Final answer:
The 40 dB loudness contour is referred to as the Fletcher-Munson Curve. These curves illustrate the perceived loudness of sounds across different frequencies and intensity levels, with the sound intensity level typically measured in decibels (dB).
Step-by-step explanation:
The 40 dB loudness contour is referred to as the Fletcher-Munson Curve. The Fletcher-Munson Curves are a set of equal loudness contours that show the loudness of sounds at different frequencies and intensities. To determine the perceived loudness of a sound, one can use these curves by locating the point that intersects the corresponding frequency and sound intensity level, then finding the loudness value in phons. When we speak of a 40 dB contour, we are referring to a perceived loudness at the 40 dB mark on one of these curves for a given frequency.
For further clarification, sound intensity level is a unitless quantity that tells you the level of sound relative to a fixed standard, often measured in decibels (dB). The mathematical expression for sound intensity level is β(dB) = 10 log10 (I/I0), where 'I' is the sound intensity in watts per meter squared and 'I0' is the reference sound intensity, typically taken as 10−12 W/m2, the lowest intensity a person with normal hearing can perceive. So, when the intensity level is at 0 dB, this indicates the threshold of human hearing.