Results of listening test for double striking as described in the text. Average score across all participants is shown with equally spaced contours between +1 (darkest blue) and (darkest red). The heavy black line shows the zero contour and the lower white line the hyperbolic fit to this contour. This corresponds to a value in (33) and represents the threshold of perception. The upper white line corresponds to the value , and in the region above this line, almost all subjects hear a double strike.
