Учебники / Auditory Perception - An Analysis and Synthesis Warren 2008

56Spatial localization and binaural hearing

source was in front or behind. If the appropriate direction was perceived, then timbre differences could not be heard, and the spectral differences were transformed into localization differences for a sound that seemed to have an unchanging timbre.

There have been a few studies concerning the effect of changing or eliminating pinna interaction with incident sound upon the detection of elevation. Roffler and Butler (1968) reported that, with normally positioned pinnae, listeners could identify with ease which source was radiating sound when two loudspeakers in the medial plane were separated in elevation by 11 . But performance in this task fell to chance level when they flattened and covered the pinnae using a plastic covering having an aperture over the opening of each ear canal. Gardner and Gardner (1973) studied the effect of occluding the cavities of the pinnae with plastic material on the ability to detect elevation in the medial plane. The accuracy of localization decreased with increased filling of the cavities, and no one portion of a pinna seemed more important than another.

Room acoustics

Plenge (1974) reasoned that listeners should not be able to localize sources within rooms unless they were familiar with both the nature of the sound and the acoustics of the room. He prepared two-channel recordings using microphones placed on a dummy head, and then played the recordings to listeners through stereo headphones. He claimed that his subjects could achieve external localization of sounds delivered through headphones when the stimulus resembled a familiar sound in a familiar room; otherwise, the source appeared to be located within the head, as is usually the case when listening through headphones. Plenge stated that only a few seconds of listening was sufficient for calibration of a room’s acoustic properties, which were stored as long as the listener remained in the room and then cleared immediately upon leaving, so that the listener could recalibrate at once for a new acoustic environment.

Most experiments studying the cues employed for sound localization have attempted to eliminate or minimize the room-specific complex influence of echoes and reverberation by conducting experiments out of doors or in anechoic rooms. In order to study effects of enclosures on localization, Hartmann (1983) reported a systematic series of experiments in a special room having variable acoustics and determined the effects of changes in reverberation time upon the accuracy of localization for a number of different types of sound, including brief impulsive sounds, steady-state tones, and noise.

Auditory reorientation 57

Auditory reorientation

In addition to the very rapid adjustment of listeners to the acoustics of a particular room described by Plenge (1974), there is evidence that listeners also can adapt to (or compensate for) long-term changes in auditory cues.

There have been reports going back to the last century that accurate directional localization may be found in complete or partial monaural deafness, indicating that a criterion shift or recalibration of the cues used to determine localization had taken place. Conductive deafness is of special interest, since it is possible to compensate for, or to remove, the blockage and restore normal hearing. If the blockage is due to a plug of cerumen (wax) in the ear canal, the procedure is especially simple and can be accomplished in a few minutes by syringing. Von Bezold (1890) reported that, when normal sensitivity was restored by removal of cerumen in one ear, there was a period of localization confusion lasting for a few weeks while adjustment to ‘‘normal’’ hearing took place. Bergman (1957) described the consequences of successful operations on people with conductive hearing losses involving the middle ear ossicles; along with an increase in acuity in one ear after the operation, there was a period of localization confusion which was not present before the operation.

Other experiments have reported that similar changes in localization could be produced experimentally by maintaining an artificial blockage in one ear for enough time to permit adjustment of localization to this condition, followed by confusion upon removal of this blockage (for summary of this literature, see Jongkees and van der Veer, 1957).

In addition to these dramatic changes in localization criteria, there is a slow recalibration that we have all experienced. During childhood, the dimensions of head and pinnae change, and our criteria for localization must change accordingly if accuracy is to be maintained.

There have been several experiments reported which have used ‘‘pseudophones’’ to alter localization cues. The first of these was by Thompson (1882), who was impressed both by Wheatstone’s discovery of the stereoscope and Wheatstone’s construction of a ‘‘pseudoscope’’ in which binocular cues to distance were inverted by interchanging inputs to the right and left eyes. Thompson designed an instrument he called the ‘‘pseudophone’’ which had reflecting flaps for directing sounds into the ears from front, back, above, or below. The experimenter could manipulate the flaps on pseudophones worn by blindfolded subjects, and so change the apparent position of sound sources. Young (1928) used a rather different type of a pseudophone consisting of two funnels mounted on the head which gathered sound that could be led to the ears through separate tubes. He studied the adaptation of localization to

58Spatial localization and binaural hearing

inversion produced by lateral cross-over of the sound-conducting tubes. Young’s work was replicated and extended by Willey, Inglis, and Pearce (1937). These studies found that while listeners could learn to respond appropriately to the reversal of sides produced by the pseudophones, accurate localization of sound sources could not be accomplished even after periods as long as a week. The lack of complete adaptation, perhaps, was not too surprising since the pseudophones were quite different from pinnae, and the transformations produced by the apparatus were not a simple lateral inversion of normal input.

Held (1955) designed an electronic pseudophone employing hearing-aid microphones. These were mounted 20 cm apart on a bar that was attached to a headband worn by the subject. The bar formed an angle of 22 relative to the line joining the subject’s ears, so that Held considered that he had shifted the interaural axis by 22 . The microphones were each used to power an ipsilateral earphone worn under a muff that served to attenuate the normal airborne stimulation. Listeners wearing this device for a day could adapt appropriately, and go through their normal daily activities without disorientation.

The ability to adapt to a changed correspondence of sensory input to environmental conditions is not restricted to hearing. As far back as 1879, Helmholtz wrote that ‘‘I have performed and described experiments in order to show that even with distorted retinal images (e.g., when looking through lenses, through converging, diverging, or laterally deflecting prisms) we quickly learn to overcome the illusion and to see again correctly . . . ’’ (see Warren and Warren, 1968, p. 237).

Changes in lateralization criteria can occur quite rapidly. Flu¨gel (1920–1921) reported that after a short monaural exposure to a sinusoidal tone, a subsequent diotic tone that would normally appear to be in the medial plane would be shifted toward the previously unstimulated ear. He attributed this shift to a ‘‘local fatigue.’’ However, this explanation was weakened by his observation that while a monaural poststimulus threshold elevation was restricted to the previous frequency, the poststimulus lateralization shift of binaural tones occurred with all frequencies. Bartlett and Mark (1922–1923) confirmed Flu¨gel’s observations, and then demonstrated that his explanation was inadequate. They found that similar after-effects were produced by listening to the same tone delivered to both ears at the same level, but with interaural phase differences causing the adapting tone to be lateralized. Following stimulation by this lateralized binaural tone, they found that a binaural test tone that was matched in level and phase at each ear was lateralized to the side opposite to that of the previous stimulus. Since the level of the adapting tone was the same at each ear, they concluded that the lateralization shift reflected an ‘‘error in judgment’’ rather than a change in sensitivity. This error in judgment was the

Estimates of distance from the source 59

result of the criterion for medial plane localization being shifted toward the side of the previously lateralized tone (assimilation). This caused a tone that would otherwise appear centered to be displaced toward the opposite side (contrast or negative after-effect). After-effects of a similar nature have been reported for a variety of perceptual judgments, not only in hearing, but with other perceptual modalities as well. Warren (1985) summarized this literature and suggested the following ‘‘criterion shift rule’’: The criteria used for evaluating stimuli are displaced in the direction of simultaneous or recently experienced values. This simple principle can help explain some of the experimental biases encountered in laboratory experiments, as well as a number of illusions occurring in speech perception. It has been suggested that the criterion shift rule reflects a continuing calibration of evaluative systems normally ensuring that behavioral responses remain appropriate to current conditions. Criterion shifts are discussed at greater length in Chapter 8.

Estimates of distance from the source

Experiments carried out many years ago demonstrated that under some conditions, listeners can estimate distances of sound sources with considerable accuracy (Matsumoto, 1897; Shutt 1898; Starch and Crawford, 1909). Other studies have shown that there is ‘‘loudness constancy’’ or ‘‘loudness invariance’’ by which listeners can compensate for changes produced by varying the distance of the listener from the source (Mohrmann, 1939; von Fieandt, 1951; Shigenaga, 1965). Let us consider the cues used by listeners in estimating the distance of a sound source.

Perhaps the most obvious cue associated with an increase in distance is a decrease in intensity. As a reasonably close approximation, the intensity of a sound reaching the listener directly from the source is inversely proportional to the square of its distance (the inverse square law), so that a twofold change in distance corresponds to a fourfold (6 dB) change in intensity. It has been recognized for a long time that intensity is a major cue to the distance of familiar sounds (see Thompson, 1882; Pierce, 1901). However, intensity is not the only cue to distance of a source; the ratio of direct to reverberant sound can be a quite important factor.

Since both reverberant and direct components can play a role in judgments of the distance of a source, a problem is encountered for experiments attempting to determine the ability of listeners to judge the manner in which sound varies with distance. If listeners are given the task of estimating the effect of, say, doubling distance, simply attenuating the signal creates a conflict of cues – the decreasing intensity of direct components (which arrive first)

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indicates an increase in distance, while the fixed ratio of direct to reverberant sound produced by attenuating the entire signal indicates an unchanging distance. However, there does appear to be a way of avoiding this dilemma, which permits a direct determination of the ability of listeners to estimate the manner in which intensity changes with distance without the influence of other cues, whether concordant or conflicting, to acoustic perspective. This can be accomplished by having the subjects generate the sounds themselves.

Warren (1968a) reported an experiment that required subjects to adjust their productions of various self-generated sounds (the vowel ‘‘ah,’’ the consonant ‘‘sh,’’ and a pitch-pipe note) so that the level at a target microphone ten feet away would remain unchanged when its distance was reduced to five feet. As will be discussed more fully in Chapter 4, the attenuations produced to compensate for halving the distance for all of the sounds were close to the theoretical value of 6 dB, ranging from 6 dB to 8 dB for sounds reaching a microphone directly from the source.

There have been a number of reports that when listeners are functioning as receivers rather than generators of sound, reverberation can function as an important cue to distance. When listening to external sources in rooms with normal acoustics, the ratio of direct to reverberant sound decreases with distance. In an experiment by Steinberg and Snow (1934), a person who was speaking at a fixed level moved back from a pick-up microphone while the gain was changed by the experimenter, so that the amplitude delivered to the listener through loudspeakers remained fixed. Even though the overall amplitude did not change, the talker seemed to recede from the listener, an effect attributed by Steinberg and Snow to a change in the relative proportions of direct and reverberant sound. Be´ke´sy (1938) reported similar results when he used equipment designed to change the ratio of direct to reverberant sound while keeping the overall amplitude fixed. Subsequently, Mershon and King (1975) and Butler, Levy, and Neff (1980) reported that noise generated under reverberant conditions seemed much further away than noise generated in an anechoic room.

Maxfield (1930, 1931) described the importance of matching what he called ‘‘acoustic perspective’’ to visual perspective in making sound movies. It was pointed out that since binaural suppression of echoes (which will be discussed shortly) was not possible with one-channel recordings, the overall level of reverberation must be decreased below the normal level to appear normal. He found it necessary to vary the proportion of direct to reverberant sound by appropriate positioning of the microphone when the camera position was changed in order to maintain realism: Maxfield gave an example of a long-shot sound track used with a close-camera shot which made it seem that the actors’

Estimates of distance from the source 61

voices were coming through an open window located behind them, rather than from their lips. He made the additional interesting observation that correct matching of acoustic and visual perspective influenced intelligibility – when a long-shot picture was seen, a long-shot sound track was more intelligible than a close-up sound track despite the fact that the increased reverberation would make it less intelligible if heard alone.

The delayed versions of a sound produced by echoes not only give rise to an acoustic perspective, but can also impart a pleasing ‘‘lively’’ character to voice and music (as opposed to an undesirable so-called ‘‘dead’’ character when echoes are much reduced). The ability of listeners to minimize interference of masking by echoes has been rediscovered many times, and it goes by a number of names including ‘‘precedence effect,’’ ‘‘Haas effect,’’ ‘‘law of the first wave front,’’ and ‘‘first arrival effect’’ (see Gardner, 1968; Litovsky, Colburn, Yost, and Guzman, 1999 for reviews). Although reduction of masking caused by echoes works best with binaural listening, there is a considerable echosuppression effect even without binaural differences. I have found that it is possible to demonstrate this single-channel suppression effect for speech by comparing a recording with a low normal reverberation with one that was identical, except that the reverberant sound preceded the direct sound (a condition that could never occur naturally). The reversed reverberation recording was derived from a master tape prepared in an acoustically dead studio with the microphone a few inches from the talker’s mouth. This tape was played in a reversed direction into a room having normal reverberation characteristics. An omnidirectional pick-up microphone used for rerecording the speech was placed within two feet of the loudspeaker producing the reversed speech. When the backwards rerecording with the newly added reverberation was reversed once again on playback, the double reversal resulted in the speech being heard in the normal direction, but with reverberation preceding the voice. The slow build-up of reverberation preceding the sounds of speech and the abrupt drop in intensity at silent pauses seemed quite bizarre and was extremely noticeable. However, with a rerecording prepared from the same master tape in a fashion which was identical, except that the reverberation was added in the normal temporal direction, the reverberation was suppressed, resulting in speech having a barely perceptible reverberant quality.

Echo suppression is of little help in understanding a speaker when there are several simultaneous conservations going on along with other extraneous sounds: the so-called ‘‘cocktail party effect’’ (Cherry, 1953). Echoes of individual voices are masked by the babble, but it can be possible to direct one’s attention to one of the speakers by ignoring the speech having different

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azimuths, timbres, and amplitudes. Interestingly, if one’s own name or some other attention-capturing word is spoken by a nonattended speaker, it may be possible to become aware of the item and shift attention to the other speaker.

Gardner (1969) tested the ability of listeners to estimate their distance from the source of a voice in a large anechoic room at the Bell Telephone Laboratories. Echoes were effectively abolished by the use of sound absorbent wedges of appropriate composition and size that were placed along the walls, ceiling, and floor. Listeners and equipment were suspended between floor and ceiling on a wire-mesh support which did not reflect sound appreciably. When a voice was recorded and played back over loudspeakers at distances varying from 3 to 30 feet, estimates were independent of actual distance (probably due to the lack of reverberation that normally occurs in rooms and covaries with distance from the source). However, when a live voice was used, an accuracy better than chance in estimating actual distances was found, especially for distances of a few feet. Gardner speculated that the ability of the listener to estimate distance when close to the talker could have resulted from audibility of nonvocal breathing, acoustic cues to the degree of effort made in producing the level heard, detection of echoes from the talker’s body during conversational interchanges, radiation of body heat, and olfactory cues. Incidentally, the acoustic cues of vocal effort are not necessary concomitants of changes in vocal level. Talley (1937) stated that professional actors in plays are capable of producing a special ‘‘audience speech’’ in which the fact that two people separated by a few feet on a stage are conversing at the level of a shout can be disguised by appropriate control of voice quality, so that the illusion is produced that they are communicating with each other rather than with the audience.

Another cue to distance comes into play outdoors with far-off sounds. Helmholtz (1954/1877) noted that consonants with their high frequencies did not carry very far, stating that, ‘‘it is interesting in calm weather to listen to the voices of the men who are descending from high hills to the plain. Words can no longer be recognized, or at most only such as are composed of M, N, and vowels as Mamma, Nein. But the vowels contained in the spoken word are easily distinguished. They form a strange series of alternations of quality and singular inflections of tone, wanting the thread which connects them into words and sentences.’’ A more mundane example of the low-pass filtering of distant sounds is the transformation of explosive broadband thunderclaps to the deep rumble of a far-off storm. In keeping with these observations, Bloch (1893), after experiments with a variety of stimuli, concluded that sounds lacking high frequency components sounded farther away than sounds containing high frequencies. Also, Levy and Butler (1978) and Butler, Levy, and Neff (1980)

Suggestions for further reading 63

reported that low-pass noise appeared farther away than high-pass noise. Coleman (1963) presented values for the absorption coefficient in dB/100 feet. for different frequencies, which showed that higher frequencies are attenuated much more than lower frequencies. He noted that this attenuation was highly dependent upon water-vapor content of the air, increasing with the humidity. The calculations he presented show that at ‘‘typical conditions’’ encountered in a temperate climate, components at 8,000 Hz may have an attenuation 3 dB greater than components at 1,000 Hz for each 100 feet of travel.

Sensory input and physical correlates

We have seen that changes in the nature of sensory input caused by shifts in the location of a source are perceived in terms of positional correlates, not in terms of the sensory changes themselves. As Helmholtz has stated ‘‘ . . . we are exceedingly well trained in finding out by our sensations the objective nature of the objects around us, but . . . we are completely unskilled in observing the sensations per se’’ (see Warren and Warren, 1968, p. 179). In keeping with Helmholtz’s statement, evidence will be discussed in Chapter 4 indicating that, when attempts are made to have subjects estimate sensory magnitude directly by judging the relative loudness of sounds, responses are based upon experience with a familiar physical scale regularly associated with the extent of stimulation. Ordinarily, experience with localization serves as the physical correlate of loudness, and evidence will be presented indicating that judgments of relative loudness are based upon experience with the manner in which stimulation varies with distance from the source.

Suggestions for further reading

A standard text by an expert: Blauert, J. 1997. Spatial Hearing: The Psychophysics of Human Sound Localization. Cambridge, MA: MIT Press.

For a detailed discussion of selected topics: Gilkey, R., and Anderson, T. (eds.) 1997. Binaural and Spatial Hearing in Real and Virtual Environments. Hillsdale, NJ: Erlbaum.