Methods and apparatus to synthesize reference media signatures based on constituent media signatures generated by metered media devices are disclosed. An example apparatus includes a collector to collect first constituent media signatures from a first media device that identify a first portion of media, a collector to collect second constituent media signatures from a second media device that identify a second portion of the media, and a synthesizer to combine the first constituent media signatures from the first media device and the second constituent media signatures from the second media device to form a reference media signature that represents the media, the reference media signature for comparison with third constituent media signatures collected from a third media device to detect consumption of the media at the third media device.