I want to share with you two websites by Carla Reese that just about say it all when it comes to composing for alto or bass flute.
What I appreciate are her explanations about the dynamic capabilities and limitations of these flutes. I am often frustrated by composers who write ensemble works and choose these flutes for their range only, rather than for their timbral capabilities. Often, I am expected to match the dynamic environment of my colleagues playing trumpet, (bass) clarinet, bassoon, and English horn – instruments that more or less match the range of these flutes. When frustrated, I have been known to peevishly remark to the composer, “a saxophone would have served your purpose better.” The inevitable response is:”but I love the color of these flutes.” “Well then write so that you can actually hear the color of these flutes”, is my usual unspoken response.
As Carla explains on her site, “Dynamic range, and projection, on the bass [and alto] flute[s] is controlled predominantly by tone colour rather than actual dynamic”. She further provides a spectral analysis that compares the sound of the C, alto and bass flutes.
If you are considering writing for bass or alto flute, especially in an ensemble situation, please read her pages!