Volume 19, Issue 5, September 1947

Methods of Measuring Speech Spectra
View Description Hide DescriptionA comparison was made of several alternative methods and devices for analyzingspeech in terms of its acoustic spectrum (a time average of the sound‐pressure level per cycle vs. frequency). A general procedure, applicable to a wide variety of analyzing instruments, consists of a comparison made between a known acoustic spectrum and an unknown spectrum(speech). For maximum convenience, the known spectrum should be a “white noise” whose spectrum level is a constant number of decibels per cycle. This spectrum is used as a standard to provide an over‐all calibration of the entire recording and analyzing system. In practice, the shape of the speechspectrum is obtained directly as the difference in decibels, in each pass band of the analyzer, between the level obtained for the white noise and the level obtained for the speech.
In order to measure conveniently the spectrum of the noise used as the standard of reference, a graphic analyzer was devised as follows: A Hewlett‐Packard wave analyzer, Type 300A, was modified to give access to the 20,000‐cycle voltage al the input to the meter system. This voltage was used to drive either of two types of graphic recorders. Continuous analyses from 0 to 16,000 c.p.s. can be made with various constant band widths.
The procedure of comparing a known with an unknown spectrum eliminates the necessity of calibrating separately the recording, reproducing, and analyzing equipment. In the analysis of speech, only the calibration of the microphone need be considered as a final correction to the data. This procedure also makes it possible to obtain accurate analyses despite the use of voice recording and reproducing equipment having unknown frequency characteristics.
Although the choice of filter cut‐off frequencies is arbitrary, it has been found most convenient to analyzespeech by dividing it up into bands that stimulate equally wide regions on the basilar membrane. This is accomplished by choosing filter cut‐offs at equal intervals along the mel scale of subjective pitch.
Within the limits of observational error the same answer is obtained when speech is analyzed by any of three different measuring systems: (a) square‐law integrator (audio spectrometer), (b) linear integrator, (c) R.C.A. noise meter and Esterline Angus graphic recorder. When, with any of these systems, a speechspectrum is determined as the difference in decibels between the two analyses — that for white noise and that for speech — the speechspectrum can be stated in terms of sound‐pressure level per cycle averaged over each of the nominal pass bands of the filters. Very narrow pass bands (5 c.p.s.) reveal details in the speechspectrum not disclosed by the wider filter bands.

Phase Effects in Monaural Perception
View Description Hide DescriptionExperiments are reported demonstrating that the enveloped‐wave shape of a complex steady‐state tone is an important factor in audible perception. In particular, it strikingly influences sensations of roughness or smoothness and is related to a sensation of apparent pitch. As envelope‐wave shape depends on the phases as well as the amplitudes of the components, these differences of sensation can be produced by changes in the phase alone of but a single component or group of components. The results provide general verification of a limitation placed by Helmholtz, on theoretical grounds, as to the degree to which his phase rule might be expected to be true. The experiments were conducted at such levels that subjective tones produced by non‐linearity in the ear are not believed to influence the results. It is suggested that these results emphasize the importance of time factors in the phenomena of aural perception.

Masking Effect of Periodically Pulsed Tones as a Function of Time and Frequency
View Description Hide DescriptionStudies have been made of the masking effect of a periodically pulsed single‐frequency tone upon a similar wave of the same or different tone. The occurrence in relative time of the masked wave was varied over the whole period of pulse repetition. The masking is greatest when the two pulses are near coincidence, and approaches threshold as the pulse separation becomes greater. A small amount of fatigue effect was noticed when the masked pulse was in the region following the masking pulse. A totally unexpected result was that the maximum masking was generally obtained when the masked pulse preceded the masking pulse by 1 or 2 milliseconds. The amount of this time difference was found to be independent of the pulse repetition rate in the region explored of 30 to 100 pulses per second but is a function of the relative frequencies under the pulse envelopes.

The Effect of Frequency Spectrum on Temporal Integration of Energy in the Ear
View Description Hide DescriptionThese experiments are designed to test the following hypothesis. The rate of the temporal integration of energy in the ear (at threshold) is dependent on the width of the frequency band of the energy to be integrated. Duration is exactly equivalent to intensity only when all the energy to be integrated is in a narrow band of frequencies. The hypothesis tested by taking advantage of the spectral distribution of energy in short tones. As a tone becomes very short, the effective band width of the energy increases. The band width of energy is essentially defined by the reciprocal of the duration of the tone. Thus as the duration of a tone decreases, not only does the total energy in that tone decrease, but the band width of energy also increases. The intensity threshold, then, has to be increased (as duration is decreased) to compensate for both effects if the hypothesis is correct. The results are in line with the predictions of the hypothesis. The width of the band necessary for maximum integration is also related to frequency and the width of critical bands.

Studies on Pitch Discrimination Masking. II. The Effect Of Signal/Noise Differential
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Reflection and Transmission of Sound by Thin Curved Shells
View Description Hide DescriptionIn this paper the reflection and transmission of sound by thin curved shells, as well as several related problems, are treated mathematically. First, one derives an inhomogeneous integral equation for the sound field in an infinite medium containing a thin curved shell of different material. The solution of the integral equation is then reduced, approximately, to the evaluation of a surface integral not too different from that obtained in the usual Kirchhoff diffraction theory. The integral is evaluated approximately and gives expressions for the pressure waves reflected from and transmitted through a thin curved shell. The reflection and transmission coefficients of the shell are obtained from these expressions. It is found that the reflection coefficient can be expressed as the product of a geometrical factor, a phase‐cancellation factor, and a reflectivity factor. When the reflection problem for a rigid obstacle is solved with the aid of the assumptions of the Kirchhoff diffraction theory, the reflection coefficient thus obtained is the product of the geometrical and phase factors of the previous solution. The geometrical factor alone is obtained as the reflection coefficient when the reflection problem is solved exactly by geometrical acoustics. The agreement among the various solutions may be considered as a partial justification for the Kirchhoff theory as well as for geometrical acoustics.
The Kirchhoff method is also applied to the problem of refraction at a curved surface. From the solutions of the reflection and refraction problems, the laws of reflection and refraction for curved surfaces are obtained. In addition, the mirror and lens laws, the conditions for point images, and the change of phase at a focus are obtained.

Microbarometric Pressures from Large High Explosive Blasts
View Description Hide DescriptionCharges detonated for Army‐Navy Explosives Safety Board tests in Idaho, October 1946, produced pressurewaves recorded by subsonic frequency microbarographs at distances 12.9 to 452 km. Observations showed both normal and abnormal signals at 182 and 292 km, no clear abnormal signals at 141 or 89 km, no signals of any kind at 872 km. In the zone of normal audibility, average wavevelocity between blast point and receiving station decreases slightly with increasing distance, and may increase slightly with charge weight; it is substantially the same as soundvelocity. No consistent travel‐time differences for the abnormal signals resulted from changing charge weight between 3.2 and 250 tons TNT. Neither normal nor abnormal signal strengths were predictable from charge weight. The largest abnormal signal properly recorded was a 3‐cycle wave train with peak‐to‐peak amplitude 220 microbars received 182 km from a 125‐ton blast. Interpolated to apex pressure perturbation, this signal amplitude eliminates shock wave supersonic velocity as a logical explanation for abnormal audibility. Incident angles of abnormal rays are not calculable. However, if one assumes 182 km as the descent distance for rays starting out horizontally, neglects wind effects, and accepts the apex temperaturesmeasured by balloons, rough calculations of lower stratosphere temperatures are possible. These establish 34 km as a minimum altitude at which ground temperature is reached.

A Continuously Variable Acoustic Impedance
View Description Hide DescriptionThe mechanical impedance of an electromagnetic speaker is affected by current flowing in the electrical elements, just as the electrical impedance is influenced by the motion of the mechanical system. This paper gives the relation between mechanical impedance and electrical load impedance, making use of the three complex speaker constants evaluated by the motional impedance method presented earlier by R. D. Fay. Measurements are given showing that the mechanical impedance of a WE555 horn driver can be varied over a fairly wide range.

Measurement of Acoustic Impedances of Surfaces in Water
View Description Hide DescriptionAn analytical and experimental investigation of the practicability of a water‐filled acoustic impedance tube is described. It is shown that by proper choice of tube material, and of diameter and wall thickness with respect to wave‐length, a satisfactory tube for measuring the impedance of underwater acoustic materials can be built. The phase velocity of sound in the tube is nearly constant in the useful frequency range but less than the velocity of sound in free water, and the variation in sound pressure along a radius of the tube is frequency dependent. Both rigorous and approximate formulae are given for the phase velocity and radial pressure dependence. These formulae are checked experimentally.

A Powerful High Frequency Siren
View Description Hide DescriptionThis paper describes the design, construction and performance of a high intensity, high frequency (3–34 kc) siren and some rather striking phenomena which occur in the intense sound field produced by it. The siren is of the usual type consisting of a rotor which interrupts the flow of air through orifices in a stator. The rotor consists of a disk, approximately 6 in. in diameter, with 100 equally spaced slots and is driven by a small hp high speed motor whose speed is varied by changing the applied voltage. The 100 corresponding holes in the stator are circular in section. For better radiation at the lower frequencies a plywood horn is mounted on the siren. The siren itself is small, and may be operated in any orientation. In its initial form, operating with chamber pressures in the neighborhood of 0.2 atmosphere, its measured efficiency was between 17 percent and 34 percent in the frequency range 3 to 19 kc with an acoustic output between 84 and 176 watts. There is a fair amount of parasitic noise, but it is negligible compared to the signal. With recent modifications, chamber pressures of about 2 atmospheres were obtained, yielding acoustic outputs of approximately 2 kilowatts, and an efficiency of about 20 percent.

Flow‐Resistance Characteristics of Fibrous Acoustical Materials
View Description Hide DescriptionThe flow resistance of a material is highly important as a factor in determining its acoustical performance in various applications, particularly in the blankets used for soundproofing of aircraft cabins. It is desirable, therefore, to know the manner in which the flow resistance of a lightweight acoustical material varies with such physical parameters as thickness, density, size and shape of fibers, and type of construction (“felting”) of the material.
From an experimental study of a large number of fibrous acoustical materials, the following empirical relation has been established for materials of high porosity (0.9 to 1.0): where K is a constant, characteristic of a particular material,S is the surface density of the blanket of material,T is the thickness, and r is the cross‐sectional radius of the fibers. The type of construction of the material determines the value of x in the exponents. This value generally lies in the range from 0.3 to 1.0.

The Properties of Felt in the Reduction of Noise and Vibration
View Description Hide DescriptionIn order to provide engineering information for the use of felt in vibration isolation, the dynamic stiffness and damping of three widely different grades of felt were measured by means of resonance curves. The dynamic stiffness, which was much greater than the static stiffness, decreased with amplitude so that the resonance curves were unsymmetrical. Except for pressures below 6 p.s.i., the stiffness modulus of felt increased with static pressure at a rate which made their ratio nearly constant. The natural frequency of a mass supported by a felt pad is, therefore, nearly independent of the static pressure load on the felt. Curves are given showing natural frequencies for 1 inch thicknesses of soft, medium, and hard felt for a range of static pressure between 1.5 and 100 p.s.i. It is shown that, except at low pressures, there is little difference in the natural frequency of hard or soft felt, and that there is a low limit of about 20 c.p.s. for 1‐inch felt.

A Slide Rule for the Study of Music and Musical Acoustics
View Description Hide DescriptionMusicians are seldom concerned with the mathematical background of their art, but an understanding of the underlying physical principles of music can be helpful in the study of music and in the considerations of problems related to musical instrument design. Musical data and numerical standards of the physics of music are readily adaptable to slide‐rule presentation, since they involve relationships which are the same for any key. This rule adjusts relative vibration rates, degrees of scale, intervals, chord structures, scale indications, and transposition data, against a base of the piano keyboard. It employs and relates several standard systems of frequency level specification.

Plate Vibration of Stringed Instruments at the Wolfnote
View Description Hide DescriptionThe principal mode of belly vibration at the wolf note was determined by direct investigation of violins,violas, and cellos. The same mode was found to be common to all three types of instrument, corresponding to a marked peak in the response curves of the instruments. Chladni sand patterns were obtained on a flat brass plate shaped like a violin; typical patterns are presented, including the mode observed on instruments at the wolfnote. Requirements and methods for improving the wolfnote are discussed. Some functions of the f‐holes, soundpost, bass‐bar, and sides are noted.

The Performance of Cup‐Mouthpiece Instruments
View Description Hide DescriptionIt is shown by a qualitative analysis that the playing of cup‐mouthpiece instruments must be associated with the existence of a pressure antinode at or near the mouthpiece, and that harmonic production in such instruments can be adequately explained by the acoustic overload in the air column.
A quantitative analysis discloses the “mechanism” by which the odd series of modes of vibration occur at frequencies related to each other by the 1, 2, 3, 4 integers of the complete harmonic series.

An Electrical Method of Measuring the Intonation of Cup‐Mouthpiece Instruments
View Description Hide DescriptionAn electrical driving system has been worked out for intonation tests on cup mouthpiece instruments. The system amounts to driving an acoustic transmission line from a closed end and measuring the resonant frequencies. It is shown experimentally that these resonant frequencies correspond quite precisely (within about 10 cents) to the frequencies normally played, except at the lower end of the scale where the player habitually corrects the defective intonation of the usual instrument. The experimental work includes tests on specially designed instruments that could not be played “in tune” with standard fingering.

Underwater Acoustical Measuring Facilities at the U.S. Navy Underwater Sound Laboratory
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Coordinates and the Reciprocity Theorem in Electromechanical Systems
View Description Hide DescriptionIt is shown that the (force‐voltage, velocity‐current) mechanical‐electrical analogy yields circuit equations for an electrostatic electromechanical transducer which satisfy the reciprocity theorem, while the (force‐current, velocity‐voltage) analogy acts similarly for a magnetic transducer. Reversing the two analogies between the two types of transducers yields (in both cases) equations which violate the reciprocity theorem in sign but not in magnitude. These conclusions indicate proper coordinate choices in applying Lagrange's equations to such systems. It is also shown that McMillan's “one‐way” four‐terminal network (see reference 1) is equivalent to a simple, class A vacuum‐tube amplifier having an amplification factor of approximately two.

High Frequency Absorption in Air and Other Gases
View Description Hide DescriptionThe absorption coefficients were measured for gases: (A) ; (B) ; (C) in normal air proportions; (D) in normal air proportions; (E) in normal air proportions, plus vapor at 37 percent Rel. Hum. at 26.5°C; (F) in normal air proportions, plus a ‐content varied from zero to 1.15 percent by volume. Let R denote the ratio of the measured absorption coefficient to that computed from the Stokes‐Kirchhoff equation. For gases (A), (B), (C), and (D), R is roughly 1.5. For gas (E), R is of the order of 10 at 15 Kc.p.s., and approaches 1.5 with increasing frequency. For gas (F) at 89 Kc.p.s., R increases with rising ‐content, attaining roughly R = 20 at 1.15 percent ‐content.

On the Design of Perforated Facings for Acoustic Materials
View Description Hide DescriptionAn approximate analysis of the acoustic behavior of perforated facings for absorptive materials is outlined and is reduced to a design chart. Only the mass reactances of the holes and of the facing material are included in the analysis. This simplified picture is adequate for many practical problems, but breaks down for certain conditions which are discussed. If the acoustic impedance of the unfaced material is known, absorption coefficients for the material with perforated facing can be read directly from the chart.