Simple Explanation

Fs is the natural "vibration frequency" of a speaker in open air, while Fb is the tuned resonance frequency when placed in a sealed or ported box.

Concise Technical Definition

Fs (free-air resonance) is the frequency at which a driver naturally resonates in open air. Fb is the system resonance frequency when the driver is mounted in an enclosure (usually sealed or ported).

Layman-Friendly Analogy

Fs is like the pitch a guitar string makes in open air; Fb is how it changes when you press it against a fret (enclosure).

Industry Usage Summary

Fs is used to determine a driver’s low-end extension potential; Fb is used in enclosure design to align box and driver behavior for desired bass output.

Engineering Shortcut

Fs = free-air resonance; Fb = box-tuned resonance (sealed or vented).

Full Technical Explanation

Fs (free-air resonance) is the frequency at which a speaker driver naturally vibrates with the least mechanical resistance when suspended in air, determined by its moving mass and compliance. It’s a key Thiele/Small parameter for evaluating low-frequency capabilities. Fb refers to the system’s resonance when the driver is installed in a specific enclosure. In sealed boxes, Fb shifts slightly higher than Fs due to air stiffness. In ported (bass-reflex) designs, Fb is largely determined by the tuning of the port or passive radiator. Accurate knowledge of Fs and Fb is crucial in speaker design, as mismatches can lead to poor bass response or mechanical stress.