An apparatus comprising an acoustic transducer arrangement configured to transmit at least one acoustic signal and configured to detect a reflection of said at least one acoustic signal, and a controller configured to determine a time-of-flight of the at least one acoustic signal, the controller further configured to determine at least a first value indicative of temperature based on said time-of-flight of the at least one acoustic signal and calibration information indicative of a relationship between time-of-flight and temperature in a space the apparatus is located.