Simple Explanation

A filter that changes the timing (phase) of audio signals without changing their volume.

Concise Technical Definition

A filter that alters the phase or phase delay of a signal without affecting its amplitude (magnitude) across the frequency spectrum.

Layman-Friendly Analogy

Like nudging different parts of a sound wave forward or backward in time without turning the volume up or down—everything stays the same loudness, just slightly shifted.

Industry Usage Summary

All-pass filters are often used in speaker crossover design, phase alignment, and digital signal processing to correct phase issues without altering tonal balance. Useful in live sound and time alignment applications.

Engineering Shortcut

All-pass = phase-only filter; gain stays flat.

Full Technical Explanation

An all-pass filter is a signal processing filter that passes all frequencies equally in terms of amplitude but introduces a frequency-dependent phase shift. Unlike low-pass, high-pass, or band-pass filters, it does not alter the magnitude response of the signal. These filters are commonly used to correct phase misalignment between drivers in loudspeakers, manage group delay, or shape phase response in analog and digital audio systems. Mathematically, the magnitude of the transfer function remains unity, while the phase varies with frequency.