Flanger Feedback Calculator
Calculate the resonance boost in dB produced by a flanger's feedback circuit from a given feedback coefficient. Use it when designing or setting up flangers to understand how much peak emphasis each feedback amount creates.
About this calculator
A flanger creates its jet-plane sweep by mixing a signal with a short, modulated delay of itself. Adding feedback — routing the delayed output back into the input — creates resonant peaks at the notch frequencies, making the effect more dramatic and metallic. The resonance gain at those peaks is calculated as: resonance (dB) = 20 × log₁₀(1 / (1 − |feedback|)), where feedback is a coefficient between 0 and 1 (exclusive). As the feedback value approaches 1, the denominator approaches zero, and the resonance gain climbs toward infinity — which is why a feedback of exactly 1 or more produces instability (infinite or undefined gain). This formula gives the theoretical peak boost in decibels at the comb-filter notch frequencies, helping you predict how aggressive the effect will sound before committing to a setting.
How to use
Suppose you set a flanger's feedback knob to 0.7 (70%). Step 1 – Enter feedback = 0.7. Step 2 – Check that |0.7| < 1, so the formula is valid. Step 3 – Calculate: resonance = 20 × log₁₀(1 / (1 − 0.7)) = 20 × log₁₀(1 / 0.3) = 20 × log₁₀(3.333) = 20 × 0.5229 ≈ 10.46 dB. A feedback of 0.7 therefore boosts the comb-filter peaks by roughly 10.5 dB — a pronounced, cutting metallic resonance. Increasing to 0.9 would yield about 20 dB of boost, an extremely aggressive sound.
Frequently asked questions
What happens to flanger resonance as feedback approaches 1?
As the feedback coefficient approaches 1, the formula's denominator (1 − |feedback|) approaches zero, causing the resonance gain to rise toward positive infinity in decibels. In practice this means the flanger enters self-oscillation, producing a sustained ringing or screaming tone at the notch frequencies rather than a sweeping effect. Most hardware and software flangers include safety limiters to prevent this. Musically, values near 0.9 are already very aggressive and are typically used for special effects rather than standard processing.
How does negative feedback change the sound of a flanger?
The formula uses the absolute value of the feedback coefficient, so negative feedback has the same resonance magnitude as positive feedback of the same size. However, the phase relationship differs: positive feedback reinforces peaks at certain frequencies, creating a bright, metallic character, while negative feedback reinforces different frequency points in the comb filter, shifting which harmonics are emphasised. Some flangers offer an 'invert' switch for this reason, giving a slightly different tonal flavour at the same resonance intensity. In terms of stability, both positive and negative feedback become unstable as their magnitude approaches 1.
What is a safe feedback range for a flanger without self-oscillation?
Practically speaking, feedback values below 0.85 are generally safe — they produce strong resonance (up to about 16 dB of peak boost) without risking self-oscillation in most circuits. The sweet spot for musical flanging is often between 0.5 and 0.75, giving 6–12 dB of resonance: prominent and characterful without dominating the mix. Above 0.9 the effect becomes extreme and feedback-heavy, useful for sound-design purposes but difficult to control in a musical context. Always start low and increase gradually when exploring high-feedback settings.