(Japan was the greatest contributor with about 40% of the data.) The study produced new curves by combining the results of several studies-by researchers in Japan, Germany, Denmark, UK, and USA. They did this in response to recommendations in a study coordinated by the Research Institute of Electrical Communication, Tohoku University, Japan. Perceived discrepancies between early and more recent determinations led the International Organization for Standardization (ISO) to revise the standard curves in ISO 226. Recent revision aimed at more precise determination – ISO 226:2003 It became the basis for a standard ( ISO 226) that was considered definitive until 2003, when ISO revised the standard on the basis of recent assessments by research groups worldwide. In 1956 Robinson and Dadson produced a new experimental determination that they believed was more accurate. The highest contour is the threshold of pain.Ĭhurcher and King carried out a second determination in 1937, but their results and Fletcher and Munson's showed considerable discrepancies over parts of the auditory diagram. The lowest equal-loudness contour represents the quietest audible tone-the absolute threshold of hearing. Loudness, being a psychological quantity, is difficult to measure, so Fletcher and Munson averaged their results over many test subjects to derive reasonable averages. Fletcher and Munson adjusted the reference tone until the listener perceived that it was the same loudness as the test tone. For each frequency and intensity, the listener also listened to a reference tone at 1000 Hz. In their study, test subjects listened to pure tones at various frequencies and over 10 dB increments in stimulus intensity. Within this range, the human ear is most sensitive between 2 and 5 kHz, largely due to the resonance of the ear canal and the transfer function of the ossicles of the middle ear.įletcher and Munson first measured equal-loudness contours using headphones (1933). The human auditory system is sensitive to frequencies from about 20 Hz to a maximum of around 20,000 Hz, although the upper hearing limit decreases with age. 5 Relevance to sound level measurement and noise measurement.
4 Headphones versus loudspeaker testing.