Heat release rate fluctuations cause recurrent problems in many steady operating combustors. Direct measurements are difficult and these fluctuations are generally inferred from optical diagnostics. An alternative acoustic method was recently developed and is validated here in the case of unconfined laminar premixed flames submitted to harmonic flow modulations. The technique relies on determining the transmission time of frequency-modulated ultrasounds propagating through the perturbed flow. The transmission of ultrasounds is altered by the perturbed interface between the burned gases and ambient air and by the perturbed flame front. A theoretical link between the dynamics of these two interfaces is derived for small low frequency perturbations. The predicted shapes taken by these interfaces are compared with two-color Schlieren images. This model is then used to determine the resulting heat release rate fluctuations and corresponding disturbances in the transmission time of ultrasounds. An analytical expression is derived for the transfer function between heat release rate and sound transmission time disturbances as a function of frequency when these conical flames are submitted to harmonic flow disturbances. Measurements made with this acoustic technique are compared to analytical predictions and to optical measurements exploiting the chemiluminescence emission from the flame. Effects of the forcing frequency and modulation level are examined. Results indicate a good match between predictions and measurements for the gain and phase of the transfer functions of the lean and stoichiometric unconfined flames investigated when the input level is not too large. Issues and perspectives are then briefly discussed to extend this heat release rate measurement technique to practical configurations. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.