Composer's Note:

The undertone series is the inverse of the harmonic spectrum. Natural harmonics are multiples of the frequency: f, 2f, 3f, etc. Undertones are divisions of the series: f, f/2, f/3, etc. Whereas overtones go higher and higher, undertones go down, clustering closer and closer together until they reach zero. This work uses a compressed undertone series by using a transposed spectrum down a perfect fourth.