Contains reports on two research projects.National Science FoundationUnited States Air Force, Cambridge Research Center, Air Research and Development Command (Contract AF19(604)-6102)United States Navy, Office of Naval Research (Contract Nonr-1841(42)

Bell, C. G.

Fujisaki, H.

Heinz, J. M.

House, A. S.

Malme, C. I.

Stevens, Kenneth N.

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XIII. SPEECH COMMUNICATIONProf. M. Halle Dr. T. T. Sandel O. FujimuraProf. K. N. Stevens Dr. W. S-Y. Wang H. FujisakiProf. J. B. Dennis Jane B. Arnold M. H. L. HeckerDr. A. S. House C. G. Bell J. M. HeinzDr. G. Rosen P. T. Brady C. I. MalmeA. SPEECH ANALYSIStWork has continued on the "analysis-by-synthesis" programs (1, 2) for the TX-0computer, with an emphasis on gaining greater precision in the matching of vowel andconsonant spectra, greater ease in manipulating test spectra, and a better understandingof various error criteria. Collaterally with the automatic resolution of spectra leadingto the development of speech-recognition systems, attention has been given to the prob-lem of semiautomatic analysis for measurement and description of speech sounds.To facilitate the study of speech spectra and the development of analysis strategies,a general program has been written for direct Flexowriter communication with variouscomputer programs. This study includes various punch-out and read-in programs forstorage or retrieval of blocks of speech data. Other investigations include programsfor calculating simple functions that have either a pair of complex conjugate poles orzeros, or a real-axis pole or zero; for combining these elemental functions to formmore complex spectra; for calculating the outputs of a filter bank of known characteris-tics when it is presented with a specified input spectrum; and several display programsfor viewing speech data and calculated spectra.These new and modified analysis techniques have been used to determine the posi-tions of poles and zeros associated with the spectra of several vowel, fricative, andnasal sounds. Our preliminary results indicate that the method can be used to locatepole and zero positions with a precision that has not been possible in any analysis meth-ods that have been used heretofore.C. G. Bell, H. Fujisaki, J. M. Heinz, A. S. House, K. N. StevensReferences1. C. G. Bell, J. M. Heinz, G. Rosen, and K. N. Stevens, Automatic resolutionof speech spectra into elemental spectra, Quarterly Progress Report No. 54, ResearchLaboratory of Electronics, M.I.T., July 15, 1959, pp. 161-167.2. C. G. Bell, F. Poza, and K. N. Stevens, Automatic resolution of speech spectrainto elemental spectra, Paper A6, Proc. Seminar on Speech Compression and Processing(AFCRC-TR-59-198), December 1959.On Leave from the Research Institute of Communication Science, University ofElectro-Communications, Tokyo, Japan.tThis research was supported in part by the U.S. Air Force (Air ForceCambridge Research Center, Air Research and Development Command) under ContractAF19(604)-6102; and in part by National Science Foundation.121(XIII. SPEECH COMMUNICATION)B. DETECTABILITY OF SMALL IRREGULARITIES IN A HARMONIC LINESPECTRUMIn a previous report (1), the detectability of small changes in the energy content ofa narrow frequency band in a white-noise spectrum was discussed. The filter circuitand psychoacoustic testing methods developed for the noise experiment have been usedto determine the differential energy change at a single harmonic frequency in an acous-tic line spectrum that would produce a detectable change in timbre.The psychoacoustic test stimuli were obtained by passing the signal from a pulsegenerator through a lowpass filter (5-kc cutoff), and then through a filter with a simpleresonance and antiresonance. The effect of the second filter was to increase ordecrease the amplitude of a single harmonic frequency by a controlled amount. Threetypes of harmonic line spectra were used in a series of 5 tests. Spectrum 1 was aflat line spectrum produced by impulses having a repetition rate of 500 pps. This typeof spectrum was used because its harmonic structure could be controlled accurately.Spectrum 2 was a line spectrum with a 500-cps fundamental, but with higher harmonicsattenuated at a rate of 6 db per octave by means of an RC network. This spectrumsimulated the output produced by certain musical instruments. Spectrum 3 was a flatline spectrum produced by impulses at a repetition rate of 150 pps. Three additionaltests were given to measure the differential intensity change for a pure tone at 300,1000, and 3000 cps. The stimulus conditions are summarized in Table XIII-1.Table XIII-1. Summary of tests.Test Number Spectrum Fundamental Harmonic Frequency(cps) (Intensity Varied) (cps)1 1 (flat) 500 2nd 10002 1 (flat) 500 6th 30003 2 (6-db slope) 500 2nd 10004 2 (6-db slope) 500 6th 30005 3 (flat) 150 2nd 3006 pure tone 300 3007 pure tone 1000 10008 pure tone 3000 3000The intensity changes used in all tests were in steps of 0. 5, 1, 2, 4, 6, and 10 db, withThis work was supported in part by the U.S. Navy (Office of Naval Research) underContract Nonr-1841(42). Reproduction in whole or in part is permitted for any purposeof the United States Government.122(XIII. SPEECH COMMUNICATION)the exception of the tests at 300 cps in which steps of 2, 4, 6, 10, 15, and 20 db werenecessary.The test stimuli were presented to the subjects by means of an Ampex tape recorderand Permoflux PDR-8 earphones. Each recorded test contained 100 stimulus groups instandard ABX presentation. A stimulus group consisted of three tone bursts, each of500-msec duration with a sound-time fraction of 1/2. In the ABX test, subjects wereasked whether the last stimulus sounded more like the first or more like the second;they were given 5 seconds in which to record their decision. Six subjects gave a totalof 48 judgments on each intensity change. The reference sound-pressure level at thetest frequency in each case was 70 db (re 0. 0002 4bar) as determined by a coupler cal-ibration of the earphones. These experimental results are shown in Table XIII-2. Theintensity change detectable by an average listener 75 per cent of the time was obtainedby averaging the results from each subject. The subjects' test results were also dividedinto two groups in order to observe the effect of musical training. The data listed underGroup I in Table XIII-2 are the averaged results from three subjects with no previousmusical training. The column under Group 2 contains the averaged results of threesubjects, two of whom are amateur violinists and one is an amateur clarinetist. Datafrom the earlier experiment (1) with the use of white-noise stimuli are included forcomparison.Examination of the results shown in Table XIII-2 indicates that:(a) Without exception, the musically trained subjects were more sensitive to changesin intensity.(b) A detectable change in second-harmonic intensity was smaller with the slopedspectrum than with the flat spectrum.(c) At the sixth-harmonic frequency, the barely discernible intensity change, ordifference limen (DL), was about the same for both the flat and sloped spectra.(d) The average difference limen for intensity was smallest at 1000 cps and largestat 300 cps.(e) The results for the musically trained subjects on tests 1 and 3 (line spectrum)agree closely with those for test 7 (pure tone). The remaining line-spectrum and pure-tone tests do not agree as well for this group, the largest difference showing up at300 cps.(f) In general, the subjects without musical training had a smaller DL for intensityin the pure-tone tests than in the line-spectrum tests, the only exception being intest 7b.(g) The two groups of subjects showed very similar DL's in the tests involving anincrease in pure-tone intensity.(h) The DL values obtained for the musically untrained subjects in the line-spectrum tests are close to those obtained previously with a white-noise spectrum of123Table XIII-2. Summary of experimental results. Data from an earlierexperiment in which stimuli with a white-noise spectrumwere used are shown for comparison.Test Number Test Frequency(cps)Intensity Difference LimenGroup 1 Group 2(db) (db)la (flat spectrum)3a (sloped spectrum)7a (pure tone)noise spectrumlb (flat spectrum)3b (sloped spectrum)7b (pure tone)noise spectrum2a (flat spectrum)4a (sloped spectrum)6a (pure tone)noise spectrum2b (flat spectrum)4b (sloped spectrum)6b (pure tone)noise spectrum5a (flat spectrum)8a (pure tone)noise spectrum5b (flat spectrum)8b (pure tone)noise spectrum1000intensity increase1000intensity decrease3000intensity increase3000intensity decrease300intensity increase300intensity decreaseAverage(db)StandardDeviation(db)±1.6±1.4±0.3±0. 5±2.3±1.4±1.6±0.94.63.02. 74.24.43.04.36.64.54.72.55.25.36.62.98.83.31.92.72.51.93.23.13.42.24.74.51.45. 12.69.82.4±2.±1.±0.±0.+2.±2.±1.±1.8.22.810.55.36. 72. 79.010. 13.814.3±2.0±0.5±1.4±5. 7±1.9±2.8(XIII. SPEECH COMMUNICATION)approximately the same bandwidth. (The values listed are the peak heights or notchdepths of a simple irregularity in a white-noise spectrum that is detectable 75 per centof the time by an average subject (1).)(i) The DL values obtained for the musically trained subjects in the line-spectrumtests are, in general, less than one half as large as those obtained previously with thewhite-noise spectrum.About twenty years ago, Loeb (2) reported the results of an experiment similar tothe one just described. In his experiment, he used apparatus designed by Nahrgang (3),to achieve independent variation of the intensity of each of seven harmonics of funda-mentals ranging from 129 cps to 1740 cps. Differential thresholds for changes in timbrewere determined as a function of changes in the intensity of one or more harmonic fre-quencies. Loeb found that the threshold for timbre change was equal to the differentialthreshold for intensity of the modified harmonics. His result agrees quite well withthe data of Table XIII-2 for the second harmonic at 1000 cps. A possible explanationfor the larger differences at 300 and 3000 cps is that Loeb used well-trained subjectsand a direct-comparison method of testing.From the present data, the average DL for intensity of a pure tone was 2. 6 db foran increase in intensity, and 3. 7 db for a decrease in intensity. These values can becompared with a value of 2. 4 db obtained by Dimmick and Olson (4) by the "same ordifferent method" of psychoacoustic testing.The ratio of the detectable energy change to the total spectrum energy is muchsmaller in the case of a harmonic line spectrum than it is in the case of a pure tone.The fact that the threshold for timbre change in a complex line spectrum is almost equalto the difference limen for intensity of the modified harmonic is remarkable. It indi-cates that, for the type of judgment required here, each partial tone is relatively freefrom the influence of its neighbors - at least, in a harmonic spectrum.It is known that the amplitude of the vibratory response of the basilar membrane toa pure tone has a broad peak (5). The "sharpening" needed to give the ear its selectivecharacteristics is generally assumed to be provided by the central nervous system. Theresults given here indicate that the auditory mechanism can indeed perform as a selec-tive filter, and that its selectivity is improved by proper training.C. I. MalmeReferences1. C. I. Malme, Detectability of small irregularities in a broadband noise spec-trum, Quarterly Progress Report No. 56, Research Laboratory of Electronics, M.I.T.,Jan. 15, 1959, p. 139.2. E. Loeb, Uber die Subjektive Wirkung von Klangfarbenanderungen, Akust. Z. 6,279-294 (1941).(References continued on next page)125(XIII. SPEECH COMMUNICATION)3. S. Nahrgang, Beobachtungen und Messungen an einem neuen Gerat zurKlangsynthese, Akust. Z. 3, 284-301 (1938).4. F. L. Dimmick and R. M. Olson, Intensive difference limen in audition, J.Acoust. Soc. Am. 12, 517-525 (1940).5. G. von Bekesy, On the resonance curve and the decay period at various pointson the cochlear partition, J. Acoust. Soc. Am. 21, 245-254 (1949).126

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