Charles W. Nixon

Human Response to Sonic Boom in the Laboratory and the Community

Present‐day estimates regarding the acceptability of sonic booms by man are derived from various observations, overflight programs, and experimental field and laboratory studies conducted both within and outside the United States. The loudness and annoyance of individual booms and their dependence on the boom overpressure and pressure‐time function as well as the complex reaction of individuals, groups, and communities exposed to sonic booms of varied magnitude and frequency are discussed. The few experiments available proving that even sonic booms of the maximum intensity presently feasible do not produce direct medical injury are described. Based on the integrated body of results of recent physiological, psychoacoustic, behavioral, and sociological studies in various countries, estimates of the effects and acceptability of regular, frequent supersonic commercial overland flight schedules are presented and discussed in terms of aircraft noise pollution in general, and of potential certification of aircra...

Experiments on the Bone‐Conduction Threshold in a Free Sound Field

Earlier investigators of the free‐field bone‐conduction threshold occluded the auditory meatus with plugs and muffs and reasoned by various arguments that the real‐ear attenuation obtained was less than the attenuation provided for air‐conducted sound. The conclusion was that the sound perceived reaches the auditory system via bone conduction, i.e., not through the auditory meatus. The present experiments are a check on these conclusions for the higher frequencies. With the auditory canal occluded, the whole skull or sections of it, excluding the muffs, was covered with a 2‐in. layer of absorbent cotton. A higher free‐field threshold with the cotton indicated that without cotton the sound was reaching the ear via bone. (The SPL over the muffs was not altered by the cotton.) Cotton over the face and back of head raised the bone‐conduction threshold by 10 to 18 db (2000 to 8000 cps). Bone conduction thresholds measured previously were confirmed from 1000 to 8000 cps. Front and back of the head were approxim...

Effects of Sonic Boom on People: St. Louis, Missouri, 1961–1962

The vicinity of St. Louis, Missouri, was exposed to approximately 150 sonic booms during a 10‐month period from July 1961 to April 1962. Ground overpressures, ranging up to about 3 lb/sq ft. were carefully measured for a series of 17 of the supersonic flights. Data obtained from over 2300 direct interviews, analyses of complaints, and engineering evaluations of alleged damage were related to information on aircraft operations and sonic‐boom overpressure measurements. Most residents interviewed indicated some interference with routine living activities, yet less than 1% filed formal complaints. Alleged building damage was superficial in nature and consisted mostly of cracks in brittle surfaces. There were no reports of direct adverse physiological effects.

Comparative Annoyance of “Approaching” versus “Receding” Sound Sources

Sound sources that continuously increase in frequency and/or intensity over a period of several seconds may be perceived by observers as approaching entities, whereas the converse is true for sources with decreasing signal characteristics. The comparative annoyance of such sounds signaling “approaching” and “receding” sources was evaluated by means of paired‐comparison judgments of several signals of 15 sec duration that continuously varied in both frequency (from 600 to 1200 or 1200 to 600 Hz) and intensity (from 70 to 100 or 100 to 70 dB). Findings clearly demonstrate that signals characterized as approaching entities were judged more annoying than all other signals assuming constant average intensity and frequency content over the signal duration. Increasing intensity influenced judgments of annoyance to a greater extent than did frequency changes. Rank order or annoyance corresponded to the hypothesis that the “approaching” characteristics of a sound source contribute significantly to its judged annoyance. The importance of these findings for the evaluation of aircraft flyover noises is discussed.

Primary Components of Simulated Air Bag Noise and Their Relative Effects on Human Hearing

Air bag inflation noise contains a positive pressure pulse caused by the air bag volume displacement and a high‐frequency noise burst associated with bag unfolding, air turbulence, and the like. Auditory temporary threshold shift (TTS) was measured for 12 discrete frequencies (125‐12‐kHz range) for volunteers exposed to (1) a positive pressure pulse of 165 dBpk, (2) a high‐frequency noise burst of 153 dBrms, and (3) (1) and (2) presented simultaneously. Results indicated that (a) the high‐frequency noise burst produced the greatest amount of TTS, (b) the positive pressure pulse produced no measurable TTS, and (c) the two components occurring simultaneously produced less TTS than the high‐frequency noise burst alone. TTS associated with air bag inflation noise is primarily the product of the high‐frequency noise. It is indicated that the positive pressure pulse reduces the effectiveness of the high‐frequency noise in producing TTS. Some implications of these observations relative to the use of air bag rest...

Quantification of the Noisiness of “Approaching” and “Receding” Sounds

This investigation represents the first phase of a research program designed to quantify subjective responses to time‐varying sounds grossly approximating those produced by aircraft flying over an observer at rest. The basic sounds employed represented “approaching” and “receding” sources that continuously increased or decreased in intensity and/or frequency over a period of 15.25 sec. The comparative annoyance or noisiness of such sounds was evaluated by means of paired comparison and individual adjustment judgments. For the three experiments conducted, the findings indicated that: (1) Signals representing an “approaching” sound were generally judged more annoying than those representing a “receding” sound—in spite of the fact that the approaching and receding signals contained the same average intensity and frequency content over signal duration. (2) Signals with time‐varying components—whether intensity or frequency—were judged to be noisier than signals with non‐time‐varying components. The maximum av...

The effect of reduced headband force on the attenuation of muff‐type protectors

The headband force of five different protectors was decreased in one‐half pound increments until the force was less than one pound. Using a dummy head, the attenuation was measured for each value of headband force. The results indicated that four of the muffs were relatively insensitive to a decrease in headband force and loss of 50% of the original force resulted in less than a 3‐dB reduction in the value of the Noise Reduction Rating (NRR). One muff, however, was very sensitive and a 50% loss in headband force resulted in roughly a 50% loss in attenuation. To verify the dummy head results, attenuation was measured at selected forces on three human subjects. Although the attenuations measured were somewhat less than those measured for the dummy heads, the effect of changing headband force produced similar results. While all protectors were unaffected by a reduction of one‐half pound in force, the effect of greater reductions varied dramatically with the make and model of the protector. Apparently, manufacturers need to set guidelines tailored to their specific models.

The Perception of Synthetic Speech in Noise

Although much information about synthetic speech has been acquired over past decades, we have been unable to find in the literature a systematic examination of the perception of synthetic speech in noise. Simpson [1] has reported that synthetic speech altitude callouts to airline pilots in widebody jet cockpit noise at a S/N of -10 dB for the first time were 99.1% intelligible and that synthetic speech voice warnings to helicopter pilots in simulated helicopter noise at a S/N ratio of -22 dB were 99.2% intelligible [2]. Nusbaum [3] has reported that perceptual confusions for synthetic CV and VC syllables were quite different than confusions observed for natural speech degraded by noise. Pisoni (personal communication) indicates that one of two synthetic speech systems with very high levels of segmental intelligibility in quiet, showed greater decrements in the intelligibility of CV syllables in noise than did the other system. Clark [4] reported little difference in the intelligibility of vowels in noise for synthetic and natural speech, whereas natural CV syllables were clearly superior to synthetic CV syllables under all noise conditions.

Attenuation Characteristics of Earmuffs at Low Audio and Infrasonic Frequencies

Progress in the development of newer and larger propulsion systems has resulted in the generation of greater acoustic energy in the low‐audio and infrasonic frequency ranges. The efficiency of muff‐type ear protectors in intense acoustic exposures of this type has been questioned. For this purpose, a study was initiated to evaluate the effective noise reduction of present‐day earmuffs in this frequency range. Subjective attenuation and noise reduction of three different earmuffs are described. These characteristics were determined (1) by physical measurements of discrete frequency signals in the range from 1 to 500 Hz inside and outside earmuffs worn by subjects and (2) by psychoacoustical or subjective measurements similar to the USASI “Real Ear Attenuation at Threshold Method.” Measurements were performed using nine subjects; and the data, including correlation between the two measurement methods, are presented in terms of mean attenuation and as a function of frequency. Results show that good present‐d...

Comparison of WG11 study variability to that of other interlaboratory studies

The variability of the data measured in the WG11 interlaboratory study was compared with that measured in three other interlaboratory studies conducted in this country, in Europe, and in the Scandinavian nations. All studies utilized a real‐ear attenuation at the threshold measurement paradigm and basically followed the procedures described in the current ANSI and ISO hearing protector measurement standards, with one major exception. The WG11 study utilized naive subjects and hearing protector fitting procedures that were relatively free from experimenter influences instead of the trained subjects and the ‘‘best fit’’ procedures required by current measurement standards. A subject fit (only hearing protector manufacturer’s printed instructions handed to the naive subjects) and an informed user fit (manufacturer’s instructions plus experimenter general comments to the naive subjects only relative to the instructions) were utilized for the WG11 study measurements. Comparisons indicate that the variability o...