Acoustic unsteadiness of sprays from pressurised metered-dose inhalers

A technique has been developed for the measurement of acoustic unsteadiness of sprays from pressurised metered-dose inhalers.An ensemble (n≥126) of acoustic measurements of sprays were obtained for two formulations with and without ethanol. Acoustic signals were analysed using the Hilbert-Huang Transform to obtain their amplitude envelopes, allowing the instantaneous energy to be determined. Ensemble statistics of each formulation’s instantaneous energies allowed the determination of time-variant unsteadiness. The performance of the technique is demonstrated using two formulations with different steadiness characteristics; internal flow pattern phase contrast visualisations are presented that show the flow pattern varies by formulation. Unsteadinesses are presented for both formulations and is approximately 50% higher for the propellant-only formulation in the first 100 ms of injection. Variation of the unsteadiness with low-pass filtering of the acoustic energy demonstrates that the unsteadinesses are more separable when short-duration fluctuations are removed.