Philadelphia, motet central Works by Bach and Tallis highlight two choral concerts.

The oddest part first: The Grinnell Singers from Iowa's Grinnell College are on tour not only with the Tallis motet, but also with a high-art program including works such as Suite de Lorca - four Federico Garcia Lorca poems heard amid some hugely imaginative descriptive musical effects devised by Finnish composer Einojuhani Rautavaara.

Increasingly fashionable U.S. choral composers like Eric Whitacre were represented by lesser-known works such as Water Night. Names I was encountering for the first time, like Frank Tichell, became ones I'd seek out again, particularly since his There Will Be Rest tapped into the Grinnell Singers' inner lives with the kind of performance that's not soon forgotten.

So what was the group doing in Northeast Philadelphia Monday night? No doubt the 50 singers and their director, John Rommereim, wondered the same thing as they faced an audience smaller than their own numbers.

A pity. Any number of more geographically accessible Center City churches have reasonable rental rates with a nearby public likely to appreciate the group's strengths, such as singing in credible Russian (Rachmaninoff and Sviridov were on the program) and a sound anchored by its soprano section. And in a program of unaccompanied works, such anchors are necessary.

Spem in alium wasn't consistently sturdy, but you could get more out of this live encounter with the piece than in more polished, surround-sound recordings of the 40-voice motet. Rommereim fanned out the singers in the auditorium's seating area to show how the piece's entrances travel spatially with some well-placed ricochets.

Just as daring, in a way, was the choir's performance of the Bach motet Der Geist hilft unsrer Schwachheit auf without instrumental accompaniment. There's historical evidence to support that approach. But Bach's six motets, another hard-to-scale peak in the choral literature, easily come off more as feats than as acts of music making. Such was the case here.

On Sunday, Choral Arts Society, as part of Bach Festival of Philadelphia, sang all six Bach motets in one program, wisely utilizing quiet organ accompaniment at St. Martin-in-the-Fields Church. Some choral directors would just dive into the music and hope for the best; Choral Arts' Matthew Glandorf had prescribed strategies to allow the singers to make music with these pieces that are densely contrapuntal but full of text-driven pronouncements. Glandorf also recognized the individual challenges of each - the motets are anything but a homogenous collection - sometimes using full chamber choirs, other times paring down the forces to a single voice on a part.

Once in a while, momentum flagged for lack of vital sound. Also occasionally, vocal balances went out of whack when individual singers seemed to be establishing the kind of musical signposts that enable good ensemble work. Still, the Bach motets aren't likely to be more satisfying, except with early-music specialist choirs, and you'd probably have to go to Europe to hear one of those.

Contact music critic David Patrick Stearns at dstearns@phillynews.com.