Tympanate moths can hear echolocating bats and take evasive action to avoid capture. A model is described to calculate the distances at which the A1 and A.2 cells of noctuid moths would detect foraging bats. The model is constructed using empirical data of the moths' tympanic response, call intensity of echolocating bats, and echo target strengths of insects. With no excess atmospheric attenuation, the distances at which moths, detect bats are very large, up to 250 m, but adding even small values of attenuation reduces the A1 cell detection distances to below 15 m. As more attenuation is added to the model the A1 detection distances are reduced, but the A2 cell distances remain relatively stable. Reduction of source level appears to be the best mechanism by which a bat can detect the moth before the moth detects the bat. This is more successful if the bat also uses a high frequency call. It is proposed that FM bats are not able to increase the frequency of their calls beyond certain limits imposed by atmospheric attenuation. This limits the options for reducing acoustic apparency to either very short duration calls or ones of low intensity. Bats using CF calls are probably not so restricted to the lower frequency ranges and so may exploit the higher ranges to reduce their call apparency.