Synthetic mid-IR galaxy spectra (3-30 μm) highlighting the impact of ISM parameters. Top: Variation of the spectrum as a function of ionization parameter (log₁₀U). Bottom: Variation with gas-phase metallicity (log₁₀Z/Z_⊙). Each line represents a mock galaxy spectrum generated with fixed ISM conditions while varying the labeled parameter. We see that the impact of these parameters on the shape of the PAH spectrum is subtle. All spectra have the same bolometric luminosity.