For the matching of two harmonically-related pitches, either unisons or pure intervals, the human ear is much more precise than any electronic device can ever be. This is because the ear can easily single out precisely those spectral elements which create the acoustic phenomena which we call “out of tune”, rejecting all other “extraneous” sonic information, such as irrelevant overtones, variations in volume, envelope decay, action noise, etc. Electronic machines cannot do this, and they must always subject the sound to all kinds of filtering and electronic trickery to make it appear as though they can “hear”. This imitation of the human capability is more or less successful, but almost never completely so.
Of the two methods of visually representing “out of tuneness”, the majority of tuning devices, both hardware and software, have the worst: an indicator needle meter showing “sharp” or “flat”. All needle indicators are by nature imprecise, because they are based on an inherently false method of evaluation: cents deviation. We do not hear tuning aberrations as degrees of proportional deviation from an ideal pitch; instead, we hear the effects of phase cancellation between the various frequencies which make up any complex sound. Our coarse sense for detuning depends upon hearing the beats between the fundamentals and overtones of the two tones, but once they are almost perfectly in tune, our evaluation shifts to the detection of timbral undulation due to the “flanging” effect. Once you learn how to be aware of flanging – something which is very easy to do – your ability to judge tuning far exceeds that which any electronic device can either detect or display. Furthermore, you can do it with no or very little latency, that is, a delay between doing something and detecting its effect. Electronic devices always need some time before they can process the signal received and convert it into the unnatural representation of cents deviation, and even in the best of conditions – for example, a pipe organ (steady volume, pitch, and wave shape) in a quiet room – the indicators almost always waver and jump. Although they seem to work well enough for guitars (and instrument which is inherently more or less always out-of-tune), for keyboard work, such meters are only for the crudest of measurements: do not use them for tuning the instrument or even only setting the temperament. A mistaken result is the rule rather than the exception.
The other type of tuning machines uses strobe meters, which either have spinning discs (such as the wonderful old Sanderson Strobo-Tuners – perhaps the best and most accurate visual tuning device ever invented) or rows or circles of flashing LED lights (as of yet, I know of no smartphone app that has mimicked a strobe). Ostensibly, they show a visual form of the beating which our ears hear. However, they are again mere electronic devices which are by nature easily confused by changes in volume level and wave form. While strobes are generally much more accurate and reliable than indicator needle meters, they also must filter and process the signal from the microphone, and in doing so they introduce many of the same problems of display jitter and false high/low indications which plague metered devices. Only when tuning tones which are poor in harmonic content – the lower octaves of a stopped flute, for example – can a strobe outperform human hearing. In this case, even though the upper harmonics we use to judge tuning are absent, the strobe can nonetheless indicate the slow phase drift of the fundamental tone, something it may take us a long time to hear. However, precisely because we CAN’T hear this detuning, the superior capability of the strobe is completely pointless; tuning is as tuning sounds.
Nature gave each and every one of us a wonderful and incredibly precise device for this sort of comparison: our ears. in almost all situations, we can very easily hear at levels of precision many times better than any artificial device is capable of. Under ideal circumstances, we can hear a detuning as small as 0,05 cents! Furthermore, we have the wonderful advantage that our ears are connected to our brains. For example, in deep bass registers, the excessive inharmonicity of the relatively short and fat strings produces an overtone content which makes it impossible to tune octaves which are completely beat-free. If you stop one overtone from beating, inevitably another will only start beating. Under such conditions, a meter will only dance and jitter; by contrast, your ears and brain can work together to decide which combination of slowly-beating overtones is the least bothersome.