Small axial fans are widely used to cool various electric devices, wherein they are often installed in a narrow duct. Experiments and compressible flow simulations were conducted to clarify the effects of duct width and rotational speed on aerodynamic characteristics and acoustic radiation. The measurements of the sound pressure level around a fan installed in a duct with different widths demonstrate that intense acoustic resonance occurs in a narrow duct at blade-passing frequency for specific rotational speeds. Moreover, the static pressure coefficient of the fan decreases at the specific rotational speed at which intense acoustic resonance occurs. Computation with artificially weakened acoustic resonance was also performed and predicted flow fields were compared with those obtained with intense acoustic resonance at the same rotational speed and duct width to elucidate the effects of acoustic resonance on flows around fans. The computational results show that flow disturbances occur near the inner walls of the bell-mouthed inlet of the upstream duct due to acoustic resonance, which prompts inflow turbulence to fans. As a result, the flow between rotor blades becomes increasingly turbulent, causing the rise in static pressure caused by fans to decrease.