Headphones Versus Loudspeaker Testing
Good headphones, well sealed to the ear, can provide a very flat low-frequency pressure response measured at the ear canal, with low distortion even at high intensities, and at low frequencies the ear is purely pressure sensitive and the cavity formed between headphones and ear is too small to introduce any modifying resonances. Headphone testing is therefore a good way to derive equal-loudness contours below about 500 Hz, although reservations have been expressed about the validity of headphone measurements when determining the actual threshold of hearing, based on observation that closing off the ear canal produces increased sensitivity to the sound of blood flow within the ear which appears to be masked by the brain in normal listening conditions. It is at high frequencies that headphone measurement gets unreliable, and the various resonances of pinnae (outer ear) and ear canal are severely affected by proximity to the headphone cavity.
With speakers, exactly the opposite is true, a flat low-frequency response being very hard to obtain except in free space high above ground or in a very large and anechoic chamber free from reflections down to 20 Hz. Until recently it was not possible to achieve high levels at frequencies down to 20 Hz without high levels of harmonic distortion, and even today the best speakers are likely to generate around 1 to 3% of total harmonic distortion, corresponding to 30 to 40 dB below fundamental. This is not really good enough, given the steep rise in loudness (rising to as much as 24 dB per octave) with frequency revealed by the equal-loudness curves below about 100 Hz, and a good experimenter must ensure that trial subjects really are hearing the fundamental and not harmonics, especially the third harmonic which will be especially pronounced as speaker cones become limited in travel as their suspensions reach the limit of compliance. A possible way around the problem is to use acoustic filtering, such as by resonant cavity, in the speaker setup.
A flat free-field high-frequency response up to 20 kHz, on the other hand, is comparatively easy to achieve with modern speakers on-axis. These facts have to be borne in mind when comparing results of various attempts to measure equal-loudness contours.
Read more about this topic: Equal-loudness Contour
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