  Synchrotron emission is one of few observable tracers of galactic magnetic
fields (\textbf{B}) and cosmic rays (CRs). Much of our understanding of
\textbf{B} in galaxies comes from utilizing synchrotron observations in
conjunction with several simplifying assumptions of equipartition models,
however it remains unclear how well these assumptions hold, and what \textbf{B}
these estimates physically represent. Using FIRE simulations which self
consistently evolve CR proton, electron, and positron spectra from MeV to TeV
energies, we present the first synthetic synchrotron emission predictions from
simulated L$_{*}$ galaxies with "live" spectrally-resolved CR-MHD. We find that
synchrotron emission can be dominated by relatively cool and dense gas,
resulting in equipartition estimates of \textbf{B} with fiducial assumptions
underestimating the "true" \textbf{B} in the gas that contributes the most
emission by factors of 2-3 due to small volume filling factors. Motivated by
our results, we present an analytic framework that expands upon equipartition
models for estimating \textbf{B} in a multi-phase medium. Comparing our
spectrally-resolved synchrotron predictions to simpler spectral assumptions
used in galaxy simulations with CRs, we find that spectral evolution can be
crucial for accurate synchrotron calculations towards galactic centers, where
loss terms are large.
