  We present a new representation of harmonic sounds that linearizes the
dynamics of pitch and spectral envelope, while remaining stable to deformations
in the time-frequency plane. It is an instance of the scattering transform, a
generic operator which cascades wavelet convolutions and modulus
nonlinearities. It is derived from the pitch spiral, in that convolutions are
successively performed in time, log-frequency, and octave index. We give a
closed-form approximation of spiral scattering coefficients for a nonstationary
generalization of the harmonic source-filter model.
