WASHINGTON - Derogatory jokes about the viola are probably waiting to be made over the fact that Jennifer Higdon's concerto for that instrument sat for five years on a waiting list before arriving at its premiere Saturday at the Library of Congress.

In truth, the Philadelphia composer was keen to give the ordinarily brooding instrument a levity other viola concertos lack, but first she had to finish her opera Cold Mountain. The concerto, written for violist and Curtis Institute president Roberto Diaz and the Curtis Chamber Orchestra, is ultimately concerned with getting down to essentials, yielding distinctive, under-the-surface strength. (Curtis will webcast a second live performance from its Lenfest Hall at 8 p.m. Monday on its website, www.curtis.edu/CurtisPerforms).

Co-commissioned by the Library of Congress, the concerto was premiered at the historically important Coolidge Auditorium, with Diaz playing the library's "Tuscan-Medici" Stradivarius viola. Written to fit on the small stage, the orchestration was even limited to percussion that wouldn't occupy much space.

Despite that, the well-laid plans didn't quite hold up. Not only was the stage so crowded that conductor Robert Spano could barely make his way to the podium, the acoustics felt brutally dry, something like Philadelphia's Academy of Vocal Arts. The musicians didn't look uncomfortable, but they played rather carefully. Even the lovely Strad viola wasn't optimum for the score's propulsive moments (Diaz plays his own Amati on Monday).

Yet Higdon's piece was unstoppable. Not as imposing as her Pulitzer Prize-winning Violin Concerto, this concerto bypasses her typical zeal for drawing new sounds from standard instruments. No frosting on this cake. But the depth of emotion in the first movement's songlike lyricism wasn't the only evidence of a mature composer at work. Subtle construction elements kept that lyricism aloft so artfully you didn't want the movement to end.

In the hectic second-movement scherzo, viola and orchestra seemed to be racing each other in separate if related pieces, often exchanging a friendly wave. The third was a spirited series of chamber music moments emerging from the orchestration; one put Diaz in a string trio that made already well-honed music even more effectively pared down.

The concert's stealth surprise was Spano conducting his own Hölderlin Songs, based on elegaic Friedrich Hölderlin poems, each an extravagant post-Mahlerian whirlpool (orchestrated by Curtis composer David Ludwig). Soprano Rachel Sterrenberg gave herself over completely, though fine points of singer and songs will likely be more easily appreciated in Monday's webcast.