Subsonic (speeds below the speed of sound)

Key characteristics

• Mach number: Subsonic corresponds to M < 1; many practical discussions distinguish low-subsonic (very small fractions of the sound speed) from high-subsonic nearing M ≈ 0.8–0.95.
• Compressibility effects: These are modest at low subsonic speeds but grow as M approaches unity, affecting lift, drag and stability.
• Absence of strong shock waves: Unlike supersonic flight, continuous shock-boundary behavior and sudden pressure jumps are not present in fully subsonic flow.
• Medium dependence: The same object can be subsonic in one medium and supersonic in another; see speed of sound for details on how it varies.

Applications and examples

Most general aviation aircraft, many helicopters, and commercial airliners operate in the subsonic or high-subsonic range during portions of flight — typical jet airliner cruise speeds are often around Mach 0.78–0.85. Subsonic projectiles and bullets do not produce the characteristic sonic boom of supersonic rounds. In acoustics, subsonic also describes vibrations or signals at frequencies below human hearing when used informally, though strictly that refers to infrasound rather than speed.

Other uses and notable facts

Outside physics and engineering, "Subsonic" is also used as a product name in computing, most commonly for media‑server software that streams audio and video to client devices. In aerospace design, ensuring efficient subsonic performance leads to wing shapes and control surfaces optimized for attached flow and low drag at those speeds.

Distinctions

Subsonic is one of several flow regimes. As speed increases, aircraft enter the transonic band (roughly M ≈ 0.8–1.2) where mixed subsonic and supersonic regions occur, then supersonic (M > 1) and finally hypersonic (roughly M > 5). Each regime has different design challenges, regulatory concerns and operational effects.