Audiology distinguishes itself by being the un-science of sound measurement. When you go to the audiologist, they produce a chart of your threshold elevations at testing frequencies resulting from your hearing test. The vertical axis, (plotted upside down), is labeled in units of “dBHL”.
If you were a scientist you might be tempted to interpret that as the elevation above the accepted threshold of hearing level at each frequency. And you would be wrong…
What the zero point of dBHL really is, is very difficult to nail down. But from reading the history, starting with the paper authored in the mid ’60’s by the Air Force, testing 27 young cadets, and using a particular model of headphone… then continuing on to the technical reference manuals for modern audiology testing equipment…
You begin to realize that the manufacturers have gone out of their way to avoid discussing any science of measurement with their audiologist customers. Instead they outline calibration procedures for the audiometers and specify some dBSPL measurements that should appear through the headphone into an acoustic coupler at specified settings of test volume.
There is some inconsistency among manufacturers. But the bigger problem is that the original Air Force headphone model is no longer available. And it wasn’t a very good headphone by today’s standards anyway.
But nobody seems to have written down a scientific exposition of an ANSI Standard audiometer calibration. Instead, you get a lot of hand-waving and some obscure mentions of SPL measurements through couplers.
Missing entirely is any discussion about the frequency response of the audiometer test headphones, and the possible (likey) disparity between measuring sound levels into a metallic acoustic coupler, compared to a human ear.
Manufacturers give no indication about whether or not they account for the offset from the absolute threshold of hearing. This could easily be done in software on the audiometers, provided the manufacturers have some notion of where their calibration levels lay with respect to a standard of hearing threshold.
From my research and experiments, I have to conclude that they do not make any such corrections. The audiogram merely reports the level setting controlled by the audiologist. Computer software seems far too modern to bother with in the audiometers.
Notwithstanding, I have done my best to ferret out vital information. And it appears that the reference standard for 0 dBHL is as follows:
This data is shown graphically here:
The gray curve at the lower end of the graph, labeled ATH, is the reference level for the threshold of hearing, as specified by ISO 226. The green curve is the table of values shown above for the 0 dBHL reference levels. And the red curve is the interpretation of those reference levels in Phon space – which is what really counts in assessing hearing.
Crescendo works entirely in Phon space, and that is where it applies the Decruitment/Hyper-recruitment EQ trims. You can’t get there with a conventional equalizer because those work in SPL space – the same space where microphones and speakers and headphones work. Our hearing operates in Phon perceptual space, not SPL physical space.
Your audiogram shows the elevation, in SPL space, of your threshold levels above the green curve, not the gray ATH curve. So this also means that your hearing is worse than shown on your audiogram.