How We Measure Response Accuracy
The accuracy score is perhaps the single most important tool for earphone design.
A perfect recording of a live performance played through earphones with 100% accuracy would produce the same sound at the eardrum as the live performance. A complicating factor is that the acoustic resonance and horn effects of the ear change a flat signal entering the open ear to an eardrum signal with the approximate frequency characteristic of the target curve. A perfect earphone will create that same frequency characteristic at the eardrum.Target Curve
Target
curves on Etymotic Research graphs indicate 100% accuracy: The open ear
diffuse-field response of the KEMAR® manikin modified to compensate for the
high frequency boost added to high-quality recordings. This modification
(approximately 5 dB at 10 kHz) is necessary to avoid earphones sounding too
bright on commercial recordings. Commercial recordings have a high-frequency
boost that compensates for the high frequency roll-off in studio monitor
loudspeakers and high-quality stereo loudspeakers and earphones.
Earphone response was measured on a KEMAR® manikin that has the same acoustic properties as the average head and ear. 25-band accuracy scores are calculated by summing the difference between the earphone response and the target response in each 1/3 -octave band from 50 Hz to 12.5 kHz.
In the 1970s, Consumers Union used an "Accuracy Score" to rate loudspeakers. Consumer Reports reported that it was possible to predict listeners' loudspeaker ratings within 8% from a calculation based on one-third-octave frequency response measurements converted to loudness in sones. The average error in loudness from a perfect system, subtracted from 100%, gives the accuracy score. Etymotic Research extended this 21-band calculation to a 25-band calculation and routinely uses the 25-band accuracy score in all earphone designs.
Other in-the-ear earphone manufacturers do not report accuracy scores, but Etymotic Research continues to test competitive products. The accuracy scores of Etymotic earphones are higher than all competitive earphones we've tested, except electrostatic headphones that cost thousands of dollars.