Many watermarking systems make use of correlation for calculating a detection metric, which means that several reference patterns are generated at encoder side and one or more of them are embedded inside the content signal, dependent on the message to be embedded. To decode the embedded message, it is necessary to discover which reference pattern was embedded at encoder side. This is determined by correlating the known reference patterns with the content signal. A watermark detector decides, depending on the size of the correlation result values, whether or not a given pseudo-random sequence was embedded. However, this does not provide correct decisions if watermarked audio is emitted by a loudspeaker and then captured with a microphone. According to the invention, it is taken advantage of the received signal echoes instead of treating them as noise. The watermark detection result is improved by integrating the correlation values resulting from echoes into the main correlation peak, thereby using correlation result amplitude values located within a predetermined neighbourhood of a correlation result peak amplitude value and exceeding a predetermined threshold.