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.