Assessment of the level of speech information protection from leakage through acoustic and vibration channels is carried out according to international and national standards and in compliance with regulatory documents. To assess its security level, regulatory documents in many countries imply the use of signal/noise ratio. However, the method has a series of significant shortcomings, which do not make it possible to determine the real state of security level. The improved objective evaluation method, which is based on determining the coefficient of residual intelligibility for a test-signal after its recovery by the methods of mathematical analysis (adaptive filtration, correlation and spectral analyses, wavelet transformation, etc.) was proposed. The coefficient of residual intelligibility is determined for each word included in a short phrase, a test signal. The analysis of frequency of using the phonemes in the Ukrainian speech was performed. It was shown that given the definition of the term "allophone" and the number of native speakers, it is possible to assume that the total number of allophones tends to infinity. To reduce the calculation complexity, we proposed the formalized approach based on the simplified linguistic model – a phoneme (a letter), a diphone (two letters), and a triphone (three letters). As a source of information, it is possible to use text documents. We proposed analytical dependences for calculating the coefficient of residual speech intelligibility and its components – coefficients of frequency of using allophones in the words of the Ukrainian language and the importance of allophone recognition for the word recognition. The interrelations of the SPC (speech privacy class) and word intelligibility W were shown. On their base, the scale of objective estimation of the degree of speech information privacy on the boundary of the controlled zone by the criterion of residual speech intelligibility was proposed