A method for determining the quantitative proportions of a plurality of granulate fractions of a blend, comprising the steps of: - providing the blend, wherein the grains of each fraction have a different density and the grain size of the grains of all fractions of the blend are comprised in the same grain grade, - transporting the blend in a conveying direction in a conveying flow with a constant flow energy, wherein grains of the blend collide with at least one impact structure configured as a waveguide and produce acoustic signals which propagate in each impact structure as structure-borne sound waves, - acquiring the acoustic signals and conversion into electrical signals with the aid of an oscillation transducer, and - signal processing the electrical signals from the oscillation transducer in order to determine the quantitative proportions. characterized by the following steps for generating a calibration function prior to starting the signal processing for a conveying flow with a constant flow energy: - testing a plurality of samples of the blend, wherein the quantitative proportions of the fractions in the individual samples are different and are determined with a conventional method or are known, as follows: - dividing the converted electrical signals from the oscillation transducer into time windows for each sample, - identifying a minimum negative intensity value of the electrical signal or a maximum positive intensity value of the electrical signal in all time windows for each sample, - determining a cumulative relative frequency for the maximum or minimum intensity values of the electrical signal for each sample, - determining an intensity value I n from the cumulative relative frequency of the maximum or minimum intensity values of the electrical signal for each sample, wherein the value "n" is the percentage proportion of the intensity value which is smaller than or equal to the intensity value I n, and the value "n" of the intensity value I n for each sample is equal to 50% or higher and is consistent for all samples, and - generating the calibration function by allocating the previously determined intensity value I n for each sample to the known quantitative proportions of the fractions of the granulates for the respective sample. - providing the blend, wherein the grains of each fraction have a different density and the grain size of the grains of all fractions of the blend are comprised in the same grain grade, - transporting the blend in a conveying direction in a conveying flow with a constant flow energy, wherein grains of the blend collide with at least one impact structure configured as a waveguide and produce acoustic signals which propagate in each impact structure as structure-borne sound waves, - acquiring the acoustic signals and conversion into electrical signals with the aid of an oscillation transducer, and - signal processing the electrical signals from the oscillation transducer in order to determine the quantitative proportions. - testing a plurality of samples of the blend, wherein the quantitative proportions of the fractions in the individual samples are different and are determined with a conventional method or are known, as follows: - dividing the converted electrical signals from the oscillation transducer into time windows for each sample, - identifying a minimum negative intensity value of the electrical signal or a maximum positive intensity value of the electrical signal in all time windows for each sample, - determining a cumulative relative frequency for the maximum or minimum intensity values of the electrical signal for each sample, - determining an intensity value I n from the cumulative relative frequency of the maximum or minimum intensity values of the electrical signal for each sample, wherein the value "n" is the percentage proportion of the intensity value which is smaller than or equal to the intensity value I n, and the value "n" of the intensity value I n for each sample is equal to 50% or higher and is consistent for all samples, and - generating the calibration function by allocating the previously determined intensity value I n for each sample to the known quantitative proportions of the fractions of the granulates for the respective sample.