 It is a matter of a shift mechanism, a “pitch-shift”, in the sense that the frequencies of the modes are all multiplied by a common factor, therefore leaving unchanged their relationships.In point of fact this effect has been encountered in practice and is utilized for obtaining the vibrato.The term “pitch-shift” is however improper in this case, since the spectrum of the partial tones of the membrane is not harmonic and as a result a pitch is not definable.