The engineer who opened the door to making recorded music customizable and portable by inventing the cassette tape, Lou Ottens, has died. He was 94.

Ottens, who worked for Philips and also helped develop compact discs, died Saturday, according to news reports from the Netherlands.

Cassettes were introduced at a 1963 radio exhibition in Berlin and eventually surpassed 8-tracks in cars in the early 1970s. With the introduction of Sony’s Walkman at the end of the decade, cassettes blossomed in the early ‘80s to become the most popular format. Interest in cassettes waned in the late 1990s, and largely disappeared in the first decade of the 21st century before seeing a revival in recent years.

Ottens, as the head of new product development for Philips in the ‘60s, developed the cassette tape as a response to the too-big and too-expensive reel-to-reel tapes. For his invention, Ottens used a small, thin wooden block as the physical model. He encouraged Philips to license the format for free, which helped increase its adoption by the industry.

"Cassettes taught us how to use our voice, even when the message came from someone else's songs, compiled painstakingly on a mixtape," documentary filmmaker Zack Taylor, director of Cassette: A Documentary Mixtape, told NPR. "So next time you make that perfect playlist on Spotify or send a link to share a song, you can thank Lou Ottens."