Volume 26, Issue 4, July 1954
Index of content:
26(1954); http://dx.doi.org/10.1121/1.1907361View Description Hide Description
The transmission of sound through thin elastic plates set in a plane infinite baffle or an infinite duct is discussed in some detail. A two‐dimensional configuration is considered in both instances. The problems then are the transmission through a strip set in an infinite baffle and transmission through a plate set in a duct supporting lowest mode propagation.
These systems make it possible to indicate in a formal manner how the boundary conditions affect transmission. The notion of bending impedance is used as a means of categorizing the mounting conditions.
Experimental investigations of the transmission through circular plates in ducts of the same cross section were carried ont. It is shown that the transmission loss varies with the manner in which the plates are mounted. The conditions under which this behavior is predictable are discussed.
26(1954); http://dx.doi.org/10.1121/1.1907362View Description Hide Description
Steady‐state flow data for a viscous, incompressible fluid are plotted as acoustic resistance against volume velocity extended to a volume velocity corresponding to a Reynolds number of one. The acoustic impedance at zero frequency is then equal to the resistance predicted from Poiseuille'sequation for laminar flow. The value of the Poiseuille resistance depends on the effective length of the pipe which in turn requires a reliable value of the end correction. By using end corrections that have proven adequate for periodic flow, the computed resistance agrees with the experimentally measured resistance in the region where resistance is independent of volume velocity or flow for orifice diameters from 0.106 cm to 1.28 cm. The resistance vsflow curve has a bend point at a Reynolds number of about ten for all orifice diameters measured. The resistance depends on the flow beyond a Reynolds number equal to ten, increasing with constant slope.
26(1954); http://dx.doi.org/10.1121/1.1907363View Description Hide Description
Vibration frequency and damping have been measured as a function of temperature in the range 4.5°K–300°K, for lead,copper,aluminum, and silver rods. Damping has been found to reach a maximum near of Debye temperature, owing to a new relaxation effect. Near the absolute zero both damping and frequency changes vanish as a high power of temperature.
26(1954); http://dx.doi.org/10.1121/1.1907364View Description Hide Description
Impedance tube methods of testing acoustical materials are restricted to frequencies below the value for which the wavelength is approximately equal to the largest lateral dimension of the tube. Above this frequency, extraneous modes of propagation interfere with measurements. The restriction is a serious one for typical acoustical materials that cannot adequately be sampled in less than 1 square foot. It has been found that the difficulty may be circumvented by using a horn to couple a large sample to a small impedance tube. Comparisons between horn and tube measurements have been made for a range of absorptive materials.
26(1954); http://dx.doi.org/10.1121/1.1907365View Description Hide Description
At low frequencies, the acoustic impedance of a right circular cylindrical enclosure (containing air, or other gases) is affected by the cooling effects of the walls. Analytical expressions for the temperature distribution have been obtained, and computations of the effect on the impedance are given in the form of plotted correction factors. These corrections are used in making absolute pressure calibrations of condensermicrophones at low frequencies.
The solution presented for the average temperature distribution applies also to the heat‐conductivity problem of a uniform volume distribution of sinusoidal heat sources, which are considered to be in phase, inside a cylindrical enclosure having isothermal walls.
26(1954); http://dx.doi.org/10.1121/1.1907366View Description Hide Description
The tapered reed, used in ultrasonic whistles, is a symmetric and homogeneous bar of rectangular cross section, uniform in the center and uniformly tapered to a sharp edge at each end. It is vibrating transversely under free‐free conditions in the plane of the taper, and in its lowest symmetric mode; i.e., it must be mounted at the nodes. We have calculated the frequencies and positions of the nodes for such bars, combining the exact methods of Rayleigh and Kirchhoff for bars of uniform and tapered cross sections, respectively. The main problem is that of joining the two different solutions at the junction of the two regions. Results were obtained for a representative range of values of the ratio of length of tapered portion to total length. The effect of the surrounding medium has not been considered.
26(1954); http://dx.doi.org/10.1121/1.1907367View Description Hide Description
The use of low velocity dyed‐liquid jets to obtain a motion picture history of the jet oscillation in a jet‐edge system is discussed. A group of photographs selected from such a history is presented, illustrating the jet configuration between orifice and edge at various intervals in the cycle of the jet motion. These photographs are then compared with the profiles predicted by a theory of one of the authors. It is found that when the actual divergence of the jet stream is accounted for, good agreement exists between the theoretical profiles and those observed.
26(1954); http://dx.doi.org/10.1121/1.1907368View Description Hide Description
High quality magnetic tape recordings were made of individual bells at the carillon location, including bells cast by Meneely and Company, Watervliet, New York; Van Aerschodt, Louvain, Belgium; John Taylor and Company, Loughborough, England; Franz Schilling Söhne, Apolda, Germany. Measurements made on the tapes with a sound analyzer, Stroboconn and an automatic level recorder give the relative tuning, the relative amplitudes at the microphone position, and the decay rates of the various components in the bell tone.
26(1954); http://dx.doi.org/10.1121/1.1907369View Description Hide Description
Raman first observed that the overtones of the Indian musical drums,Mridanga and Thabala, form a sequence of five natural harmonics, and that they result from nine modes of vibration, some of which are (approximately) degenerate. After a brief review of the previous work on the subject, a theory of these drums is developed in the present paper on the basis that the drumheads of these instruments can be regarded as circularly symmetric membranes with a radial step discontinuity the density. The eigenvalues and eigenfunctions of such a composite membrane are obtained, and it is then shown from a numerical calculation that for suitable ratios between the densities and radii of the two parts, the frequencies and the modes of vibration are in accordance with Raman's observations. Measured values of the frequencies of the first nine modes confirm the approximate degeneracies and the harmonicity indicated by the theory and establish the adequacy of the composite membrane as a mathematical idealization of these drumheads.
26(1954); http://dx.doi.org/10.1121/1.1907370View Description Hide Description
Articulation scores for nonsense syllables and for monosyllabic, disyllabic, and polysyllabic words were obtained as a function of the cut‐off frequency of low‐pass and high‐pass filters and also as a function of signal‐to‐noise ratio at different noise levels. Results indicate that eliminating all frequencies above or below 1600 cps does not impair the intelligibility of words seriously. The intelligibility of a word is a direct function of the number of syllables in the word and monosyllabic words are more intelligible than nonsense syllables. The relation between the intelligibilities of each word type and nonsense syllables is not the same when the system is impaired by filtering as it is when the system is impaired by noise. The implications of the lack of consistency from one experiment to the other for the generality of the concept of articulation index is discussed.
26(1954); http://dx.doi.org/10.1121/1.1907371View Description Hide Description
Autocorrelation techniques provide a method of displaying speech sounds in a form which differs significantly from conventional sound spectrograms. The autocorrelation function ψ11(τ), of a signal I(t) is defined by . Delay τ introduced in the speech signal appears as ordinate in these displays, time t as abscissa, and magnitude of the resulting correlation function ψ11(τ) is shown by the density of the pattern. Characteristic features of the displays are discussed and evaluated in terms of the mechanism of the correlation process. Data considered are limited to the ten spoken digits one, two, ⋯ nine, “oh.”
26(1954); http://dx.doi.org/10.1121/1.1907372View Description Hide Description
The full benefits of modern communication engineering will be realized only when we have enough quantitative knowledge of the transmission properties of the human communication channel to guide technological development. This deficiency is a limiting factor in the effort to achieve economy of transmission through more efficient use of the time and frequency dimensions in peace time telephony and television; it has also become of serious concern in our inability to extract adequate information from marginal signals in military applications.
This paper uses concepts in information theory and psychophysics to provide a basis for the quantitative measurement of certain communication properties of the human being. Part 1 of the paper discusses testing techniques in which binary signals were used for measuringinformation transmitted over a human communication channel. Part 2 proposes a model of this channel for investigating the factors which influence this information rate. The model involves three parameters: the signal level, the noise level of the human circuits, and the discriminant level of the human circuits. Some illustrative experimental measurements of these parameters are included.
26(1954); http://dx.doi.org/10.1121/1.1907373View Description Hide Description
The experiments described here continue a series reported in this Journal earlier. A number of objective tests are described, directed towards the general problem concerning the faculty we possess of perceiving only one acoustic world under normal conditions (in spite of having two ears) and yet, if our ears be stimulated by different signals, artificially, we can attend to one or the other of them. The tests have been made with continuous speech (readings from light fiction) and the results assessed statistically. The first tests aim at measuring the reaction time r required to “switch the attention” from one ear to the other, as assessed by perception of the words of the message. A second set of tests show that we perceive only one speaker, as a “gestalt,” when the ears are stimulated by similar messages but with a delay between them exceeding 20 times that ever experienced in real life by virtue of binaural directivity.
26(1954); http://dx.doi.org/10.1121/1.1907374View Description Hide Description
The combination of the physical tones of the frequency ratio 9:13 makes the secondary pitch 5 clearly audible; but neither the difference tone pitch nor the summation tone pitch, nor any so‐called aural harmonic is audible. The theoretical discussion is based on the hydraulic (nonvibratory) mathematics of cochlear mechanics.
26(1954); http://dx.doi.org/10.1121/1.1907375View Description Hide Description
Frequency modulation within a carrier frequency pulse in the sonic microwave region is measured in this paper by comparing any span of several carrier cycles with every other such span along the length of the pulse. The method is especially suitable for modulation in the range of 0.1 to 5 percent.
26(1954); http://dx.doi.org/10.1121/1.1907376View Description Hide Description
Measurements of ultrasonic propagation of longitudinal waves in pentachlorobiphenyl were made over a large range of viscosity at frequencies in the range 7.5 to 52.5 Mc. The data at low viscosities demonstrated the presence of both shear and compressional viscous processes. Measurements in the relaxation region indicated that a single relaxation time theory could not explain the results. Several possible explanations are discussed. The data at very high viscosities showed a frequency independent absorption loss per wavelength, which indicated the possible presence of a nonrelaxational type of hysteresis.
The thermal coefficient of the high‐frequency velocity was measured and found to be about twice the coefficient of the low‐frequency velocity. The thermal coefficients of the instantaneous elastic moduli are compared to the low‐frequency elastic moduli. It was concluded that the differences in the temperature dependence of these moduli could be related to the changing degree of order in the liquid.
26(1954); http://dx.doi.org/10.1121/1.1907377View Description Hide Description
The ultrasonicabsorption coefficients for longitudinal waves in glycerol and pentachlorobiphenyl are measured over the frequency range from 7.5 to 70 Mc. The measurements are made in the range of viscosities from 105 to 107 poise so that the ultrasonic frequencies used are well above the main relaxation frequencies associated with viscous flow processes in these liquids. The results indicate that the absorption coefficient per cm can be represented by the expression , where B and H are constants and f is the ultrasonic frequency. The constants B and H in both liquids are found to decrease with increasing temperature, exhibiting an “apparent” activation energy of about 24 kcal/mole and 12 kcal/mole for the pentachlorobiphenyl and glycerol respectively.
It is concluded that the results found here indicate that both terms B and Hf found in a could best be explained by assuming that one or more distributions of relaxation times exist.
It is suggested that ultrasonic hysteresis in both liquids and solids have a common origin indicating that the hysteresis effect in liquids is not related to the viscous flow mechanism causing the absorption maximum at lower frequencies and viscosities. A possible mechanism offered for the hysteresis loss is the coupling of the acoustic energy into heat energy due to the anharmonicities in the lattice structure.
- LETTERS TO THE EDITOR
26(1954); http://dx.doi.org/10.1121/1.1907378View Description Hide Description
Results are reported on recent investigations to test the possibility of detecting braintumors by ultrasonic scanning. It is concluded that though compensated ultrasonograms (sound shadow pictures) may contain some information on brain structure, their are too sharply “noise” limited to be of unqualified clinical value.
26(1954); http://dx.doi.org/10.1121/1.1907379View Description Hide Description
26(1954); http://dx.doi.org/10.1121/1.1907380View Description Hide Description
Pekeris obtained a solution for the transmission of sound in a 2‐layered liquid half‐space by dealing with the Hankel transform of the potential function. It is shown that in satisfying the boundary conditions at the interface his method involves an unjustified interchange of a differentiation and an integration.