WM-40

The WM-40 was a stretchable compact cassette Walkman introduced in 1984 within the WM-10 family, adding auto-reverse playback and Dolby B noise reduction to the expandable body format. Its metal-and-plastic shell extended for cassette loading and closed back down into a slim portable shape, while the front-mounted controls and manual tape selector kept operation simple despite the compact mechanism. Powered by a single AA battery, it was one of the smallest players of its time to combine miniaturization with a more convenient feature set.

Sony was moving the tiny expandable concept beyond novelty into something easier to live with. The WM-40 was clearly aimed at users who wanted the smallest possible player but had little interest in manually flipping the cassette every time a side ended. It sits in the useful middle where the engineering still feels clever, but the priorities were already shifting toward everyday practicality.