Standardimnon of lung function tests in paediatrics

First and second latching selectors receive tone signals from the tone generators and are operatively coupled to a keyboard for respectively deriving first and second tone signals having frequencies determined by the depressed keys in the keyboard. The frequencies of the first and the second tone signals are respectively divided by first and second frequency dividers respectively having predetermined dividing factors of, for example, 2 and 3, so that the first and the second frequency divided tone signals (i.e., the signals at the outputs of the frequency dividers) have a frequency ratio which is close to but not exactly equal to an integer. The mixed resultant tone exhibits an ensemble effect as produced by a multi-series tone generator.