(written June, 1997)
The main concept of the concerto is the interesting sounds that result from a contrast between two performing groups of different masses. The quintessential concerto composer was Vivaldi, who boasted he could write them faster than his copyist could copy them out. He did indeed write over 500 concertos in his life, so Vivaldi's braggadocio is not so far away from the truth as one might think.
We think of the "different masses" of the concerto as being typically a lone virtuoso soloist opposed to the large, rich sound mass of a full orchestra, but there are interesting variations on this scheme. I recently heard some wonderful music by Schütz entitled Symphoniae Sacrae III which are what you might call vocal concertos; a singer, or more often a group of vocalists, are the soloists, opposed to both a larger chorus and an orchestra, which has its own instrumental counterpoint. All told, a beautiful, layered texture.
Schütz's scheme of vocal soloists, chorus, and orchestra was an interesting precedent for the finale of Beethoven's Ninth Symphony, which employs the exact same tripartite structure. Beethoven's solo vocal quartet passages are customarily an intonation nightmare in concert, as the four opera soloists don't realize their usual aria style of singing creates an awful mess when three other big voices are similarly, and simultaneously, bellowing away.
The idea of using a group of soloists as the "small mass" instead of a single soloist is of course not unique to Schütz but was a common form in early music. Bach's Brandenburg Concertos remain some of the most remarkable music ever written to my ears. More recently, Arvo Pärt's Passio utilized for the narrator's role of the Evangelist not the usual male soloist but a vocal quartet of soprano, countertenor, tenor, and baritone, an interesting twist.
Once, however, you begin to think of the concerto idea as this contrast of large and small masses of sound, you can hear concertos everywhere. The call-and-response aspect of African music, which filtered through to American ragtime and jazz, is just another version of the same idea. The field call holler in folk music; or to move indoors, the response of the congregation to its leader. Big-band music revealed its sophistication in using different masses of sound and playing them off against each other in diverse ways.
Turn on the radio and hear a great old Motown tune, and what's the texture? A lead singer, the trio of back-up singers (three women, maybe, if the soloist was male, or three men if the soloist was female, to better differentiate the layers of sound), and some sort of band as a third, rhythmic and accompanimental element. If you figure in his influence on pop music, I think you could make a case that in several ways Vivaldi was the most influential composer of all time.
Yes, if you want to, you can hear concertos everywhere...