The sound of several dozen young men singing “Who’ll join the union?” in three-part harmony rolled through the rehearsal room at Southwest High in Minneapolis like a freight train pulling into a station.

Their Nikes dripping pools of melted snow on the floor, midmorning eyelids still sleepy, the guys listened intently to Adrian Davis of Roosevelt High, one of several Twin Cities choir directors warming them up for a no-nonsense session with David Morrow of Atlanta’s Morehouse College yet to come.

“Sing with your mouth closed, your teeth open on that note,” Davis said. “Don’t just do it, overdo it. Then you sound like bosses.”

And basses. The authority booming from those collective reverberating vocal cords gently warned: We may have skinny 16-year-old ribcages, but you’d better be ready to join that union.

The young singers are part of a new 200-member male chorus that will join the adult VocalEssence ensemble, the Morehouse College Glee Club and guest conductor Morrow on Feb. 21 at Orchestra Hall. The annual Witness concert, staged by VocalEssence, pays tribute to the cultural contributions of African-Americans. The program this time includes classic American spirituals, the Grieg composition “Brothers, Sing On!” and a newer work based on President Obama’s 2008 victory speech.

The Morehouse Glee Club is one of the most prestigious in the nation. It has performed for several U.S. presidents and can be heard on the soundtracks of films by alumnus Spike Lee. Morrow is only the third director in its 105-year history.

“It’s the only all-male ensemble of its kind at a black college in the country, and it inspires such loyalty that each of the conductors was a former student and member who came back,” said G. Phillip Shoultz III, who was appointed associate conductor of Vocal­Essence last spring and will share conducting duties with Morrow at the concert.

“They sing not only gospel and spirituals but nontraditional European music, and part of their mission is to produce community and business leaders, not just singers.”

At rehearsal in Southwest’s auditorium, Morrow came off as a friendly drill sergeant, hustling the lollygaggers into their seats and barking, “The word is ‘made’! Don’t sing ‘may.’ That’s what comes between April and June.”

Shoultz, who goes by Phil but is affectionately known as “GPS” at VocalEssence to set him apart from director Philip Brunelle, calls his own conducting style “high-octane.”

He said, “Young people want to have fun but also want to work hard, to be engaged, not get their time wasted.”

He uses a lot of movement in his rehearsals, with liberal references to proven interest areas of teenage boys — sports and food.

“My personal workout includes a lot of boxing, so I’ve put together a warmup with jabs and hooks and uppercuts to get those bodies going. Each body shot has a different sound. Then to teach about pharyngeal space, I talk about biting into an apple and holding it to help them unlock the sensations necessary for free, natural, efficient singing. They complain at first, but when they hear how they sound, it’s invigorating.”

Shoultz and Brunelle hope that the new multi-school teen chorus, which includes some boys who have never been in a choir, will spark wider interest in choral singing, which has a rich history in Minnesota.

They have a convert in Antoine Ferguson, a junior who sings in a Minneapolis alt-rock band called the NightStones with his buddies, but wants “to better my skills.”

He also likes the “holiness feeling” of singing spirituals with a large group.

“When I sing ‘If I Got My Ticket,’ it feels like the Lord is saying, ‘Yeah, you can come. Come on along.’ ”