Confused about an amp's Notch filter / feedback

[LiveMusic]

Sorry... dumb rookie questions.

 

What causes feedback? I'm sure there are lots of reasons. But is being too close to the amp a problem? How about playing in a real small room?

 

Playing Taylor 310BCE with Fishman preamp onboard going through Fender Acoustamatic Junior. If it matters. Anyway, the amp has a notch control for feedback. I don't know whether to increase it or decrease it if feedback is a problem. I can't tell much difference. If I have the volume jacked up so I can pick fingerstyle, I get feedback when I hit the "A" base note. I got something outawhack here.

 

Ya know, I don't understand something. You spend what I consider a considerable amount of money on a guitar and an amp and they hardly tell you beans about how to operate the dang thing. I mean, how the heck does somebody learn how to twist all these knobs without having a mentor show you? It should be pretty simple to write up a few pages on how to operate what you bought. Wouldn't have to be a big, thick manual. A few pages would help a lot. (Fender's manuals suck.)

[aframe9999]

Feedback occurs when a particular frequency coming from a speaker is reintroduced back into the amplification via a mic or pickup or whatever. It keeps amplifying itself (like a vicious circle) until you get that annoying squeal.

 

A lot of factors influence feedback. How close the mic (or pickup) is to the speaker, where it's placed in relation to the speaker, acoustics of the room, what the ceiling is like (in the case of mics and monitors.....sound bounces off the ceiling from the monitor and into the mic) etc. etc.

 

Feedback will occur at only a very specific frequency. If you can find what that frequency is by means of an EQ or Notch filter, and turn that frequency down, the feedback will go away.

 

A 31 band EQ is used on a PA to control feedback in the monitors. The system is turned up until you begin to hear the beginnings of feedback. The pitch or tone of the squeal is where the feedback is occuring in the frequency spectrum. (How high or low it is.) sometimes it will be an earsplitting high squeal, or a low rumbling, or somewhere inbetween. One of the sliders on the eq will be close to the problem frequency. so turn that one down. if its a real high squeal, it might be like 10K or 16K. if it's a low rumble, it might be like 125 or 250. A 31 band EQ can turn down any number of frequencies at once. the problem is there are a lot more frequencies than just 31. so when you turn down, say 10K, you're turning down a much bigger chunk of sound than just 10k. You're also turning down the frequencies AROUND 10K as well. In other words, if you have a pizza and cut it in 8 slices and take one piece out, you're taking a significant amount of the pizza away. If you take the same pizza and cut it into 1000 pieces, each piece you take out leaves much more pizza.

 

So here's the difference in a Notch Filter. A notch filter does the same thing as an EQ except it takes a much thinner slice of sound out. A much more specific frequency. But usually it will only do one frequency. So you're taking one piece of pizza that has been cut in 1000 pieces. But you're only allowed to take out 1.

 

So, When you get feedback in your guitar, figure out where the feedback is occuring in the frequency spectrum. (is it high, low, middle whatever) and adjust the notch filter so that specific freqency is cut. That should eliminate the problem. However, if you're too close to the amp, or playing very loud, the problem won't go away with just a notch filter.

 

Hope this helps....

 

 

 

------------------

Andrew

 

My Band:

http://www.encoreentertainment.net/rockcandy.htm

[Mile]

Problem with acoustic guitar is that it has that hole in it. Inside guitar vibrations from strings get amplified and you got yourself nice and loud sound (unlike electric guitar). Now all frequencies get amplified: wanted and unwanted. If you try to jam with bass guitar sometimes you'll get that low rumble out of your guitar (I know my started shaking when my brother hit specific note on bass guitar). I think that there are little rubber or plastic covers for sound holes... Get one and try if it eliminates feedback.

Oh, and tell what you achieved.

[WoodSound]

Dear LM: If you have the Fishman "blender" with mic

and transducer, definitely FIND the Manual which

is shipped with your Taylor and follow it to the

letter. You won't have proper acoustic tone unless

you set up your pre-amp controls properly. Most

acoustic floor amps are set up to receive a piezo

signal already, so if there is a passive/active

switch, set it on passive. Set your guitar volume

control to between 1/2 to 3/4 turn from "zero".

Set your amp gain very low, just above "zero."

You should be able to sit in front of your amp

now and play normally and listen to your tone.

Adjust the EQ and brightness to a satisfactory

acoustic tone (very small settings needed).

Once happy with this, move to the SIDE of your

amp and bring up the amp volume a little until

satisfied. Most acoustic amps perform well at

lower volumes(opposite the electric variety.)

Avoid soundhole baffles if you can, they dampen

the vibrations and spoil the tone. If you have

the blender pre-amp (with mic) and not the

set-up manual, send me an E-mail and I'll walk

you through it and give you a ballpark startup

setting. Let us know how you are doing. Bob Wood.

WoodSound9@aol.com

[dave251]

"Feedback" occurs when a frequency from the source(your guitar) is IN PHASE with the nearest or strongest loudspeaker. That is, the frequency at which the speaker diaphragm and your guitar top are moving in the same direction at the same time. You can move closer or farther away from the speaker and the feedback should subside AT THAT PARTICULAR FREQUENCY. You may get resonance exageration or feedback at another frequency. Open back speaker cabs exascerbate the situation. Non-linear(ie, non-flat response) speakers can also cause problems. If your amp has a phase invert switch, try that...it will help for that particular location/relation between the guitar and speaker.

 

An acoustic guitar has two basic feedback nodes: the top resonance(usually up around 180hz to 250hz) and the box resonance, which will be in the 90hz to 125hz range, depending on woods and body size. Your "A"not sounds like the latter.

 

I usually try to control box resonance through the phase switch, and the top resonance through just enough eq to get the job done. Also, try putting the amp behind you, with yourself in between the guitar and monitor.

 

The notch filter "sweeps" the audio spectrum with a "dip" shaped curve...thus you will need to locate your problem frequency and set the control at that point.

 

An internal mic will give you more problems than the saddle 'ducer, and will be much more usable the larger the room and stage area you perform in.

 

Dave

 

www.electrocoustic.com

[WoodSound]

I forgot to mention in earlier post that the

Fishman Blender pre-amp has a phase reverse switch

at the on-board controls. See Fishman's manual for

advice using it. Flip it back and forth when in your

stage/practice position: the "warmer" sound is

usually the correct one. I find blending in about

25 % of the mike warms the sound nicely, but can

start mike feedback when you go higher.