Of course, digital piano makers have put more effort into copying the tone of the acoustic piano than any other aspect. How they've done this is beyond the scope of this article (see "Digital Piano Basics" for this information), but one interesting attempt is that of adding a soundboard to the digital. The Kawai CA-91, introduced in 2006, with its Soundboard Speaker System; and the Yamaha CGP-1000 Clavinova in 2007, with its Hybrid Active Soundboard System, both use an actual piano soundboard, set in motion by transducers, to augment the conventional speakers and impart a more natural tone to the instrument.
The latest entry in the hybrid arena is also the first instrument to be formally named a Hybrid Piano. Yamaha unveiled its AvantGrand series in 2009. The AvantGrand elevates the digital piano to a new level with a number of hybrid technologies, first of which is a real piano action. As mentioned above, this eliminates any discussion of whether or not it feels like an acoustic piano action — it is one. (However, whether or not the action feels right is still a legitimate topic of discussion.) This action controls the digital voices through the use of optical sensors, which measure the velocity of the keys and hammers without physically contacting any part of the action.
All three AvantGrand models have grand-piano actions, but whereas model N3 is also shaped like a grand, the cabinets of the lower-cost N1 and N2 are closer to that of a vertical piano (which brings up the interesting observation that the decision of whether to call a digital piano a "grand" or a "vertical" is not a simple one). In 2012, Yamaha introduced the model NU1 Hybrid Piano, the first digital piano with a real vertical-piano action.
One aspect of the traditional acoustic-vs.-digital argument that changes with the addition of a real action is the digital's advantage of rarely needing maintenance. While the AvantGrand and NU1 models will never need to be tuned, eventually their actions will require some degree of adjustment or regulation. (We'll bet the piano technician will be surprised when, on arriving to regulate an action, he or she finds the "piano" is a digital.)
But there's more to the feel of an acoustic piano than its action, and this brings us to the last attribute of acoustic pianos that designers of digitals have attempted to copy: the intangibles. With the AvantGrand, one "intangible" — the vibrations generated by the strings and transmitted throughout the instrument — has been made tangible. Yamaha has added this ingredient to the N2 and N3 by connecting transducers to the action to send the appropriate frequency and degree of vibration to the player's fingers. This is where the experience of playing becomes a bit . . . spooky. Not unlike an amusement-park ride that convinces your brain that you're dodging asteroids while hurtling through space when you are, in fact, fairly stationary, the AvantGrand's Tactile Response System quickly convinces you that you're feeling the vibrations of nonexistent strings.