avfilter/avf_showspectrum: finally fix log scaler

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/avf_showspectrum.c

@@ -581,7 +581,7 @@ static int plot_spectrum_column(AVFilterLink *inlink, AVFrame *insamples)
                 a = pow(a, 0.20);
                 break;
             case LOG:
-                a = 1 + log10(FFMAX(FFMIN(1, a), 1e-6)) / 5; // zero = -120dBFS
+                a = 1 + log10(FFMAX(FFMIN(1, a * w), 1e-6)) / 6; // zero = -120dBFS
                 break;
             default:
                 av_assert0(0);