Experimental studies are conducted to investigate the effect of internal chamfer at the mouth of Hartmann resonators. Studies involve a range of nozzle pressure ratios from 4 to 6, and chamfer angles 15°, 30°, and 45°. Further, the effects of cavity length and stand-off distance are also considered. The spectra, directivity, and acoustic power characteristics are studied in detail. Detailed numerical simulations are carried out to capture the flow oscillations inside as well as at the outside of the mouth of the chamfered cavities. Computations show flow diversion in chamfered cavities and explain the shift in the directivity observed experimentally. The fundamental frequency of cavities with 15° and 30° chamfers is observed to be higher than that of regular cavities. Resonance is intensified by the presence of chamfer resulting in higher overall sound pressure levels of chamfered whistles. Thus, chamfered Hartmann whistles are found to emit more than twice the acoustic power of a regular cylindrical whistle. The tonal quality of sound is analyzed using a new metric termed as "resonance index". © 2010 Elsevier Ltd.