A device for indicating a beat level L, the device including an analyzer which responds to a reference frequency .OMEGA. and a harmonic coefficient B to extract from an evaluation signal E a first acceleration signal .gamma..sub.B at a frequency equal to the product of reference frequency .OMEGA. multiplied by harmonic coefficient B; and a second acceleration signal .gamma..sub.B-1 at a frequency equal to the product of reference frequency .OMEGA. multiplied by the harmonic coefficient minus one; and it further includes a control circuit CC which receives acceleration signal .gamma..sub.B, .gamma..sub.B-1 to produce beat level L.