These 18" passive radiators have a cast aluminum basket with a 1" (25mm) flat MDF disk, a cardboard former (to which you can add up to 400g of mass*), and a large half-roll surround which up to supports 3" peak to peak.

PR1821 Specifications - Bare

Fs: 3.65 Hz
Qms: 67.72
Vas: 1840L
Cms: 0.9
Mms: 2100g

PR1821 Specifications - With 400 grams added weight

Fs: 3.3 Hz
Qms: 74
Vas: 1840L
Cms: 0.9
Mms: 2500g