Beat detection is an important MIR research area. Due to its growing usage in multimedia applications, the need for systematic ways to evaluate beat detectors is growing too. This research tests RhythmExtractor2013, a pipeline offered by Essentia, an open-source music analysis library used in research and industry. The annotated test samples, taken from four open-source datasets - GTZAN, Ballroom, SMC MIREX and MDB Drums, had tempo transformations (uniform, randomized, incremental and decremental tempo changes) applied to them and put to test against the aforementioned extractor. F-measure was chosen to calculate the extractor’s accuracy. The results show, that the accuracy is affected mostly by the presence of steady rhythm and drums, but also by the window size during the result calculation process, with the worst scores appearing when the samples are slowed down.