Communication by speech is the most direct connection between human beings. Noise disturbs this connection because it masks some components of acoustical features and therefore reduces the information content of speech. Diminishing of intelligibility does not only prevent communication but can bring forth dangerous situations. The paper discusses the mechanism of speech production and the most important peculiarities of speech as information. The chief goal of the treatment is the redundancy of speech which is the most important factor to preserve information in the presence of noise interference. In connection to this the acoustical data of intelligibility measured chiefly by the author, will be given. A new computation method for the determination of intelligibility percentage in the presence of noise interference will be presented.