Volume 31, Issue 3, March 1959
Index of content:
31(1959); http://dx.doi.org/10.1121/1.1907711View Description Hide Description
A high‐intensity sound test chamber capable of producing 145 db and having a working area of ft is described. The chamber uses the principle of reverberant sound to test electronic packages used in aircraft and missiles. Double wall construction and use of Aquaplas for panel damping provides the necessary sound insulation. Complete instrumentation and the loudspeakergenerator will be described.
31(1959); http://dx.doi.org/10.1121/1.1907712View Description Hide Description
The effect of the frequency of occurrence of words upon their intelligibility in noise was examined under two conditions: (1) in unknown message sets where the specific words under test were initially unknown to the listener; and (2) in known message sets where the specific words under test were known to the listener. Substantial effects of word frequency are observed with unknown message sets, but not with known message sets. In known message sets, the prime factor determining intelligibility is the phonemic interconfusability among the words. In unknown message sets, it is suggested that the important determinant of the intelligibility of a word is its frequency of occurrence relative to the frequencies of the words with which it might be confused.
31(1959); http://dx.doi.org/10.1121/1.1907713View Description Hide Description
Intelligibility is defined as a property of speech communication involving meaning. Since only meaningful units have normal linguistic distributional properties, only meaningful units have the phoneticproperties of actual speech. An auditory intelligibility dialect ratio is defined for a listener relative to a specified message. An analysis is presented of the Harvard PB lists, and a set of 10 lists of 50 monosyllables each is presented having an almost exact first‐order phonemic balancing.
Measurement of Ultrasonic Wave Velocities and Elastic Moduli for Small Solid Specimens at High Temperatures31(1959); http://dx.doi.org/10.1121/1.1907714View Description Hide Description
An improved buffer rod‐specimen unit is described for measuring longitudinal wavevelocities in solids at temperatures up to 350°C. The method is particularly useful in determining elastic moduli of single crystals since very small path lengths in any prescribed direction can be employed. The use of molten metals to provide coupling at still higher temperatures is discussed.
Two methods involving mode conversion at an impedance discontinuity are described for measuring shear wavevelocities. Application to crystals of cubic symmetry is detailed, with a note on possible extension to less symmetrical crystals of any orientation.
Data for fused silica and single crystalgermanium are listed for temperatures up to 300°C in illustration of the methods described.
31(1959); http://dx.doi.org/10.1121/1.1907715View Description Hide Description
An experimental program was conducted with a model ramjet combustor directed towards understanding the mechanisms which maintain high‐frequency combustionpressure oscillations (commonly referred to as “screech”) in the air‐breathing jet engine. Results show that pressure oscillations characteristic of transverse acoustic modes occur as fuel‐air mixtures approach stoichiometric values. The particular mode excited is related to orientation of the flame fronts in the duct, which in turn are functions of the particular flame holder geometry. It is shown that heat release associated with the flame fronts occurs in high‐impedance zones for the modes excited. High‐speed motion pictures of the combustion zone during screech show that the flame fronts are serrated, the number of serrations being proportional to screech frequency. The serrations are interpreted to be related to vortex shedding from the flame holder lips. High‐amplitude oscillations corresponding to the fundamental longitudinal mode occurred when conditions of intense screech were achieved. This mode transference is attributed to increased heat addition in the forward portion of the duct, a position favorable for driving longitudinal modes. Preliminary results show that high‐turbulence levels produce screech at lower values of fuel‐air mixture.
31(1959); http://dx.doi.org/10.1121/1.1907716View Description Hide Description
Absorption and velocity of sound in the range 0.5 to 10 Mc have been measured for solutions of hemoglobin under various conditions of temperature, concentration, and chemical environment. The hemoglobins of several species of mammals and different chemical forms of hemoglobin have been studied.
In the absorption per wavelength only minor variations with frequency have been observed. Dispersion in the velocity of sound of these solutions has been measured. It has been possible to relate quantitatively the magnitude of the absorption and dispersion through relaxation theory by assuming broad distribution of relaxation times.
31(1959); http://dx.doi.org/10.1121/1.1907717View Description Hide Description
Beta‐quartz, the high‐temperature form of quartz, which exists from 573°C to 850°C, is examined for use as a piezoelectric device. The formulas for the piezoelectric constants and elastic constants are given for rotated reference frames. By suitably rotating the crystal plate, it is possible to piezoelectrically excite lengthwise extensional, face shear and thickness shear modes of vibration. Thickness extensional modes cannot be excited. It is thus possible to make both high‐ and low‐frequency piezoelectric resonators and transducers out of β‐quartz for very high temperature applications.
31(1959); http://dx.doi.org/10.1121/1.1907718View Description Hide Description
The piezoelectric relations and equations of equilibrium for an elastic spherical shell permanently polarized in the radial direction are derived. The solution for the radial displacement corresponding to uniform pressures on the inner and outer surfaces of the shell and a given voltage difference between the surfaces is obtained.
31(1959); http://dx.doi.org/10.1121/1.1907719View Description Hide Description
The problem of the propogation of harmonic signals in a slightly rough duct is investigated. The analysis made tractable by the assumption that the heights of the roughness peaks are small compared to the average separation of the duct walls. Both acoustic and electromagnetic waves are treated. In the case of a duct with harmonic roughness, it is found that waves are not scattered upstream if the frequency of the transmitted signal exceeds certain critical values related to the natural modes of the wave guide and the wavelengths of the wall roughness. It is also found that the scattered signals are amplified whenever the frequency of the transmitted signal is equal to any of a series of critical frequencies determined by the natural modes of the guide and the spectrum of the wall roughness. Statistically rough walls are also treated, and specific results worked out for roughness functions having Dirac delta function correlation exponential correlation.
31(1959); http://dx.doi.org/10.1121/1.1907720View Description Hide Description
If the criterion is adopted that a schematic diagram of a system must give a complete representation of all the physical or engineering principles involved including a specification of the characteristics of each part and how the various parts are joined to form the system, it follows that perfect couplers must be included the list of items available to anyone creating schematic diagrams. It is shown by examples that perfect couplers produce their action by the imposition of constraints and that the set of constraints for a given coupler imposes the condition of no power loss or gain. Conversely, if a conceptual device is known to be a perfect coupler and some constraints are known, then the remaining ones can be deduced. It is shown that perfect couplers may be of two types and that examples of each type are common. It is further pointed out that perfect couplers present no problem if an investigator is always aware of their possible existence. The discussion closes by showing that couplers can be dealt with mathematically in a systematic manner.
31(1959); http://dx.doi.org/10.1121/1.1907721View Description Hide Description
The acoustical damping system described in this paper uses a diaphragm, coupling chamber, and a resistive element in the form of a plug of porous sintered metal. A major feature of the development is the use of pressurization to match the acoustic impedance of the coupling chamber to that of the plug. Critical requirements which must be satisfied by the diaphragm, the chamber, the plug, and its acoustical termination are presented. The theoretical frequency response is presented in dimensionless form, and expressions for the dimensionless parameters are given in terms of physical constants of the instrument. Experience with instruments designed according to these principles and manufactured in quantity by Statham Instruments, Inc., has confirmed the theory.
Synchronism of Neural Discharges and Their Demultiplication in Pitch Perception on the Skin and in Hearing31(1959); http://dx.doi.org/10.1121/1.1907722View Description Hide Description
The role of the volleys in the sensation of “pitch” on the skin was investigated. It was shown that the existence of rotating skin sensations does not insure perfect synchrony between sinusoidal vibrations and the discharges in the neural pathway. Experiments seem rather to indicate that, just as vibration loudness is a complex result of summation and inhibition, so is pitch sensation on the skin also influenced by summation and inhibition of a large group of end organs. One experiment showed that the pulse rate experienced on the skin is much lower than the pulse rate of the stimulus. This suggests a sort of “demultiplication” in the rate of the neural discharges along the neural pathway. This is one possible way in which synchronism may be maintained between the stimulus and the neural discharges, even though the frequencies transmitted are lowered.
31(1959); http://dx.doi.org/10.1121/1.1907723View Description Hide Description
Wires have been permanently implanted on the round window of the cochlea in cats. The voltage output of ears responding to sound stimulation has thus been made continuously available in unanesthetized animals for periods up to 4 months. By cutting the middle earmuscles of one ear and comparing its responses with those derived from the normal ear on the opposite side, it has been shown that the muscles: (1) do not appreciably influence absolute sensitivity, (2) contract to intense stimuli within 15 msec of their delivery to either ear, (3) attenuate transmission of tones between at least 500 and 3000 cps, and (4) significantly protect the ear against damage from intense sounds. Spontaneous contractions sporadically and intermittently introduce a transmission attenuation of several decibels in the resting normal cat. The stapedius muscle is much more important than the tensor tympani in producing these effects.
31(1959); http://dx.doi.org/10.1121/1.1907724View Description Hide Description
Electrical responses to low‐intensity repetitive clicks and bursts of noise were recorded from the cat auditory cortex. Responses decrease in size as the repetition rate is increased, and for rates higher than 50/sec are smaller than the “ongoing” activity of the cortex and become difficult to detect visually. An electronic average response computer was used to detect responses to high rates of stimulation. Stimulus‐locked responses to clicks and bursts of noise were found for rates up to 200/sec in unanesthetized cats, but only up to 100/sec after anesthetization.
31(1959); http://dx.doi.org/10.1121/1.1907725View Description Hide Description
The loudness of multitone complexes and bands of white noise was measured as a function of band width at levels between 5 and 35 db above threshold. When account is taken of the changes in the loudness that are due to the nonuniformity of the ear's sensitivity at low levels, the spreading of energy appears to have the same general effect on the loudness of both line spectra and continuous spectra centered at frequencies as low as 400 cps and as high as 5000 cps. At 5 db SL the spreading of energy does not increase the loudness, and may even decrease it. Above about 10 db SL, the spreading of energy in a complex sound over more than a critical band increases the loudness. As the sensation level is raised up to 35 db, the spreading of energy becomes more and more advantageous to loudness. The critical band defines the limits within which the spreading of energy leaves the loudness of a complex sound unchanged. The widths of the critical bands that were measured in these experiments on loudness summation below 35 db are approximately the same as those measured in experiments on loudness summation at higher levels and also in experiments on phase, threshold, and two‐tone masking.