ESDU 90005 derives semi-empirical equations for the resonance frequencies that occur at subsonic speeds in wind-tunnels with walls having slots or circular perforations. Results from the equations for the first resonance frequency (which can limit the usefulness of the tunnel for dynamic testing) are presented graphically for plenum chamber depth-to-height ratios of between 2 and 4. Comparisons between predictions by the method and both test data and finite element calculations suggest the accuracy may be within 10 per cent at Mach numbers below 0.8 but may fall to 20 per cent at higher subsonic Mach numbers. A Fortran program of the method is available on disc (uncompiled) in the software volume as ESDUpac A9005. Examples illustrating the use of the hand calculation method and the program are included.