The YAMAHA P115 digital piano provides some additional functions, mainly in the ISO APPS, and the sound effects .
The P-115 has 192 voices polyphony (versus 128 in older versions of the piano, in the P-105) and improved tone reverberation and reverb effects.
Other upgrades to the P-105 and P-115 include additional drum rhythms, speaker on/off (while using aux to disable the onboard speaker line out) and sound enhancements,
Also, the P115 adds some EQ and volume.
The P-115 has dual mode just like the P-45, with the ability to split the keyboard into two halves (each with its own "MIDDLE C"),
This way you can practice together on the same piano.
Finally, the Yamaha P115 can use the built-in 2-track recorder to record songs and outputs using USB/host out of practices or ideas.
This foreign website points out in detail the difference between YAMAHA P115 and YAMAHA P105,
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