The “Missing 6 dB” of Tillman, Johnson, and Olsen Was Found - 30 Years AgoTo Tom Tillman, with Gratitude When I arrived at Northwestern University as a new doctoral student in the fall of 1971, I was in awe of the fact that I was going to be in what most people then considered to be the center of audiology. The people I would work with were those who wrote most of the texts and research I had read. Little did I realize that I would also be in the midst of a period of transition.The old Speech Annex was being replaced with the new Frances Searle Building to house the School of Speech. The Northwestern way of connecting pieces of equipment using coax cable with banana plugs routed at right angles along the edges of the equipment was giving way to MAC Panel plug boards and plug wires. The use of calculators to run statistical analyses was giving way to computer punch cards and the SPSS software package. The system of storing subject information on McBee keysort cards was being replaced with newly developed computer database systems. The use of transformers to match different signal sources was being replaced by a new semiconductor called an operational amplifier. A newly developed pressure pump was changing the way we understood middle-ear function.I was caught between the tradition of the Northwestern way of doing things and the emerging technology of computers, new facilities, new electronics, and new diagnostic tools. Naturally, I was nervous. I had so much to learn but confused about the right path to follow. Tom Tillman was a source of direction and inspiration for me. He helped me to see the value in both the traditional and the modern. He taught me to embrace change without disregarding the past. Tom helped me to understand that to move into the future we must not lose sight of our past. I will always to be grateful to Tom for his insight, his ability to analyze a problem and see a solution, and the help he gave me in finding the path to my eventual graduation in the summer of 1975.Michael G. Block, Ph.D. Missing Tom Tillman I've missed Tom; really missed having him answer the phone when I had a knotty academic or research problem. And I miss the reminder that one of my idols had become one of my friends.I had called only a few weeks before Tom died when a Ph.D. student discovered that the way she set the gain on the digital hearing aid she was using could arguably have been set a better way. She had originally checked it three ways, including the old-fashioned oscillator-voltmeter method, but its high-frequency gain for dynamic speech sounds was higher than expected (and higher than might have been optimum). Tom listened to the account, to her willingness to redo half of her data collection, and said “As you go along in every research project, you encounter ways that you could have done it better. But the time comes when you need to stop improving and complete what you started. I think that is where you are here.” You can imagine her relief (and mine). The answer was all the more welcome because of Tom's absolute integrity; he would never have given such an answer to make a student (or advisor) feel better at the expense of good research.Tom genuinely liked and cared for students. Tom is almost the sole reason I have a Ph.D. Over 30 years ago, I called Tom about an article he had written with R.M. Johnson and Wayne Olsen on the “Missing 6 dB.”1 I thought the experimental findings were correct, but didn't think anything was “missing.” Tom not only wasn't defensive, but invited me to Northwestern to discuss the article. At the end of that discussion, he enlisted a graduate student, Michael Block, to carry out the additional experiments we devised to settle the question. Some eight years later, I decided that my preconceptions about the type of people who obtain Ph.D.s couldn't be entirely correct, and-with Tom's encouragement-enrolled at Northwestern where Tom became my Ph.D. advisor.The next time I saw Tom under pressure was when a check on the levels of the recorded stimuli used in some perceptual masking experiments indicated that the reported “8 dB of perceptual masking” was an artifact of a complicated calibration procedure (not Tom's). To explain, it turned out that the real-speech masker was ∼8 dB more intense than the speech-envelope-modulated speech-spectrum noise masker. Tom had been associated with many of the early perceptual masking findings, and might have been expected to be defensive. I watched Tom during the meeting as the new findings and calibration checks were described. He listened intently and at the end said, “We should check this new finding and, if it holds up, report it so that no one else will make the same mistake.” Other colleagues expressed concern for their reputations and for that of Northwestern. Tom always focused on the search for truth; I never saw him flinch in light of possible personal consequences.After my own dissertation committee had listened to my third Ph.D. proposal and rejected it as “not interesting” and worse, I wrote a blistering three-page retort, explaining the committee's abysmal lack of insight. I sent all five copies to Tom to distribute, gave up on my Ph.D., and went back to work full time. Tom waited quietly for six months, then invited me over for a visit whereupon he confessed that he had never distributed my diatribe and wondered if finishing my research (and Ph.D.) wasn't more important than being right? Tom could be just as gentle when necessary as he could be rigid as iron when facing poor science or questionable ethics. As a scientist, a writer, a teacher, and a mentor, Tom had few peers. But mostly I will miss him as a trusted friend.Mead C. Killion, Ph.D.
02 April 2004 (online)
This article describes the experiments undertaken to track down the missing 6 dB that Tillman, Johnson, and Olsen reported in 1966. In keeping with earlier findings of other authors, we found that nothing was really missing. We made probe-microphone recordings of the original stimuli and obtained the same differences reported by Tillman, Johnson, and Olsen once we took into account differences between anechoic-chamber and test-booth sound fields. Historically, these differences have been rediscovered by each new generation, with the same subtle experimental errors having to be uncovered anew.
Missing 6 dB - thresholds - earphone thresholds - sound-field thresholds - probe microphones
- 1 Tillman T W, Johnson R M, Olsen W O. Earphone versus sound-field thresholds sound-pressure levels for spondee words. J Acoust Soc Am. 1966; 39 125-133
- 2 Sachs R, Burkhard M. Earphone Pressure Response in Real Ears and Couplers. Project 20021. Franklin Park, IL; Knowles Electronics 1972
- 3 Tillman T W, Olsen W O, Killion M C, Block M G. MAP versus MAF for spondees: Nothing's really missing. Presented at the Annual Convention of the American Speech-Language-Hearing Association November 17-20, 1973 Detroit, MI;
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- 10 Killion M C. Revised estimate of minimum audible pressure: Where is the missing 6 dB?. J Acoust Soc Am. 1978; 63 1501-1508
- 11 Shaw E AG. Ear canal pressure generated by a free sound field. J Acoust Soc Am. 1966; 39 465-470
- 12 Zwislocki J J. An Ear-Like Coupler for Earphone Calibration. Syracuse, NY; Institute for Sensory Research, Syracuse University 1971 Report LSC-S-9
- 13 Wever E G, Lawrence M. Physiological Acoustics. Princeton, NJ; Princeton University Press 1954
- 14 Dirks D D, Stream R W, Wilson R H. Speech audiometry: earphone and sound field. J Speech Hear Dis. 1972; 37 162-176
- 15 Wiener F M, Ross D A. The pressure distribution in the auditory canal in a progressive sound field. J Acoust Soc Am. 1946; 18 401-408
Michael G Block, Ph.D.
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