SOLVED: Acoustic Bass Feedback Problem

[Fat Fingers]

I’ve learned a lot from TB and this is my humble attempt to reciprocate a little in the hopes to help somebody, somewhere down the road.

I have an acoustic 4-string Tacoma Thunderchief with the LR Baggs active preamp (with notch filter, but no phase switch). Recently, I have been gigging with it as my sole instrument in a 4-piece relatively loud honky tonk band. No matter where I stand in reference to my amp and cabinet, I can’t increase preamp gain to a reasonable level (either onboard or outboard preamp) without initiating an epic feedback drone that would put giant Malaysian cicadas to shame.

So to diagnose in the downtime, I stood directly in front of my cabinet and cranked gain on the amp until the speaker started talking back to me (careful with the ears ). Then I backed it off a little and played notes until I found where the feedback was the strongest – B3 on the 9th fret of the D-string…ok, so looking at a chart of musical note frequencies: that is… um… 246.94 Hz.

I downloaded a free audio spectrum analyzer app to my phone and had it listen to my ‘bad B3’ to confirm my offending frequency. The app I used is called, “Audizr”.

Red is peak signal strength while green is real time signal strength when I took the snapshot.

There it is: 246 Hz. Sweeping the notch filter slowly on the onboard preamp would never work to eliminate the feedback and I am not sure why. My amp head does have a semi-parametric mid, but the frequency selector knob seemed to have too wide of a bandwidth (very low Q) and would change the tone too much when cut around my bad B3.

The only fully parametric EQ (with control over Q) that I had was a filter in the Zoom B3n multi-effects bass guitar processor. So, I loaded up the ‘BassPEQ’, set frequency to 250Hz and cut the gain to -12.0.

___________Q = 0.5_____________________Q = 16

Playing a B3 after PEQ shows that it yields to the peak at 129 Hz or B2. You can see my 'bad B3', the next peak to the right of the 129 Hz peak, getting shorter with higher Q factor. And by the grace of the Sound Gods… no more feedback with either Q. There didn’t seem to be much difference in tone either. I will have to play around with the Q a little more to see if I notice anything. I also have an HPF loaded on the Zoom at 80 Hz ('LowEQ').

I am guessing that this bad “B”lood resonance is inherent to the instrument. I also wonder whether an upright bass may feedback at a lower Hz due to its more cavernous volume. Maybe someone who knows more will ‘chime-in’, but at a different frequency please .

Before trying to solve this problem: Whenever the lead guitarist would play a ‘screaming A’ chord, I could feel the body of the bass rumble and hear the feedback start to loop until I grabbed the strings and muted the instrument. Close enough to the ‘bad B’, I guess.

Praise the Parametric Princess of Sound: This bass sounds great with this band!

 

[Wasnex]

The Tacoma does tend to feedback fairly easy when it's amplified. The body panels are very thin and resonant, thus easy to excite. This is why the instrument is so loud acoustically. The amp also pumps the strings so they want to run away and ring.

Between phase, notch, and variable mid control on my instrument's Fishman preamp, I can get the bass to sound really nice, and loud enough to bury good acoustic guitars, but not loud enough for a rock set with electric guitars. If I need to play that loud, I'll use a solid body electric.

I have done a lot of work as an audio tech. Before every gig, I ring the system for feedback using either a 31 band graphics or fully parametric EQs. The following indented section was cut and pasted from one of my recent posts:

When I am amplifying a string bass, I tune it for feedback the same way I tune a PA system. Increase the gain until the bass is almost on the verge feedback and make a mental note of any frequencies that ring out. The next steps of the process varies depending upon if I am using a 31 band graphic or a parametric EQ.

With a 31 band graphic I usually run through and boost all of the frequency bands two at a time. The goal is to identify the hottest 4-5 frequency bands and dip them 6-12dB. Usually I go through three adjacent bands before I decide to make a cut. For example, slowly boost 100Hz and 125Hz then move both bands back to zero. Next, slowly boost 125Hz and 160Hz then move both bands back to zero. If you get the same frequency feeding back between all three bands you know to cut the middle band; but sometimes it doesn't work this way. Sometimes you get a different frequency each time you boost two bands.

After covering three adjacent bands, make a judgment regarding how much headroom before feedback is available and which band is closest to the frequency with the worst problem. You may decide no cut is needed, to cut one frequency 12db, to cut two frequencies 6dB, or to cut three frequencies 3dB. With practice, it will become intuitive.

With a parametric EQ, the process is a bit different. Increase the gain till the bass is on the verge of feedback. Next select a medium, narrow band, apply about 6dB of boost and slowly sweep the frequency control. Keep one hand on the gain control so you can cut it quickly if the feedback gets really bad. Note the frequencies that are most prone to ringing and eliminate them by selecting narrow band filters and dipping them 6-12dB.

If a bunch of frequencies ring close together you can use a wider band filter, but you don't want to cut as much. Try 3-4dB. Same goes for hotspots on the instrument that develop too much sustain as you increase volume, try using a gentle broad band cut.

After dialing in the EQ, it's a good idea to boost the volume to see how much gain before feedback you have built into the system. Turn up and play a bit louder than you expect to for a few minutes to make sure you have plenty of headroom and the tuning works well. Sometimes you might decide to zero out the EQ and try again, especially the first few times you tune a bass this way.
​

Some additional ideas:
1. The frequencies often relate to the overtone series so the instrument may have a tendency to feedback in octaves, fifths, thirds, etc. I believe sub harmonics may be important as well, so cutting an octave below a problem frequency can sometimes help.
2. The feedback is related to the resonance of the instrument, but also the distance between the instrument and the amp, and also may be related to resonance in the room...In other words you may need to change the EQ if you move around or when you play in a different place.
3. You can buy a rubber plug for the sound hole of some acoustic instruments that will greatly help with this problem. I even tracked down a source for a sound hole plug for the Tacoma ThunderChief a long time ago, but the links seem to have gone dead.

FYI, I get the impression that not many people do this, especially not to this degree. IMHO it's a very useful strategy for increasing volume and improving tone .

​

 

[Fat Fingers]

That adds quite a lot.
I hope to enter the realm of amplified upright soon and will be making use of your methods .

 

[Wasnex]

That adds quite a lot.
I hope to enter the realm of amplified upright soon and will be making use of your methods .

You may be able to play without any EQ, but tuning the instrument this way tends to make the response more even, and clear up/focus the tone as long as you don't go overboard. It takes a bit of practice to develop a feel for how much EQ to use. Too much and you kill your tone.

If you need to play louder, you will need to use more EQ. Think of it like a balancing act where you are trying to keep enough gain before feedback to play, but not apply so much EQ that the instrument sounds lifeless. 4 to 5 deep narrow band notches and 1 or 2 shallow narrow band cuts is pretty much the limit. Use less if you can.

Using a spectrum analyzer is a great value added tool .

 

[RustyAxe]

My Thunderchief gives me fits around D on the A string. Drove me nuts until I got an Empress ParaEQ and now I can tame those wolf notes.

 

[Fat Fingers]

Between phase, notch, and variable mid control on my instrument's Fishman preamp, I can get the bass to sound really nice, and loud...

Does the phase switch on your Fishman preamp negatively reduce bass response when engaged or does it affect bass tone? Asking because I was considering whether to try "fdeck's HPF pre" as my LR Baggs Miratone does not have the phase switch.

 

[Fat Fingers]

My Thunderchief gives me fits around D on the A string. Drove me nuts until I got an Empress ParaEQ and now I can tame those wolf notes.

Nice on the Empress. I have the Tacoma E6 preamp and I am guessing that Wasnex has the Tacoma E4 preamp. Which do you have? Thay are listed on this Fender support page:

What kind of electronic options did Tacoma offer?

 

[RustyAxe]

Nice on the Empress. I have the Tacoma E6 preamp and I am guessing that Wasnex has the Tacoma E4 preamp. Which do you have? Thay are listed on this Fender support page:

What kind of electronic options did Tacoma offer?

No idea what the model number is. It's a Baggs, with the phase button. Oval brass plate. Bass,Mid,Treble,Vol, phase ... like in this pic (from Reverb, not mine)

 

[Fat Fingers]

This would be mine (also not my pic). Sounds like yours was the E3 preamp. Do you use the phase switch to help control feedback and does it change tone?

No idea what the model number is. It's a Baggs, with the phase button. Oval brass plate. Bass,Mid,Treble,Vol, phase ...

 

[Wasnex]

Does the phase switch on your Fishman preamp negatively reduce bass response when engaged or does it affect bass tone? Asking because I was considering whether to try "fdeck's HPF pre" as my LR Baggs Miratone does not have the phase switch.

The phase switch simply flips polarity 180 degrees. The effect it produces depends on distance between the amp and instrument. Keep in mind wavelength varies with frequency, so the result depends upon the phase angle that occurs after the sound travels the distance between the amp and instrument.

If you want to get down into the weeds, the speakers and electronic processing also introduce group delay and phase lag that varies with frequency...so it's not necessarily the actual distance, but the effective distance for a given frequency, if that makes sense.

I am not an engineer so I may not be explaining it correctly or perfectly. In my experience, the phase switch is a very useful feature with acoustic electric instruments.

 

[Wasnex]

View attachment 3234245
This would be mine (also not my pic). Sounds like yours was the E3 preamp. Do you use the phase switch to help control feedback and does it change tone?

I'll answer directly this time: The phase switch helps with feedback and changes tone.

 

[Wasnex]

View attachment 3234245
This would be mine (also not my pic). Sounds like yours was the E3 preamp. Do you use the phase switch to help control feedback and does it change tone?

Couldn't find a picture of mine but it's a Fishman Prefix Plus. It works okay, but wouldn't say it's excellent.

I believe the attached manual is correct.

 

[RustyAxe]

Do you use the phase switch to help control feedback and does it change tone?

Yes. Minimal effect on tone.