The reduction of the vibration amplitudes of a system subjected to external exciting forces can often be achieved by a suitable alteration in the mass or stiffness of the system. The natural frequencies of the system are thereby changed and are no longer in the neighbourhood of the peak harmonic components of the exciting force. This procedure can be shown to have little effect on the problem of jet efflux excitation (unless a really radical change of stiffness is made, such as could only be obtained by a complete change in the type of construction). This is due to the jet efflux pressure field having a fairly flat pressure spectrum and therefore containing harmonic components of all frequencies (over its range of flatness) which are more or less equal in magnitude. However, despite this flatness the spectrum of the generalised force exciting a particular mode has a large peak at the frequency where the sound wave of that frequency has the same wavelength as the structural mode (the coincidence effect).