Учебники / Auditory Perception - An Analysis and Synthesis Warren 2008

36Spatial localization and binaural hearing

externalized sound images. It is conventional to use the term ‘‘lateralization’’ to describe intracranial sidedness of sound images produced through headphones, and to use the term ‘‘localization’’ to describe positioning of externalized images. The localization of a source in space involves both dichotic cues based upon differences in the input to each ear, and monoaural cues for which the stimulus to one ear alone can provide information concerning location of the source.

Binaural perception of azimuth

In 1876 and 1877, Lord Rayleigh demonstrated that the azimuth of the source of complex sounds such as voices could be identified accurately, as could high frequency sinusoidal tones. He attributed this localization ability to interaural intensity differences caused by the head’s sound shadow. It was not until some 30 years later that Rayleigh (1907) appreciated that interaural phase differences could also be used for determining the azimuth of low frequency pure tones. He proposed at that time his influential ‘‘duplex’’ theory which considered that high frequencies are located laterally by interaural intensity differences while low frequencies are located laterally by interaural phase (or time) differences.

Figure 2.1 shows the analysis of Woodworth (1938) of how interaural pathlength differences (leading to interaural time and phase differences) arise when a source is to the right or left of the medial plane. This figure shows the additional path length as due to two segments: a straight line portion, d1, and the curved portion following the contour of the skull, d2. The total increase in path distance to the further ear, DD, is described by the expression:

where r is the radius of a cross-section of the head considered as a circle and h is the azimuth of a distant source measured in radians. This formula can be converted to give values for time delay, DT. If we consider that the radius of the head is 8.5 cm, and the speed of sound is 34,000 cm/s, then the difference in time of arrival of sound at the two ears is given by the expression:

Interaural phase differences can furnish cues to azimuth only at low frequencies. When a sinusoidal tone is about 650 Hz, its wavelength (about 50 cm) corresponds to about twice the path-length increment to the further ear with sources at azimuths of 90 or 270 from the medial plane. Hence, there will be a 180 interaural phase difference (that is, a pressure peak in one ear will occur

40Spatial localization and binaural hearing

of front-back reversals were noted (that is, sources on the same side differing

by plus or minus the same number of degrees from either 90 or from 270 were confused), but confusions of right and left were almost never made. The greatest errors occurred at about 3,000 Hz, with error scores dropping at higher and lower frequencies. Subsequently, Sandel, Teas, Feddersen, and Jeffress (1955) used listeners seated in an anechoic room with head movements restricted by a clamp. Broadband noise and sinusoidal tones were heard alternately, each fading in and out gradually to minimize transient clicks. Since broadband noise is localized with greater accuracy, it was used as an acoustic pointer, and was adjusted to correspond to the apparent azimuth of the tone. Sandel et al. (1955) reported that the tonal frequency producing poorest accuracy in localization was about 1,500 Hz, a value an octave lower than that reported by Stevens and Newman. As in the earlier study, front-back confusions were quite common. This is not surprising if we consider that the interaural time delays are equivalent for such positions. The question that needs answering seems to be how discrimination between positions in the front and back quadrants on the same side of the medial plane can be made at all.

There appears to be two main ways in which front-back errors can be minimized when dealing with real sources in everyday life: one involves asymmetrical effects of the pinnae which will be discussed subsequently, and the other involves head movements. Changes in time of arrival correlated with lateral head-turning can disambiguate the direction of continuing sounds. For example, if the sound is on the right side, rotation of the head to the right will decrease the interaural time difference for sources in the front quadrant and will increase the interaural time difference for sources in the back quadrant.

Minimal audible angle

Mills (1958, 1972) measured the minimum angle permitting lateral resolution of tones, calling it the minimal audible angle by analogy to the wellknown minimum visual angle. Listeners sat in an anechoic chamber with the position of their head fixed by a restraining clamp. Pairs of sinusoidal tone pulses were presented, and listeners were requested to tell whether the second pulse came from the right or left of the first (the source was moved in the short interval separating the pulses). Figure 2.4 shows the results obtained. The minimal audible angle was the least when measured as variation from 0 (straight ahead). Since listeners tend to orient their head toward the source of a sound of interest, the source is usually brought into the region of most accurate localization. Also, it often is possible to fixate a sound source visually near the medial plane, so that the auditory cues to localization can be

Binaural beats 41

Figure 2.4 The minimum audible angle between successive pulses of a tone as a function of the frequency of the tone and the azimuth of the source ( 0 , * 30 ,

~ 60 , ~ 75 ). (From Mills, 1972.)

compared, and if appropriate, superseded by the more precise visual localization. In the medial plane, the minimum auditory angle is about one degree for high and low frequencies, being greater for frequencies from 1,500 to 2,000 Hz. (The minimum discriminable visual angle is about one minute of arc.) The high minimum auditory angle at midrange frequencies is in keeping with the duplex theory considering that judgments of azimuth are based on interaural phase differences at low frequencies and interaural intensity differences at high frequencies with neither cue acting effectively at midrange.

Binaural beats

When sinusoidal tones are mistuned slightly from unison and introduced separately into the two ears, binaural beats may be observed which are characterized by changing lateralization or fluctuation in loudness. For example, if a tone of 500 Hz is presented to one ear and 500.5 Hz to the other, a listener may hear a phantom source which moves from one side to the other and back again each 2 s, with any lateralization associated with the side of leading phase. Patterns of loudness maxima may also occur with beat periods longer than 1 s, but they are not very noticeable and show no consistent relation to phase angle. For fast rates of binaural beating (a few per second), lateralization shifts become difficult to hear, and listeners generally report a

42Spatial localization and binaural hearing

loudness fluctuation having a diffuse intracranial location (Lane, 1925). The upper frequency limit for the production of binaural beats by sinusoidal tones is uncertain. These beats are generally considered to be strongest for frequencies from about 300 to 600 Hz, becoming difficult to hear at 1,000 Hz. However, Wever (1949) claimed that by using ‘‘very slight frequency differences’’ he was able to obtain binaural beats (or lateralization shifts) up to 3,000 Hz. Wever also referred to an unpublished study by Loesch and Kapell indicating that subjects could hear binaural beats up to 2,500 Hz after about 10 hours of practice. Since there was evidence of increasing frequency limits with increasing practice time at the end of the experiment, Wever stated that ‘‘it seems certain’’ that further improvement would have occurred with further practice. It appears that the extent of practice and frequency separation (or beat rate) employed could be responsible for much of the variability in reports from different laboratories concerning the upper frequency limit for binaural beats.

Binaural beats were of importance in the development of the duplex theory of sound localization, since they helped convince Lord Rayleigh (1907) that interaural phase differences could indeed be detected. Rayleigh’s earlier doubts appear to have been based upon the observation of Helmholtz and others that changing the phase relations between harmonic components of complex tones delivered to the same ear generally could not be detected.

Thus far, we have been considering the effect of time-of-arrival delays upon perception of laterality for sinusoidal tones. Rather different results are obtained when either brief clicks or extended complex sounds are heard.

Detection of interaural delays for clicks and for complex sounds

Klumpp and Eady (1956) obtained thresholds for detection of interaural time differences using a delay line to deliver a variety of different sounds through headphones.

We have already discussed their report that the lowest threshold value of 10 ls was obtained with a 1,000 Hz tone, with the detection of time delay becoming impossible at frequencies of 1,500 Hz and above. When they presented a single click with a nominal duration of 1 ls, an interaural delay threshold of 28 ls was found, but a 2 s burst consisting of 30 such clicks had a delay threshold of only 11 ls, indicating that integration of information from the individual clicks of a burst took place. When broadband noise was used, a threshold value of 10 ls was found, with approximately the same threshold observed for a 150–1,700 Hz noise band. Other noise bands with different bandwidths and frequency ranges had somewhat higher thresholds.

Detection of interaural delays for complex sounds 43

It might be thought that spectral components of noise below 1,500 Hz were required for detection of time differences, since Klumpp and Eady (1956) had found that phase differences for sinusoidal tones of 1,500 Hz and above were ineffective in producing lateralization. As discussed in Chapter 1, low frequency tones produce a phase locking of neural response, so that an interaural temporal comparison of fibers with the same low characteristic frequencies locked to the same phase angle could provide the temporal information for lateralization of broadband noise. However, Klumpp and Eady found that a narrow-band noise containing frequencies from about 3,000 to 3,300 Hz could be lateralized (although the threshold was at the relatively high value of 60 ls). It would appear that temporal information carried by the high frequency noise could be used for lateralization, despite the original formulation of the duplex theory that considered temporal information to be restricted to low frequency components. Henning (1974) provided an interesting demonstration of the nature of this temporal information. He introduced interaural time delays using a 3,900 Hz tone that was sinusoidally amplitude modulated at 300 Hz (that is, the amplitude envelope of the 3,900 Hz tone was varied in a sinusoidal fashion 300 times a second). Henning found that interaural time delays in the amplitude pattern could be detected with this stimulus at approximately the same threshold delay as found for a 300 Hz sinusoidal tone, despite the absence of any low frequency spectral components (spectrally, this amplitude modulated stimulus can be considered as consisting of three sinusoidal frequencies: the center frequency of 3,900 Hz, and two sidebands of 3,600 and 4,200 Hz respectively). McFadden and Pasanen (1975) used binaural beats to provide another demonstration that complex waveforms consisting of tones well above the classical limit of 1,000 Hz could give rise to dichotic temporal interactions. When a 3,000 Hz tone was presented to both ears and mixed with a 3,100 Hz tone at the left ear and a 3,101 Hz tone at the right ear, the difference in the interaural envelope frequencies (100 versus 101 Hz) produced binaural beating at a rate of 1/s. Binaural beats were noted even when the spectral components in the two ears were rather different as long as the envelopes were close in frequency. Thus 2,000 and 2,050 Hz in one ear would cause binaural beats of 1/s when heard with 3,000 and 3,051 Hz in the other ear.

Most experiments dealing with the limits for detection of interaural temporal delay have studied the lower limit for lateralization (the shortest detectable delay). But where does the upper limit lie? As we shall see, this limit is surprisingly high – at least one-half second for broadband noise delivered first to one ear and then to the other. At these long interaural delays, the sound is no longer lateralized to the leading ear.

44 Spatial localization and binaural hearing

As the interaural delay for broadband noise is increased from the threshold of about 10 ls reported by Klumpp and Eady (1956), an increase in the extent of lateralization is perceived for the fused single image up to delays of several hundred microseconds. The greatest possible delay attributable to interaural path-length differences is roughly 650 ls, and as would be expected, listeners hear a single lateral source at this delay. However, if the delay is increased to about 2,000 ls (2 ms), this sharp single lateral image starts to become diffuse (Bilsen and Goldstein, 1974). Blodgett, Wilbanks, and Jeffress (1956) reported that sidedness is lost for broadband noise with interaural delays of about 9.5 ms. The delay range producing diffuse lateralization is of interest because it appears possible to hear either lateralization to the leading ear or a pitch equivalent to that of a sinusoidal tone of l/s Hz (where s is the time delay in seconds), but not both. Fourcin (1965, 1970) used a variety of stimuli which permitted his listeners to hear this interaural delay pitch. The simplest condition involved broadband noise from two generators delivered to each ear. The noise from one generator was presented diotically (identical presentation in each ear), while the noise from the second generator had an interaural delay of a few milliseconds. Fourcin found that an interaural delay of s s produced a pitch matching that of a sinusoidal tone of 1/s Hz. Later, Bilsen and Goldstein (1974) reported that it was possible to hear a faint pitch of l/s Hz with noise from only a single source delivered to one ear several milliseconds after delivery to the other. (Pitches were produced for delays ranging from about 3 ms through 12 ms, corresponding to 333 and 83 Hz respectively.) Warren, Bashford, and Wrightson (1981) confirmed Bilsen and Goldstein’s findings, but found that their subjects had an appreciably longer delay limit for the perception of pitch (50 ms delay corresponding to a pitch of 20 Hz). But, in addition, Warren et al. (1981) reported that an infrapitch periodicity could be detected at much longer interaural delays, up to a lower limit of at least 500 ms (corresponding to a repetition frequency of 2 Hz). These long delay periods could be matched in repetition time with periodic pulses delivered simultaneously to both ears (as could pitch-range interaural delays). In addition, for delays greater than 200 ms (corresponding to frequencies less than 5 Hz) the repetition period could be matched by the rate of finger tapping without the need for employing a second auditory stimulus. As we shall see in Chapter 3, mixing an ongoing diotic noise with its own restatement after a delay of 500 ms also can be perceived to repeat at 2 Hz, and the ability to detect such long-period repetition has implications for theories concerning neural periodicity analysis.

Curiously, although listeners can detect long interaural delays for broadband noise, they cannot tell which ear received the noise first. After perception