The Alto or Octave Bassoon is an old instrument that has recently been revived along with the Tenor Bassoon. It is pitched one octave higher than the standard Bassoon. As the instrument is currently constructed, it has very limited keywork and range. Thus, technique is severely limited. The range is only two-and-a-half octaves, an octave less than the Bassoon. The bottom range is not fully chromatic lacking the bottom B-natural and C-sharp.
In sound, the Alto Bassoon produces a curious buzzy sound. It is almost comical in the wrong hands. I personally have played on one of these instruments, and can vouch for some of the characteristics. When I played it at a soft dynamic in a slow passage, the sound was quite charming.
The instrument has yet to be used in any significant way. Even historical literature is lacking. I feel that bassoonists and composers are waiting for an instrument to be built with fully modern keywork and range. It can easily serve as the top voice in a bassoon ensemble. It will mix well with the saxophones. Curiously, it is this instrument that shows beyond a shadow of a doubt that the bassoon and oboe families are at best distant relatives (i.e. the Bassoon is not the bass member of the oboe family). When an Alto Bassoon and a Bass Oboe are compared side by side, as I have done, their sounds are completely different, even if their ranges are exactly the same. Oboes get more sonorous as they descend; bassoons get reedier as they ascend.
Demonstration of an Alto Bassoon