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JBL VRX 918 Subs - Blown driver ------ not!


Paul Johnson:
JBL rig, new in a theatre this year. Flown each side, a VRX918, and underneath 3 932's, with another pair of 918's and another pair of 932s on the floor, with the 932s toed in to fill in a little hole, where normally some stage edge fills would have been, but the space for these was needed for other things.

It's a seasonal venue, so opened in July, ran through to October. Only a few shows in (loud ones, it must be said) we had an extra loud one in. Next day was a one man and a mic show, and resetting everything I heard a nasty noise. Playing music with the level up revealed a very rough sound to bass on certain notes - best I could describe was like an added harmonic to the bass guitar - like when you have a dodgy fret, or a buzzy bridge saddle. Lower the volume, it cleaned up. raise it and the rough sound got worse. I tracked it to the flown sub, and because we just didn't have time to take it down, I disconnected it and added an extra floor sub (non-JBL) which got us through to the end.

We did the derig and returned everything to the store. I'd ordered a new driver, so today kept the morning spare to swap the driver. Clearly I am too old and useless. I marked the speaker with some orange tape, and by the time they'd been loaded and unloaded, it had gone! The plan was to plug in the cable, tip the box up so the cone pointed upwards and I could whip the grill off and prod the cone. I set up a track with alternate 20Hz tone, kick and bass and an amp. First three performed fine. I looked at the faulty one and I whipped the grill off and did the test. It too passed, No nasty noises.

Skip to the chase. Well worth remembering this. One of them had 3 of the 4 Allen bolts holding the casters on 3 or 4 complete turns loose. With the weight off the casters and the speaker pointing forwards, certain notes shook the casters so badly it was them making a crazily loud noise - far louder than I'd have ever imagined.
Tightening them cured the issue in 3 minutes! Such a small thing with a really big impact. I'd noticed one of the British speaker manufacturers refuses to attach castors to his products - insisting on dollies. Maybe he knew best!

Mike Caldwell:
I've traced a few blown speakers down to a loose screw or some mechanical vibration in a speaker cabinet.

Tim Hite:
There was a gig out in Pioneertown CA. We'd already sound checked and everything was fine. I walked away from A rig to do a wedding ceremony and came back to horrible 60Hz noise from my rig, so I started trying to troubleshoot.

I pulled all cables from the stage boxes and couldn't find the offender. Powered off the monitors and that didn't help. After powering down my amp racks and disconnecting mains power the noise was still there.

Turns out that cicadas in the trees beside the stage whine at ~60Hz


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