An impedance tube with two microphones offers the possibility of determining the sound absorption coefficient spectrum of foam materials quickly and cost-effectively. The calculation method and the measurement process itself are described in Standard DIN EN ISO 10534-2 and ASTM-E 1050-08, respectively. Despite the standardization, there are strong deviations between the calculated sound absorption coefficients obtained in measurements of the same sample. From the literature, the influence of the diameter of the sample on the determined sound absorption spectrum is known but discussed in a controversial manner. Round robin tests, i.e. tests to determine the sound absorption coefficient at identical materials in different laboratories, show, that there are different results in determines sound absorption between the labs. Various effects are mixed in that setting, which explains why no explicit parameters with influence on the sound absorption spectrum can be derived. Therefore, in that thesis, the experimental measurement process is examined at a polyurethane foam to systematically determine the sound absorption spectrum in the impedance tube. Parameters are identified that influence the result of the sound absorption spectrum: diameter of the sample, sample thickness, mounting conditions and cut of the sample, as well as the expertise of the person that cuts the sample. To verify all factors, different full-factorial designs of experiment were set up. Besides the graphic evaluation and visual comparison of different averaged sound absorption coefficient spectra for the assessment of influence of above mentioned parameters on the sound absorption coefficient spectrum, single-number values are calculated as target values to be used for the calculation of the analysis of variance. From the architectural acoustics, the α_w-value is known as a single-number value for indicating the sound absorption of an absorber. Furthermore, the classical parameters of a curve analysis are used as target value: the arithmetic mean, mean absolute deviation, variance, skewness, and kurtosis. The analysis of variance indicates whether parameters have an influence or not. A subsequent effect evaluation shows the true extent of the tested parameters’ influence. The visual comparison of the absorption spectra allows detailed statements about the influences of factors on the spectrum in certain frequency ranges. By calculating single-number values from the sound absorption spectrum and using effect evaluation results, the influence of certain parameters can be quantified. The analysis of variance shows that all factors have a significant influence on absorption, independent of the chosen target value. Calculating the effects, however, shows that the diameter and even the mounting direction, for example, have no practice-relevant influence on the single-number values of the calculated absorption, even though it also shows that the effect of the human factor is not negligible. The results of the calculated absorption spectra are compared with the results of the calculated absorption coefficient spectra from material models. The thesis shows that a systematic confrontation with factors that influence the sound absorption spectrum will allow for evaluation of the results under the given uncertainty.