Introduction

Real acoustic feedback appears when the sound waves from the cabinet hit the strings with enough energy (accomplished by playing loud enough) and with the right phase (accomplished by positioning the guitar at the correct distance from the cabinet). The difficult part with acoustic feedback on moderate volumes is therefore to get feedback on the desired notes—it is easy to get feedback, but if you for example need to get it on the high D, you have to find the exact distance so that the string vibrations and sound waves don’t cancel each other. The rumour is that when Robert Fripp recorded the feedback parts on Bowie’s Heroes, he made markings all over the floor in order to find the right position for different notes...

The Acoustic Feedback simulator is a mathematical model of real acoustic feedback. We have modeled how the sound waves hit the strings, how the strings interact with the amplifier and how the sound waves travel through the air back to the guitar again. But the only thing we couldn’t model is the distance between the guitar player and the cabinet (computer?). So to solve this we made an “auto-positioner”, pretty much like a Brian Eno running around with the cabinet so that Robert Fripp didn’t have to move between the different markings on the floor.

So with all these parts put together, we present to you the Acoustic Feedback plug-in!

 

Getting Started

  1. Make sure that you have a good guitar signal in to your audio interface. Use a line box or DI with a high impedance input (more than 500 kilohms).

  2. If the guitar signal is too low (with peaks lower than 12 dB), use a volume plug in or compressor to boost the signal.

  3. Insert the Acoustic Feedback on your guitar track in your favourite host software, and set all controls to the 12 o’clock position.

  4. Insert Vintage Amp Room or your favorite amp modeler after the Acoustic Feedback.

  5. Try some single string notes, such as the E on the 9th fret on the G string. Strike the note and wait for the feedback to appear. If it takes too long time, increase the Feedback knob. If the feedback is too fast and aggressive—decrease it instead. Bend the note to see how that affects the feedback.

 

Playing Techniques

The basic techniques in order to get the most natural sound from the Acoustic Feedback simulator are:

- Don’t push it. If there isn’t any energy left in your strings (ie. the strings have nearly stopped moving) the plugin won’t be able to feedback on the correct note. End your notes after a while by muting the strings or pick another note. Don’t let them ring infinitely.

- No hum or noise. Noise interferes with the feedback and shortens the lifetime of it. A short sustain (ie. dead strings) will also shorten the feedback lifetime.

- Play nice, single string and always end your notes deliberately. Sloppy playing is not rewarded!

- Notes played on low-numbered frets usually have longer sustain than notes that are played high up (for example above 15th fret).

- No chords!

If you do have a sloppy playing style, you will have to increase the Tolerance control to make the effect trigger more easily. The downside is that it doesn’t always end very naturally.

The feedback effect volume is independent of the input guitar volume! If you have a low guitar input volume, you will need to lower the Mix control in order to get a good balance between the clean guitar signal and the feedback effect.

 

User Interface

user-interface.jpg

  1. Mix

  2. Feedback

  3. Tolerance and Tolerance indicator

Mix: The mix between the DRY (original) and WET (feedback) guitar sound.

Feedback: Adjusts how aggressive the feedback is.

Tolerance: Adjusts how easy it is to achieve feedback (and how tolerant it is with sloppy playing).

Tolerance Indicator: When the light is on, you got full feedback. When it starts to fade it’s time to end the note.

Mix

Controls the balance between the feedback and the original signal. Outputs only the clean guitar sound when set on dry, and only the feedback effect when set to WET.

Set this control so that the balance between the guitar and the feedback sounds natural. This knob is usually set somewhere in the green area.

Try playing around with Mix set fully on WET You can get some pretty interesting results from that...

Feedback

Controls how fast the feedback builds up. This corresponds in the real world to the volume of the real amplifier. Low settings will give subtle harmonics, and high settings will give an aggressive feedback.

Feedback = SUBTLE (Yellow area): The feedback will usually not buildup and the effect will be very subtle.

Feedback = NATURAL (Green area): The feedback will have a slow buildup time and act quite naturally, although it might be hard to get feedback on all notes.

Feedback = WILD (Red area): The feedback will have a faster buildup time and it will be quite easy to get the feedback effect on most notes, but sometimes it won’t sound very natural.

How the Feedback control works depends greatly on your guitar, preamp, volume, what note you play etc.

Tolerance

Controls how easy it is to maintain the feedback while changing notes, bending, doing vibratos etc. This corresponds in the real world to a combination of the distance between the speaker and guitar, but also the volume of the amp. When set at the lowest setting, it will only feedback on stable notes, and when set at the highest setting, it will feedback on most things, even noise or background hum.

Tolerance = MODERATE (Yellow): Feedback only on very stable notes.

Tolerance = NORMAL (Green): Feedback on vibratos, bends and slides. This usually gives the most natural sounding feedback.

Tolerance = RAMPANT (Red): Wild and unpredictable feedback. Sometimes.

If you find it too hard to get feedback, you can do one of three things:

  1. Increase the Feedback control (to make the effect appear faster).

  2. Increase the Tolerance control (to make the effect more tolerant towards note changes and different playing techniques.

  3. Increase the input volume by increasing the gain of your preamp or using a volume plugin before Acoustic Feedback.

The timbre of the feedbacking tone will change slightly if you change the Tolerance control during a feedback This can sound abrupt and unnatural, so it’s better to only change the Tolerance when no note is feedbacking.

Tolerance Indicator

The Tolerance Indicator indicates when a feedback is on its way, and when the feedback is about to die. If the Tolerance Indicator starts to fade, make sure that you end your note deliberately, before any strange sounds appear. This is specially helpful when you have a high Feedback or Tolerance setting.

Harmonic Selector (Deluxe only)

harmonic-selector.jpg

Controls which harmonic that will dominate the feedback. In the real world, this can sometimes be controlled by changing the distance from your guitar to the amplifier. In a world built up by zeros and ones, this is done by pushing a button (or five).

16’ = SUBOCTAVE. This knob will add a subtle harmonic one octave below the fundamental. If only this harmonic is selected, the plugin will never start a feedback, but if it’s selected in conjunction with other harmonics you’ll get a very growly feedback. Use this as a you would use the “nitro” button in your favourite car racing video game.

8’ = FUNDEMENTAL. This will give you feedback on the fundamental, ie., the same note as you are playing.

5 1/3’ = ONE FIFTH ABOVE FUNDEMENTAL. This will give you a feedback on the fifth above the fundamental. Works best in conjunction with the 16’ button.

4’ = ONE OCTAVE ABOVE FUNDEMENTAL.

2 2/3’ = ONE OCTAVE AND A FIFTH ABOVE FUNDEMENTAL.

 

Using MIDI

Although the Acoustic Feedback doesn’t have any built-in MIDI support, it supports automation, and is therefore easy to control via MIDI in a number of hosts. The most effective (and fun) way to control the Acoustic Feedback is by using an expression control pedal.

Expression Pedal

It is very easy to control the feedback effect by assigning the Feedback control to an expression pedal (a MIDI volume pedal).

Connections

If you don’t have an expression pedal connected to your system, you will need:

  1. Expression pedal. The cheap ones cost about €30 and work perfectly fine.

  2. Expression pedal input. These connectors are usually found on MIDI keyboards, so take a look at the back of your keyboard to see if you have a 1/4” stereo jack with “Expression” or “Foot Control” written on it.

Connect the expression pedal to the MIDI device, and use the MIDI control section in your host software to assign the control number and MIDI channel to the feedback effect.

Feedback control

  1. Assign the expression pedal to the Feedback control.

  2. Set the Tolerance control to 1 (Natural).

  3. When ever you want to get the feedback started, push the pedal (hence increasing the Feedback control).

  4. The feedback effect will decrease as soon as you lower the Feedback control.

MIDI Automation and Touch-Ups

To really get the sound you want without controlling it with an expression pedal, you might want to use the automation feature in your audio sequencer.

  1. Start out by setting the Acoustic Feedback on a setting that you find works most of the time and record your guitar track.

  2. For parts that you don’t want any feedback on, set the Feedback control to 0%.

  3. On parts where you desperately need feedback, but your current setting doesn’t give it to you, set the Tolerance to MAX, and gradually increase the Feedback during the duration of the note you’re playing.

  4. Just before the note ends, do a quick fade out with the Feedback control, and if necessary set the Tolerance control back to it’s previous settings.

This way you can have “uncontrollable” settings, but by just using them on specific notes, you will avoid troublesome note endings and strange noises.

Sometimes it is easier to use the Mix control to fade the effect in and out, but the Feedback control will always give more realistic results.

 

FAQ

I don’t get any feedback? What am I doing wrong?

It could depend on a lot of things, but you can start by setting the all controls fully clockwise; Mix = WET, Feedback = WILD and Tolerance = RAMPANT. Then you will only hear the feedback effect with the most aggressive settings on the feedback algorithm. Make sure that you have enough input gain on your guitar signal, and strike a single note, for instance the open B string, and listen for the feedback.

When you have learned to get the feedback, decrease the controls one by one until you get a natural sounding feedback.

The feedback is too aggressive and doesn’t build up in a realistic way

If the feedback comes too fast, decrease the Feedback control. It can be a bit difficult to find that sweetspot since it depends on your guitar, but it’s usually located somewhere in the green area.

The feedback doesn’t blend in with the sound

This has probably to do with one of two things:

  1. The Mix control is set too high so the feedback effect is too loud compared to the guitar signal. Decrease the Mix control.

  2. The Feedback control is set too high, so that the feedback doesn’t build up naturally. Decrease the control.

The feedback doesn’t stop when I change notes

If the Tolerance switch is set at RAMPANT, lower it to NATURAL or MODERATE. The Tolerance controls how easy the feedback aborts when you change notes.

If it doesn’t help, and you really want the feedback to abort, make sure that you strike the strings when you change notes. Hammer-ons or pull-offs are not always enough to abort the feedback effect.

The feedback ends in mysterious ways

Decrease the Tolerance control and make sure that you end your notes deliberately. If you mute the strings using your left hand, make sure that you don’t end up with a harmonic instead. If you for example play on the 13th fret, you might end up with a 12th fret harmonic when you mute the strings, which will cause the plugin to feedback on the 12th fret harmonic note.

How to feedback on chords?

You don’t. Well, sometimes it works, sometimes it doesn’t. If you need a feedback on a chord, you could record it by using two takes. On the first take you play the chord without the feedback effect, and on the second take you play a single string from the chord with the feedback effect. With some creative mixing you can make this sound very realistic.

How do I control what harmonic the effect feedbacks on?

There aren’t any “intelligent” algorithms to decide harmonic, it’s more of a “survival-of-the-fittest” kind of thing. The strongest harmonic content of the note from your guitar usually survives in the end. The basic rule is that lower notes usually have a high harmonic feedback, and high notes have a low harmonic (or the fundamental) feedback.

How do I get that big Woodstock sound?

The Acoustic Feedback together with the White Amp from Vintage Amp Room is a good start, you can make the amp sound a little larger by setting the mic in a far-field position, but to really get that arena feeling you will have to add some stereo slap-back delays and a bit of stadium reverb.

 

Extended Features

A whole range of added control can be accessed with Extended Features. Please see the separate "Extended Features" manual for more information.

 

Credits

Ulf Kilberg – Mathematical modeling and research
Niklas Odelholm – Modeling and sound design
Oscar Öberg – Modeling
Torsten Gatu – Interface and framework programming
Arvid Rosén – Framework programming
Per Connman – Prototype modeling
Ulf Ekelöf – 3D rendering and graphics