Dealing with Feedback from Amplified Acoustic Guitars

Acoustic guitars have a magical quality that the sound of the woods, the bracing, the construction and the internal architecture can all contribute to a different tonality. I can hear the difference between my Boucher dreadnought and my Martin dreadnought, or a Taylor Parlor and a 1932 Gibson Parlor. These differences in tones are far more real than we find in electric guitars where the majority of the tone comes from the pickups and the electronics.

However acoustic guitars are designed to be played acoustically where all the sound is coming from the guitar body itself. Unfortunately, when one gets to gigs of a certain size and with other instruments, the acoustic needs some form of amplification.

The way it was done for decades was a microphone pointed towards the end of the fretboard. This worked but because it was very dependent on the body resonance, took tweaking to prevent feedback at certain frequencies. It also locked the player in a particular position so makers started looking for ways to amplify a guitar via an internal pickup system that could feed an acoustic specific amplifier or go direct to a PA.

Different pickup methods had an impact on the potential for feedback, and this resulted in many companies going with very low output, DC powered pickups using the piezoelectric effect to capture the sound and provide a high impedance feed to the amplifying equipment.

Why Feedback Happens

Feedback occurs because an acoustic guitar is designed to amplify vibration acoustically. When amplified sound from a speaker re-enters the guitar body, especially at its natural resonant frequencies (often around 90–110 Hz for many dreadnoughts), a loop is created:

speaker → guitar top/air cavity → pickup → amplifier → speaker.

This loop quickly grows into audible feedback. This is a reality of physics, not a flaw in the guitar or the pickup per se. To address this, different types of pickups were tried, each with its own strengths and weaknesses.

Pickup Systems

Different pickup systems vary dramatically in how prone they are to feedback:

• Internal microphones produce the most natural sound but are highly sensitive to air movement and resonances inside the body. Think of a gooseneck condenser microphone inside the body.

• Soundboard transducers pick up vibrations from the top and are susceptible to resonant feedback. Think of contact pickup mounted to the top or bottom of the bridge plate.

• Under-saddle piezos are more resistant because they are mechanically isolated from airborne resonance. This is the most common deliverable because of incredibly low cost to make, that they are readily powered by a 9V battery but are very low output with poorer signal to noise ratios.

• Magnetic soundhole pickups are the least prone to feedback because they sense string vibration directly. Think of images that you have seen of pickups visibly clamped across the soundhole, evident in many photos of John Lennon’s preferred acoustic guitars.

The more a pickup “listens” to the guitar body or cavity, the more likely feedback becomes. Translated, the more accurate the pickup is relaying the true acoustic tone of the guitar, the more likely you are to get feedback. It’s one of those cases where inexpensive acoustics made of less resonant plywoods and with more simple piezos rarely feedback. It’s a good experience for many players, but some find the sound weak and nasally and are unhappy. The workaround is an external acoustic preamplifier device as I have written about before such as the excellent options from LR Baggs.

Stage Scenarios

In a stage setup, there are likely speakers to deliver sound to the audience. Less common with the advent of in-ear monitors are foldback or wedge stage monitors pointing at the musicians so they can hear themselves. Stage speakers and stage monitors particularly push the amplified sound back into the guitar body where it can be picked up again by the pickup and even vibrate the guitar top depending on stage volume. This is most prevalent where there is a live drummer or other amplified instruments.

Addressing the Problem

For years you would see a lot of work in stage setup to keep the speakers far from the amplified acoustic guitar, but smaller stages don’t allow for that. Also common was to see a thick rug rolled out for the acoustic player to stand on that extended in front of the player. This rug deadened audio reflections from the stage itself and worked quite well, but is a challenge for small bands or single instrumentalists to deal with.

An inexpensive and very effective solution is the soundhole plug. It’s basically a rubber plug that covers the soundhole and that prevents external sound from entering the guitar body. Tommy Emmanuel has used these to superb effect because his guitars use internal condenser microphones that are very susceptible to feedback.

Another option that is more expensive but is tweakable to different venues is an EQ pedal. The better ones also include a dedicated selector called a notch filter as well as a high impedance to low impedance converter. If you play live a lot, this is my recommended solution because it is the most flexible, although getting it nailed may take a few more minutes at soundcheck.

Using an Equalizer Pedal

Most better acoustic amplifiers have this function built in, although many don’t which makes them overpriced underpowered PAs. A pedal takes away any responsibility from the amp or the PA as most PAs don’t have decent equalization at all.

Feedback often emerges when certain frequency bands are boosted—especially:

• Low mids around 100 Hz (body resonance)

• Upper mids around 700–1.2 kHz (soundboard “honk”)

• High frequencies if internal mics are used (air resonance)

Improper EQ can excite resonances that would otherwise remain controlled. In this scenario EQ should always be subtractive not positive. Here are some suggested EQ settings that are excellent places to start. While graphic EQ pedals will work, my experience tells me that parametric EQ pedals are better because you can control the width (the “Q”) of the cut.

Cut the resonant low mids

• A narrow cut around 90–120 Hz often removes the primary feedback frequency.

Manage upper-mid “hot spots”

• Dips around 700–1,000 Hz can tame boxiness and suppress feedback.

Avoid unnecessary boosts

Flat EQ is often better than boosting “problem” frequencies for warmth or presence.

Some EQ pedals and the aforementioned acoustic amps and dedicated preamps will include a notch filter that seeks out feedback loops and makes an effort to suppress it once you set it in the feedback range.

If you are hearing rumble or find a loss of clarity, check to see if your pickup system or preamp or pedal offer phase inversion. This simple switch flips the phase of the output single. It won’t help feedback but can make a big difference with rumble or muddiness.

Recommendations

My personal recommendation for an EQ pedal is the EQ2 from Source Audio. I always use one when recording or when I very rarely play live. There are others, some less expensive but they lack the true parametric capability as well as the ability to store multiple EQ presets, important if you switch guitars in a set.

If you want a preamp that does impedance conversion with dual notch filters as well as a built in tuner and 5 band EQ, the LR Baggs Venue is what I recommend. It lacks the preset capability of the EQ2, but I find the impedance switching a critical function for live play as well as in the studio with long cable runs. And its preamp is the best in the business. As a generalization, I like an external preamp for acoustics even if the internal system is a really good one like Taylor’s Expression System/2 or the highly tweaked PRS/Fishman system in the PRS SE acoustics. The lower the quality of the guitar pickup, the more important an external preamp becomes. There is no such thing as a free lunch.

I hope that you found this article informative and useful. Until next time.