discö Rice · March 02, 2012, 12:26:52 PM

Quote from: Mr. Foxen on March 02, 2012, 12:10:01 PM
Is the vent hole only the thickness of the wood, or is there a shelf inside? It needs to be close to a foot long for a sensible tuning for a cab that size, if it is shorter than about 6 inches, then that will be your problem, the speaker is unloaded at low frequency and is acting like it isn't in a box at all.

Watts aren't very relevant, unless the amp is totally broken and shorted and it won't be doing much like music then. The amount the speaker is moving is going to be causing you problems, the idea of a ported box is that it is a tuned resonator and the port loads the speaker at a certain frequency and stops it moving so much whilst producing more output. If the speakers own resonance isn't suited to the box one they'll break very easily, it is part of the Thiele/Smlal specs that decent speakers are provided with, but people like the utterly useless 'wattage, voice coil diameter, magnet weight' specs that don't tell you anything but are less hard to understand.

Yeah it's just a hole in the front panel. I would've thought that would allow it to aspirate and move more freely, but I guess that's bad. I didn't take anything out of that box, so I guess there hasn't been a legitimate vent in there the whole time I've owned it. Sounds like something I can build pretty easily, though. You're saying about a foot deep? How thick should the walls of the vent be?

Mr. Foxen · March 02, 2012, 01:10:08 PM

You need to tune it appropriate to the woofer. Without a woofer spec can't really say. Felt about inside made sure its just a slot?

I just redid the calculation with a 1" deep vent and it should be fairly happy down to 40hz which is loads low enough for the purpose, but feeding it lower frequencies than that will rapidly cause upset.

Easy way to lower tuning would be close off half the hole, is an external job that way. Any more effort an you are best off getting an actual bass cab. Which you are pretty much definitely better off doing than buying an 18" speaker anyway.

Moving too freely is the problem, it is moving far enough to destroy itself. The whole idea of a box is to control that.

Discö Rice · March 02, 2012, 01:48:11 PM

Wow, that makes sense, Foxen. I've had a thorough look around inside, and there is definitely nothing but a hole in the front. She plays in B standard, which puts her lowest fundamental at about 30.8 hz, so adjusting the port is a definite need. Out of curiosity - what would sealing it completely do?

In related news, I called the place I got the speaker from, and they're going to refund it (that's a fucking relief). I'm calling Peavey tech support later to see exactly what they might have in their inventory that might be a better match.

Mr. Foxen · March 02, 2012, 01:54:03 PM

Peavey tech support probably can't do box calculations either, they'll happily pull something out their ass though. Basically as soon as they don't ask for all those same things, and spend some time doing the math, they are BSing.

Sealing the hole totally makes any woofer safer, but not as bottomy. With a box that big, that is probably fairly happy, stuff below 50hz it pretty irrelevant to tone anyway.

But seriously, an 18" sub alone is not very suitable for electric bass.

Discö Rice · March 02, 2012, 02:13:37 PM

haha no that's not the only rig she's running. She has a channel splitter sending a signal to a GK400RB that powers a vintage 8x10, and the other signal is going through the afore mentioned 700RB Mk.II, which powers that 18. No crossover involved, just has all the highs and mids EQ'd out on the sub.

Mr. Foxen · March 02, 2012, 02:26:17 PM

A high pass tuned to the box is the safest thing, its how proper PA subs are used. Everything is kept within its passband. Cutting lows to the 8x10 is also beneficial because you on't end up with two sources of low end that cancel each other out and make a massive mess of comb filtering.

Edit: the 8x10 also explains why not hearing the speaker killing itself. Kind of got to question how much it is contributing if you can't hear it farting out.

Discö Rice · March 02, 2012, 03:36:18 PM

We're both thinking "Fuck all this - just get a 2x15 legitimate bass cab and call it a day.".

Mr. Foxen · March 02, 2012, 03:59:46 PM

Quote from: Discö Rice on March 02, 2012, 03:36:18 PM
We're both thinking "Fuck all this - just get a 2x15 legitimate bass cab and call it a day.".

Best way is decide if like the sound of the 8x10 alone, but want louder, if so, get another the same. If its more bottom you need, might be better replacing the 8x10. 2x15 isn't necessarily gonna be bottomier, although sealed 8x10s do strong tend to not be bottomy.

Discö Rice · March 02, 2012, 04:12:44 PM

Quote from: Mr. Foxen on March 02, 2012, 02:26:17 PM
Edit: the 8x10 also explains why not hearing the speaker killing itself. Kind of got to question how much it is contributing if you can't hear it farting out.

Well, it's a small room we're in. Honestly we heard some slightly growly tones, but the horrific screech of death didn't happen until it was too late. As far as how much it contributes, before the last speaker blew, it was absolutely crushing. Elephantine. I think I just need to replace that speaker with the exact same speaker that was in it in the first place and get an appropriate amp.

Mr. Foxen · March 02, 2012, 04:44:20 PM

A lower powered amp won't help matters, watts still aren't the issue. I'd lower the tuning either way by blocking off half the port, you can do it with chunks of styrofoam and experiment some. Not eqing loads of bass in is more likely to help not breaking it, flat bass, cut treble. With the current tuning the bass boost is probably about where the breaking frequency is, so bad plan.

Discö Rice · March 02, 2012, 06:31:05 PM

The cabinet didn't change, and the amp didn't change, just the speaker, which is the only thing that died. So it stands to reason that the speaker replacement choice was the problem, not the stuff that worked before and still works.

Mr. Foxen · March 02, 2012, 07:04:13 PM

Thought the previous speaker died too, hence the replacement?

Discö Rice · March 02, 2012, 07:41:38 PM

It did, but only after years of abuse. It was definitely excessive excursion that killed the newer, more powerful 4 ohm speaker, and it makes sense that the vent was too large for it, but the lower wattage 8 ohm speaker that was in it before took a hell of a lot of punishment and sounded incredible with that cab the way it is. I just thought a lower wattage amp might take some of that spikey SS clipping badness out of the equation. Maybe a limiter would do the same?

inductorguitars · March 02, 2012, 07:54:54 PM

Quote from: Discö Rice on March 02, 2012, 07:41:38 PM
definitely excessive excursion that killed the newer,

Did you check the amp for DC?

Mr. Foxen · March 02, 2012, 08:54:58 PM

Quote from: Discö Rice on March 02, 2012, 07:41:38 PM
It did, but only after years of abuse. It was definitely excessive excursion that killed the newer, more powerful 4 ohm speaker, and it makes sense that the vent was too large for it, but the lower wattage 8 ohm speaker that was in it before took a hell of a lot of punishment and sounded incredible with that cab the way it is. I just thought a lower wattage amp might take some of that spikey SS clipping badness out of the equation. Maybe a limiter would do the same?

SS clipping just sounds really bad, the thing with it harming woofers especially is a myth with little bits of truth, in that it can exceed the stated output of an amp (because the output rating is clean), and the corresponding compression can overheat a voice coil if you keep pushing it, but so will compressed clean sound. All not very relevant, and a lower powered amp will just clip more. A voltage limiter is the correct way to stop clipping , but you need to know the voltage limits of your cab at its passband, and you won't and don't have a passband. The better way to keep it from breaking is an appropriately set high pass filter to you don't throw more power in the lows to your amp at all, so you don't eat all its headroom and don't push the woofer harder than necessary.