Low Frequency hum - wtf - help!

Coldsummer

Experienced
I've recently noticed a problem with either my AxeFX or possibly another part of my system and quite honestly I don't have a clue why it's happening and even less of an idea of how to fix it. I'm hoping one of you guys might have an answer because it's driving me mad.

So basically, whenever I hit a B note (it's worse on the 2nd fret A string but also occurs on the 7th fret low E) and hold the note it creates a really low frequency, cyclical hum which gets louder and louder. I've only noticed it recently and it happens on different presets (more noticeable on higher gain ones) and with different guitars so I've ruled out a single preset or particular guitar as the source. In which case, it must be part of my overall set-up or maybe I've inadvertently changed a setting somewhere although I don't think I've altered anything. It's got me really stumped.

My system is; AFX II XL output 1 XLR > MOTU 8-Pre audio interface > stereo ADAM A7X FRFR monitors. The AFX is connected to my iMac by USB and the 8-Pre is connected to the iMac by Firewire. The 8 Pre is my default audio interface/output device.

Any ideas chaps???
 
Oscillating may be a better description than cyclical? My technical vocabulary isn't that great I'm afraid.
 
Is this live in different venues or in a single room, coz it sounds like a resonant frequency and you may be getting a standing wave building. Especially if it's around one particular note.
 
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It's in a single room at home. I'll get some headphones and see if it makes any difference. Thanks for the pointers.
 
I've had the same problem before. It's just the room's poor acoustics accentuating a certain frequency (or range of frequencies) giving you a low feedback. It goes away with headphones. Funny thing about it is that it has changed everywhere I've lived. In my dorm, it was A (~110Hz), in my first apartment it was B (~123Hz), and now it's D (~145Hz). It's a pain in the ass, but no fault of your gear. Put some bass traps or broadband panels in the corners and it will tame it, depending on the quality of panels you have.
 
Bass traps are good, but you will need to spend money to get some that work, the cheap foam ones don't another good idea is, if you can break up the walls with irregular surface. Ideally make it so you're walls aren't square then standing waves can't bounce back and forward and build. I know it sounds expensive but often a fake drywall wall with Rockwool is cheaper than a decent bass trap, although ideally you would have both.
Room design for playing/recording music is an art to do properly and costs a fortune. But you would be amazed with what you can do with a couple of mattresses covered in thick blankets.
 
If it's a 'new' problem in a previously usually fine room then it's possibly something in the room has been moved/changed. I had a problem with A notes - it was an acoustic in a rack nowhere near the speakers ... but sounded as if it was coming from the speakers. I had recently cleaned a few guitars and re-racked them so it was just that small movement of the acoustic that made it resonate.

I shifted the acoustic slightly so it wasn't resting in the rack in the same way and it disappeared.
 
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