On the assumption of the additivity of steady and acoustic flow characteristics, an equation was developed for the time variation of a siren port area required to produce a sinusoidal wave. If low efficiencies can be tolerated the port opening may be kept small and the operating pressure made large compared with the acoustic pressure; then the equation reveals the well‐known fact that the proper port area variation is nearly sinusoidal. For high efficiencies the operating pressure must be made to approach the peak acoustic pressure and the port area variation must depart widely from a sinusoid if a sinusoidal pressure wave is to be generated.A siren was designed and built to verify the assumptions. The output signal was found to be sinusoidal at the design operating pressure and to depart from sinusoidal as expected at operating pressures either higher or lower. Efficiency at the design pressure was between 35 and 45% which is between 70 and 90% of the theoretical maximum efficiency for a sinusoidal sound generator of the siren type using a dissipative valving mechanism.The derived equation might be used to determine the port area variation of a siren for generating chosen nonsinusoidal wave form by substituting, within limits, the expression for the desired wave in place of the sine function.