What’s a sonic shell?

Two halves of a plastic cassette shell are normally joined in one of two ways, by screws or sonic welding. Sonic refers to a welding method often used for plastics. Ultrasonic welding creates a solid-state weld using high-frequency acoustic vibrations applied to plastics held together under pressure. No connectors, soldering materials or adhesives are required.

Although shells screwed together can be repaired more easily, welded shells are popular because they are rigid, professional in appearance, and a bit more reliable than a shell joined together at a few selective points. Professional music releases are generally recorded on sonic shells.