Vocal harmonics refers to the structure of frequencies found within the sound of a person's voice. These harmonics are different from person to person, and also depend on the speaker's state of mind. Furthermore, vocal harmonics can have a strong influence on how a listener psychologically responds to a person's speech.
This subject is not to be confused with transdimensional harmonics, a field principally concerned with the study of extradimensional energies and intelligences.
Method of Identification[]
Clanks and other constructs have a variety of methods for identifying people biometrically, including facial recognition ✣ , smell ✣ , and genetic analysis of blood ✣ and hair ✣ . Analyzing vocal harmonics is yet another of these methods. For instance, Castle Heterodyne has been able to identify ✣ Agatha Heterodyne using her vocal harmonics, which it apparently recognizes as similar to those of her mother ✣ .
More discriminating systems can go beyond simply recognizing a speaker's voice, and can even determine which of several personalities is in control of a person's body. For instance, when Gilgamesh Wulfenbach was under the partial control of a mental overlay of his father, Tarvek Sturmvoraus was able to detect a "secondary resonance" ✣ in his vocal harmonics, a sign that an extra mind was present and influencing his actions. A highly trained ear can apparently do this even without special equipment, since Lady Vrin instantly recognizes ✣ a copy of Lucrezia Mongfish residing in the mind of Agatha Heterodyne, even though she knew that Agatha had been in control of the same body only minutes before. While Vrin was able to partially resist Agatha's commands, Lucrezia's slightly different voice was able to kill Vrin with a single word.
It is not clear whether the Heterodynes as a family can be identified by vocal harmonics alone, since this has never been explicitly stated. However, as mentioned before, it is clear that the Castle is able to recognize certain family resemblances between voices, and it is also known that sparks in the family instinctively heterodyne, a form of humming that involves complex manipulation of their vocal harmonics to produce novel frequencies.
Command Harmonics[]
The command harmonics are a part of a speaker's vocal harmonics that are of particular interest to sparks. When a spark gives a command, particularly if they are in the madness place, their command harmonics form a structure known as a command wave ✣ , which can subconsciously influence the listener into doing as they say. It is unknown whether non-sparks are capable of generating strong command harmonics, since this has not been seen to date.
This influence is most famously used by sparks who use a command voice to utterly control their creations, such as the Other, who used it to control her slaver wasps and revenants. However, it is also apparent that sparks' voices and natural charisma are able to strongly influence others, especially those who are natural minions ✣ , to the point that it can seem like mind control ✣ even without explicit control. Gilgamesh Wulfenbach is especially good at using unhinged rants for this purpose; he once converted Captain Vole from a state of murderous defiance to fearful submission ✣ with nothing more than a couple of hits and a crazed speech. Perhaps even more impressively, he once delivered an ultimatum ✣ to Ardsley Wooster that was so horrifying that for years Wooster found himself occasionally terrified by the thought that Gil would "melt England" ✣ .
Machine Equivalent[]
Some machines appear to be controlled by an electronic equivalent of command waves. Du Quay used a piece of Castle Heterodyne and a telluric wave conductor to broadcast such command waves ✣ through the telluric current under Paris, temporarily allowing it to take control of the city. It's possible that the Castle uses such waves to control its own subsystems in Mechanicsburg (and/or the seneschals ✣ ).