Origin of term spectrogram I'm writing up a manuscript and I use the term spectrogram quite a bit.  I've defined a spectrogram as the magnitude squared of the short time Fourier transform of my signal.  It's an estimator for a time-varying spectrum of a non-stationary process.  I'd like to cite a paper for the origin of the term but I'm not sure what to cite.  Could anyone give me some advice?
IIRC, etymology of "Spectro" is "of radiant energy". "Gram" is derived from the Greek word gramma, γράμμα which is a written learning.