Examples of mixture of 'almost periodic' signals/data in nature or practice I am working on algorithms to separate mixtures of 'almost periodic' signals. One such example is composite ECG consisting of maternal and fetal signal. 

Can anyone help with examples of such signals? The signal can also be from internet data.
The sound produced using the lowest pitched strings of large stringed music instruments often includes slightly inharmonic overtones (due to finite string thickness and stiffness), which renders the waveform of the note pitch not quite, but "almost periodic".  You can record these sounds from the bottom octaves of most (physical, stringed) pianos, and bass guitars, etc.