Walking into Rockefeller Chapel on Friday night, I felt like I was trespassing at a members-only event. It was a concert by the Hilliard Ensemble, performing music by 15th-century composers Guillaume Dufay and Josquin Desprey. The audience, like the music, was a bit specialized.

If you’ve never heard of either composer, don’t worry. A lot of people haven’t. But everyone who had heard of them was in attendance at this concert. Waiting for the show to begin, I entertained myself by eavesdropping on my fellow audience members. One group behind me was arguing about the pronunciation of old French and the correct spelling of Desprey. The woman next to me was swapping professional updates with a friend (“Are you doing S.S. Cyril and Methodius?” “No, I have to get up for work at seven.” “Ugh.”).

Dufay is one of the best-documented pre-16th-century composers. A peripatetic court composer, he stood as the key figure in the creation of a broadly accepted European mainstream in musical style. The program was from his Missa Se la Face ay Pale, a mass based on a “highly unusual” ballade he had written many years before. Desprey, a student of Dufay, is not as well known and was represented on the program by a collection of religious settings.

Founded in 1974, the Hilliard Ensemble is now considered one of the world’s premiere chamber ensembles, both in new and old music. Friday night’s performance proved that their reputation for excellence in interpreting early works is richly deserved.

The ensemble takes its name from the British miniaturist painter Nicholas Hilliard, and it is particularly appropriate—the music of Dufay and Desprey is as closely textured as one of Hilliard’s miniatures. Near-infinite amounts of tiny, carefully textured details fill out the parts of each of the members with obsessive care. The simple tunes are elaborated by turns and runs, each part independently complicated, yet all woven together like a close-weave tapestry. Beautifully constructed, beautifully sung.

Like most beautiful and detailed things, however, early music is best appreciated if you already know a lot about it. This is intellectual music, and is not exactly accessible. Personally, I think it’s a bit unfair to ask us to sit tight for 45 uninterrupted minutes of soothing, slow-ish music. There’s a reason so many people around me were dropping off, and it wasn’t necessarily because they were uncultured.

The second half of the program sped up a bit, enlivened by graceful, simple settings from the twelfth-century St. Martial manuscript and a lovely, soaring countertenor line in Josquin’s Ave Maria. But it was still just a bit slow. The man beside me woke up and chose to spend the second half reading his book rather than napping.

The setting was also something of a problem. Sacred music tends to sound best in churches, but Rockefeller is a big place, and four men just aren’t enough to really make the place sing—no matter how close you are to the performers. As it was, the sound got a bit lost and stayed concentrated under the nave instead of spreading out to fill the audience.

Having the Hilliard Ensemble on CD is a good deal more convenient than having them in front of you. At least you can turn the CD off if you get sleepy, and take the time to listen to the same parts over and over again to understand the complexities of the music. That’s what much of this early music needs: a lot of time and concentration to be really rewarding.

On the other hand, I’m no medievalist. Maybe I should have asked the lady at the end of my row what she thought?