Index of content:
Volume 8, Issue 3, January 1937
8(1937); http://dx.doi.org/10.1121/1.1915889View Description Hide Description
In a previous abstract, a method was described for measuring the sound absorptivity of small specimens. Sound from a loudspeaker travels down a cast‐iron tube 8 inches in diameter, strikes the specimen under test at an angle of 45 degrees and is reflected down a side tube where it is measured. By measuring the sound reflected down the side tube from a thick glass plate, and by measuring the sound in the side tube when an exponential horn is substituted for the specimen, it is possible to arrive at the reflectivity and consequently the absorptivity of the specimen. The earlier apparatus has been redesigned and measurements have been taken on a large number of materials. For the frequency of 500 c.p.s. the method has given absorptivities for commercial materials that are in satisfactory agreement with those determined by reverberation measurements.
8(1937); http://dx.doi.org/10.1121/1.1915890View Description Hide Description
8(1937); http://dx.doi.org/10.1121/1.1915891View Description Hide Description
The relation between tone frequency and position of stimulation on the basilar membrane has been calculated from data on differential pitch sensitivity. The calculations involve assumptions concerning the choice of the upper and lower pitch limits of hearing and the choice of tone levels which should be used in obtaining differential pitch sensitivity data. It is shown that for quite different assumptions the positions of stimulation for tones in the range from 500 to 10,000 cycles are not greatly affected. Outside this range the positions depend on the assumptions. The calculated positions for tones of 1000, 2000 and 4000 cycles fall, respectively, at points on the membrane about and of its length away from the helicotrema. The calculated positions are compared with positions obtained from post‐mortem studies of human cochlea and with positions obtained from electric response measurements on the cochlea of anesthetized guinea pigs. The differences between various methods for the most part are no larger than calculated differences between observers.
8(1937); http://dx.doi.org/10.1121/1.1915893View Description Hide Description
A subjective scale for the measurement of pitch was constructed from determinations of the half‐value of pitches at various frequencies. This scale differs from both the musical scale and the frequency scale, neither of which is subjective. Five observers fractionated tones of 10 different frequencies at a loudness level of 60 db. From these fractionations a numerical scale was constructed which is proportional to the perceived magnitude of subjective pitch. In numbering the scale the 1000‐cycle tone was assigned the pitch of 1000 subjective units (mels). The close agreement of the pitchscale with an integration of the differential thresholds (DL's) shows that, unlike the DL's for loudness, all DL's for pitch are of uniform subjective magnitude. The agreement further implies that pitch and differential sensitivity to pitch are both rectilinear functions of extent on the basilar membrane. The correspondence of the pitchscale and the experimentally determined location of the resonant areas of the basilar membrane suggests that, in cutting a pitch in half, the observer adjusts the tone until it stimulates a position half‐way from the original locus to the apical end of the membrane. Measurement of the subjective size of musical intervals (such as octaves) in terms of the pitchscale shows that the intervals become larger as the frequency of the mid‐point of the interval increases (except in the two highest audible octaves). This result confirms earlier judgments as to the relative size of octaves in different parts of the frequency range.
8(1937); http://dx.doi.org/10.1121/1.1915895View Description Hide Description
A quarter of a century ago William Haskell developed an organ pipe which is like an ordinary open pipe with a smaller tube, closed at the top, inserted in it. This pipe gives the full series of harmonic partials usually obtained from an open pipe of length equal to the sum of the lengths of the outer tube and the inner one. The theory of the pipe is here developed and checked experimentally.
8(1937); http://dx.doi.org/10.1121/1.1915896View Description Hide Description
In view of a paper by Meyer and Klaes the author examines again certain arguments against the view that the strike note of a bell is a difference tone. The work of Meyer and Klaes is summarized, and four new tests are reported. The results of all four tests are opposed to the hypothesis that the strike note is a difference tone. In a very tentative way the possibility is suggested that in some bells the strike note may be the second partial tone of the bell. This is not true for the bells which the author has examined most carefully. For these bells he still believes that the strike note, aside from the octave in which it lies, is determined by the fifth partial tone of the bell. As to the octave, the work of Meyer and Klaes suggests that this may be determined by the difference tone from the fifth and seventh partials, this difference tone being produced by the nonlinear response of the auditor's ear. An appendix gives some information regarding harmonic overtones from tuning forks.
- PROGRAM OF THE SIXTEENTH MEETING OF THE ACOUSTICAL SOCIETY OF AMERICA
8(1937); http://dx.doi.org/10.1121/1.1901990View Description Hide Description
Three problems are discussed: The insulation from air‐borne sound by multiple walls. Structure‐borne sound transmission.Measurements of vibrations by electrical apparatus.
It is known that a single wall vibrates almost as a mass, provided its lowest natural period is much lower than the sound frequencies. Therefore a series of single walls with air‐spaces between them act as a mechanical low pass filter, with a cut‐off frequency and sound dispersion. But these properties are only present, if the component vibrations in the air‐space, parallel to the wall are damped. A multiple wall of this kind with a low cut‐off frequency has a very large transmission loss.
Building materials propagate sound very well. Their small transmission losses (hysteresis) are investigated by a resonance method. To obtain a loss of 1 db material lengths from 10–100 m or more are required. Materials such as rubber or cork are used to avoid the sound propagation in buildings. Their dynamical properties(elasticitymodulus and loss factor) are tested by a special electrodynamical vibrometer.
With a mechanical apparatus the displacement or the acceleration of the ground is determined. The method, which measures the velocity amplitude (obtained from a low resonance‐frequency electrodynamical apparatus) is to be preferred. An easily portable apparatus of this kind has been constructed. The average value of the vibrations of buildings due to traffic is 3.10−3 cm/sec. The average vibration frequencies can also be determined. In the horizontal direction buildings oscillate mainly in two ways, as a whole or with one node.
8(1937); http://dx.doi.org/10.1121/1.1901991View Description Hide Description
Instruments for sound analysis may be grouped into five classes—graphic, resonance, heterodyne, stroboscopic, and diffraction analyzers. The operation of each type of instrument is briefly described and examples of analyses performed by the various methods are presented.
From the point of view of analysis sounds may be classified roughly into four groups:
(1) Steady state sounds or sounds which may be maintained at constant fundamental frequency, constant intensity and unvarying quality for long enough to carry out the analysis.
(2) Sounds which are essentially transient in nature.
(3) Sounds which may be maintained constant on the average but whose frequency, intensity and wave form are modulated at a constant frequency.
(4) Noise, or sounds which are entirely random in form but which are continuously maintained.
The sound spectra corresponding to each group are described and the application of the various types of instruments to the analysis of these spectra is discussed.
8(1937); http://dx.doi.org/10.1121/1.1901992View Description Hide Description
Violintones covering the greater part of the playing range of the instrument have been subjected to harmonic analysis. The analyses included normal tones and “harmonics,” which were studied with a variety of technical playing conditions obtaining, for example, variable point of contact between bow, and string, sympathetic vibration of unstopped strings, constant pitch but with change of string, variable pitch on each string, and recording in a music studio and a “dead” room. The results show (1) that change in point of contact between bow and string effects a characteristic variation in harmonic structure; (2) that harmonic constitution varies with the introduction of sympathetic vibration of open strings; (3) that not only do tones of the same pitch played on different strings vary in harmonic structure but also different pitches played on the same string; (4) that the lower natural harmonics are not essentially different in structure from fully‐stopped tones of the same pitch and same string; (5) that the higher natural and artificial harmonics are not pure tones but approach a much greater degree of purity than do most normal tones.
8(1937); http://dx.doi.org/10.1121/1.1901993View Description Hide Description
Recently materials for direct recording and reproducing work have been improved so that they are now suitable for many uses. These materials, as they are available on the market, are classified chemically into five groups and measurements are given of frequency characteristic, surface noise, life, distortion, etc. These data have been taken with both lateral and vertical recording. A short aural demonstration will be given of typical recording of materials of each group.
8(1937); http://dx.doi.org/10.1121/1.1901994View Description Hide Description
The purpose of the study was to determine whether violinists, in unaccompanied performance, typically play in either the natural or the equally tempered musical scale, and if not, whether they tend systematically to expand or contract musical intervals as compared with their theoretical magnitudes. Six professional violinists participated in the investigation. Unaccompanied performances of three standard violin selections were recorded by an oscillographic technique. The average fundamental frequency of the main body of each of the tones was measured, and interval extents then were computed. The major part of the study was limited to an analysis of five intervals—major and minor seconds, major and minor thirds and perfect fourths. Tests of reliability of measurement showed that the largest expected error of measurement (±3 SDs) for any given frequency was approximately 0.03 tone, but in at least 78 percent of the cases, measurements were statistically significant to 0.01 tone.
The major findings were: (1) The six violinists typically performed in neither the natural nor the equally tempered scale. (2) As compared with both natural and equally tempered intonation, major seconds and major thirds tended to be expanded, minor seconds and minor thirds on the average were contracted, and perfect fourths tended to approximate the theoretical scale values for that interval. (3) The average extent of each of the five intervals approximated its theoretical magnitude in Pythagorean intonation.
8(1937); http://dx.doi.org/10.1121/1.1901995View Description Hide Description
Research designed to measuretimbre as a function of harmonic structure, frequency level, and intensity level is in progress. This paper, the first of a series of projected reports, includes a description of apparatus, a brief discussion of problems of measurement, and a summary of some of the data already secured.
The apparatus consists mainly of an electrostatic generator, described in part elsewhere. This instrument produces complex tones constituted of desired combinations of any or all of sixteen consecutive harmonics, the frequencies, intensities, and phase relationships of which are subject to control. Each harmonic is reasonably free from stray components. A calibrated Western Electric 555 receiver is used with the generator. The current flowing through the receiver (current being proportional to the r.m.s. value of pressure developed in the ear canal) is measured with a vacuum tube voltmeter. As many as five different test tones, each to be paired a required number of times with a suitable standard tone, may be set up in advance for random presentation to a listener. When adjustment is complete, a test series may be run more or less automatically. To avoid undesirable transients which arise when tones are interrupted suddenly, the circuit is arranged to start and stop the tones gradually. Considered as a whole, the apparatus meets experimental requirements fairly adequately.
Several problems arise in connection with attempts to measuretimbre. One of the most perplexing of these is the choice of meaningful criteria (preferably a single criterion) in terms of which two timbres can be differentiated. Other important problems have to do with choosing suitable standard tones, with finding methods of obviating possible changes in pitch and loudness when harmonic structure is altered, and with deciding whether individual harmonics should be adjusted in terms of intensity level, sensation level, or loudness level. It is admitted that rather arbitrary decisions were sometimes made.
Most of the investigations to date have dealt with the measurement of what are called masked absolute thresholds and masked differential thresholds. When a single harmonic is raised in intensity level until it is barely perceptible in the presence of other harmonics, its intensity level at that point is called its masked absolute threshold. The amount that a single harmonic, already at or above its masked absolute threshold, must be raised in intensity level in the presence of other harmonics to give rise to a just perceptible change is called its masked differential threshold. Masked absolute and masked differential threshold values are held to be measures of the sensitivity of the ear to timbre differences. Data at hand show that discrimination varies with both frequency level and intensity level, in addition to its variation with harmonic structure. Particularly close is the relationship between masked differential thresholds and harmonic structure. It is likely that masked absolute thresholds vary with phase, but evidence on this point is too fragmentary to warrant a definite conclusion.
8(1937); http://dx.doi.org/10.1121/1.1901996View Description Hide Description
Although hearing impairment is ordinarily measured for tones of threshold intensity, attention has recently been turned to the measurement of impairment for tone levels above threshold. It has been found that some people may have an appreciable hearing impairment for threshold sounds yet hear normally for loud tones. Others show the same hearing impairment at all levels of sound intensity.
Loudness studies indicate that the variable or recovery type impairment should occur if the impairment is due to an atrophy of nerve fibers, and hearing losses calculated on this assumption are quite similar to observed losses. From this viewpoint the hearing loss caused by the presence of a masking noise should be of the variable type which was found to be the case.
Thus the often made statement that deafened people hear better in noisy places turns out to be true when the deafness is of the variable or recovery type. The implication of the statement that the noise as such improves the hearing however, is wrong.
8(1937); http://dx.doi.org/10.1121/1.1901997View Description Hide Description
An observer listens with one ear to a combination of two pure tones. One is a loud 100‐cycle tone, the other is a weaker 200‐cycle tone. A phase relation between the two tones can be found such that the loudness of the two tones together is about one decibel less than the loudness of the fundamental alone. This is an example of the steady tone phase effect, in which a fundamental and one of its harmonics are heard together, the phase relation between the tones, as well as the pressure level of the harmonic, being subject to arbitrary control by the observer.
This effect was studied with a fundamental of 100 cycles kept at a sound pressure level of 104 db above 0.0002 dyne/cm2. Harmonics up to the fifth were used, with particular emphasis on the second and third. Measurements of the phase of the harmonic sound pressure giving minimum loudness averaged approximately 180° for the second harmonic and 0° for the third (epoch angle of cosine functions relative to fundamental). Loudnessmeasurements at these phase settings showed the loudness of the combined fundamental and harmonic to be about 1 db less than that of the fundamental alone. 180° from the phase of minimum loudness the loudness of the combined tones was about 1 db greater than that of the fundamental alone. The variation of quality with phase relation was also studied.
Interesting variations between observers were investigated and attributed partly to psychological and physiological differences in individuals. But consideration of all the data obtained points to the conclusion, supported by theory, that the phase effect is a more complicated phenomenon than has commonly been assumed. Since this effect has been the basis for many studies of subjective tones, the concept of subjective tones is analyzed and interpreted in a new light.
8(1937); http://dx.doi.org/10.1121/1.1901998View Description Hide Description
Problems as to the nature and origin of the aural (subjective) harmonics,tones which are heard, but which are not present in the sound wave outside of the ear, have concerned musicians and scientists for many years. It is customary to assume that these harmonics arise because of non‐linearity in the functioning of the middle ear. Their magnitudes have been measured by indirect means, such as the method of best beats, but there has been no adequate direct study.
We have used several means to attack this problem. (1) The minimum amount of distortion (2nd harmonic) which the ear can detect in a tone was studied by determining the thresholds for the second harmonic when it is added, outside the ear, to a pure fundamental in various phase relations. (2) Since it is possible to pick up from the cochlea an electrical potential which gives a good indication of the functioning of the auditory mechanism, we made an analysis and measurement of the harmonic components present in the cochlear response of animals under normal and certain abnormal conditions. (3) The phase relations of the harmonics in the cochlear response were determined with the aid of a cathode‐ray oscillograph. (4) Externally generatedharmonics were added to the fundamental in various phase relations both for human and animal subjects.
The results show that, for purposes of analysis, we can distinguish two sources of aural harmonics. The odd harmonics arise, from non‐linearity, when the amplitude of vibration approaches the elastic limit of the ear. The even harmonics are due to an asymmetry produced by tension of the muscles of the middle ear, and consequently, their magnitudes can be experimentally controlled.
Professor Hallowell Davis aided in the work on animals.
8(1937); http://dx.doi.org/10.1121/1.1901999View Description Hide Description
A subjective scale for the measurement of pitch was constructed from determinations of the half‐value of pitches at various frequencies. This scale differs from both the musical scale and the frequency scale, neither of which is subjective. Five observers fractionated tones of 10 different frequencies at a loudness level of 60 db. From these fractionations a numerical scale was constructed which is proportional to the perceived magnitude of subjective pitch. In numbering the scale the 1000‐cycle tone was assigned the pitch of 1000 subjective units (mels).
The close agreement of the pitchscale with an integration of the differential thresholds (DL's) shows that, unlike the DL's for loudness, all DL's for pitch are of uniform subjective magnitude. The agreement further implies that pitch and differential sensitivity to pitch are both rectilinear functions of extent on the basilar membrane.
The correspondence of the pitchscale and the experimentally determined location of the resonant areas of the basilar membrane suggests that, in cutting a pitch in half, the observer adjusts the tone until it stimulates a position halfway from the original locus to the apical end of the membrane.
Measurement of the subjective size of musical intervals (such as octaves) in terms of the pitchscale shows that the intervals become larger as the frequency of the mid‐point of the interval increases (except in the two highest audible octaves). This result confirms earlier judgments as to the relative size of octaves in different parts of the frequency range.
8(1937); http://dx.doi.org/10.1121/1.1902000View Description Hide Description
The well‐known fact that the ear generates an electric potential in response to stimulation by a sound wave has its counterpart in the fact that, when an alternating current is passed through the head, an auditory sensation results. The observer hears a tone whose pitch is determined by the frequency of the alternating current.
Measurements were made of the power needed to elicit a sensation using currents of various frequencies. The power, rather than the simple voltage or current was measured, because the body presents a complex impedance to the current. The total impedance, when the electrodes are applied in a standard manner, decreases with increasing frequency. The power factor varies slightly with frequency. When the power is increased about 20 db above the threshold value, the threshold of electric shock—a stinging sensation—is reached. This effect severely limits the size of the auditory area under electrical stimulation.
The amount of distortion present makes it difficult to understand speech when the observer is connected directly to the output circuit of a radio set, but music can be readily identified. The rectification producing this distortion appears to be largely electrical.
It is probable that the alternating current sets up vibrations in the inner ear due, perhaps, to a residual charge on the basilar membrane which makes it behave as a condenser microphone.