Any discussion of feedback must begin by examining resonance. This phenomenon often plays a large role in the generation of feedback. One definition of resonance is: increasing the intensity of sounds by sympathetic vibration. In an acoustic guitar the resonance of the top and body cavity help create the sound by ‘acoustically amplifying’ the string vibrations. If the overall resonance is ‘even’ across a wide range of notes, the loudness of all the notes will be even. A resonant peak is a note that, due to uneven resonance, is louder than the other notes. If the instrument, loudspeaker, room or stage floor have ‘resonant peaks’, feedback will occur prematurely. If a speaker cabinet has a resonant peak around ‘A 440’ (and everything else is ‘flat’), then ‘A 440’ is the note that will feedback first, as you turn up the volume. Some resonant peaks in instruments are part of the character of the instrument, and it is usually desirable to maintain them, however, resonant peaks in loudspeakers and rooms are generally not desirable and should be avoided.
When choosing an instrument you want to amplify, listen carefully for any obvious resonant peaks. Also, consider that large instruments with lots of surface area ‘hear’ the speakers more efficiently than smaller instruments (an upright bass will probably feedback at lower volume than a ukulele). Responsive guitar tops ‘hear’ the speakers more efficiently than ‘stiff’ guitar tops, so in general stiffer tops feedback less. Feedback occurs when the sound coming from a speaker excites the top of the instrument to the point where the top of the instrument feeds the sound of the speaker back to the speaker (via the pickup or microphone).
Loudspeakers and Loudspeaker Cabinet Design
Loudspeakers and the cabinets that they are installed in can have a significant effect on the threshold of feedback. Most loudspeakers have a characteristic known as ‘free air resonance’. The cone of a speaker is kind of springy and prefers one particular frequency to all of the others. This ‘resonant peak’ can be dampened by good speaker cabinet design (the air trapped in the cabinet acts as a brake on the speaker cone). In a Fender Deluxe ‘electric guitar’ amp the back of the cabinet is open, this is great for electric guitar but causes feedback at low volume with an acoustic instrument because the resonance of the speaker has not been ‘dampened’ by the cabinet.
In order to make speakers more efficient (and thereby use less amplification) some speaker cabinet designs incorporate ‘tuned ports’ or ‘folded horns’. Although this does improve efficiency, if the design is less than optimum it may lower the volume before feedback because of decreased dampening on the speaker. A well designed, fully sealed cabinet (sometimes called infinite baffle) tends to have the highest volume before feedback but also requires the most power. Using tuned port for PA and infinite baffle for monitors may be the best of both worlds for the acoustic musician.
It is impossible to generalize in this area, there are excellent tuned port systems and infinite baffle systems on the market today, therefore, it is advisable to listen carefully to a new loudspeaker system for ‘even loudness’ of every note on your instrument (no resonant peaks). At low volume, try running a number of scales and listen for any notes that ‘jump out’ more on the speaker than they do on the guitar.
Loudspeaker Cabinet Placement
Ideally, PA Speakers should be placed so that they are in front of the performer and above the head height of the audience, pointed down at the audience.
If your amp is your monitor and your PA, try placing it at your side, a little behind you (so you can hear it) and point it at the audience. Try to keep your amplifier up off the floor, position it at audience head height or above if possible. Experiment with speaker placement, sometimes moving a speaker a few feet to the left or right can have a profound effect on feedback, especially in odd-shaped rooms.
Room Acoustics
The shape, furnishings and size of the venue have a significant effect on feedback. Hard surfaces and low ‘hard’ ceilings increase the likelihood of feedback. Parallel walls and ceilings that are parallel to the floor can lead to the production of a standing wave. (A standing wave is a particular note that bounces back and forth between the parallel surfaces producing a ‘peak’ that can lead to feedback). Loudspeaker placement relative to your position in the room and the volume at which you play become even more significant if the room is working against you.
There are a few things that can help. For example, try to avoid pointing your speakers at hard reflective surfaces (or the bartender). Avoid placing your speakers (or yourself) in the corner of a room. If you find yourself in a small club having to turn down (because of feedback) to the point where the people in the back cannot hear you, try running a spare amp or speaker right out into the room. Reasonably priced, powered extension speakers are available. This method covers a larger area with less volume and can increase audio fidelity, (in bigger venues, it can lead to audio-delay problems). If the stage floor is ‘boomy’ then it becomes all the more important to try to keep your amp and or speakers up off the stage, perhaps on a chair with something soft like a cushion between the chair and amp. If you play upright bass try to ‘de-couple’ your amp from the stage floor. If your amp is heavy, some thick, soft rubber feet may be a good investment.
EQ can be very effective in eliminating feedback, but like most things there is a trade off. With acoustic instruments too much EQ can produce an ‘artificial’ or ‘processed’ sound. Continually comparing the ‘amplified sound’ to the ‘acoustic sound’ of your instrument when making adjustments to EQ will help preserve the true sound of your instrument.
Before attempting to reduce feedback with EQ, place your guitar and speakers in the position where they are to be used (preferably with someone holding the guitar). Be ready to mute the sound incase any feedback takes off uncontrollably. Turn up the volume to just before the point of feedback. Sounds that are louder than a loud conversation, over time may damage your hearing.
Removing ‘resonant peaks’ with a graphic equalizer: Slowly turn each frequency control up and then down, if any of them cause feedback then dip the level of that frequency. 10th octave graphic equalizers are preferred (to 3rd octave) for feedback control. 10th octave means that there are 10 controls for each octave, as there are 13 notes in an octave, you can turn down almost one single note without affecting the adjacent notes very much.
Removing ‘resonant peaks’ with a parametric equalizer: For each band (low, mid, high) set the notch width to fairly narrow, gradually boost the level while sweeping the frequency, if a particular frequency causes feedback then dip its level. Experiment with the width control to achieve the narrowest notch possible without affecting the sound quality (in some equalizers really narrow notches can adversely affect fidelity).
When dipping the level of a feedback frequency, basically turn down that frequency until the feedback stops, continue (turning down) until you hear the EQ effect the instrument’s tone and then back up a little.
For more information on using EQ to help control feedback download the Owners Manual (PDF) for the Pro Acoustic Mix DI(PAMDI). Using the PAMDI’s tunable EQ to deal with feedback is on page 15. There is a ‘Frequency to Pitch’ chart on page 16 that helps determine exactly what pitch the feedback is at.
If all else fails, don’t forget the old tried and tested sound hole plug, it may get you through a gig that would otherwise be lost to the dreaded feedback.