Volume 29, Issue 12, December 1957
Index of content:
29(1957); http://dx.doi.org/10.1121/1.1908757View Description Hide Description
The physical form of the acoustical field at normal complex modes in rooms can be represented as a set of elementary curvilinear ducts, each carrying a single stationary wave, satisfying individually the boundary conditions at the walls. In spite of the differences in their lengths, all the ducts resonate at the same nominal frequency, the differences in their flares adjusting the phase velocity along their axes.
This theory is confirmed by the analysis derived from the theory of horns. An experiment on the computed model of the duct yielded with good accuracy the calculated frequency of resonance.
29(1957); http://dx.doi.org/10.1121/1.1908759View Description Hide Description
During the last year the ARF Acoustics Section has been moved into new quarters. In these new facilities a reverberation room has been built for use in the acoustic research of that group. The room has a volume of approximately 3000 cu ft and a reverberation time of about 5 sec. The four walls and ceiling of the room have been splayed. Measurements indicate that the room has a relatively uniform reverberation time with respect to frequency. Data are presented to show the ambient noise level in the room, the steady‐state sound energy distribution within the room, and the reverberation time as a function of frequency and location within the room.
29(1957); http://dx.doi.org/10.1121/1.1908761View Description Hide Description
Twenty subjects, age 20 to 58 years, were clinically screened to exclude gross otological pathology. Their auditory thresholds were then determined both by Békésy audiometry and by conventional individual pure‐tone audiometry by the method of limits, in 2 db steps. Measurement of the threshold of hearing by either method gave essentially similar results, and the reliabilities of the methods were of the same order for all frequencies tested. When both tests were repeated after an interval of one week, mean improvements of 1 to 2 db were found at all frequencies by the Békésy method, and in all but one by the individual pure tone method. In both methods, however, improvements were significant only at 1000 and 3000 cps.
29(1957); http://dx.doi.org/10.1121/1.1908763View Description Hide Description
As a part of an investigation on the hearingacuity of Air Force personnel, pure‐tone audiometric tests and job, noise, and medical history questionnaires were administered to 996 Bergstrom AFB flight‐line personnel. For 16 Air Force Specialty groups mean hearing losses (better and worse ear) at 4000 cps ranged from 1.8 db to 20.7 db reaudiometer zero. Worse‐ear hearing losses at 3000 cps and at 4000 cps in nine Air Force Specialty groups (N=854) did not differ significantly from one group to another. Significant positive correlation was found when 4000‐cps hearing loss was correlated with age and when 4000‐cps hearing loss was correlated with length of noise exposure. However, when the positive hearing lossvs exposure correlation was adjusted for the positive relationship between age and exposure, no remaining statistically significant relationship existed between 4000‐cps hearing loss and length of noise exposure. Infrequent use of ear protection is interpreted as evidence indicating a real need for effective noise and ear‐protection indoctrination of personnel exposed to high‐level on the job noise.
29(1957); http://dx.doi.org/10.1121/1.1908766View Description Hide Description
During recent years a program of work has been in progress at the National Physical Laboratory with the objective of providing improved data on several aspects of subjective acoustics. In connection with standards for audiometry,measurements have been made of the threshold of hearing for pure tones by earphone listening, and these have since been extended to the case of listening in free field. Latterly a redetermination of the equal‐loudness relations for pure tones has been completed aimed at resolving discrepancies between former determinations and providing an improved basis for the establishment of a standard set of contours. These results apply to a large team of otologically normal observers, and cover the range from 25–15 000 cps and up to 130 db in sound pressure level. The results of this investigation enable the equivalent loudness of any pure tone to be expressed by simple formulas with coefficients varying smoothly with frequency. Considerable attention has also been devoted to the determination of the loudness scale, i.e., the function relating loudness level in phons to the magnitude of the loudness sensation expressed on the sone scale. An assessment of the experimental evidence has led to the formulation of a simple relation which appears adequate for practical purposes in noise measurement. Investigations are continuing on the determination of loudness levels of complex sounds from their objective spectra.
29(1957); http://dx.doi.org/10.1121/1.1908768View Description Hide Description
Audiograms were obtained on 1200 naval enlisted men at NAS Cecil Field, Florida. Although hearing losses were slightly greater among men who were or had been exposed to noise from reciprocating engines and jets without afterburner, this is probably due to the fact that these men have also been exposed to more gunfire. Follow‐up audiograms taken on 220 of these men (all men in three squadrons whose planes had afterburners) revealed no additional hearing loss after seven months of moderate exposure. A similar negative result was found in a study of flight‐deck personnel during a three‐month cruise aboard an aircraft carrier.
29(1957); http://dx.doi.org/10.1121/1.1908770View Description Hide Description
When exposed to sounds of sufficient intensity Peking ducks respond in a characteristic manner combining head shaking and tail twitching. The response rate (number of responses per unit‐time) increases with intensity up to a certain level, but falls with further increase in intensity. There is no response at intensities lower than a certain critical value which may therefore be described as the irritation threshold. Prolonged exposure to sound leads to a more or less exponential drop in response rate but recovery is rapid during rest periods. Measurements on about thirty ducks indicate that the threshold has a broad minimum at about 500 cps and the average value at this frequency is about 70 db [all sound pressure levels are given with respect to 0.0002 d/cm2] with a range of 65–85 db. There is some indication that the threshold is slightly lower for warble tones. At 15 and 18 kc no consistent responses have been obtained up to 115 and 120 db, respectively. Similar experiments with chickens and geese showed fewer responses and hence much less accurate results. It has also been shown that hungry Peking ducks are greatly discouraged from taking food placed in a low‐frequency sound field at 100 db intensity.
29(1957); http://dx.doi.org/10.1121/1.1908772View Description Hide Description
When exposed to high‐intensity sound, ringbilled gulls do not, to any extent, show the head‐shaking reaction that was previously observed for ducks. They do, however, show a great tendency to cringe or crouch and are also subject to a temporary change in the rate of heart beat. These reactions are used to define and measure an acoustic “irritation threshold” at different frequencies between 100 and 3000 cps. The threshold determined with reference to the cringing reaction is remarkably reproducible over a period of weeks (having an average minimum value of about 85 db at 400 to 500 cps) but rises considerably during the first eleven months of the bird's life.
29(1957); http://dx.doi.org/10.1121/1.1908774View Description Hide Description
Changing the time separation between two trains of filtered pulses, each at the same basic repetition rate, makes it evident that there is a pitch which is correlated with the magnitude of the time separation. When both trains of pulses are at 30 db sensation level, decreasing the intensity of one train of pulses by 10 db results in a noticeable asymmetry in the production of the “time‐difference” pitches. That is, the “time‐difference” pitches are much stronger when the pulses of the less intense train are in the second half of the interval between the pulses of the more intense train. No differences in spectra of the stimuli have been found which could explain this asymmetry.
29(1957); http://dx.doi.org/10.1121/1.1908776View Description Hide Description
The acoustic method of impedance measurement at the eardrum, described in the preceding paper, has been improved and applied to the investigation of the effect of some pathological changes in the middle ear on the measured impedance. The investigation shows that the otosclerosis and the interruption of the ossicular chain can be easily detected by means of the impedance measurement. An electrical and mathematical model of the dynamics of the middle ear is developed.
29(1957); http://dx.doi.org/10.1121/1.1908778View Description Hide Description
The use of the constant‐ratio rule to predict the confusion matrices for each of two five‐item subsets given the confusion matrix for a ten‐item master set is tested with naive subjects. Ninty percent of the predicted cell entries (expressed as proportions) deviated by less than 0.05 from the obtained cell entries. The predicted articulation score for the first subset was 67.9%, and the obtained articulation score was 68.9%. For the second subset the predicted and the obtained articulation scores were 78.4% and 82.6%, respectively.
29(1957); http://dx.doi.org/10.1121/1.1908780View Description Hide Description
The Lombard, or voice reflex, effect results in speech with characteristics different from those of speech that is normally produced. This change of characteristics can be demonstrated as an effective way to combat noise interference during reception. It also demonstrates the advisability of control of the production of speech by the speaker himself, in addition to that offered by equipment, during audiological evaluations.
Fifteen naive speakers read words and sentences while noise was being delivered to their headsets. There were five noise conditions. Their speech was recorded, with the noise being kept out of the recording channel, and then limited. Noise was then added to the recording in such a way as to produce a constant speech‐to‐noise ratio. The result was played to 200 American listeners. Results indicate that at a constant speech‐to‐noise ratio of reception speech produced by a talker with masking noise in his cars becomes more intelligible as the masking level rises to a given value. The change in intelligibility throughout the range investigated suggests an application to audiological testing as well as a device for use in voice communication.
Speech Communications at High Noise Levels: The Roles of a Noise‐Operated Automatic Gain Control System and Hearing Protection29(1957); http://dx.doi.org/10.1121/1.1908782View Description Hide Description
Two aids for hearing conservation, a noise‐operated automatic gain control system and an insert ear protection, were evaluated in terms of their effect upon speech intelligibility. At high noise levels, these aids not only do not interfere with the speech intelligibility, they may substantially improvespeech intelligibility while affording hearing protection.
29(1957); http://dx.doi.org/10.1121/1.1908784View Description Hide Description
The effects of high sound levels on wide‐band speech in noise roughly parallel the effects of high levels on filtered speech. The Articulation Index (AI) concept encompasses both findings if it is assumed that the effect of high sound levels is to produce an effective change in AI.
29(1957); http://dx.doi.org/10.1121/1.1908786View Description Hide Description
This symposium on sound level meters is essentially a report of Committee Z24‐W‐18 of the American Standards Association, which for three years has been evolving a new standard for sound level meters. This new standard is more than a revision of the current one (Z24.3‐1944), since early in the deliberations of the committee it was decided that the standard warranted a completely new draft.
All persons participating in the symposium are either members of Committee Z24‐W‐18 or have been closely associated with the committee activity. In addition to the participants, other committee members have done yeoman work in the preparation of the draft through its numerous revisions. These other members are Mr. George Bonvallet, Dr. Arnold Peterson, Dr. Paul Veneklasen, Dr. Richard Wells, and the late Dr. Paul Geiger.
29(1957); http://dx.doi.org/10.1121/1.1908796View Description Hide Description