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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Lounge => Topic started by: Jared Bartimus on April 12, 2011, 01:58:50 PM
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A campus ministry program I help out purchased a used MP418SP a few months back and we have been having problems with popping ever since. When the system is run anywhere near its limit we get a popping sound in the 80-100hz range. Before I had assumed we were sending out too much in the lower ranges and bottoming out the driver.
The frequency was determined by playing the 80hz warble from the Bink Audio Test CD and then using individual test tones to narrow it down. Low cut on the channel strip was turned off and the main eq's had, I believe, a 40hz low cut engaged. Every time the tone would hit the middle of the range the sub started popping. EQ was set flat for everything below about 120hz and there were no significant changes above that point.
It started popping both when pushed fairly hard at that frequency and when I was moving the fader relatively quickly at a lower level. I don't know if that part was an issue with the sound board or something to do with the sub.
Is the original assumption that we are doing something wrong and causing it to bottom out probably correct or was the driver maybe somehow already damaged when we got it? It doesn't seem right that the sub has extreme difficulty keeping up with a pair of JRX115's that are being powered to the same combined wattage. I know we aren't going to be getting a strong hit from the kick drum with only a single 18" sub in a room as big as we are in but it had trouble getting over stage volume even with just two people singing and an acoustic guitar.
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It sounds like over excursion, but if it's happening with quick fader moves at low levels (or with things that should not be creating sub bass, like an acoustic act) then the problem may lie elsewhere. Check your cable connections, maybe one of them has gotten a little corroded.
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I thought it was overexcursion myself, but shouldn't that get worse at lower frequencies? At 110hz it is fine and at 70hz it is fine but at 80-100hz it isn't. If I remember correctly pretty much every connector in the sytem has been checked within the past 6-9 months due to other issues we had been having.
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I thought it was overexcursion myself, but shouldn't that get worse at lower frequencies? At 110hz it is fine and at 70hz it is fine but at 80-100hz it isn't. If I remember correctly pretty much every connector in the sytem has been checked within the past 6-9 months due to other issues we had been having.
Can you find another MP418 to compare with? I don't think it's the loudspeaker itself...
If it were, it would be more likely to bottom out at lower frequencies (particularly at/below the box tuning freq) and that would be at levels below maximum power.
I suspect what you're hearing is a cabinet issue... a broken glue joint, failed fastener, etc that resonates at 70Hz.... or there is something loose in the box that starts moving when the box resonates at 70Hz.
Perhaps Art Welter can share his considerable experience with such things....
Have fun, good luck.
Tim Mc
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Can you find another MP418 to compare with? I don't think it's the loudspeaker itself...
If it were, it would be more likely to bottom out at lower frequencies (particularly at/below the box tuning freq) and that would be at levels below maximum power.
I suspect what you're hearing is a cabinet issue... a broken glue joint, failed fastener, etc that resonates at 70Hz.... or there is something loose in the box that starts moving when the box resonates at 70Hz.
Perhaps Art Welter can share his considerable experience with such things....
Have fun, good luck.
Tim Mc
Thanks for the suggestions so far. It sounds like it is coming from the driver itself but it could still be an issue in the cabinet near the driver.
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It very well could be overexcursion. This is a ported cabinet. The tuning frequency would be somewhere in the 35-40 hz range I'm guessing. The excursion is controlled at that frequency so the woofer hardly moves at all. Above and below the tuning frequency is a different story all together. I would imagine that the speaker is bottoming out around 90 because that would be the highest point in it's operating range before the low pass filter takes over (I'm guessing around 125 ish?). Also the port tuning keeps excursion under control down to the 40 hz range and the HP on the graph will prevent problems below tuning.
Therefor it seems that around 80-90 hz would be the only place where excursion is relatively left unchecked on this particular system. It wouldn't be hard to drive it into overexcursion.
Buy more subs if you want it that loud.
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It very well could be overexcursion. This is a ported cabinet. The tuning frequency would be somewhere in the 35-40 hz range I'm guessing. The excursion is controlled at that frequency so the woofer hardly moves at all. Above and below the tuning frequency is a different story all together. I would imagine that the speaker is bottoming out around 90 because that would be the highest point in it's operating range before the low pass filter takes over (I'm guessing around 125 ish?). Also the port tuning keeps excursion under control down to the 40 hz range and the HP on the graph will prevent problems below tuning.
Therefor it seems that around 80-90 hz would be the only place where excursion is relatively left unchecked on this particular system. It wouldn't be hard to drive it into overexcursion.
Buy more subs if you want it that loud.
How much does the frequency response graph relate to the actual driver excursion? I am bad at reading these graphs but it looks like 90hz is actually almost the lowest output before the crossover kicks in.
http://www.jblpro.com/mpro/PDF/MP418SP.pdf
I do agree that more subs are needed, but until that becomes a possibility I wanted to make sure we weren't just fighting a lost cause to do an already damaged driver/cabinet.
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Does NOT sound like "over excusion" i have 4 of these cabs,and they can be pushed very hard! If you where pushing these cabs hard enouph to "pop" the drivers the amp would go into protect mode before damage occured. Look at the clip indicators on the back of the cab to see if they're lighting up when its being pushed...if they are,that's you problem! if not, the problem is somewhere else??? just sayin'...
A campus ministry program I help out purchased a used MP418SP a few months back and we have been having problems with popping ever since. When the system is run anywhere near its limit we get a popping sound in the 80-100hz range. Before I had assumed we were sending out too much in the lower ranges and bottoming out the driver.
The frequency was determined by playing the 80hz warble from the Bink Audio Test CD and then using individual test tones to narrow it down. Low cut on the channel strip was turned off and the main eq's had, I believe, a 40hz low cut engaged. Every time the tone would hit the middle of the range the sub started popping. EQ was set flat for everything below about 120hz and there were no significant changes above that point.
It started popping both when pushed fairly hard at that frequency and when I was moving the fader relatively quickly at a lower level. I don't know if that part was an issue with the sound board or something to do with the sub.
Is the original assumption that we are doing something wrong and causing it to bottom out probably correct or was the driver maybe somehow already damaged when we got it? It doesn't seem right that the sub has extreme difficulty keeping up with a pair of JRX115's that are being powered to the same combined wattage. I know we aren't going to be getting a strong hit from the kick drum with only a single 18" sub in a room as big as we are in but it had trouble getting over stage volume even with just two people singing and an acoustic guitar.
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Does NOT sound like "over excusion" i have 4 of these cabs,and they can be pushed very hard! If you where pushing these cabs hard enouph to "pop" the drivers the amp would go into protect mode before damage occured. Look at the clip indicators on the back of the cab to see if they're lighting up when its being pushed...if they are,that's you problem! if not, the problem is somewhere else??? just sayin'...
It is right near the edge of clipping but it does it without the clip lights coming on. In theory that cabinet is rated for about 2400 peak and the internal amp is only putting out about 600W peak. I believe the only design difference is a slightly larger cabinet to make up the room taken by the amp. I am sure it is still possible to bottom out the driver with the right frequency though.
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The fact that it is recreatable with low level rapid fader movement suggests it could be another issue. You may even try to borrow or rent a nother cabinet of the same. If it reproduces in both you probably have some other issue, If not its possible you got a bad one "out of the box".
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Sounds almost like a 48v problem with the mixer? or dirty faders? or possibly the EQ? you could try contacting a Crown tech. to see what they say? i dont think JBL would be of any assistance... i have never experienced this issue with my MP418sp's i LOVE these cabs and you just cant find these animals anymore...they were "one of a kind" and sound great!!!
JBL's biggest mistake was discontinuing them!
The 4ohm output is 660 per channel.
A campus ministry program I help out purchased a used MP418SP a few months back and we have been having problems with popping ever since. When the system is run anywhere near its limit we get a popping sound in the 80-100hz range. Before I had assumed we were sending out too much in the lower ranges and bottoming out the driver.
The frequency was determined by playing the 80hz warble from the Bink Audio Test CD and then using individual test tones to narrow it down. Low cut on the channel strip was turned off and the main eq's had, I believe, a 40hz low cut engaged. Every time the tone would hit the middle of the range the sub started popping. EQ was set flat for everything below about 120hz and there were no significant changes above that point.
It started popping both when pushed fairly hard at that frequency and when I was moving the fader relatively quickly at a lower level. I don't know if that part was an issue with the sound board or something to do with the sub.
Is the original assumption that we are doing something wrong and causing it to bottom out probably correct or was the driver maybe somehow already damaged when we got it? It doesn't seem right that the sub has extreme difficulty keeping up with a pair of JRX115's that are being powered to the same combined wattage. I know we aren't going to be getting a strong hit from the kick drum with only a single 18" sub in a room as big as we are in but it had trouble getting over stage volume even with just two people singing and an acoustic guitar.
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A campus ministry program I help out purchased a used MP418SP a few months back and we have been having problems with popping ever since. When the system is run anywhere near its limit we get a popping sound in the 80-100hz range. Before I had assumed we were sending out too much in the lower ranges and bottoming out the driver.
The frequency was determined by playing the 80hz warble from the Bink Audio Test CD and then using individual test tones to narrow it down. Low cut on the channel strip was turned off and the main eq's had, I believe, a 40hz low cut engaged. Every time the tone would hit the middle of the range the sub started popping. EQ was set flat for everything below about 120hz and there were no significant changes above that point.
It started popping both when pushed fairly hard at that frequency and when I was moving the fader relatively quickly at a lower level. I don't know if that part was an issue with the sound board or something to do with the sub.
Is the original assumption that we are doing something wrong and causing it to bottom out probably correct or was the driver maybe somehow already damaged when we got it? It doesn't seem right that the sub has extreme difficulty keeping up with a pair of JRX115's that are being powered to the same combined wattage. I know we aren't going to be getting a strong hit from the kick drum with only a single 18" sub in a room as big as we are in but it had trouble getting over stage volume even with just two people singing and an acoustic guitar.
Hi, going through this thread and thinking about this box sight unseen, it might seem that you could have two unique problems.
One being a dirty fader. Moving it might send out low frequency noise that might make the woofer significantly.
Then the woofer driver I think might be damaged. I have replaced ported woofers that did pop around 90 or 100 Hz, exactly as you describe, and at a much lower output than they should have been capable of. They looked fine and measured fine with an ohmmeter. It turns out in my case of the speakers I was working on, they had been overdriven and the voice coil formers were damaged.
So, to summarize... The woofers might be damaged and could be popping like that for any signal that triggers them to move a certain distance. I had the ones I was working on reconed.
Then there might be one or more dirty/oxidized faders that are sending out enough low frequency rumble to make the woofers to move the distance required to make them "pop". They probably need cleaning/replacing.
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Hopefully I don't miss any of the suggestions. This is easier than multiple quotes.
48v problem: phantom is disabled. Overheads are powered by batteries because we had so much trouble with the snake in the past. That has since been resolved but it is easier to just stick with the batteries.
Dirty fader: hadn't thought about that being the reason for the problem when actually moving it. I thought the noise was in sync with the sine wave but I may have been incorrect. It is doubtful they have ever been cleaned unless there was a specific problem. The board is a mackie 1604-vlz pro.
EQ: It was flat below I believe 120hz with no significant cuts/boosts anywhere else when I was testing with the tones. Low cut at, I believe, 40hz (I need to check to verify that).
Driver/cabinet damage: This is the one that seems to be most commonly suggested, and the situation I am hoping is NOT the case. Reconing would probably be $100 + labor and a new driver would be around $500 + labor from what I have read. I wish actual potential driver damage had been investigated right when the sub was purchased rather than 6 months later. Myself and everyone else just assumed it was something to do with how we were running the system and would just keep an eye on it a few times during practice to make sure none of the changes we made at FOH were pushing it past the edge.
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Over excursion is not a function of frequency. It is more directly related to amplitude and velocity. However, frequency can have some effect on amplitude and velocity by means of air damping within the cabinet as mentioned earlier.
It is also possible that this speaker has been reconed by the previous owner. If it was NOT done by a professional, the cone could have been set at an incorrect depth within the magnet gap. Its neutral resting position must be at a depth where, the center of the voice coil (half its depth) aligns with the center of the top plate. In other words, if the voice coil is just a little too deep, it can bottom out frequently. Also, shims must be used to center it side-to-side within the gap to prevent scraping and shorting.
Another possibility is spider and surround fatigue which can result in diminished mechanical damping.
Any one of these things could be wrong and we would never know.
Again, you'll just have to try another speaker cabinet to narrow down the problem by process of elimination.
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It very well could be overexcursion.
I like the analysis, but not the conclusion. Note OP mentioned this was having a hard time keeping up with JRX115's.
I own an MP418SP/MP418S pair in my C rig. While they aren't VT4880's, one should have no issue keeping up with a pair of JRXanythings.
I'd lean toward the aforementioned structural resonance hypothesis, until shown otherwise.