2. post-mortem(examination ): autopsy, an examination of a dead body, esp. by cutting it open, to discover the cause of death 3. infra-sound Dangerous, destructive and harmful sound waves whose frequency is lower than 20 Hz and not audible to human ears2. post-mortem (examination): autopsy, an examination of a dead body, esp. by cutting it open, to discover the cause of death 3. infra-sound: Dangerous, destructive and harmful sound waves whose frequency is lower than 20 Hz and not audible to human ears