Maybe I am too sceptical about it, but is it possible that the beats were present in the external squeal of the brakes. It seems highly unlikely to me that a brake produces a pure tone. Sorry if I should miss the point. stefan Stefan Muenkner | stefan.muenkner@uni-tuebingen.de Universitaets HNO Klinik | Silcherstrasse 5 | voice +49 7071 29 83824 D-37073 Tuebingen | fax +49 7071 29 84163 Germany |