Britain's King's Singers have recorded popular music before, in among their usual Renaissance and contemporary fare, but the 2013 release Great American Songbook marked a departure from their earlier work in several ways. What you'll think of it may depend on how attached you are to the classic King's Singers a cappella sound, but there's no question that the group deserves credit for pushing its own boundaries after nearly half a century in existence. The album, as promised, consists of classics of American song from the Broadway and Tin Pan Alley era; the largest group is by Cole Porter, the sexual nature of whose lyrics takes on a somehow disembodied quality when sung by this group, and by Rodgers & Hart. The first new wrinkle lies in the arrangements, by British composer Alexander L'Estrange. They're unusually elaborate and well-tailored to the voices of the Singers, who have remained startlingly consistent in their sound over the years despite numerous changes in personnel. L'Estrange has a way of breaking the melodies down into individual atoms and distributing them among different singers, bringing to mind forms of contemporary composition. The second novelty here is the presence of an orchestra on disc two. The King's Singers have performed and recorded with orchestras before on many occasions, but the a cappella/orchestra division over two discs is more unusual. As it happens, they seem less enthusiastic about the orchestral music this time around; the second disc clocks in at just over 31 minutes, just slightly more than half the length of the first. The arrangements are less complex, and the group seems unenthused by the flaccid playing of the South Jutland Symphony Orchestra. The action is on the first disc, which does represent one of the King's Singers' more complex pieces of work. Finally there is the sound; Great American Songbook seems to contain more than the usual quota of multi-tracking and other electronic tweaking, although you don't really learn anything from the booklet other than a credit for "wonderful post-production magic." In all, an interesting new direction for a veteran a cappella vocal group.