Index of content:
Volume 31, Issue 4, April 1959
31(1959); http://dx.doi.org/10.1121/1.1907726View Description Hide Description
A musical instrument is proposed with the following elements: a tone generator capable of reproducing the attack and decay transients and the steady state of the tones of conventional musical instruments with correctly proportioned partials, the frequencies of those associated with one note being integral multiples of a low frequency; keyboards sensitive to the speed with which a key is depressed and the distance to which it is depressed, so that the intensity of each note may be expressively controlled at all times; a unit for the controllable generation of massive choruses from solo voices by producing narrow, complex spectra about each partial; an intensity control unit; environment modification equipment to alter the apparent size of the room within which the music is heard and the apparent size and location of the source, and to distribute and diffuse the sound; a preset unit in which sequences of timbre,vibrato,choral massiveness, environmental characteristics, and other factors may be established before performance and quickly and easily called forth during performance. The features include tones of familiar character and quality, realistic choraleffects, expressive touch, environmental control, flexibility, and ease of playing. Numerical results are appended to demonstrate the feasibility of construction.
31(1959); http://dx.doi.org/10.1121/1.1907727View Description Hide Description
Far‐field noise measurements from model tests of a shielding flap type jet noise suppressor are presented in the form of radiation patterns and frequency spectra. The tests were conducted using a cold air jet issuing from the nozzle in such a manner as to attach to and flow along the surface of a shielding flap. Whereas other proposed noise suppressors provide generally symmetrical noise radiation patterns, the present device skews the pattern in a beneficial way such that large noise reductions are obtained in the downward direction. These large noise reductions are thought to result from both the acoustic shielding of the flap and the benefits of flow attachment.
31(1959); http://dx.doi.org/10.1121/1.1907728View Description Hide Description
An attempt is made to increase the intelligibility of speech sampled in time (interrupted speech) by filling in the blank spaces with repetitions of the speech samples (reiterated speech). The experiment involves the evaluation of these processes of speech distortion in quiet and in noise with two duty cycles (speech‐time fractions), with various rates of interruption of the speech signal, and with various buildup and decay times in the switching operation. The results indicate that (1) there is little difference between the intelligibility of reiterated speech and interrupted speech in quiet, (2) reiterated speech is more intelligible than interrupted speech in noise if signal measurements are based on instantaneous amplitudes of the speech samples, and (3) buildup and decay has little effect on the intelligibility of interrupted speech when the entire buildup phase is confined to the “on” time of the switching period.
31(1959); http://dx.doi.org/10.1121/1.1907729View Description Hide Description
Many different instrumental techniques for deriving an energy property of the speechwave have been employed in communication studies. There is as yet, however, no satisfactory method of calculating the loudness of the various types of complex quasi‐periodic sounds that appear as time functions in speech. This paper does not present a method of making such loudness calculations, but suggests that any such method would be inadequate for deriving physical correlates of accent or stress judgments about speech. The study reports volume indicator and instantaneous amplitude observations on sustained vowels produced under various conditions of speech effort, and on vowels produced in CNC syllables in a carrier phrase with stress held constant. The untested theory is proposed that the perception of linguistic stress is based upon judgments of the physiological effort involved in producing vowels. The relative amplitude and spectrum of the vowel provide the chief basis for judgment of the physiological effort.
Strain Gauge Measurement of Output of Magnetostrictive Ultrasonic Transducer—Pitfalls of Optical Measurement31(1959); http://dx.doi.org/10.1121/1.1907730View Description Hide Description
An easily fabricated, bifilar wire strain gauge wound helically about the hilt of an ultrasonicmagnetostrictive transducermeasures amplitude faithfully and is more convenient than optical methods. Pitfalls in the usual optical method are illustrated and discussed.
Study of Temperature Variation of Ultrasonic Velocities in Some Organic Liquids by Modified Fixed‐Path Interferometer Method31(1959); http://dx.doi.org/10.1121/1.1907731View Description Hide Description
A simple, rapid, and accurate method of determining temperature coefficient of ultrasonic velocity in liquids is presented. The method is based on a modification of the fixed‐path double‐crystal interferometer. Temperature variation of ultrasonic velocity in a few organic liquids have been studied by employing this method.
31(1959); http://dx.doi.org/10.1121/1.1907732View Description Hide Description
The usual solutions describing the propagation of continuous waves in solid plates and cylinders fail to predict the observed velocity and attenuation, and secondary signals, in the propagation of a narrow‐band pulse of compressional waves of carrier frequency of the order of 10 Mc/sec. It is shown that all these factors may be accounted for if a slightly different form of solution is chosen, to fit more closely the conditions under which the propagation takes place. Both plates and cylinders are treated theoretically, and methods of obtaining approximate solutions of the modified frequency equations are discussed.
The results of this analysis agree closely with those derived by other, less exact methods, which commence with the assumption that the solid medium behaves very similarly to a fluid.
The relation of this work to the propagation of pulses at low frequencies is also discussed.
A method of analysis similar to that employed here is also necessary to predict experimental results in the propagation of pulses in multilayered wave guides of any configuration, mechanical or electromagnetic.
31(1959); http://dx.doi.org/10.1121/1.1907733View Description Hide Description
A two‐receiver steerable correlation system intended to process signals from point sources at a large distance from and in a common plane with the receivers will give a predicted source direction which is generally different from the actual source direction when the source distance from the receivers is not large. The exceptions are broadside or end‐fire sources for which the predicted and actual directions are equal. Apart from these cases, the predicted direction is always closer to broadside than the actual direction. Other results include the fact that a given predicted direction corresponds to a source location at any point on a corresponding hyperbolic arc.
31(1959); http://dx.doi.org/10.1121/1.1907734View Description Hide Description
Two detection systems having multiple inputs each consisting of nonstationary noise and a stationary random signal are analyzed and compared on a signal‐to‐noise ratio basis. In both systems the inputs are divided into two groups and the sum of each group is formed. In the first system the resulting two wave forms are multiplied directly and the product averaged. In the second system the two wave forms are strongly clipped prior to multiplication, forming a polarity‐coincidence correlator. Previous studies have shown the latter system to be slightly inferior for stationary noise. The results of this paper show that the latter system may be quite superior in certain types of nonstationary noise.
31(1959); http://dx.doi.org/10.1121/1.1907736View Description Hide Description
A siren has been developed which permits the generation of a sound pressurewave with harmonics more than 15 db below the fundamental over a broad frequency range and over a range of power output exceeding 40 db. The large dynamic range was obtained by the use of a shutter which changes the maximum open area of the siren ports and simultaneously alters the time variation of area so that the output signal remains substantially sinusoidal at any shutter position. The shutter can be operated rapidly to provide a means for programed amplitude modulation of the output signal with a minimum of harmonic distortion.
31(1959); http://dx.doi.org/10.1121/1.1907737View Description Hide Description
In this paper a method is described for calculating the effect of the finite‐amplitude propagation on the travel times of pressure waves from explosions at high altitudes to the ground. Numerical results are presented for explosions of 2‐lb and 4‐lb H. E. charges at altitudes ranging from 15 to 95 km. Vertical propagation is assumed in the calculations. This work has been carried out in order to improve the systematic accuracy of the rocket grenade experiment for upper atmosphere temperature and winds. The significance of the finite‐amplitude propagation in this experiment is discussed.
31(1959); http://dx.doi.org/10.1121/1.1907738View Description Hide Description
An analysis is given of a solion acoustic pressuredetector which has a direct current output proportional to the logarithm of the pressure input. An approximate method is used which avoids solving the Navier‐Stokes equation describing the fluid flow and the convectiondiffusionequation for calculating the current.
The calculation consists of (1) solving the zero flow “transient” diffusionequation for the reducible component in the electrochemical redox system, (2) using this transient solution to calculate the output current, with the time spent inside the detector orifice as the independent variable, and (3) relating this time to the pressure by using the known acoustic impedance of the detector orifice.
The calculated results show that the logarithmic response is obtained through a combination of diffusion and hydrodynamic effects. For the ideal case of constant impedance as a function of volume flow, the logarithmic response is degraded.
31(1959); http://dx.doi.org/10.1121/1.1907739View Description Hide Description
The latency of the well‐synchronized whole‐nerve action potential (AP) in the cochlea of the guinea pig includes both neural and traveling‐wave delays. The latency varies with the intensity and with the frequency of the acoustic stimulus. Selective local interference with the response was produced by (1) selective masking, (2) local fatigue of sensory elements, and (3) local injection of drugs. Such interference the basal turn prolonged the latency of the AP response. Low‐frequency tones can reduce the N 1 response to click stimuli without changing the modal latency. Apparently nearly all of the well‐synchronized AP is due to neurons that arise in the first turn of the cochlea, regardless of the frequency of stimulation. The traveling‐wave delay appears as a significant component of latency only in the smaller AP responses that remain after the impulses arising in the first turn are suppressed. Information about the time of onset of an acoustic signal can be delivered to the central nervous system primarily by the best‐synchronized or modal portion of the AP response.
31(1959); http://dx.doi.org/10.1121/1.1907740View Description Hide Description
Previous investigations on the auditory localization of sounds have established that interaural differences in time and in intensity are both important. There have also been attempts to set temporal and intensive differences against each other, so that a lead in time at one ear accompanied by an increase in intensity at the other ear would cause a sound to be localized in the midline. On the basis of hypotheses suggested by physiological observations, two experiments were carried out. The first showed that the intensive difference required to offset a difference in time at the two ears depends upon the level of the sounds. Clicks at low level require smaller differences in intensity to offset a given temporal difference than do clicks at a high level. The relation between intensive differences (decibels) and temporal differences (microseconds) is not linear at any of the levels studied. In the second experiment, high‐frequency masking noise interfered with reception in the basal turn of the cochlea. Although this noise did not mask the clicks, it produced a temporal lag that required compensation by a time delay at the opposite ear. The relations between these psychophysical results and physiological findings are discussed.
31(1959); http://dx.doi.org/10.1121/1.1907741View Description Hide Description
Twenty passive sonar recordings were ranked by 50 subjects in terms of their aurally perceived (psychological) characteristics on seven different dimensions. Correlations were run between octave band sound pressure level measurements of the sounds and their rank orders on the seven psychological dimensions. Meaningful relationships were found between ranks on certain psychological dimensions and energy concentration within certain octave bands.
Also, an analysis was made of the manner in which the twenty sounds clustered within the space defined by the seven psychological dimensions. Analyses of these sound clusters revealed that the rhythmic beat pattern of the stimulus is a principal attribute upon which sonar men base their judgments of “similarity.”
31(1959); http://dx.doi.org/10.1121/1.1907742View Description Hide Description
In recent years there has been an increased emphasis on medicolegal problems involving the assessment of hearing loss due to noise exposure. One of the attendant difficulties, however, has been the problem of separating the factors of noise exposure and of age as contributors to hearing disabilities. The present study was performed to determine the normal thresholds of hearing for pure tones for an age‐stratified sample of subjects drawn from a population exposed to minimal levels of industrial noise. Audiometric measurements from 250 to 8000 cps were made on a total of 500 subjects divided into four age groups: 18 to 24, 26 to 32, 34 to 40, and 43 to 49 years, inclusive. The results of this study indicate that, in general, only minor differences (less than 5 db) exist between the average thresholds of right and left ears, but that women have more sensitive hearing than men and show less intersubject variability. This sex difference is independent of age and is more marked at the higher frequencies. For both men and women, there is a decrease in hearing sensitivity with increasing age and a progressive spreading of the loss from the higher to the lower frequencies. Men are more affected than women, with the hearing loss occurring at an earlier age and producing a greater degree of auditory impairment. The threshold curves of the present study are considered to yield a more valid estimate of the “pure” effects of age on hearing than previously available. It is recommended that future audiometric standards be specified independently for men and for women according to particular age levels.
31(1959); http://dx.doi.org/10.1121/1.1907743View Description Hide Description
This study investigated the effects of 4 band widths of noise of different center frequencies presented at three sensation levels on the size of the differential threshold (ΔI) for intensity. Eleven untrained subjects and three trained subjects served in a study employing the method of limits. Analysis of the data showed that ΔI's for a low octave band (127–225 cps) of noise were larger than those for a middle octave band (1040–2080 cps) and high octave band (4080–8160 cps) of noise. The middle and high bands in turn yielded ΔI's that were larger than those for a wide‐band noise. The magnitude of the ΔI's for the octave noise bands were tentatively related to the average ΔI for those frequencies contained in the band. Comparison of the ΔI's for wide‐band noise to those obtained by other experimenters showed close agreement.
31(1959); http://dx.doi.org/10.1121/1.1907744View Description Hide Description
The index of detectability d′ was estimated from data collected with various psychophysical procedures, specifically, the forced‐choice method with different numbers of alternatives, and the yes‐no method. The estimates were, in all instances, quite comparable. This result is comforting in view of the fact that none of the other indices extant provide a unification of data collected with different procedures. This result is also somewhat surprising view of the fact that the calculation of d′ assumes perfect memory, and forced‐choice experiments were conducted with as many as eight temporal alternatives.
31(1959); http://dx.doi.org/10.1121/1.1907745View Description Hide Description
The use of repeated presentations of a given signal event as an experimental technique in psychoacoustic studies provides information about several general properties of the hearing process. From the relationship between the gain in detectability that results from additional observations and the type of signal and noise employed, inferences can be made about: (1) the observer's ability to integrate over time, (2) the amount of noise generated by the auditory system, (3) the nature of the process of frequency analysis, and (4) the observer's mode of dealing with uncertainty as to signal frequency. The first set of experiments permitted five observations of each signal where the signal consisted of a pulsed tone, of known frequency, in noise. Both variable noise, i.e., noise that is statistically independent from one presentation to another, and constant noise, i.e., noise that is exactly the same on each of the five presentations, were used. With variable noise, the detectability index d′ improves, as predicted, as the square root of the number of observations. The use of constant noise, which results in less improvement, provides an estimate of the portion of the total noise affecting detection that is of internal origin. The results under different levels of external noise indicate that internal noise is proportional to external noise. A second set of experiments employed signals whose frequencies were unknown to the observers, and signals comprised of several widely spaced frequencies. Their results are discussed in relation to three alternative models of the process of frequency analysis.
31(1959); http://dx.doi.org/10.1121/1.1907746View Description Hide Description
The growth and recovery of TTS in normal observers following exposure to octave‐band noise is shown to follow the same course as that after broad‐band noise: both are linear in log time. Rate of growth varies with frequency of exposure band and test frequency, being greatest at 4 kc following exposure to 2400–4800 cps or 1200–2400 cps, less at lower test frequencies and octave bands. The time for total recovery apparently is a function of the initial TTS. The results support present damage‐risk criteria for continuous noise, which (1) suggest ear protection when octave‐band levels exceed 85 db SPL and (2) require it in levels above 95 db SPL.