spectral-density

Spectral density quantifies how signal power or energy is distributed across frequency. Remove the concept of frequency and spectral density ceases to operate — it literally measures distribution 'across frequency.' The removal test passes: the concept stops operating operationally.

Spectral density analysis derives from the Fourier transform: the power spectral density is computed as the squared magnitude of the Fourier transform of the autocorrelation function (Wiener-Khinchin theorem). The Fourier transform existed first and provided the mathematical foundation from which spectral density concepts emerged.

Spectral density analysis is designed for the purpose of measuring how signal power distributes across frequencies. Its entire utility in engineering is to provide a measured quantity — the spectral density — that serves measurement tasks in audio, RF, and vibration analysis.

A power spectrum is a specific kind of spectral density — it quantifies the distribution of signal power across frequency. A competent speaker calls it 'a type of spectral density.' Nearest kind is spectral-density.

power-spectral-density IS a specific kind of spectral-density: it measures power distribution across frequency for stationary signals. PSD is a specific instance of the broader spectral-density concept. Specific→general per Law 9.