Imagine singing alone. Without instruments or music, standing before 15 fierce critics who judge each tone against countless others.

Better not be flat.

Hundreds of Yardlings and sophomores were herded through endless a cappella auditions last week, some singing for up to six hours each night.

And most didn't make the final cut.

Experience, talent and pizzazz mean little when singers don't meet the eclectic demands of Harvard's elite groups, whose selections are limited by space and their need for particular vocal parts.

"We're restrained by what [parts] we need to take," says Thomas E. Flores '98, a member of the Callbacks who judged many of this year's auditions.

In any given semester, an a cappella group will take an average of only three or four new members, leaving the vast majority of talented College singers without a company.

The Opportunes, a popular co-ed group, auditioned nearly 80 singers for just two spots, says member Tara B. Purohit '99.

And single-sex groups like the Radcliffe Pitches and the Harvard Krokodiloes each saw more than 50 auditionees this year, each adding just six new members.

"It was kind of insane," says Matthew S. Trent '00, a newly anointed member of the Callbacks. "Going through the process was really grueling."

Auditions begin with preliminary tryouts, which narrows the field to roughly half the initial number, say group members.

Following preliminary tryouts, there are as many as three more nights of callbacks for those who make the final cuts.

"Three out of the four nights I ended up singing six hours straight," Trent says. "Its a lot to expect out of singers.

Tryouts make for an especially difficult second week for first-years who are still adjusting to college life, says Melissa K. Gruber '00, a new member of the Callbacks.

"A lot of people felt that there could have been one less callback," she says.