Experimental investigations are carried out on Hartmann whistles to explore the effect of external chamfer at the cavity mouth. The acoustic performance depends upon the cavity length, jet-to-cavity-spacing and external chamfer angle (15, 30, and 45). The modifications in spectral and directional characteristics of external chamfered Hartmann whistles are studied in detail and are compared with a regular cavity. The frequencies are observed to attain a minimum value at a chamfer angle of 30 along with modification in the acoustic spectra. In general, it is noticed that the external chamfered whistles are directive at an emission angle of around 45 where it is around 37 for a regular whistle. Numerical simulations portray the flow/shock oscillation features in external chamfered cavities. Simulations depict intense flow diversion at the mouth of chamfered cavities and elucidate the directivity shift as well as the enhancement of acoustic power observed experimentally. Thus, it is observed that Hartmann whistles with external chamfering could radiate acoustic power up to 2.3 times that of a non-chamfered whistle. © 2013 Elsevier Ltd. All rights reserved.