Simple Explanation

A type of digital filter that uses both current and past sounds—including its own output—to shape audio.

Concise Technical Definition

A digital filter that uses feedback by incorporating both past input and past output values to compute each new output sample.

Layman-Friendly Analogy

Like a recipe that reuses a little of the previous meal to flavor the next one—each result is partly based on earlier ones.

Industry Usage Summary

Widely used in audio DSP (e.g., EQs, crossovers) due to efficiency; however, less predictable phase and stability than FIR filters.

Engineering Shortcut

Recursive digital filter: output = function(past outputs + inputs).

Full Technical Explanation

An IIR filter (Infinite Impulse-Response filter) is a type of digital filter characterized by its use of both past input samples and past output samples to compute the current output. This recursive structure allows IIR filters to achieve a desired frequency response with fewer coefficients than an FIR (Finite Impulse-Response) filter, making them more computationally efficient. However, because they use feedback, they can have nonlinear phase response and require careful design to ensure stability. IIR filters are commonly used in real-time audio applications like equalization, tone shaping, and active crossovers where efficiency is critical.