“White, black, and other.” I am sure that he would be astonished to learn how complicated that issue has become.Īfter the conclusion of World War II, and inspired by the Beasley Survey, hearing specialists in the United Kingdom (UK) decided to carry out their own survey. I asked Beasley what advice he could give me on how to categorize ethnicity in reporting such data. Parenthetically, I was working at the time on a study comparing the audiograms of Houston firemen of various ethnic origins. We had a very pleasant chat in which he revealed that he went into his survey knowing a good deal about epidemiological surveys but not so much about testing hearing. I was at the Baylor College of Medicine at the time, and he was working on a project at the University of Texas Health Science Center in a nearby building. I had the pleasure of meeting Willis Beasley in the early 1960s in Houston. This study has since become known as the USPHS Survey of 1935–36, the “American Survey,” or simply the “Beasley Survey.”These data became the basis for what then came to be called the “American Standard” for audiometer calibration ( Figure 2). Results were collated, analyzed, and made available to audiometer manufacturers in the United States. Large samples were stratified geographically, crews were assembled and trained in the use of the audiometer, then sent out to key locations nationwide. In 1935, the USPHS commissioned a young epidemiologist, Willis Beasley, to design and carry out a national survey of the SPLs corresponding to the average thresholds of large samples of the entire normal-hearing population of the country ( Figure 1).īeasley did an excellent job. Plus, in any event, whose numbers should the manufacturers adopt?Īt this impasse the United States Public Health Service (USPHS) stepped in, determined to survey the hearing of citizens nationwide, in order to supply the necessary average SPLs at each test frequency. Unfortunately, no one knew what the average SPLs at each test frequency should be, because no single individual could hope to gather and survey the large group of listeners necessary to calculate useful averages. Just tell us what the different SPLs should be at the different frequencies and we will insert appropriate resistors at each frequency. Users of audiometers complained to the manufacturers that 0 dB on the hearing loss dial ought to hold for all test frequencies. Throughout the 1920s and well into the 1930s, clinicians had to calibrate their own audiometers by testing a small number of people without hearing complaint, averaging those data, and noting what the correction on their hearing loss dial should be at each test frequency. The various iterations of what those numbers should be is the complicated story of the quest for audiometric zero. What we now call “0 dB HTL” is linked to a series of SPL numbers that change as frequency changes. However, this is a number whose corresponding sound pressure level varies with frequency. Early in the 1920s a distinguished otolaryngologist, Edmund Prince Fowler, seeking a method for expressing hearing loss as a percentage, proposed an audiometric format in which the actual average threshold SPL was defined as 100% of “average normal hearing” at each test frequency.This later became 0 dB sensation units, and eventually 0 dB hearing threshold level (HTL). All actual thresholds of listeners were expressed in decibels relative to this measure.īut this straightforward method of displaying audiometric data did not prevail. ![]() In the early days of audiometry during the 1920s his measured auditory thresholds were all expressed as sound pressure levels (SPL) relative to a defined physical reference level of 0.0002 dynes/square centimeter. ![]() And that is the way it would have been for hearing if the renowned physicist, Harvey Fletcher,had had his way. That is characteristic of most measures of bodily function, a range-not a number. If your number is in that range, relax! Otherwise obsessing about where it is within that range will only drive your blood pressure up!” I once asked a physician friend “What is average normal blood pressure?” “First of all,” he replied, “blood pressure is not a number it is a range. Hearing Science | July 2019 Hearing Reviewīy James Jerger, PhD What is “0 dB” on the audiometer and how was it standardized? Here is a brief history of the evolution of audiometric zero, the conflicting studies and controversies surrounding it, and how “0 dB” on the audiometer was ultimately established and standardized. As with most sciences, there are some notable quirks in audiology, and the zero decibel point on the audiometer is one of them.
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