Here's a 20cm sub that is just 6.25cm deep.

Now you can boost your car audio with the 500 Watts of pure, solid bass of a superior sub without a complex installation.

Designed for enclosed use, the TS-SW841D combines the interlaced glass and carbon fibre reinforced IMPP (Injection Moudled Polypropylene) double cone and our unique damper-less air suspension excursion control system.

A typical sub uses a damper to control the woofer’s cone excursion. This air suspension system creates a sealed air space between the double cones, effectively replacing the damper and providing superior compliance linearity - regardless of your volume input - for reproducing exceptionally clear, low distortion bass sound.