MANSFIELD, MASS. — LISTENING to Natalie Cole sing songs made famous by her father Nat King Cole, one can imagine two completely different reactions running through the audience.The older listeners would be reminiscing; associating these songs with their dating, marrying, and raising a family. The younger members, who weren't even born when Nat was alive, would be looking at his daughter, resplendent in a '50s style iridescent purple dress, and wondering what this R&B star was doing with the Boston Pops and singing songs containing the word "darling." With her new album, "Unforgettable," a lush and tender tribute to Nat's songs, Natalie has found a way to musically bridge the generation gap. The album resurrects the tight, big band rhythms, swelling strings, and romantic lyrics of '40s and '50s songs, and adds a contemporary feel. Released on Father's Day, "Unforgettable" has been No. 1 on the pop Billboard chart for five weeks and has sold 1.2 million records. At the Great Woods outdoor amphitheater, older listeners "aahed" the songs they saw the gentle man with the wide grin sing on his '50s TV show: "Route 66," "Mona Lisa,Straighten Up and Fly Right," and "Smile," the song penned by Charlie Chaplin, who recommended that Nat record it. Electra marketing vice-president David Bither suggests the album is filling a void for older listeners who have not been well-served by the music industry. But Natalie's appeal is cutting across generational lines. "The response from retailers has been that it's not just the 40-60 crowd," says Mr. Bither. "Younger people are buying it for themselves and for their parents." And alternative stores that normally might be playing contemporary bands like REM, are playing it. "Unforgettable" is coming at a time when standards are being being rediscovered. Linda Ronstadt may have kicked things off by joining Nelson Riddle & Orchestra in 1986 to do standards. More recently, Harry Connick Jr., whose voice has an uncanny resemblance to the young Frank Sinatra, achieved fame with the soundtrack for "When Harry Met Sally" and continues to produce standards. Sinatra's and Nat King Cole's early work is being reissued. And the resurrection of swing and ballroom dancing around the coun try is creating a desire for danceable standards. All the decisions - for the marketing, the photography, the music and the video - came out of the special quality of the album. It's both high-tech and "high touch": classic, warm, affectionate. George Hurrell, the legendary 86-year-old Hollywood photographer, agreed to come out of retirement to shoot Natalie in the dramatically lit style of the '40s. Natalie sings a "duet" with a tape of her father singing "Unforgettable." Director Steve Barron licensed the original record from Capitol Records, took everything off except Nat King Cole's voice, added a contemporary arrangement using his voice as an immovable element, and wove in Natalie's voice. The technical feat "would have been impossible 10 years ago," says Bither. IN the marketing campaign, "Unforgettable" was played for record store owners, who tend to be slightly older and would remember Nat King Cole. The campaign was supposed to continue into the fall, but the success of the song swept past it. The album represents a departure from Natalie's career in the R&B and pop worlds; she's known for high-energy dance tracks. Her first album, "Inseparable," in 1975, launched her career; three albums went gold and she's won three Grammys. But growing up with Nat, she was raised on music by Nancy Wilson, Ella Fitzgerald, Count Basie, and Lambert, Hendricks & Ross. Over the years, she's woven some of her father's songs into her concerts. Natalie is said to have thought about doing this album for most of her career but wanted to wait until the time was right.