Marty's Sony Walkman (on the right.)
The Sony Walkman was a popular audio tape player sold in the 1980s.
Marty McFly listened to his rock music on his Walkman. Mr. Strickland confiscated any Walkmans he came across and engaged in a execution-style session placing them in a vise and crushing them during detention for violators.
Marty used his Walkman as a torture device by playing an Eddie Van Halen guitar solo which woke up George McFly. Disguised as a space alien named "Darth Vader" Marty threatened to melt his brain with the device.
Behind the scenes
According to the novelization, Marty's fourth Walkman was confiscated by Strickland and crushed when he served detention before the Battle of the Bands. Interestingly enough, Marty has another throughout the movie Back to the Future. This could be explained by the assumption that Marty simply had many others.
