Biomedical subjects

W M Hartmann

Publications and source records attributed to W M Hartmann.

At least 19 recordsLinked to original sources

PubMed1999

Identification and localization of sound sources in the median sagittal plane.

The ability of human listeners to identify broadband noises differing in spectral structure was studied for multiple sound-source locations in the median sagittal plane. The purpose of the study was to understand how sound identification is affected by spectral variations caused by directionally dependent head-related transfer functions. It was found that listeners could accurately identify noises with different spectral peaks and valleys when the source location was fixed. Listeners could also identify noises when the source location was roved in the median sagittal plane when the relevant spectral features were at low frequency. Listeners failed to identify noises with roved location when the spectral structure was at high frequency, presumably because the spectral structure was confused with the spectral variations caused by different locations. Parallel experiments on sound localization showed that listeners can localize noises that they cannot identify. The combination of identification and localization experiments leads to the conclusion that listeners cannot compensate for directionally dependent filtering by their own heads when they try to identify sounds.

Adult↗

PubMed1998

The masking-level difference in low-noise noise.

In experiment 1 NoSo and NoS pi thresholds for a 500-Hz pure tone were obtained in a low-fluctuation masking noise and a high-fluctuation masking noise for six normal-hearing listeners. The noise bandwidth was 10 Hz. In agreement with previous investigations, the NoSo thresholds were lower in low-fluctuation noise than in high-fluctuation noise. For three listeners, NoS pi thresholds were similar for the two types of noise, while for the other three listeners, Nos pi thresholds were higher for low-fluctuation noise than for high-fluctuation noise. In experiment 2, the masker was created by amplitude modulating a 500-Hz pure tone by a 0-10-Hz low-pass noise. The degree of masker fluctuation was controlled by adjusting the average modulation depth (100%, 63%, 40%, and 25%). The signal was a 10-Hz-wide noise centered on 500 Hz. Results were similar to those of experiment 1: for the NoSo conditions, signal detection improved with decreasing degree of fluctuation, and for NoS pi conditions, the results were subject dependent. For three listeners, NoS pi thresholds were again similar in the two types of noise, while for the other three listeners, NoS pi thresholds were again higher in low-fluctuation noise than in high-fluctuation noise. The results showed that a high degree of masker fluctuation sometimes facilitates NoS pi detection. It is possible that the binaural detection mechanism utilizes the relatively good signal-to-noise ratios that occur in the low power or "dip" regions of fluctuating masker waveforms.

Adult↗

PubMed1998

The pitch of a mistuned harmonic: evidence for a template model.

A harmonic of a periodic complex tone can be heard out as a separate entity if the harmonic is slightly mistuned from its correct frequency. Pitch matching experiments show that the pitch of such a mistuned harmonic differs systematically from its frequency. The shift in pitch is found to be an exaggeration of the frequency mistuning. This article considers two classes of model for the pitch shift. In the first class are tonotopically local interaction models which attribute the pitch shift to interactions between the mistuned harmonic and neighboring harmonics, where the neighborhood is established by peripheral filtering. The second class of model attributes the pitch shift to a contrast between the mistuned harmonic and a broadband harmonic template. This article describes six pitch matching experiments using complex tones having spectral gaps, strategically chosen to compare local interaction and template models. The results show that when a competition is set up between local interactions and a template, the template proves to be dominant. A parallel between the pitch shifts of mistuned harmonics and periodicity pitch, also attributed to a harmonic template, is seen as the frequency range of the mistuned harmonic is changed. Tonotopically local influences are evident in several experiments, but they are of secondary importance.

Adult↗

PubMed1998

On the source-identification method.

The source identification method is a standard psychophysical procedure for studying the ability of listeners to localize the source of a sound. The method can be described in terms of a statistical model in which listeners' responses are determined by the width and bias of an internal distribution. This article presents a theoretical study of the method, particularly the relationships between the average experimental observables, rms error and variability, and parameters of the internal distribution. The theory is tested against source-identification experiments, both easy and difficult. Of particular interest is the experimental dependence of observable statistics on the number of sources in the stimulus array, compared with theoretical predictions. It is found that the model gives a good account of several systematic features seen in the experiments. The model leads to guidelines for the design and analysis of source-identification experiments.

Adult↗

PubMed1998

Sound localization in the median sagittal plane by listeners with presbyacusis.

In Experiment 1, a group of listeners with substantial hearing loss due to presbyacusis and a group of listeners with normal hearing were given three localization tests: a frontal plane test in which they judged whether sounds came from the left, overhead, or the right; a sagittal plane test in which they judged whether sounds came from directly in front, overhead, or behind; and an elevation test in which they judged the vertical position of sounds coming from in front. The two groups performed similarly on the frontal plane test, which chiefly depended upon their ability to use binaural localization cues. They performed differently on the sagittal plane and elevation tests, for which the predominant localization cues were spectral. The listeners with presbyacusis were substantially less accurate than those with normal hearing in both of these instances. They had particular difficulty judging source elevation, rarely scoring much above chance. Follow-up testing of a group of subjects in the early stages of presbyacusis showed localization performance that was intermediate to the other two groups, but far more like that of the normal-hearing listeners. In Experiment 2, additional tests were run with the following conditions designed to encourage improved performance by listeners with presbyacusic hearing loss: (1) filtering of stimuli to preclude masking of more informative high-frequency components by low frequencies; (2) simplification of the elevation test and greater spatial separation of its loudspeaker sources; and (3) use of hearing aids. Conditions 1 and 2 had no appreciable effect on performance; condition 3 significantly improved presbyacusic listeners' ability to localize in the sagittal plane, particularly when sounds came from the front.

Adult↗

PubMed1997

On the Duifhuis pitch effect.

An effect discovered by Duifhuis [J. Acoust. Soc. Am. 48, 888-893 (1970)], wherein an omitted high harmonic of a periodic complex tone is found to have an audible pitch, is extended to a variety of new broadband signal conditions. The effect is found to exist for flat spectra and spectra decreasing at 6 dB/octave, independent of phases as long as they are constant. The effect exists for alternating phases and Schroeder phases. It can generate a missing-fundamental pitch. Pitch and loudness matching experiments support the status of the omitted harmonic as an objective tone in the signal. Further experiments using narrower bands challenge the traditional explanation for the effect, which attributes it to short-term frequency analysis by peripheral auditory filters. Instead, the experiments suggest that different peripheral channels must be combined, maintaining some phase information, to generate the effect.

Adult↗

PubMed1997

Psychophysical and physiological evidence for a precedence effect in the median sagittal plane.

A listener in a room is exposed to multiple versions of any acoustical event, coming from many different directions in space. The precedence effect is thought to discount the reflected sounds in the computation of location, so that a listener perceives the source near its true location. According to most auditory theories, the precedence effect is mediated by binaural differences. This report presents evidence that the precedence effect operates in the median sagittal plane, where binaural differences are virtually absent and where spectral cues provide information regarding the location of sounds. Parallel studies were conducted in psychophysics by measuring human listeners' performance, and in neurophysiology by measuring responses of single neurons in the inferior colliculus of cats. In both experiments the precedence effect was found to operate similarly in the azimuthal and sagittal planes. It is concluded that precedence is mediated by binaurally based and spectrally based localization cues in the azimuthal and sagittal planes, respectively. Thus, models that attribute the precedence effect entirely to processes that involve binaural differences are no longer viable.

Acoustic Stimulation↗

PubMed1996

On the pitches of the components of a complex tone.

The pitches of the harmonics (numbers 1, 2, 3, 4, 5, 7, 9, and 11) of a complex tone were measured in a matching experiment. The harmonics to be matched were mistuned (8% or less) either positively, or negatively, or not at all. For all mistuned harmonics and all listeners the matching pitches were found to be exaggerations of the mistunings, i.e., the data exhibited pitch shifts with the same sign as the mistunings. This result is shown to be contrary to place models of pitch perception, such as the spectral pitch algorithm of Terhardt, in which pitch shifts are caused by the interaction of excitation patterns for the individual harmonics. An alternative model, in which pitch is determined by neural timing, also fails to account for the data. However, a hybrid model, combining effects of excitation pattern interaction with neural timing, does agree with most of the data.

Adult↗

PubMed1996

On the externalization of sound images.

Listeners perceive the sounds of the real world to be externalized. The sound images are compact and correctly located in space. The experiments reported in this article attempted to determine the characteristics of signals appearing in the ear canals that are responsible for the perception of externalization. The experiments used headphones to gain experimental control, and they employed a psychophysical method whereby the measurement of externalization was reduced to discrimination. When the headphone signals were synthesized to best resemble real-world signals (the baseline synthesis) listeners could not distinguish between the virtual image created by the headphones and the real source. Externalization was then studied, using both discrimination and listener rating, by systematically modifying the baseline synthesis. It was found that externalization depends on the interaural phases of low-frequency components but not high-frequency components, as defined by a boundary near 1 kHz. By contrast, interaural level differences in all frequency ranges appear to be about equally important. Other experiments showed that externalization requires realistic spectral profiles in both ears; maintaining only the interaural difference spectrum is inadequate. It was also found that externalization does not depend on dispersion around the head; an optimum interaural time difference proved to be an adequate phase relationship.

Humans↗

PubMed1996

Pitch, periodicity, and auditory organization.

The perception of pitch forms the basis of musical melody and harmony. It is also among the most precise of all our human senses, and with imagination, this precision can be used experimentally to investigate the functioning of the auditory system. This tutorial presents auditory demonstrations from the zoo of pitch effects: pitch shifts, noise pitch, virtual pitch, dichotic pitch, and the pitches of things that are not there at all. It introduces models of auditory processing, derived from contemporary psychoacoustics and auditory physiology, and tests these models against the experimental effects. It concludes by describing the critical role played by pitch in the important human ability to disentangle overlapping sources of sound.

Cochlea↗

PubMed1993

Auditory demonstrations on compact disk for large N.

The popular compact disk entitled Auditory Demonstrations, sponsored by the Acoustical Society of America, includes a number of demonstrations that lead to quantitative results. Those demonstrations are evaluated here in the context of a sizeable class in a lecture room. Demonstrations concern masking, loudness, and pitch; specifically they are numbers 2, 3, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 20, and 25. The evaluations find that most of the demonstrations successfully illustrate psychoacoustical principles in a classroom context; others are less successful or require special circumstances for success. Explanations for success and failure are offered, together with some suggestions for optimizing the chances of success.

Acoustic Stimulation↗

PubMed1993

On the origin of the enlarged melodic octave.

The perceptual octave is larger than the physical octave, i.e., most listeners perceive that two tones are an octave apart when their frequency ratio is greater than 2. This result is known as the octave enlargement effect. There are two theories for the effect, one of them a central template theory, the other a peripheral timing theory. In principle, it is possible to determine which theory is better by discovering whether or not octave enlargement occurs for centrally generated dichotic pitches such as the Huggins pitch. Experiments show that octave enlargement does indeed occur for Huggins pitch. This is the result predicted by the central template theory, but it can be argued that the result does not entirely eliminate the timing theory. A detailed examination of the two theories shows that each requires revision in order to make the octave enlargement prediction follow logically from its premises. The central template theory requires the auditory system to differentiate excitation caused by different harmonics of a complex tone on some basis other than place of excitation. Neural synchrony is suggested as a basis. The timing theory, originally formulated in terms of a neural interspike interval timing, can be made internally consistent by replacing the neural interspike-interval circuit by a neural autocorrelator.

Acoustic Stimulation↗

PubMed1993

Auditory spectral discrimination and the localization of clicks in the sagittal plane.

Experiments show that the ability of human listeners to localize an impulsive sound in the medial sagittal plane (front, overhead, rear) deteriorates as the level of the sound increases. This negative level effect is strong for clicks but does not appear for broadband noise. It is conjectured that the negative level effect arises because the tonotopic excitation pattern is broadened for intense impulsive sounds. As a result, the spectral peaks and valleys, which are caused by anatomical filtering and which normally code for localization in the sagittal plane, are less recognizable. Filtered click discrimination experiments using headphones also show a negative level effect for clicks, but not for noise, and support this conjecture.

Acoustic Stimulation↗

PubMed1992

Structural alterations of an ambiguous musical figure: the scale illusion revisited.

The scale illusion (Deutsch, 1975) shows the importance of frequency range in the perceptual organization of a sequence of notes. This paper includes three experiments on the scale illusion. Experiments 1 and 2 demonstrated that if the structure of the pattern of notes used in the original scale illusion study is altered slightly, by adding or subtracting a pair of notes from the ends of the sequence, there is a significant decrease in the rate of frequency-based responses, suggesting a weaker illusion. Experiment 3 investigated two features of the note patterns that may have led to this change. Specifically, it asked whether the decrease in the strength of the illusion is due to (1) the nature of the notes at the extremes of the frequency range and/or (2) the nature of the notes at the crossing point of the two scales. While both sources were found to affect the strength of the scale illusion, the former had a greater influence.

Adult↗

PubMed1991

Periodic signals with minimal power fluctuations.

In this article, Pumplin's algorithm [J. Pumplin, J. Acoust. Soc. Am. 78, 100-104 (1985)] is used to find periodic waveforms with minimal power fluctuations. Starting with a particular power spectrum, the algorithm can find a set of phases for the harmonics such that the variance in waveform power is a minimum or near a minimum. Such optimized waveforms are smooth and tend to have very small crest factors. The particular power spectra chosen for study include narrow- and wideband spectra, with emphasis given to signals that are useful in research in psychoacoustics and in physiological acoustics.

Algorithms↗

PubMed1991

Turning on a tone.

It is possible to choose the starting phase of a pure tone in a way that minimizes the onset noise when the tone is turned on abruptly. A spectral model shows that when the tone has a low frequency, minimum onset noise is expected for a starting phase of zero (turning on a sine tone) but when the tone has a high frequency, minimum onset noise is expected for a starting phase of +/- 90 deg (turning on a cosine tone). Listening experiments confirm the above expectations and show that the transition between low- and high-frequency domains is sharp and depends upon both the electroacoustical transducer and the individual listener.

Adult↗

PubMed1990

Hearing a mistuned harmonic in an otherwise periodic complex tone.

The ability of a listener to detect a mistuned harmonic in an otherwise periodic tone is representative of the capacity to segregate auditory entities on the basis of steady-state signal cues. By use of a task in which listeners matched the pitch of a mistuned harmonic, this ability has been studied, in order to find dependences on mistuned harmonic number, fundamental frequency, signal level, and signal duration. The results considerably augment the data previously obtained from discrimination experiments and from experiments in which listeners counted apparent sources. Although previous work has emphasized the role of spectral resolution in the segregation process, the present work suggests that neural synchrony is an important consideration; our data show that listeners lose the ability to segregate mistuned harmonics at high frequencies where synchronous neural firing vanishes. The functional form of this loss is insensitive to the spacing of the harmonics. The matching experiment also permits the measurement of the pitches of mistuned harmonics. The data exhibit shifts of a form that argues against models of pitch shifts that are based entirely upon partial masking.

Adult↗

PubMed1989

On the minimum audible angle--a decision theory approach.

The minimum audible angle (MAA) technique is a well-known psychoacoustical paradigm often used in the study of localization of sound. A difficulty with this paradigm, however, is that, in terms of decision theory, it is subject to two quite different interpretations. Although it is normally regarded as involving a discrimination task, the present work suggests that it is more likely to be an absolute identification task. Because of this difference in interpretation, it appears that previous work has overestimated the ability of listeners to localize sources of sound.

Acoustic Stimulation↗