Subsonic means traveling slower than the speed of sound. At sea level on a standard day, the speed of sound is about 761 mph (1,225 km/h), so anything moving below that threshold is subsonic. The term comes up most often in aviation and firearms, but it applies to any object moving through air or another medium.
How Subsonic Speed Is Measured
Speed relative to sound is expressed as a Mach number. Mach 1.0 equals the speed of sound. Anything below Mach 1.0 is subsonic. A car on the highway at 70 mph is traveling around Mach 0.09. A commercial jet cruising at 580 mph is around Mach 0.85. Both are subsonic, just at very different points on the scale.
The speed of sound itself isn’t fixed. It changes with air temperature. In cold air at high altitude, sound travels slower than it does in warm air near the ground. At sea level and 68°F (20°C), sound moves at about 767 mph (344 m/s). At 32°F (0°C), it drops to roughly 741 mph (331.5 m/s). This means an aircraft flying at the same airspeed can be closer to Mach 1.0 at altitude than it would be near the ground, simply because the sound barrier is lower in colder air.
Subsonic Flight in Commercial Aviation
Every commercial airliner you’ve flown on is a subsonic aircraft. Most modern jets cruise between Mach 0.80 and 0.85, roughly 500 to 560 mph at cruising altitude. The Boeing 747, one of the fastest subsonic jetliners ever built, cruises at Mach 0.85. Newer widebody jets like the Boeing 787 and Airbus A350 fly in the same range.
Airlines fly subsonic for good reason: fuel efficiency drops sharply as you approach and exceed the speed of sound. Aerodynamic drag increases dramatically near Mach 1.0, and the engines burn far more fuel to overcome it. The Concorde, the most famous supersonic airliner, used older turbojet engines with high fuel consumption. Its thin delta wings and long narrow body could only carry a small payload relative to its fuel load, which made ticket prices enormous and the economics unsustainable. Subsonic jets, by contrast, optimize their cruising speed just below a point called the drag divergence Mach number, where drag is still manageable and fuel efficiency stays high.
What Happens Near the Sound Barrier
Even though a plane is flying below Mach 1.0 overall, the air flowing over its wings accelerates faster than the aircraft itself. This means pockets of air on the wing surface can reach the speed of sound before the plane does. The speed at which this first happens is called the critical Mach number, and it typically falls around Mach 0.8 for most airframes.
Once air hits sonic speed on parts of the wing, small shock waves begin forming. These cause a sudden increase in drag, a drop in lift, and changes in how the aircraft handles. Pilots in early jet aircraft sometimes called this “shock stall” because the aerodynamic changes felt abrupt and alarming. The speed range between roughly Mach 0.8 and Mach 1.2 is called the transonic zone, a tricky in-between region that’s neither purely subsonic nor fully supersonic. Engineers design commercial aircraft to cruise just below this boundary, getting as much speed as possible without entering the turbulent transonic range.
Subsonic Ammunition
In firearms, subsonic has a very specific meaning: any round traveling below approximately 1,125 feet per second (about 767 mph). That’s the speed of sound at sea level. Most standard rifle and pistol cartridges are supersonic, meaning the bullet breaks the sound barrier and creates a small sonic crack as it flies. Subsonic ammunition is specifically loaded to stay below that threshold.
The main advantage is noise reduction. When a bullet breaks the sound barrier, it produces a sharp crack that no suppressor (silencer) can eliminate, because the sound comes from the bullet itself, not the gun’s muzzle. Subsonic rounds avoid this crack entirely. Pairing subsonic ammunition with a suppressor reduces the overall noise level significantly more than using a suppressor with standard supersonic ammo. This makes subsonic rounds popular for hunting, where hearing protection matters, and for tactical applications where a reduced sound signature is valuable.
The tradeoff is energy. Slower bullets carry less kinetic energy at impact, which can reduce stopping power and effective range. A typical subsonic .300 Blackout round, for example, leaves the muzzle at around 1,000 feet per second, well under the 1,100 fps ceiling considered necessary to qualify as subsonic. That’s considerably slower than a standard supersonic rifle round, which might exit at 2,500 to 3,000 fps.
Subsonic vs. Infrasonic: A Common Mix-Up
People sometimes confuse “subsonic” with “infrasonic,” but they refer to completely different things. Subsonic describes the speed of an object. Infrasonic describes the frequency of a sound wave. Infrasound is any sound below 20 Hz, the lower threshold of normal human hearing. You can’t really “hear” infrasound in the traditional sense. Below about 10 Hz, people perceive individual pulses rather than a continuous tone. Low-frequency sound between 20 and 200 Hz is audible but often felt as much as heard, like the deep rumble of thunder or a subwoofer.
The confusion is understandable because “sub” means “below” in both cases, and both terms involve sound. But subsonic is about how fast something moves through air, while infrasonic is about how low-pitched a sound wave is. A subsonic aircraft isn’t producing infrasound (necessarily), and an infrasonic rumble has nothing to do with breaking the sound barrier.