Different equal-loudness contour studies disagree substantially in shape

Evidence

Figure 9.5(a) shows a sample from the current ISO standard (ISO 226, 2003), the result of an international collaboration initiated to address errors in the 1987 edition of the standard that was based on Robinson and Dadson (1956) data. Figure 9.5(b) shows a comparison of it and contours from four earlier studies showing how very different they are in shape. These are obviously not hard engineering data. The curves are averages over many listeners and there were large inter-subject differences. — Toole, Floyd E, Sound reproduction- the acoustics and…, 906