S 21a is a compact, high-performance self-powered touring subwoofer, designed to complement our FR-X, ARES 8 and ARES 12 series in sound reinforcement applications scaling to full stadium shows.

S 21a houses a long excursion 21″ neodymium driver and combines front diffusion, bass reflex, and transmission line design for maximum efficiency and outstanding SPL.

Rental companies favor S 21a as an alternative to industry-standard 2×18” subs, saving half the truck space and around 40% of the weight. The subwoofer can be deployed standalone, stacked or flown, making it a versatile, high-earning hire stock staple.

An advanced AUDIOFOCUS SCiO Class D amplifier module with integral DSP and versatile presets allows S 21a to be quickly deployed in any configuration, including cardioid and end-fire.

S 21a is built with pride in Belgium and coated in tough, weather-resistant polyurea to safeguard your investment. The cabinet front is protected by a rigid metal grill, backed by acoustically transparent foam.


60 kg
28 Hz - 100 Hz
8 Ohms
Radiation direct bass-reflex
Omni 360°
Pole Socket
Config cardio possible
Made in Belgium




Tech S21a

Technocal Sheet S21a


A4M SCIO User Manual