Different notation systems are available but indication of range instruments currently use the scientific (logarithmic) notation where C3 frequency is twice C2 frequency which is twice the C1 one and so on… Reference point is A 440 Hz but octave identifying numbers start on C
This system,supposed unambiguous,is unfortunately very different in France from USA and many other countries(England and Germany have slight difference from US but are basically similar to USA )
A 440, an international reference !,refers to A4 in USA while labelled La3 in France which may lead to misunderstanding in orchestration .
Futhermore –French notation starts on negative number(-2 ) and does’nt include 0. The starting point(1) Is C (16.35 Hz) below the bass clef Here Octaves notation is a geometric progression whose common ratio is 2; so a base value (55Herz ) is multiplied or divided by 2 and so does each following term . French notation indicates the power of 2.
Since 2 °=1 it cannot be used – American notation is based upon another logic since it starts on Zero (An A note below would be no perceptible ( C < 20 Hz) Misunderstanding may occur in translated book where translator might not be aware of those different systems. It would be better to refer to frequencies.