Psychoacoustic headphones distort sound unless free-field equalized to compensate their band-pass response

Evidence

These problems can mostly be overcome if sounds are presented via earphones. One advantage is that the earphones usually used in psychoacoustics show very little nonlinear distortion (less than 0.1 % or -60 dB) in the frequency range of interest. The frequency response of earphones has to be measured on real ears, because current couplers can produce misleading results. … If the earphones are used without equalizer then it must be kept in mind that they act like bandpass filters and change the sounds presented. This means that both the tone colour and the loudness are dramatically affected, particularly with broad-band sounds. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 105

Categorization

• QUESTION: Which equalization standard, free-field vs diffuse-field, is considered correct, and how much does the choice bias loudness judgments in practice?