bluuebunny/crossref_metadata_2025 huggingface dataset:
Properties of Linear Systems Used in Vibration Measuring Instruments
Authors: Trimmer John D.
Published: September 2005

Properties of simple mechanical and electrical systems are summarized, with special reference to certain ones useful in vibration measurement. It is shown that several physical systems, including the mechanical “seismographic” system used in vibration pick-ups and the electrical circuit of the resistance coupled amplifier, are governed essentially by a single analytical expression. Information is presented in the two equivalent forms: frequency characteristics of amplitude and of phase time (converted from phase angle); and indicial response, i.e., the response to forcing by a step function. These results are discussed in relation to the fundamental possibilities of vibration measurement by the seismographic method and to the determination of the parameters of particular instruments. The concepts of instrument element and instrument sensitivity, as used in the instrumentation courses at M. I. T., are introduced to systematize the discussion.
Power Transmission Loss in Exponential Horns and Pipes with Wall Absorption
Authors: Phelps William D.
Published: September 2005

The differential equation governing the propagation of sound in horns and pipes in general with wall absorption is formulated. Expressions for the pressure and power transmission loss in exponential horns and cylindrical pipes are given. Families of curves showing the power transmission loss in horns and pipes with various wall absorption coefficients are computed. Measurements on the power transmission loss in pine wood and firwood horns, and the absorption coefficients of pine and fir are shown.
Absolute Noise Level of Microphones—a Definition Proposed for Discussion
Authors: Baerwald H. G.
Published: September 2005

With high quality microphones, fluctuation noise becomes a problem. Analyzing the situation shows that the usual conception of the term “signal-to-noise-ratio” becomes ambiguous here; thus a gap appears, requiring the introduction of a definition. This should be based on the properties of the ear in order to characterize noise performance as perceived. A corresponding definition aiming at a nonambiguous noise rating is introduced, explained, and applied to different types of microphones—dynamic, condenser, and piezoelectric (sound cell type). Merits and weaknesses of the definition, and of possible modifications, and relations to a recently proposed definition of microphone efficiency are discussed. The new quantity termed “absolute noise level” or “absolute deafness” is defined as the loudness level of the noise heard at the acoustical end of an imaginary transmission system which, connected to the microphone terminals, would faithfully reproduce any sound signal present at the microphone, when the ambient sound fields at both microphone and listener locations are zero. This noise level represents the “hearing loss” introduced by listening via microphone-transmission-system instead of directly. The numerical value of absolute deafness (in Phon) is obtained from the microphone response and resistance curves, by a process based on Fletcher-Munson's work.
Absolute Calibration of Microphones
Authors: Cook Richard K.
Published: September 2005

A tourmaline crystal disk was used both as a microphone (direct piezoelectric effect) and as a source (converse piezoelectric effect). By application of a principle of reciprocity to the acoustic measurements, the piezoelectric modulus of tourmaline under hydrostatic pressure can be obtained. The piezoelectric modulus d33+2d31 so determined is compared with accepted values for the modulus, and was checked by testing-machine measurements of the modulus d33. A condenser microphone was calibrated by means of the tourmaline disk. The same principle of reciprocity was applied to data obtained by using a condenser microphone as both source and microphone, in order to secure an absolute calibration of another condenser microphone. The results of these pressure calibrations are compared with the results of thermophone, pistonphone, electrostatic actuator, and Rayleigh disk measurements on the same microphone.
Sound Measuring Instruments from the User's Viewpoint
Authors: Huber Paul
Published: September 2005

An analysis has been made of the characteristics of sound measuring equipment as used in the field. The limitations and advantages of the various types of microphones, circuits and component parts, are discussed. These limitations are directly involved in the attainable accuracy of measurement, especially when the conditions under which the measurement is made differ from those in which the instrument was calibrated.
Sound Level Meter Performance
Authors: Tweeddale J. E.
Published: September 2005

In the past decade, the increasing use of sound level meters in industry has fully demonstrated their value. However, with this increasing use, there has grown a more critical attitude with respect to their relative performance. A fundamental criticism, particularly in the case of acceptance measurements, has been the frequently observed lack of agreement among sound level meters of different manufacturers though calibrated to the same reference level and fulfilling the performance requirements of the A.S.A. This paper discusses the factors responsible for this condition based on experience in the development of sound level meters and their application to industrial problems. Suggestions are advanced relative to both procedures and standards that should aid materially in clarifying and improving the existing situation.
Shock Waves in Air and Characteristics of Instruments for Their Measurement
Authors: Thompson L.
Published: September 2005

The condensation cycle of a shock wave is not adiabatic. A transient state exists while the head of the wave passes a point in the medium which may not be capable of any analytic statement with respect to pressure and temperature at the wave surface—though density, particle velocity and wave velocity are unique and can be measured. The velocity of flow of the medium may be very great and, in general, finite waves change in form with propagation. Shock waves are pulses of great intensity and extraordinarily short times of condensation, and may have velocities many times the normal velocity of sound. The acoustic power ratings of sources of shock are especially high because of the short times of condensation. The shock pulse and supersonic waves of large amplitude are related somewhat as ultra-short d.c. transients and c.m. wave trains of radiofrequency. An equation has been obtained representing experimental results for times of wave displacement, and velocity, at points in the fields of shock. Functions for compression (Δp/p0) and condensation (s = Δρ/ρ) have been obtained by utilizing the Hugoniot-Rankine “dynamic” equation of condition and the theory of shock as a surface of discontinuity. In the case of plane waves the function for velocity in terms of condensation can thus be compared with the equation by classical treatment. Measurement of Δp and s for shock waves of great intensity is a means for studying the development of the excitational energies in the various molecular degrees of freedom, since the results should identify the effective ratios of the specific heats for intervals down to 10−10 or 10−11 second. Simultaneous measurements of velocity of flow and wave pressure offer a basis for establishing the form of any transient relation which may exist between p and ρ, applicable to these systems. Methods for measurement of Δp (piezoelectric technique) and ρ (filament microphone and optical techniques) are considered. All direct measurements of pressure in shock systems are ballistic in nature, even with piezo gauges. The problem of interpreting measurements of over-all blast effects with slow (mechanical) gauges is therefore essentially of the same type.
Damping of Normal Modes by Small Absorptive Areas: Exact Solution of a Simplified Case
Authors: Pellam J. R. , Bolt R. H.
Published: September 2005

The method previously developed for analyzing the diffraction of plane waves striking an absorbing strip in an infinite plane wall is applied to the corresponding problem in a finite enclosure. A particular case is selected such that component waves, which in the general case are summed to form a resultant wave, may be studied individually as isolated normal modes. A semi-cylindrical room is employed; its concave wall tends to “focus” sound upon the center of the opposite diametric wall on which the absorbing panel is centered. If the wave equation is set up in elliptic cylinder coordinates with the edges of the panel as foci, the boundary conditions are readily satisfied. The theory is strictly applicable to a semi-elliptical room only, but holds well in the semi-cylinder for strip widths up to one-third the diameter. The experimental enclosure has a 35-cm radius, a vertical “height” of 10 cm, and is constructed with highly reflective metal surfaces. The selective damping of various normal modes is investigated for strip widths varying from 5 to 25 cm, and over a wide range of normal acoustic impedances. The results substantiate the prediction that the effective absorption coefficient goes through a maximum when the ratio of strip width to wave-length is, for most modes, of the order of one-half.
Precision Measurement of Acoustic Impedance of Sound Absorbing Materials
Authors: Beranek Leo L.
Published: September 2005

A brief survey is made of the limitations of existing methods for the measurement of the normal acoustic impedance of sound absorbing materials. A more exact method has been developed which utilizes a cylindrical tube terminated at one end by a rigid wall at whose center the sound is admitted and at the other end by a movable piston on which the absorbing sample may be mounted. The acoustic impedance is computed from the widths and positions of pressure-length resonance curves, the pressure being measured at the tube wall at the source end. An exact mathematical analysis is provided which includes correction for the effects of dissipation in the tube and distortion of the sound field in the vicinity of the source. In the frequency range of 100 to 8000 cycles, using two tubes of different diameters, measurements are made to show the various types of impedance curves available for practical applications, and to show variations between different samples of the same material. The effect of sample mounting conditions is discussed in connection with the correlation of small tube and full scale reverberation chamber measurements.
On Non-Uniform Acoustical Boundary Conditions in Rectangular Rooms
Authors: Maa Dah-You
Published: September 2005

An exact solution is obtained for the behavior of sound in a rectangular room whose walls are covered non-uniformly with acoustical materials. The sound field in the chamber is decomposed into normal modes of vibration, each having its normal frequency, decay constant, and pressure configuration affected by the distribution of acoustical materials. The normal modes of vibration are considered to be composed of traveling waves undergoing both specular reflection and diffraction. In most cases the non-uniform boundary may be replaced, through a perturbation method, by an equivalent uniform boundary whose acoustic admittance is simply related to the properties of the actual boundary. Additional effects of diffraction, when not negligible, may be superposed. The diffraction patterns for several simple cases are computed and compared with the predictions of conventional free wave analysis. The present theory is adequate for precalculation of sound decay curves in rectangular rooms for any given arrangement of acoustical materials. It can, in addition, be used for guidance in selecting distributions of material leading to particular results, such as linear decay curves or maximum “apparent absorption coefficient.”
Room Noise at Subscribers' Telephone Locations
Authors: Seacord D. F.
Published: September 2005

This paper presents information in regard to room noise at subscribers' telephone locations based on the results of recent surveys made by Bell System engineers. Data are given which describe the magnitude of room noise, its variation from place to place and from time to time. Information is also presented covering the magnitude and variation of outdoor noise, as measured at the street curb in front of the subscribers' premises, which in many cases is an important contribution to room noise.
Sound Control Apparatus for the Theater
Authors: Burris-Meyer Harold
Published: September 2005

The development of sound control apparatus for use in the legitimate theater involves provision for: (1) Ultimate flexibility of control, i.e., the audience must hear any sound from any apparent source or sources or no source or a moving source, at any apparent distance, at any pitch or intensity, with any quality, and with any predetermined reverberation. (2) Portability and adaptability of equipment. The apparatus must be trouped easily and work well in any theater. (3) Simplicity of installation and operation; similarity to lighting control apparatus. The first experimental apparatus designed to fill all these requisites is now in use. It has been developed through its application of the principles of sound control to legitimate productions and other dramatic enterprises. It involves numerous essential variations from standard PA, Radio, Motion Picture and theatrical practice.
An Improved Apparatus for the Direct Measurement of the Absorption of Sound in Gases
Authors: Leonard Robert W.
Published: September 2005

Apparatus has been designed and built for measurements in absorptive gases such as carbon dioxide and nitrous oxide. Measurements are made of the sound pressure by a microphone as it is moved away from a piston source vibrating in a large baffle. The resulting pressure-distance curves are recorded photographically and yield the absorption coefficient, after a correction, owing to divergence, is made. The source is a magnetostrictive rod driven by an improved Pierce type oscillator. The range of frequencies covered is 22 to 112 kc. Reflections from the walls of the measuring chamber are reduced materially by specially designed glass baffles. A magnetic drive is employed to move the microphone, eliminating packing glands and providing a constant volume chamber for the measurements. A milliampere meter movement is used in the recorder, making possible comparatively rapid recordings. The microphone was built up from a single bender type Rochelle salt bimorph element. The apparatus has been used in measuring the absorption of sound in carbon dioxide and nitrous oxide.
Effects of Water Vapor on Supersonic Dispersion in CO2
Authors: Telfair D.
Published: September 2005

Measurements were made at atmospheric pressure and 28°C with an acoustic interferometer from which velocity and absorption in CO2 were computed. Ten frequencies, covering the range 284 to 1595 kc/sec., were used, and at each frequency the dispersive region was passed through by taking measurements at various H2O concentrations. At each frequency, μ (the absorption per wave-length) rises with increasing humidity to a maximum of about 0.28 and then drops slowly, but at lower frequencies it also passes through a shelf or minor peak before the H2O concentration reaches its maximum value. The drop in velocity accompanying the large absorption peak is about 10 m/sec., and approximately a 1 m/sec. drop occurs near the minor peak. The water vapor concentration, hm, which produces maximum absorption, increases linearly with frequency for both absorption maxima, but at different rates. For the major peak fm = [60+8(104)hm] kc/sec. For the minor peak fm′ = [50+2(104)hm′] kc/sec. Since, on the basis of the relaxation time theory, a shift of the dispersive region to higher frequency means a more rapid transfer of energy to and from the lagging vibrational states, these results may be interpreted as indicating that the average lifetime of a quantum of vibrational energy in CO2 is markedly decreased by the presence of a small percentage of H2O molecules. Some of these facts are now published in another journal but have not been presented before to this society. A new apparatus is now being constructed which will cover a range of (−150) to 450°C and zero to 2 atmospheres pressure.
A Mechanical Playing Device for Brass Wind Instruments
Authors: Martin Daniel W.
Published: September 2005

For an investigation of the steady state performance of a musical instrument, it is desirable to have a mechanical means of playing it. Human variables influencing test conditions are thus reduced, and the tone produced will be reasonably constant in intensity, quality and frequency. Brass instruments require a mechanical lip which, when driven by normal blowing pressures, will vibrate in the conventional frequency range in much the same manner as the musician's lip does. Apparatus has been constructed to try to reproduce the physical playing system fairly closely. A simple means of controlling the tension given to the lips by the surrounding muscle has been tried with some success. A short demonstration is contemplated.
Experiments Connected with End Corrections for Organ Pipes
Authors: Jones Arthur Taber
Published: September 2005

This study was carried out with rectangular wooden pipes in which the fundamental natural frequency was found by resonance. In most cases the pipes employed were not organ pipes. The pipe to be examined was excited by a telephone receiver set into one side of the pipe, and the sound was picked up through a small bore brass listening tube screwed into another side of the pipe and connected by rubber tubing to the observer's ear. The telephone receiver was fed from an audiofrequency oscillator, the frequency of which was varied to obtain the maximum resonant response. The resonant frequency was examined (a) with plates extending at different angles from one edge of one end of the pipe, (b) with one end of the pipe closed except for slits of different sizes, (c) with one end closed except for a slit of chosen width at different distances from an edge of one end, (d) with circular holes of different sizes near the middle of one wall of the pipe. In addition the fundamental natural frequency of each of a number of organ pipes from several organ builders was compared with the frequency obtained by blowing the pipe.
Normal Hearing in the General Population as Measured on Seventeen Western Electric 2A Audiometers
Authors: Beasley Willis C.
Published: September 2005

During a clinical study of deafness and aural disease, which was carried out in 1936 by the U. S. Public Health Service in twelve cities, measurements of auditory acuity were obtained on 4,662 males and females of various ages who stated at the clinical interview that they had never observed any difficulty whatsoever with their hearing in either ear. Seventeen audiometers, eighteen 552W air receivers, and nineteen 700B bone receivers were involved in these tests. Through the cooperation of the Bell Telephone Laboratories calibrations were obtained on the air receivers, so that it is possible to state the performance characteristics of the receivers relative to laboratory standards. Similar, but less satisfactory, calibrations were obtained also for the bone receivers. The output of the audiometers was measured regularly during the course of testing. At each clinic, the same type of sound-insulating booths was used. Comparisons in this report will be given between the physical calibrations of instruments and the nature of the distributions of threshold acuity measurements on persons of the same ages who were measured at the various clinics. Data will be given also showing the influence of general noise level in the booths on the obtained average normal thresholds.
Combination Tones: Their Nature and Origin in the Auditory Mechanism
Authors: Wever E. G. , Bray C. W. , Lawrence M.
Published: September 2005

The electrical responses of the cochlea were used to study combination tones as produced through distortion in the ear. Guinea pigs were stimulated with two pure tones under two conditions, (1) with a loudspeaker when the ear was intact, and (2) with a mechanical vibrator applied to the head of the stapes after removal of all the more peripheral portions of the middle ear. The parts removed included drum, malleus, incus, and tensor tympani muscle, and in some of the experiments the stapedius muscle. Combination tones obtained from normal and operated ears were similar, and it is concluded that the peripheral mechanism of the ear transmits vibrations with high fidelity. The results thus do not support the assumptions made by Helmholtz and others that there is a significant degree of nonlinearity and asymmetry in the movements of the drum and ossicles. Distortion in these structures is not wholly excluded, but it is shown to be unimportant in relation to the distortion that appears beyond the stapes. We consider the most probable source of distortion to be the processes of the inner ear through which mechanical vibrations are transformed into electrical effects.
The Attenuation of Sound in Ducts
Authors: Beranek Leo L.
Published: September 2005

The experimental data on the attenuation of sound in ducts published recently by Hale J. Sabine is re-examined in the light of Morse's theory. It is shown that the data can be more consistently explained by Morse's theory than by the experimental formula given in Sabine's paper. It is seen that the specific normal acoustic impedance is a satisfactory designation for the absorbing properties of the duct lining. Small tube impedance measurements on the lining material are made and the values compared with those computed from Sabine's data.
Propagation of Sound in the Atmosphere—Time Fluctuations in Intensity
Authors: Delsasso L. P. , Knudsen V. O.
Published: September 2005

Measurements conducted on the propagation of sound over a dry lake in the Mojave Desert revealed large fluctuations in intensity, even in calm air and for short distances The fluctuations increased with a rise in frequency. For a point source on the surface of the lake, the intensity of a 250-cycle tone at a distance of 50 m was almost uniform, whereas a 4000-cycle tone showed rapid time variations of intensity—within only a few seconds the intensity varied as much as thirty-six-fold. An exponential sound source (4000 cycles), 75 cm above the lake and directed horizontally, exhibited not only great intensity fluctuations but also surprising changes of wave form. Even during intervals of calm, late at night, when there was no observable wind or temperature gradient (up to a height of 10 m), many of these extreme fluctuations occurred. These irregularities of the propagation of sound will be shown in a series of oscillograms. The irregularities have been simulated, qualitatively, by producing convection currents of the air in a soundproof, constant temperature room.
Method of Measuring Audiofrequencies
Authors: Lewis Don , Griffith Paul E.
Published: September 2005

We have recently developed a method of measuring audiofrequencies which has proved to be quite accurate as well as fairly rapid. The apparatus consists of a vacuum-tube oscillator which controls a multivibrator whose fundamental frequency is 100 c.p.s.; a beat-frequency oscillator equipped with an incremental-frequency control calibrated in cycles per second over a 100-cycle range: a G.R. Type 636-A wave analyzer; and a cathode-ray oscilloscope. The measurement of frequency is made in the following manner: (1) With the incremental-frequency condenser set at zero, the beat-frequency oscillator is adjusted to the unknown frequency through the use of Lissajous figures on the oscilloscope. (2) The wave analyzer is then set to the multiple of 100 cycles which is nearest to the frequency being measured, as shown by the main dial of the oscillator, and adjusted exactly by tuning it to one of the harmonics of the multi-vibrator. (3) The analyzer is then switched to the oscillator and the incremental-frequency dial (which reads from −50 to +50 cycles) is adjusted until a maximum deflection is obtained. (4) The unknown frequency is the frequency at which the analyzer is set modified by the reading on the incremental-frequency dial. The accuracy of the method depends primarily upon the constancy of the multi-vibrator and the accuracy with which it is calibrated. Checks which have been made show that the standard error of measurement is less than 0.25 percent.
On the Problem of Stringing Scales for Very Small Pianoforte
Authors: White William Braid
Published: September 2005

In designing what is called the scale or string plan of a piano, four factors have to be considered: frequency, length, unit weight and tension. All these are interlocked with each other, so any change in one involves corresponding changes in one or all of the others. The element of frequency is of course fixed. In the case of the very small piano, the length, in the low frequency part of the scale at least, is so circumscribed that very little manipulation is possible. Hence any effort to adjust the four elements so as to produce good tonal effects becomes mainly the question of how far unit weight and tension can be readjusted to fit the requirements of the case. The paper presents the author's conclusions on the problem.
An Analysis of Tone Production Factors in Flute Playing
Authors: Small Arnold , Waldrop William
Published: September 2005

This study represents an attempt to (1) analyze those factors in tone production which the flute affords and those which the player controls, and to (2) quantify their effect upon the intensity and pitch of the flute tone. The control of tone production was accomplished by the alternate use of two blowing mechanisms, one constructed in the Iowa Laboratory and the other loaned by Dr. Robert Young of the Conn Ltd. Laboratory. The factors found to effect tone in either or both of the respects studied were distance of lip opening to edge of blow hole, air pressure, size of lip opening, and angle of the air stream with edge of blow hole. Measurements of the effects will be presented. These results indicated that the most basic acoustical factor involved in flute tone production was the velocity of the air stream at the blow hole edge (and also probably the percentage of the stream entering the tube), the above mentioned factors simply being means of varying this velocity. The control of the air particle velocity by these means was therefore studied by the use of a facing or impact tube as employed in the Pitot tube, the results of which will also be reported.
Three Electrical Musical Instruments
Authors: Hammond Laurens
Published: September 2005

Mr. Hammond described the technical features of the Novacord and Solovox and recent developments in the Electrical Organ. The three instruments were played as a trio using music especially arranged for the demonstration.
The System Concept in Electroacoustical Systems
Authors: Knowles Hugh S.
Published: September 2005

The analysis of the behavior of an electro-acoustic device is often much simplified by assuming idealized boundary conditions. The more rigorous consideration of the device as part of a generalized dynamica system however, frequently materially changes the emphasis on various design factors and may even show that the simplifications vitiate the analysis. The special case of an electro-acousto-electrical transducer in which the principle of reciprocity applies and in which the acoustical link is a bounded rectangular continuum is discussed. Extension of the nomenclature and methods of analysis of the passive electrical quadripole to this case is suggested. The simplification in instrumentation which results from the use of the electro-acousto-electrical quadripole is emphasized.
Bone Conduction Threshold Measurements: Effects of Occlusion and Masking Devices
Authors: Watson N. A. , Gales R. S.
Published: September 2005

As shown by many investigators, occlusion of the external ear canals produces a lowering of the bone conduction threshold for normal ears at low frequencies. The present investigation shows that complete occlusion is not necessary to produce some threshold shift but that complete occlusion is necessary to give a maximal shift at a given frequency—also that if occlusion is complete, variation of the mass of the occluding plug does not cause a corresponding variation of the shift, the shift remaining almost constant. Small enclosures, placed over the ears and pressed tightly against them or the adjacent portion of the skull, coupling a small closed volume of air to the air of each external ear canal, also produce a lowering of the bone conduction threshold; but large enclosures, lined with absorptive material, do not. A similar, but smaller, shift arises for an enclosure over one ear. Earphones with rubber ear cushions to prevent air radiation have been used by some investigators to introduce thermal masking noise into one ear by air conduction while testing the acuity of the other ear by bone conduction. Such a masking device produces the type of bone conduction threshold shift. mentioned above if used on a normal ear or on one with small loss, and frequently causes a spurious bone conduction threshold for the ear being tested. A masking device has been developed which will introduce thermal masking noise into one ear by air conduction without causing a bone conduction threshold shift, thereby making possible accurate bone conduction threshold measurements on the opposite ear.
Consideration of Audiometers and Hearing Aids
Authors: Carter Howard A.
Published: September 2005

The Council on Physical Therapy of the American Medical Association, assisted by a group of consultants specializing in physics and in otology, has investigated and reported on audiometers and hearing aids. This paper deals briefly with the requirements for acceptance of audiometers and hearing aids, and with the methods by which the Council accepts the instruments.
Ball Bearing Noise and Vibration
Authors: Buhl W. T.
Published: September 2005

Bearing noise was investigated in the case of a fractional horsepower motor using microphone, analyzer and noise meter. The resulting noise spectrum showed prominent notes which could all be attributed to the motor. The bearing noise was found to be essentially unpitched. A different type of analysis was used which made possible a correlation of the noise with the bearing vibration. The problem revolves itself into unpitched vibration transmitted through the machine and radiated as noise. The characteristics are determined mainly by the machine size and rigidity although the source of ball bearing noise is the vibration of the bearing and can be reduced by making the bearings smoother running or isolating the vibration.
The Analysis of Pulses by Means of the Harmonic Analyzer
Authors: Shankland Robert S.
Published: September 2005

The analyses of a variety of periodic phenomena in acoustics, optics and electricity require the evaluation of the Henrici integral for the values of the coefficients of the Fourier series. An equally important group of non-periodic phenomena, including the transmission of acoustic pulses in air and solid materials, the passage of pulses through amplifiers, etc., requires the evaluation of the Fourier integral to give the complete spectrum of the vibrations making up the pulse. The analyses of such problems are especially important in studies of the starting and stopping of sounds. The solution of these problems can be carried through directly and with ease by drawing the pulse to several scales of abscissae and analyzing the graphs with the Henrici analyzer which is capable of giving thirty components. The detail in the resulting analysis depends simply on the number of curves drawn to different scales which are traced. The method is also of general use in other problems where the Fourier integral appears, including the analysis of radiation pulses and x-ray and electron diffraction patterns.
Experiments in the Analysis of a Mechanically Produced Cornet Tone
Authors: Martin Daniel W.
Published: September 2005

Harmonic analyses of the tone of a cornet played mechanically are made in a location (on a framework extending over the edge of a roof) which is a good approximation to a free field. The nearest surface which can reflect sound from the source to the receiver is 40 feet away. Measurements on the radial dependence of intensity from a small source indicate that reflected sound is negligible. The microphone can be moved continuously in both azimuth and radius in order to study the directional characteristics of the horn. Preliminary measurements show that for high pitches, the total intensity changes very little until the azimuth is nearly 60°, when a sharp decrease occurs. Consideration of the geometry of the bell of the instrument would predict this effect. Furthermore, it is apparent that the spectrum changes appreciably as the angle with the axis of the horn increases, due presumably to the greater directional effect in the higher harmonics. Comparison of the analysis of the mechanically produced tone with the tone produced at the same pitch by a cornetist, shows reasonable agreement.
Corrections on the Measurements of the Velocity of Sound in Gases
Authors: Pielemeier W. H. , Hardy H. C.
Published: September 2005

Many of the discrepancies that have been reported on the velocity of sound in gases are a result of not making the proper corrections to standard conditions. A rigorous derivation of the velocity equation gives V = [RTM(f+gRhCv∞)]12, where R, T and M have the usual meaning and Cv∞ is the specific heat at constant volume and infinite dilution. The quantities f, g and h are dimensionless and differ slightly from unity. They are functions of both temperature and pressure and are a measure of the imperfection of the gas. As an example of the application of the above equation, a discussion of the discrepancies of the measurements in air will be given. A list of the corrections and their magnitudes will show that neglect of many of these has caused considerable error in reported findings. Corrections on the reported values of V0 for air give a weighted mean of 331.47±.06 meters-sec.−1, while a calculation of the value from spectrographic data, the Beattie-Bridgman equation of state, and the latest values of the fundamental constants gives 331.45±.05 meters-sec.−1.
Tracing Distortion in the Reproduction of Constant Amplitude Recordings
Authors: Sepmeyer Ludwig W.
Published: September 2005

An analysis is made of the third harmonic tracing distortion generated in the reproduction of constant amplitude recordings. Curves are shown which give, for an assumed spectral energy distribution of speech and music, the peak distortion for constant amplitude recording and for the newly adopted commercial transcription characteristics. In some cases this distortion may be as high as 5 percent at 3000 cycles and 23 percent at 8000 cycles. Comparison is made with the tracing distortion arising in the reproduction of constant velocity recordings. The problem of reducing the tracing distortion to acceptable limits is considered. Experimental verification of the theoretical predictions and the effect of record unevenness upon the tracing distortion are discussed.
Lip Vibrations in a Cornet Mouthpiece
Authors: Martin Daniel W.
Published: September 2005

High speed photographs of a cornet player's lips were made in rapid succession to obtain a record of the separation of the lips as a function of time, and of the shape and size of the orifice at different fundamental frequencies. Pictures were taken not only along the axis of the mouthpiece, but at right angles to it, to permit observation of the extent of lip vibration along the axis. A mouthpiece is used which is specially constructed with a Lucite rim and window. Lip separations of as much as 2.8 mm at low frequencies and axial amplitudes of 1 mm are observed. The phase at which maximum separation occurs, taking zero phase at the point of zero separation, is seen to shift in going from the lowest to the highest notes played. Fourier analyses of the separation-time curves are made by the Runge method, to obtain amplitude values for the first five harmonic components. The area of the opening is obtained as a function of the lip separation, enabling one to plot area-time curves.
Theatrical Uses of the Remade Voice, Subsonics and Reverberation Control
Authors: Burris-Meyer Harold
Published: September 2005

In testing the remade voice, subsonics and reverberation control in legitimate theatrical production, the following requirements have been established: (1) Only such apparatus and operation techniques have been employed as fit the physical limitations of standard theaters, and which can be easily handled according to standard production practice. (2) Only such phenomena have been tested as have seemed to be demanded by existing scripts of recognized merit. Arrangement of apparatus, operation technique and critical appraisal of results of tests conducted in the Stevens Theater will be discussed.
The System Concept of Electroacoustical Systems
Authors: Knowles Hugh S.
Published: September 2005

The analysis of the behavior of an electroacoustic device is often much simplified by assuming idealized boundary conditions. The more rigorous consideration of the device as part of a generalized dynamical system, however, frequently materially changes the emphasis on various design factors and may even show that the simplifications vitiate the analysis. The special case of an electroacousto-electrical transducer in which the principle of reciprocity applies and in which the acoustical link is a bounded rectangular continuum is discussed. Extension of the nomenclature and methods of analysis of the passive electrical quadrupole to this case is suggested. The simplification in instrumentation which results from the use of the electroacousto-electrical quadripole is emphasized.
Absolute Calibration of Microphones
Authors: Cook Guy S. , Cook Richard K.
Published: September 2005

A moving-coil transducer was used both as a microphone and as a sound source in a free field to obtain an absolute free field calibration of a condenser microphone, by application of the principle of reciprocity. The free field calibration is compared with an absolute pressure calibration of the same condenser microphone, also obtained by means of the principle of reciprocity, and is compared with a free field Rayleigh disk calibration of the condenser microphone. The alternating air flow around a Rayleigh disk placed in a resonant tube was studied by means of illuminated smoke particles in order to explain qualitatively the systematic differences between reciprocity calibrations and Rayleigh disk calibrations of the condenser microphone.
Phase Distortion in Electroacoustical Systems
Authors: Wiener Francis M.
Published: September 2005

In the course of an investigation of phase distortion in electroacoustical systems the phase shift vs. frequency characteristic of a standard miniature condenser transmitter was measured by application of the principle of reciprocity. Measurements were made in a closed cavity and in free space. In analogy to the free field response the phase of the pressure in the undisturbed sound wave was taken as reference. By application of a principle of similitude the phase shift due to diffraction and cavity resonance was determined experimentally and found to be in agreement with theory. The phase characteristics of a number of commercial microphones of representative types were investigated by comparison with the standard condenser microphone. Difficulties were encountered at wave-lengths comparable with the dimensions of the microphones due to the uncertainty of microphone position with respect to the source. Relative response curves were obtained as a supplement and general check on the experimental method. The phase characteristics of a number of electrodynamic loudspeakers are presented.
Production and Control of Acoustic Characteristics
Authors: Wolf S. K.
Published: September 2005

In recent years there has been an increasingly active search for an electroacoustic system for producing and controlling reverberation and associated phenomena. This paper describes an electro-magnetic method of producing and controlling reverberation by the use of a magnetic tape recording system. It consists of magnetically recording a sound pattern on steel tape. The signal is picked up from the tape at frequent split-second intervals and reproduced at any desired level or characteristic. The tape is arranged for driving in an endless helical loop. An obliterating head which continuously obliterates the record is placed just before the first recording head. The phenomena of reverberation and the various methods which have been suggested by others for synthetically controlling reverberation, such as the electro-optical, electro-mechanical, mechanical recording, and the reverberation-chamber methods, are discussed. The paper also outlines other uses for the magnetic tape system in the study of acoustic phenomena both synthetically and analytically.
The Modulation of Sound Decay Curves
Authors: Watson R. B.
Published: September 2005

The irregularity of decay curves has long been noted as a difficulty in the measurement of the reverberation time of a room. It has been suggested that these irregularities may be an important factor in the evaluation of the subjective acoustic qualities of a room. The departures from an exact exponential decay arise from three causes: (1) Beats are produced by interference among the resonance frequencies of the room. (2) Multiple decay rates result from the shape of the room or the non-uniform distribution of absorption on the walls. (3) The direct waves from the source and its first images are important for large or highly absorptive rooms, and at high frequencies. To investigate these phenomena, objective measurements have been made on nine rooms of widely differing sizes and shapes. Apparatus has been designed which compensates for the exponential slope of the decay, leaving only the remaining variations to be recorded by a high speed level recorder. Experimental data are presented for the nine rooms.
Sound Absorption Coefficients and Acoustic Impedance
Authors: Brown Richard L. , Morse Philip M.
Published: September 2005

The relation between normal acoustic impedance and the various types of absorption coefficient has been discussed and specific formulas for the coefficients in terms of acoustic impedance have been published. Contours of constant absorption coefficient plotted as a function of the real and imaginary parts of acoustic impedance plotted on a linear scale are families of circles for both the normal coefficient, the coefficient which in the Sabine reverberation formula usually determines the initial rate of sound decay for rectangular chambers and low frequencies, and for the familiar free-wave absorption coefficient for any single angle of incidence. Similar contours for the Sabine or statistical absorption coefficient, which determines the rate of decay for completely diffuse sound, are shown to be very nearly circular. These plots, together with measured impedance curves or the equivalent electrical circuits of a sound-absorbing structure, are useful in designing materials and mountings for optimum sound-absorbing properties. The curves give theoretical explanation of the empirical value found by Sabine for the average cosine of the angle of incidence for frequencies above 500 c.p.s. Graphs of the absorption coefficients plotted as functions of magnitude of impedance for various values of impedance angle show that for angles greater than 45° the treatment of a complex impedance as a real impedance of the same magnitude introduces an error of greater than −20 percent in computing absorption coefficients.
The Effects of Pressure in the Middle Ear Upon the Electrical Responses of the Cochlea
Authors: Wever Ernest Glen , Bray Charles W. , Lawrence Merle
Published: September 2005

The effects of increased air pressure in the middle ear cavity upon the cochlear responses to sound stimuli have been studied in the cat. The general result of increased pressure is a reduction in the magnitude of the response. The effects are independent of the middle ear muscles since they occur in curarized animals. The amount of reduction in the responses varies with the amount of pressure and the frequency of the stimulus tone. The role of the eardrum and other parts of the ear in the effects will be discussed.
Synchronization in the Higher Auditory Pathways
Authors: Coakley J. D.
Published: September 2005

If frequency of neural discharge aids us in discriminating pitches, it is reasonable to expect this frequency to appear in the subcortical centers. Examination of the potentials in the thalamus of the cat indicates that synchronization is present at this level. It has also been observed in the radiation fibers which pass into the cerebrum. Accordingly it may be concluded that synchronization persists throughout the subcortical part of the acoustic system. No definite upper frequency limit for synchronization has been found. In nearly all preparations it is possible to observe the impulses as far as 2000 to 3000 cycles; in some animals they can be observed as high as 4000 cycles. This is approximately the range in which similar observations have been made in the eighth nerve. Throughout this range the frequency of the nerve impulses is precisely the same as that of the stimulus. Since synchronization is observable throughout the subcortical part of the acoustic system—from cochlea to cerebrum—it seems likely that this is one of the means by which we distinguish one tone from another. Our results indicate, however, that synchronization is not the only possible determinant of pitch, for fiber specificity can also be observed.
General Theory
Authors: Fletcher Harvey
Published: September 2005

The general requirements are discussed for an ideal recording-reproducing system as determined by the characteristics of hearing of a typical group of persons listening in a typical concert hall or theater. Quantitative values are set down as ideal objectives. Although microphones, loudspeakers and amplifiers which had been developed for the stereophonic transmission system were available for meeting these objectives, no recording medium was known which would record the wide dynamic range of intensity levels which the objectives indicated was necessary. However, this wide intensity range objective was met by using a compandor in the electrical system. A general discussion is given of the reasons for choosing the particular compandor used, for using variable area rather than variable density on the recorded film, for using three instead of a greater or lesser number of channels. A general description of the stereophonic sound film system is given, including the enhancement feature. This feature makes it possible to re-record from the original recording, at the same time making any desirable changes in the dynamic range or frequency response in each of the three channels.
Mechanical and Optical Equipment
Authors: Wente E. C. , Biddulph R. , Elmer L. A. , Anderson A. B.
Published: September 2005

The same mechanism is employed for propelling the film in both recording and reproducing. To permit recording of the longer orchestral selections without interruption, the machines are designed to handle film in 2000-foot lengths. Special features of the film-propulsion system for obtaining great uniformity of speed at the translation points are described. The three signal- and one control-channel currents are recorded by means of light valves of identical construction. All four tracks are exposed while the film is passing over a free-running supporting roller, mounted on the same shaft with a new type of internally damped roller. In reproduction, each track is exposed through an objective of high aperture to light from an incandescent source. After passing through the film, the light from each track is carried by a glass rod to a photoelectric cell.