Subsonic Cartridges
A subsonic cartridge is designed to fire its bullets at velocities slower than the speed of sound (1128 ft/s at 70°F) to avoid the sonic crack caused by the bullet breaking the sound barrier. This allows the cartridge to be sound suppressed relatively easily. Additionally, in some cases, subsonic loads are often intended to avoid the turbulent transonic zone entirely, maximizing potential accuracy.
Read more about this topic: Whisper (cartridge Family)