LR Baggs iBeam Onboard - Manual (Page 8)

Wavelengths of feedback frequencies are very long (figure13). If this frequency
is a guitar body resonance, the addition of energy from these two waves will
increase the total energy and can cause feedback. When these frequencies are
generated in time by one-half wavelength apart (see figure 14) they are OUT
of phase and they will subtract (see figure 15).
If this frequency is a guitar body resonance the subtraction of energy from
these two waves will decrease the total energy and can reduce or eliminate
feedback. So, to invert the phase at any particular frequency you can either
move one half wavelength (4 to 6 feet) away from or closer to the speaker or
just stand where you are and push the Invert button. It is likely that you will
not be exactly in or out of phase at whatever frequency is troubling you. If you
are physically between in phase and out of phase you will in effect be in
“neutral” and reversing phase electronically will have little or no effect on
feedback. You may still need to “fine tune” your position to achieve optimum
cancellation.
5.4 Feedback: It is likely that a room/speaker resonance will be first to
instigate feedback, energizing the guitar air cavity resonance (130-160 Hz) The
guitar top resonance (190-240 Hz) is usually the next to go off. As you increase
the volume these frequencies usually go off successively in order from the
lowest to the highest.
The phase invert control is your first line of defense against feedback. Usually
it is the air cavity resonance that goes off first. The optimum phase can
effectively damp this resonance. To find the optimum phase raise the volume
until your guitar just starts to feedback. Push the invert button. But be
prepared, feedback could get instantly worse! You never know if you are in a
good or bad phase until you push the button. If the feedback gets worse,
invert phase again and know you have already used one of your tools. The only
thing you can do with phase at this point is to move around a bit to optimize
the cancellation. Note: Phase is only useful as a feedback weapon if you are
relatively stationary. If you plan to move around on stage the Phase control
will be of no benefit unless you can push the button every time you move 4
feet!
Now that you have “fired” your first weapon, are you as loud as you need to
be? If yes, you’re done. If no, then keep raising the volume until the next
resonance (usually the guitar body air resonance) just starts to go. Slowly
sweep the frequency with the notch filter until it “grabs” and damps this
feedback. This effect will be easy to detect and obvious as this feedback
frequency will instantly vanish when you hit it. This should take care of the top
resonance. With the phase and notch in their optimum positions, you will find
you can play quite a bit louder without feedback.
When all else fails and you still are not loud enough you can simply turn down
the bass and even increase the treble for more “cut”.
The louder you play, the more critical your physical position is relative to the
monitors and the more precisely tuned the notch must be. Also be aware that
the guitar feedback frequencies will drift slightly as the guitar is warmed by
your body and the stage lights.
1 2 . S P E C I F I C A T I O N S
Size: 4" x 2.6" x 2.5”
Weight (including battery): 6oz
Battery: single 9v
Current consumption: 2.6mA
Battery life: 200 hrs (alkaline)
Bass +- 6dB @ 100Hz
Treble: +- 6dB @ 10kHz
Signal to noise: -90dB unweighted
Input impedance: 10 megohms
Output impedance: 620 ohms
Notch filter: off to 320hz
s p e a k e r
fig. 14
fig. 15: phase cancellation
g u i t a r
c o m b i n e d g u i t a r / s p e a k e r