Ambient music is a dangerous proposition for even the most studied of musicians. The combination of elongated drones, stillness and washes of synth has to be measured precisely, or the whole soufflé is going to collapse. For the most part, Matthew Cooper, the musician who records under the name Eluvium, has the formula down pat. 

Yet on his latest collection, Nightmare Ending, far too many of the songs fail to cohere. The otherwise lovely solo piano pieces "Caroling" and "Entrende" feel like throwaway interludes. "Happiness," the album's closer, floats peaceably by, trailing ghostly guitar and organ yet offering no color or substance to make you want to chase after it. The unintended effect of these flat moments is that the great tracks on Nightmare are even more amplified. The slow, tidal-wave build of buzz and fluttering piano chords on "Rain Gently" sounds epic by comparison. "Sleeper," with its use of what sounds like a prenatal heartbeat with a rhapsodic, lullabylike melody, is a song you can envision curling inside of for a warm nap.

What may have been Nightmare's undoing was trying to pour too much new material into this release. Stretching 14 songs over two CDs was a gamble to begin with. Like all the filmmakers he's been working with, Cooper needed an unmerciful editor to pare down this set to its most effective moments.

HEAR IT: Nightmare Ending is out Tuesday, May 14, on Temporary Residence.