In recent years, the acoustic bass has grown less and less willing to remain a "thump-thump-thump" in the background of jazz combos. Bassists want to prove that they can offer more than rhythemic support, that they can, in fact, provide the melodic and improvisatory axis for the rest of the band to turn on.

They also want to prove that their huge, low-octave instrument has a wider range of sound than most people think. The whale's belly hollow of the bass produces an echoing resomance that approaches the tone of the saxophone. More than most other strings, it is capable of the textural shadings required for jazz. Though the capacities of the bass in jazz have been recognized ever since Duke Ellington arranged special solos for the bassists in his orchestras, never before have so many bassists won so much attention.

Perhaps the most dramatic example of this trend is Dave Holland's Emerald Tears (ECM-1-1109), two sides of unaccompanied bass solos. The idea may seem uninviting at first, but Holland remains interesting throughout, purusing reflective but restless melodies through his instrument's low range.

Holland is probably the most original bassists in jazz today, consequently his work is not always as accessible as the more popular and quite talented Ron Carter and Charlie Mingus. In concert, he has achieved tones by scraping his bow and sliding his fingers that fusion players need banks of electronic equipment to duplicate. Holland's sound, though, is much more under his control and that much more poignant.

His improvisations are true inventions; he can maintain startling melodic runs of 16th and 32nd notes without lapsing into the refuge of familiar patterns. His balladic plucking and bowing can become ramantic without sacrificing a jazz syncopation. He runs the highest notes of his instruments into straining clusters. He incorporates percussive blows against the instrument into his note-playing. Above all, he proves how the deep, somber tones of the bass can touch the most private of moods.

Holland joins guitarist John Abercromble and percussionist/pianist Jack DeJohnette on Gateway 2 (ECM 1-1105). The eloquent melancholy of Holland's own "Reminiscence" is enhanced by DeJohnette's sensitive cymbal work. But Abercrombie's repetitive and echoed fusion guitar playing mars the 16-minute group composition, "Opening," and Holland's "Nexus." Abercrombie is much better on his own "Sing Song," where his restrained melodic leads duet lyrically with Holland's energetic bass.

Holland's most impressive record as a composer and a leader remains his little-known 1973 Conference of the Birds (ECM 1027 ST). Here he leads Sam Rivers and Anthony Braxtoa through flute and saxophone duets built on his own breathtaking melodies. Though Holland has supported Braxton, Chick Corea and Miles Davis on stage and on record, his most striking collaboration has been with Rivers.

Bassist Charlie Haden has been the strong second voice in both the Ornette Coleman Quartet and the Keith Jarrett Quartet. To highlight his voice even more, Haden recorded a series of duets of his own compositions with his favorite musicians in 1976.

Released as two records last year, they bolster the contention of many that Haden is every bit as major a talent as the leaders of his two quartets. The duets with Coleman, Jarrett, Paul Motian and Alice Coltrane are on Closeness (Horizon/A&M SP-710). The duets with Don Cherry, Hampton Hawes, Archie Shepp and Coleman again are on The Golden Number (Horizon/A&M SP-727).

The most promising newcomers on bass are Malachi Favors from Chicago's Association for the Advancement of Creative Musicians and Fred Hopkins from the New York left scene. Hopkins, who has recorded with David Murray, Air and the Wildflower series, is already a master of the weeping, aliding blues notes on the bass.

But the bassistwho has consistently topped the Downbeat critics' poll since 1975 is Ron Carter. Carter was part of the classic Milos Davis Quintet with Herbie Hancock and Wayne Shorter, and remains influenced by Davis' flary but calculated style. In contrast to Holland's European and avant-garde influences, Carter attacks the bass with a passion, playing each note hard and true, but seldom pausing in his quick, melodic leads.

Before Carter took over the Downbeat bass-poll, Richard Davis led it from 1967-74. Davis is versatile enough to have played mainstream jazz with Thad Jones, symphonic orchestra with Igor Stravinsky, avant-garde jazz with Eric Dolphy and rock 'n' roll with Laura Nyro.

On his new album as a leader, Fancy Free (Galaxy GXY-5102), he opts for laid-back mainstream jazz. Supported by fusion players Eddie Henderson on trumpet and Billy Cobham on drums, Davis keeps the setting fairly conventional and mostly acoustic.

The trend for bassists to move out front should be perfectly suited to Davis, who has at times played brilliantly. He can give each note in his leaping solos a special accent without ever slowing down. Furthermore, he can shade key notes with string-squeezing in mid-phrase without ever losing his strong swing.

Before Davis, Mingus led to Downbeat bass poll for years. Last year, Mingus won in the Downbeat poll again, but as a composer not as a bassist. This is obviously where the legendary musician has been focusing his energy. His album last year, Three or Four Shades of Blue (Atlantic SD 1700), was a masterful construction of five old and new Mingus compositions in a nine-man setting.

His newest album, Cumbia & Jazz Fusion (Atlantic SD 8801) retains the enlarged group format but is drawn out of classic be-bop into foreign motifs. Each side of the record is a different film score for Italian movie producer Daniele Senatore. The title piece was written for a film about cocaine traffic between New York and Colombia and is full of Latin American percussion and brass. The second side, "Music for Todo Moro," is for a film of the same name and is full of appropriate European art music influences.

Mingus doesn't play with the flair of some of the younger bassists, but his sense of timing is so sure he can change time signatures on you and never break his calm, confident tone. More than anyone else, Mingus first proved that the bass could be a vital tool in jazz composition and bandleading. Even as his beneficiaries now swarm over the scene, the old man continues to hold his own.