Secondly is the range of response for the pitch and volume. As with orchestral instruments, one would like at least three octaves of pitch range for performing the melodic lead of most compositions, allowing, for instance, the player to accompany the vocals. Taking the concept of sensitivity and range in conjuction, a pitch range of three octaves executable over one foot would be reasonable. Most of my designs provide seven octaves over one foot, and this additional range provides some versatility. Other considerations, as well, are important, and among these is linearity.
Linearity addresses the degree of spatial evenness among the tone intervals.
In practice, absolutely perfect linearity is impossible to achieve, although most Theremins are sufficient in this respect. Defining the volume response uses a term unfamiliar to most musicians, namely, the decibel. While, perhaps, these are the most essential Theremin parameters, there are also many others which distinguish one instrument from another. Among these, stability is paramount. The Theremin offers a formidable design challenge since the physical principle of operation involves the precise measurement of minute differences in capacitance.