Audio hallucination (sub-octave)?

jaywa · #1 02-04-2009, 06:43 AM

Last night I was rehearsing at a church that just recently upgraded their sound system to some nice JBL gear. I was using my Line6 Lowdown 150 amp at average stage volume on a typical church "stage" (hollow box about 30 inches off the floor). Well basically every note I played the whole night I was hearing that note, and then almost as loud, the note below it. Even down to low Es and Fs. Very unnerving. The guitarist noticed it too and asked if I was running some kind of effect (I wasn't).

Is this sub-octave effect some sort of audio trick the room is playing on me, or should I push the issue further with their sound tech? I asked two guys at the church if they had any kind of octaver effect engaged on their system and they said they did not. I am just "guesting" there so I don't want to be too aggressive on the point but it was driving me nuts!

FWIW, the main P.A. cluster at this church is all flown from the roof (including the sub - when will churches realize that subs belong on the floor), in the center of the room with one three-way cabinet flown on either side to cover the wings. The room is about 350 capacity and is an octagon shape.

Any insights would be appreciated.

CincyBassMan · #2 02-04-2009, 07:28 AM

Some of your problem could be the subs on the ceiling, but I think most of the problem is the hollow stage and your amp acoustically coupled. Try getting your amp off the stage floor.

Russ

megadan · #3 02-04-2009, 08:20 AM

Quote:

Originally Posted by CincyBassMan

Some of your problem could be the subs on the ceiling, but I think most of the problem is the hollow stage and your amp acoustically coupled. Try getting your amp off the stage floor.

Russ

Agreed. I've never heard of this before, but try lifting and decoupling your amp from the floor and see what happens.

jaywa · #4 02-04-2009, 08:43 AM

Quote:

Originally Posted by megadan

Agreed. I've never heard of this before, but try lifting and decoupling your amp from the floor and see what happens.

OK, I will... I had the amp is in "kickback" mode with only four rubber feet in contact with the stage, but if that is still enough to cause a problem I'll experiment a little.

Jools4001 · #5 02-04-2009, 09:43 AM

Since the Lowdown has a built in Octave Divider are you Sure you didn't have it engaged accidentally?

As a user of Line 6 stuff I know how easy it is to flip inadvertantly to a program that has an effect applied

cdawzrd · #6 02-04-2009, 10:12 AM

Subs can be flown.

jaywa · #7 02-04-2009, 10:30 AM

Quote:

Originally Posted by Jools4001

Since the Lowdown has a built in Octave Divider are you Sure you didn't have it engaged accidentally?

As a user of Line 6 stuff I know how easy it is to flip inadvertantly to a program that has an effect applied

Yes, I engaged the onboard Octaver and noticed the effect was coming out of my amp speaker, then turned it off and the effect quit coming out of my speaker but was still present in the house (or at least, onstage).

pikester · #8 02-04-2009, 02:20 PM

Sometimes, if a waveform is similar over a number of cycles, your brain can perceive that it's actually a waveform of lower frequency. This doesn't generally tend to occur in natural sounds though.

A little bit of research says that some loudspeakers produce sub-harmonics, although it doesn't really say why.

http://en.wikipedia.org/wiki/Subharmonic
http://www.sfu.ca/sonic-studio/handb...bharmonic.html

The resonance of being on a hollow box probably doesn't help matters much either.

jaywa · #9 02-04-2009, 02:46 PM

It had a definite psychological effect on me (as they say extreme low frequencies can)... I was very grumpy by the end of the rehearsal.