Lou Ottens was fiddling with a reel-to-reel tape recorder one night in the early 1960s, trying to thread a wafer-thin piece of magnetic tape through mechanical guides so that he could listen to . . . something. He would later recall that he was probably trying to play a work of classical music, though he couldn’t be sure.

What he did remember was the hours he spent futzing with the machine before arriving at work the next morning with an idea. Mr. Ottens, the head of product development at Philips’s electronics factory in Hasselt, Belgium, told his team they needed to develop an audio device that was smaller, cheaper and easier to use than the reel-to-reel tape recorder.

As a result, they invented the cassette tape, a compact, plastic-encased sound machine that helped democratize music, making it easier for millions of people to hear, record and share songs. In its wake, Mr. Ottens became affectionately known by his peers as the brilliant engineer who — fortunately for everyone else — just couldn’t work a reel-to-reel.

“The legend that came from this, which of course is not very flattering for Lou, is that the cassette was born from the clumsiness of a very clever man,” his Philips colleague Willy Leenders later said, in an interview for the 2016 documentary “Cassette.”

Mr. Ottens, who died March 6 at 94, unleashed a sonic revolution with the Compact Cassette, which Philips unveiled at a Berlin radio exhibition in 1963. Billions of cassettes were sold before he spearheaded another advance in electronics, working on a Philips team that jointly introduced the compact disc with Sony in 1982. One of his daughters, Arine Ottens, said he died at an elder care center in Duizel, the Netherlands, but did not give a cause.

With blank cassettes, listeners could record their favorite songs from the radio or from vinyl records, creating the first mix tapes — on literal magnetic tape — decades before digital playlists were shared on streaming services such as Spotify. Internet outages never stopped the music, although listeners did face occasional analog issues, such as having to wind the tape with a pencil when the cassette got stuck.

The tapes also were used to record telephone messages, books, early hip-hop songs and moments of artistic inspiration, as when Rolling Stones guitarist Keith Richards drowsily hit “record” on his Philips cassette player one night and woke up to hear “Satisfaction,” along with “forty minutes of me snoring.”

Mr. Ottens’s cassette tape was about half the width of RCA’s tape cartridge, which had been released in 1958, and ran at half speed, requiring less tape and further cutting down on size. To perfect the tape player’s dimensions, he made a wood block that could fit in the side pocket of his tweed jacket, explaining that he wanted the cassette player to be not just portable, but “pocketable.”

“There was a lot of worry the sound quality would be bad. It’s like trying to paint a masterpiece on a postage stamp,” said “Cassette” director Zack Taylor, who credited Mr. Ottens with striking a balance between size and quality.

In a phone interview, he added that Mr. Ottens convinced Philips executives to share the company’s cassette technology after flying to Japan to meet with Sony, which said it was preparing to release a rival model. In doing so, he helped establish a uniform standard that ensured cassettes sold in one country would work in another.

“I can be credited for the idea, and a number of ideas in it,” Mr. Ottens later said of the cassette tape. “But the draftsmen, the electrical designers and the industrial designers, they have done the work. I have done nothing special.”

Lodewijk Frederik Ottens was born in Bellingwolde, the Netherlands, on June 21, 1926. Both parents were schoolteachers, and his father later directed the regional employment office in Hilversum, where Mr. Ottens grew up.

As a child, he passed the time playing with a Meccano model construction set. He progressed to more advanced tinkering as a teenager during World War II, building a radio during the German occupation that enabled his family to tune in to Radio Oranje, a London broadcaster that delivered speeches from exiled political leaders such as Queen Wilhelmina.

Mr. Ottens later served in the Dutch air force, although he was stationed on the ground because of poor eyesight. He studied at what is now the Delft University of Technology, supporting himself by working half-days as a draftsman at an X-ray equipment factory, and joined Philips after graduating with a mechanical engineering degree in 1952.

Two decades later, he was named technical director of Philips’s audio division. A research team at the company’s NatLab research facility in Eindhoven was working on an optical disc project when Mr. Ottens asked them to begin developing “an audio-only version of the disc,” according to Robert Barry’s history book “Compact Disc.” “By all accounts, they were not especially keen.”

As he had with the cassette tape, Mr. Ottens insisted that his team make the disc smaller and smaller — in a word, compact. “He said it could and must be smaller — it must have the size and convenience of a cassette player,” audio engineer François Dierckx told Barry.

The end result ultimately measured 12 cm (4.75 inches) across, although Mr. Ottens would have preferred it half a centimeter smaller, according to the Dutch newspaper NRC Handelsblad.

His wife of 46 years, Margo van Noord, died in 2002. In addition to his daughter Arine, survivors include two other children, Nelly and Jan Ottens; four grandchildren; and a great-granddaughter.

Mr. Ottens retired in 1986 and later said he had little affection for cassette tapes, even as hipsters and millennials helped reignite sales, with British music labels launching an international Cassette Store Day — inspired by Record Store Day — to promote the format as a throwback alternative to vinyl, CDs and streaming.

“The cassette is history,” he told Time magazine in 2013. “I like when something new comes.”

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