The piano is for music

First things first: the piano is of course a musical instrument, not just a box of strings. As a musician, its most important qualities that are changeable through fine tuning, are its intonation and the beauty of its tone. Many people think that piano tuning is just about "getting the notes to the right pitch", as if pitch were the only quality of musical notes. In fact, the distinguishing beauty of a piano in good tune does not lie in the pitches of its notes at all. It is already taken for granted that that the notes should be at the right pitch. The distinguishing feature of excellence, as far as the sound is concerned, lies in the beauty of its tone. So it is, that the mark of excellence in piano tuning does not lie merely in the production of notes "at the right pitch". It lies in tone production. Tone production is possible largely (but not exclusively) because over most of the compass there is more than one string to each note. In fact, there are up to six strings producing the tone of every interval, octave, double octave and triple octave, etc. Collectively, the intonation and tone of the whole complex network of notes, tempered intervals and octaves, determines the tone of the instrument as a whole.    

Tone production is nevertheless probably the least acknowledged, and least understood part of the art of piano tuning. Master tuners, of course, understand it "instinctively" and "intuitively" from years of experience through which they develop an audiophile appreciation of piano tone, its behavior, and how to control it. Consequently they know how to produce it at its best, on any given instrument, even if they can't explain precisely how they do it.

Terrible theory

Unfortunately, and most misleadingly, piano tuning has often been "described" as though it were just a question of applying certain aural techniques in order to produce certain frequencies of vibrations in the strings. This comes from 19th-century "idealised" theory that often still gets reiterated today as though it were a complete, modern acoustical theory, when actually it is no such thing. It is a relatively crude model, that has its uses, but whose level of approximation to the real complexity of piano tone, renders it unable to describe piano tuning except in an equally crude way.  

In learning to tune, students of tuning do in fact learn acoustical techniques, based on the simplified theory, because unless we are born with an audiophile knowledge of piano tone and how to produce it in tuning (which of course no one has been born with), then when we are learning, we have to start somewhere.

But this is only the beginning of knowledge of piano tone, as tone. It is a long ladder that has to be climbed, but it is only a ladder on the ascent to the art of tone production in tuning. This does not mean master tuners abandon the technical merit that the training in tuning demands. It just means they go beyond it, because just as it is in fine art, technical merit is essential, but still only the foundation. Ultimately it's all about beauty of tone and intonation. This is not something that can be pinned down to a fixed "scientific" model, and meaningfully measured. It is not wholly created by applying the basic techniques. There is no one formula for it.

What ETDs do

The best ETDs replace aural techniques with acoustical measurements, using theory more sophisticated than the 19th-century theory, in order to try to produce results as close as possible to those produced by the aural techniques (as applied by the expert). Thus, they are said to "emulate" aural tuning. The success is variable, but can be good, as far as emulating the results of aural technical merit is concerned. However, the basic aural tuning techniques are not the only thing required even just for a working tuning. Also required is tuning stability, which requires techniques and knowledge in its own right. Stability means the beginning of the tuning has not deteriorated by the end of the tuning. It means that on a piano in good condition, the tone has not deteriorated a couple of days afterwards, just because the piano has been played. It means notes can be played hard, without a deterioration in their tone. This is not something an ETD can directly assist with. Some pianos do obligingly provide a relatively stable tuning without the need of much input on the part of the tuner. Many do not. 

ETD tuning using a top quality ETD can be capable of producing a good immediate emulation of technical merit in aural tuning over parts of the compass. To a certain standard, this can be over the whole compass. ETD tuning can be "supplemented" with aural tuning, but then the result is not really entirely due to the ETD, so the result cannot be entirely credited to ETD tuning.

Obviously, an ETD does not tune by hearing and appreciating tone and intonation. It relies on the application of a theoretical model representing good tone and intonation. The result is usually "supplemented" with some aural tuning in order to bring it to an acceptable standard.  

Actual ETD results compared to aural tuning will be discussed here, in due course.