Calibrate the headphones you actually wear.
The EarEqual Plug measures the individual headphone transfer function at the blocked ear-canal entrance, on the actual listener, with the actual headphone fit and seal.
Your headphone, on your head, at a defined reference plane.
The measured HpTF includes the headphone's interaction with your head, pinna and concha, including fit and seal. The ear-canal transfer downstream of the blocked entrance is not part of that measurement.
This reference plane matters because conventional headphone targets defined at the eardrum or on artificial-ear systems cannot simply be copied across unchanged.
Measure, define the target, make the filter.
Measure HpTF
Insert the Plug, put on the headphones, and measure each ear in REW or similar software.
Choose the target
Use your own measured acoustic target for the most individual result, or start with the EarEqual Reference Target.
Calculate EQ
The correction is the target divided by the measured HpTF.
Apply
Use the resulting filter in the EQ or convolution system you already use.
Measure the target at the same reference point.
If you measure a loudspeaker or another acoustic reference with the Plug and then measure the headphones with the same Plug, both numerator and denominator contain your own upstream head/pinna acoustics at the same physical reference.
EarEqual Reference Target v0.1
EarEqual also provides a preliminary universal blocked-entrance target derived from published human blocked-ear data with broad listening-balance shaping. It is a starting reference, not a claim that one target is correct for every listener.
What if you want the headphone itself approximately linear?
With a flat target at the blocked entrance, the correction reduces to:
When the Plug is removed for normal listening, your own ear canal is physically back in the playback path. This should not be described as literally "pre-HRTF", because the measured HpTF already contains headphone interaction with your pinna and concha.
Blocked measurement and normal listening are not always identical loads.
The remaining technical caveat is the change in acoustic loading between measuring with the entrance blocked and listening with the Plug removed. Some pressure-sensitive headphones, including some closed-back designs, can change response with that load. Headphones that are relatively insensitive to ear-load changes can show much smaller error.