Учебники / Auditory Perception - An Analysis and Synthesis Warren 2008

Figure 7.1 Anatomical structures involved in speech production. The subglottal system delivers air under pressure to the larynx. The vocal folds in the larynx are capable of generating broadband sounds. The supraglottal system can spectrally shape the sound produced in the larynx by altering dimensions along the vocal tract, and by opening or closing access to the nasal passages. The supraglottal system is also capable of adding some hisses and pops of its own.

The subglottal system

The glottis remains open during normal breathing without phonation. Inhalation is accomplished through expansion of the chest cavity, largely through contraction of the diaphragm and the external intercostal muscles. Exhalation is accomplished mainly through elastic recoil when the contraction of the muscles used for inhalation ceases. However, during more active breathing, including that accompanying speaking, the passive compression of air in the lungs following active inhalation is supplemented by contraction of the internal intercostal and abdominal muscles. The air passes from the lungs through the windpipe or trachea up to the larynx. During speaking, the subglottal pressure is traditionally described as equivalent to about 5 to 10 cm of water (that is, equal to the force per unit area exerted by a water column of this

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height), although during very loud shouting it can rise to the equivalent of

60 cm of water. The average flow rates of air while speaking are roughly 150 to 200 cm3/s, and since the average inhalation during speech is about 500 to 800 cm3, about 3 to 5 s of speech or about 10 to 15 words are produced by a single breath in normal speech. Delicate muscular control of the subglottal pressure and flow rate appropriate to particular articulatory gestures is essential for generating the normal amplitude patterns of speech.

Production of all English phonemes is powered by subglottal compression of air. However, there are languages in which some of the phonemes are produced by different means. A rapid downward movement of the closed glottis is used to create implosive phonemes in some American Indian and African languages, and an upward movement of the closed glottis can be used to produce ‘‘ejective’’ pops and hisses with phonetic significance. Movements of the tongue to suck air into the mouth are used to produce clicks in several languages of southern Africa.

The larynx

The larynx is homologous with a valve that protects the lungs of the lungfish from flooding, and is used generally for phonation or sound production by terrestrial vertebrates (Negus, 1949). The human larynx is at the top of the trachea and houses vocal folds consisting of two strips of ligaments and muscles attached to a firm cartilagenous support. During swallowing, the epiglottis moves down to seal off the larynx as solid and liquid material is moved into the esophagus. During normal breathing without phonation, the glottal opening between the vocal folds remains in a half-open position, although it can be opened more fully during rapid intake of breath. During voicing or phonation, the vocal folds open and close rhythmically to produce puffs of air. These puffs correspond to a buzz-like periodic sound which furnishes the raw acoustic substrate from which vowels and voiced consonants are shaped. The harmonic components of the buzz decrease in amplitude with increasing frequency, so that there is roughly equal power per octave. During whispering, the vocal folds are kept in a nearly closed position, creating turbulence that produces a hiss-like sound, which is shaped by the supraglottal region in the same manner as the glottal buzz of voiced sounds. Whispered speech also can be produced by other constrictions leading to hisses at positions near the laryngeal end of the vocal tract.

The opening and closing of the vocal folds are not synchronous with muscle twitches, as once thought by some investigators but, rather, follow from the effects of rapid air flow through the glottis. During normal phonation, prevoicing changes in muscle tension cause the vocal folds to move close together.

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The increase in the velocity of air moving through this constricted passage produces a drop in air pressure, and Bernoulli’s principle (that is, an increase in a fluid’s velocity causes a decrease in its pressure) causes the vocal folds to be drawn together, and to close. The closure results in a build-up of subglottal pressure, forcing the vocal folds open, releasing a puff of air which causes the cycle to repeat for as long as phonation is maintained.

Much information about the operation of the vocal folds has been obtained by direct observation. By placing a mirror at the appropriate angle at the back of the throat, it is possible both to illuminate and to view glottal configurations. Since movements are rapid during phonation, a succession of pictures taken at high speed has been useful in studying the cycle of opening and closing during voicing.

The rate at which the glottis opens and closes is the voice’s fundamental frequency, and determines the pitch. The rate of glottal vibration is under the control of several sets of muscles which can raise the pitch by stretching and hence increasing the tension of the vocal folds. The intensity of the voice can be controlled by changing both the subglottal pressure and the portion of the glottal period during which the vocal cords are open. Decreasing the portion of the cycle during which the vocal folds are open results in a louder sound as well as a change in quality associated with a greater vocal effort.

The fundamental frequency of the voice is about 120 Hz for an adult male and about 250 Hz for an adult female. Children may have fundamental frequencies as high as 400 Hz. The low frequency in adult males is associated with thickening and lengthening of the vocal folds resulting from hormonal changes occurring at puberty. The range of frequencies employed in speech by male and female adults is usually a little more than one octave, with a modal frequency about 1/3-octave above the lower limit of this range.

The trained voice of a singer exhibits exceptionally fine control of pitch, loudness, and quality of phonation. There are three sets of intrinsic laryngeal muscles, which are ‘‘synergistic’’ in terms of fundamental frequency and intensity (Hirano, Ohala, and Vennard, 1969); while untrained people often change intensity when they change their fundamental frequency, trained singers can alter intensity and frequency independently. Singers deliberately introduce an instability or periodic fluctuation in pitch called the vibrato, which is centered upon the frequency corresponding to the sung note. (The usual rate of this fluctuation is about six or seven per second, and the frequency excursion as large as one semitone.) Interestingly, the pitch extracted by a listener from this rapid succession of glissandi involves not a moment-by-moment evaluation but, rather, an averaging over time (in keeping with the obligatory perceptual integration of events during this time period, as described in Chapter 3). The almost universal use of the vibrato in singing is a fairly recent development in

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our culture: as late as the nineteenth century, the ability to sing notes with a fixed pitch was an admired ability of trained singers. Vibrato is not the only vocal ornamentation used by operatic singers: they occasionally use tremolo which consists of an irregular amplitude modulation superimposed upon the frequency modulation of the vibrato to convey particular emotional moods.

Trained singers usually are capable of producing a range of pitches well beyond that used in speech. In order to cover this range, they employ a number of different production modes, each corresponding to a different ‘‘register.’’ Thus, the falsetto register (used by male singers in reaching their highest notes) involves a tensing of the vocal folds so that only a portion of the edges vibrate. The ‘‘laryngeal whistle’’ register (used by female singers in reaching their highest notes) does not involve vibration of the vocal folds, but instead, a whistle is produced by air passing through a small opening in the otherwise closed vocal folds. One of the goals of voice training is to permit transitions between different registers to be made smoothly and, ideally, without the listener being able to detect a discontinuity in the passage from one produc-

tion mode to another. A trained soprano may be able to cover the range of fundamental frequencies from 130 through 2,000 Hz by employing the socalled ‘‘chest register’’ below 300 Hz, the ‘‘head voice’’ from about 300 to 1,000 Hz, the ‘‘little register’’ from about 1,000 to 1,500 Hz, and the ‘‘whistle register’’ above 1,500 Hz. (For a discussion of the various registers and theories concerning their production see Deinse, 1981.)

Although people are quite unaware of the mechanisms employed for changing pitch in speaking, it might be thought that singers would have insight into their methods for pitch change, since so much time and effort is devoted to studying and practicing pitch control. Yet, only a rather vague and ambiguous vocabulary is available for describing the registers and qualities of the singing voice. The location of the muscles employed and their exquisitely delicate control through acoustic, tactile, and proprioceptive (muscular) feedback is accomplished with little or no direct knowledge on the part of the performer, providing an example of what Polanyi (1958, 1968) has called ‘‘tacit knowledge.’’ However, the ability to describe and to respond to instructions concerning the positions and movements of the articulatory organs increases as we move upward along the airstream from the larynx to the tongue tip and lips. The basis for this increasing awareness will be discussed later when we deal with the perceptual status of the phoneme.

The vocal tract and articulation of speech sounds

The buzz produced by the larynx becomes speech only after being modified by its passage through the vocal tract. During this trip, phonemes are

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formed by the spectral shaping of the laryngeal buzz, and by obstructing the airflow. A highly simplified model of the vocal tract considers it to be a tube, like an organ pipe, which is open at one end. In a male, the length of this tube is about 17.5 cm. As an initial simplifying approximation, this tube can be considered to have a uniform cross-section. The lowest resonant frequency of such a tube has a wavelength four times the tube length, corresponding to about 500 Hz for a male. Those spectral components of the broadband laryngeal source having frequencies of about 500 Hz are increased in intensity by this resonance to produce the first formant band. Additional resonances occur in the tube at odd-numbered integral multiples of the first formant, so that the second formant is at 1,500 Hz, and the third formant at 2,500 Hz. The vocal tracts of women are somewhat shorter, and the wavelengths associated with their corresponding formants are on the average about 80 to 85 percent of the male’s. Formants are associated with standing waves (which correspond to fixed regions of high and low pressure), with all formants having one pressure maximum at the glottis and one pressure minimum at the lips. The first formant has only this one maximum and minimum, the second formant one additional maximum and minimum, and the third formant two additional maxima and minima, each spaced regularly along the uniform tube in this simplified model of the vocal tract.

Of course, the vocal tract is not a uniform tube. The length of the tube can be changed by movement of portions of the tract and, in addition, cavities and constrictions can be created at the back of the throat and within the mouth. A decrease in cross-sectional area at a region of a formant’s pressure-amplitude minimum lowers the formant’s frequency, whereas an increase in area at this region raises its frequency. Corresponding cross-sectional changes at places of pressure maxima have the opposite effect in changing the formant’s frequency.

Vowels differ in the frequencies and relative intensities of their formants. Figure 7.2 shows the complex positional adjustments of the highly flexible tongue that must be learned in order to produce the vowels that are shown. The procedure of preceding the vowel by /h/ and following it by /d/ minimizes the influence of adjacent phonemes by coarticulation, and produces vowels much like those produced in isolation. Table 7.1 gives the formant frequencies for the vowels shown in Figure 7.2 as measured by Peterson and Barney (1952) for men, women, and children. The words used in speaking the vowels are given above their phonetic symbols. The single vowels shown are sometimes called monophthongs to distinguish them from diphthongs consisting of a pair such as /iu/ occurring in the word ‘‘few.’’

Several cycles of a periodic sound such as a vowel are necessary to establish the fundamental and the line spectrum (see Figure 1.3.1), and so it might be

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Table 7.1 Averages of fundamental and formant frequencies and formant amplitudes of vowels by 33 men, 28 women, and 15 children

From Peterson and Barney, 1952.

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Figure 7.2 Tongue positions for vowels in the words: (1) heed, (2) hid, (3) head, (4)

had, (5) hod, (6) hawed, (7) hood, (8) who’d. (From MacNeilage and Ladefoged, 1976.)

thought that a vowel could not be identified from a single glottal pulse. However, there have been reports that single glottal pulses, and even segments excised from single pulses, can be used to identify vowels with accuracy not much below that found for extended statements (Gray, 1942; Moore and Mundie, 1971). Vowel recognition from a single pulse (or portions of that pulse) probably is based upon the distribution of spectral power in bands corresponding to the formants of extended vowel statements. Observations in our laboratory have indicated that conditions resembling those producing temporal induction (see Chapter 6) can enhance the vowel-like characteristics of a single glottal pulse and facilitate its recognition. The procedure works well when the glottal pulse is preceded and followed by 1 second bursts of pink noise (that is, noise with equal power per octave) separated from the glottal pulse by no more than 5 ms of silence, and presented at a level about 10 dB above that corresponding to the extended statement of the vowel from which the single pulse was excised.

Table 7.2 lists the English consonants by place and manner of articulation. All consonants involve some type of obstruction of the vocal tract. When accompanied by glottal vibration, they are voiced; otherwise they are

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Table 7.2 Classification of English consonants by place and manner of articulation

Based on Zemlin, 1998.

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P

1.

2.

R

3.

TIME (ms)

Figure 7.3 Waveform of the word ‘‘poor.’’ Three successive 85 ms segments are

shown. (From Fletcher, 1953.)

unvoiced. Places of articulation are the lips (labial), teeth (dental), gums (alveolar), hard palate (palatal), soft palate or velum (velar), and vocal folds or glottis (glottal). Stop consonants involve complete closure of the vocal tract. When the closure is followed by an audible release of air it is sometimes called a stop plosive or, simply, plosive. The length of time between the hiss-like release of air and the onset of voicing (the ‘‘voice-onset time’’) can determine whether a stop consonant is considered voiceless or voiced. Incomplete closure results in a fricative. When a stop is followed by a fricative, an affricative is produced (such as /tS/ in ‘‘choice’’ and /dZ/ in ‘‘judge’’). The glides /r/, /w/, /j/, and /l/ are produced by rapid articulatory movements and have characteristics in some ways intermediate between those of consonants and vowels. The nasal consonants are generated by blocking the passage of air through the mouth at the places characteristic of the consonant (see Table 7.2), while lowering the velum to permit release of air through the nasal passages.

Visual representation of speech sounds

The most direct way of representing the acoustic nature of speech is through its waveform. The waveform describes pressure changes over time. By construction of devices that record these changes in some mechanical, magnetic, or digital fashion, we can use this record to recreate a more or less perfect copy of the original sound. Figure 7.3 shows the waveform corresponding to the word ‘‘poor.’’

Although visual representations of waveforms can accurately depict the acoustic signal, they are of little use in identifying speech sounds. One reason

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is that the waveforms do not reveal the component harmonics or formants in any recognizable fashion. Waveforms of complex sounds do not correspond to the pattern of stimulation at any receptor site, since stimulation takes place only after segregation of spectral components. It is possible to give an approximate description of the pattern of acoustic stimulation at individual

receptor sites along the organ of Corti using multiple tracings corresponding to the output of a bank of bandpass filters representing successive critical bands. Such tracings have some use in studying the perception of periodic

and would represent only a rough approximation of local stimulus patterns. A more convenient and conventional depiction (but one that does not take into

account critical bandwidths), is produced by a sound spectrograph.

The sound spectrograph was developed at Bell Telephone Laboratories during the early 1940s. The device consists of a series of narrow-band filters that are used successively to scan repetitions of a recorded speech sample. A sound spectrogram shows three variables: time is shown along the horizontal axis, frequency along the vertical axis, and intensity by the darkness of the tracing. Figure 7.4(a) shows a spectrogram of the phrase ‘‘to catch pink salmon.’’ The formants of vowels are seen as dark horizontal bars. The silences of the stop consonants are shown as the vertical blank areas, and the fricative consonants correspond to the dark regions lacking discrete bars.

A group of workers at Haskins Laboratory developed a ‘‘pattern playback’’ which can optically scan a sound spectrogram and recreate the original sounds. Using this device, one can hand paint patterns on a transparent base and play back the optical pattern to produce intelligible speech. Hand-painted versions shown in Figure 7.4(b) were produced by first drawing the most obvious features of the original and then modifying the patterns while listening to the results until, usually by trial and error, the simplified drawn spectrograms were fairly intelligible. In the example shown in Figure 7.4, both the original and the handpainted versions could be used with the pattern playback to produce an understandable acoustic rendition of ‘‘to catch pink salmon.’’ Pattern playback not only permits editing of sound spectrograms to determine which components are necessary for intelligibility, but also permits synthesis of acoustic patterns with desired characteristics. Although this device has provided useful information and is of value in illustrating the acoustic features of importance to speech perception, today computer software allows us to synthesize and analyze speech with considerably greater ease and flexibility.

In the early years of the sound spectrogram’s use, it was hoped that it would provide a ‘‘visual speech’’ permitting deaf individuals to see and comprehend