"That's it," Robert Simpson remarks assertively. "Everyone sing. Just. Like. Them."

In this rehearsal, Simpson is unapologetically determined to rouse his choir to achieve a feathery, soaring melodic line with lilting forward motion. Not with a heavy forced thrust, but rather with an organic rise and fall that follows the musical shapes as they wave intrinsically across bar lines.

As natural as gravity and as impossibly delicate as weightlessness.

"Do that," he smiles.

A select few sopranos grasp what it takes to get there. Now it's time for the remaining Houston Chamber Choir singers to find their way. It isn't easy. The reverberant acoustics of St. Philip Presbyterian Church are a joy and a problem. The sound is rich — but clarity is at risk. Articulation and diction must append coherence to running passages that swirl about sequential harmonic progressions.

"Bach is the supreme master," he tells me. "His music is an inspiration and a challenge. Very few works demand such subtlety of musicianship. As such, Bach is the true test of an artist's abilities."

"Like climbing Mount Everest," I offer. "Everything else are hills and valleys."

"Right," he agrees.

Back to Bach

It's been a few seasons since Houston's oldest professional chamber choir programmed the music of German authority Johann Sebastian Bach. Recent concerts have been dedicated to Bologna's religious spectacles through the oeuvres of Giovanni Paolo Colonna, new commissions and a Leonard Bernstein retrospective.

But rather than curating a-back-to Bach musicale for baroque's sake, it was Simpson's desire to devise a slant so his devout audience could encounter a different entry point into the period's performance practice. In conversation with classical composer and pianist Paul English and jazz trio (bassist Jeffry Eckels and drummer Dean Macomber), a dialogue of Bach's timelessness emerged by combining three of his motets with contemporary genres — jazz and improvisation.

The outcome is Houston Chamber Choir's "Bach and All That Jazz," set for 7:30 p.m. Friday at Lone Star College–Montgomery and 7:30 p.m. Saturday at St. Philip Presbyterian Church.

With musings of the scene where an era-misplaced Beethoven loses it while "destroying" an electric synthesizer in a typical American mall in the 1989 film Bill & Ted's Excellent Adventure, I couldn't help asking: If Bach were alive today, what milieu of today's mishmash of music styles would appeal to him?

Somehow Madonna, Justin Bieber and Carly Rae Jepsen didn't make the cut — though I am certain Bach could formulate, if challenged, a wicked prelude and fugue on themes from "Call Me Maybe."

Simpson didn't hesitate to respond.

"Improvisation is one of the fundamentals of the baroque aesthetic," he elaborates. "That's true for musicians — vocalists, instrumentalists, keyboardists and composers alike. Embedded in the complex polyphonic textures is a spontaneity that nods to jazz."

Having grown up in the 1960s, a time when many experimental compositions sourced their material from classical oldies, the concept of recontexting the development of music history came naturally. English, a seasoned and Rice University-trained classical composer and notable local jazz figure in the Houston scene, was Simpson's choice to for this musical tête-à-tête.

Simpson and English first collaborated five years ago in reviving Duke Ellington's last composition, Third Sacred Concert, then a forgotten work that was premiered at Westminster Abbey in London in 1973. After Ellington's death, the score had survived with the late Barrie Lee Hall, a trumpeter in Ellington's band who was — serendipitously and luckily — living in Houston.

Simpson was eager to craft another venture with English, and Bach, and logically jazz, became the proper conduit. English was charged with responding to and reflecting on a traditional performance of Bach's motets.

Bach then and now

In contrast to the cantatas, which rely heavily on instruments, Bach's motets are almost purely vocal, written as training etudes for his school's choir. A basso continuo lays the harmonic groundwork and anchors voices. Whether one is of the school that the term motet comes from the Latin "to move" or from the French "word," both descriptors are applicable to these choral works in every sense of the definition.

Der Geist hilft unser Schwachheit auf (The spirit comes to help my weakness); Jesu, meine Freude (Jesus, my joy); and Singet dem Herrn ein neues Lied (Sing to the Lord a new song) are deeply emotive, are based on psalms and encompass intricate counterpoint and constantly evolving harmonies.

"And at the heart of Bach is dance and movement," Simpson says.

As I watch the choir rehearse, they can't help sway from side to side, feeling the impetus of the anacrusis as it leaps across to the stressed beats. If the singers' eyes stay fixated on the page for too long, Simpson reminds the group that emoting isn't effective if faces and eyes are buried in the written music.

"If you can't sing the music without a sense of complete abandon, Bach will die on the vine," he says. "You have to be creating and recreating when you are signing. Just like the spirit of jazz."

Yet that creation isn't left up to the complete whim of the artist. There's a structure and an implied way of Bach that overwhelms as one studies his technique. Although English is very familiar with Bach's methodology, he parsed through the motets to internalize the patterns that differentiate these pieces from the others.

"As an analogy to language, you cannot have a good conversation if you don't know the idiom," English says. "If you are going to improvise genuinely on themes by Bach, you need to understand his framework, his ideas, the harmonies, the motives and the words."

"The words?" I ask. "Isn't your trio purely instrumental?"

"Yes," he answers. "The text is more than just words. It gives reason to the harmonics, to the dynamics, why it's soft here and loud there, why there's a break at the end of this line, why the articulation is the way it is — for example. Though the instrumental notes, I suppose you can say are abstract, what we are communicating is tangible idea.

"You cannot ignore what is being said. Bach is telling a story — from start to finish."

When English decides to pick up a motivic idea, he expects his trio to recognize it and veer in that direction. If it's a rhythmic subject or an affect, it needs to be acknowledged. If there's good communicative synergy between them, what ensues is a melange of bass improvisations, melodic exchanges and harmonic play — with new material discovered and forged every time.

"Bach and his contemporaries stretched harmonies to their limits, to the point that Classicism pulled back," English says. "Bach's ability to masterfully organize consonance, dissonance and resolutions — the main concept behind western music — is what opens possibilities for jazz improvisation, and surprise us again and again."

About that surprise: Simpson promises something special and unscripted during the concert.

"I am not going to give it away, but it involves singing, listening and talking through Bach," he hints.

___

The Houston Chamber Choir presents "Bach and All That Jazz" on Friday at 7:30 p.m. at Lone Star College–Montgomery and on Saturday at 7:30 p.m. at St. Philip Presbyterian Church. Tickets range between $10 and $30 and can be purchased online or by calling 713-224-5566.