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It is difficult to localize the source of a tone in a room because standing waves lead to complicated interaural differences that become uninterpretable localization cues. This paper tests the conjecture that localization improves if the listener can move to explore the complicated sound field over space and time. Listener head and torso movements were free and uninstructed. Experiments at low and high frequencies with eight human listeners in a relatively dry room indicated some modest improvement when listeners were allowed to move, especially at high frequencies. The experiments sought to understand listener dynamic localization strategies in detail. Head position and orientation were tracked electronically, and ear-canal signals were recorded throughout the 9 s of each moving localization trial. The availability of complete physical information enabled the testing of two model strategies: (1) relative null strategy, using instantaneous zeros of the listener-related source angle; and (2) inferred source strategy, using a continuum of apparent source locations implied by the listener's instantaneous forward direction and listener-related source angle. The predicted sources were given weights determined by the listener motion. Both models were statistically successful in coping with a great variety of listener motions and temporally evolving cues.

Monaural noise edge pitch (NEP) is evoked by a broadband noise with a sharp falling edge in the power spectrum. The pitch is heard near the spectral edge frequency but shifted slightly into the frequency region of the noise. Thus, the pitch of a lowpass (LP) noise is matched by a pure tone typically 2%-5% below the edge, whereas the pitch of highpass (HP) noise is matched a comparable amount above the edge. Musically trained listeners can recognize musical intervals between NEPs. The pitches can be understood from a temporal pattern-matching model of pitch perception based on the peaks of a simplified autocorrelation function. The pitch shifts arise from limits on the autocorrelation window duration. An alternative place-theory approach explains the pitch shifts as the result of lateral inhibition. Psychophysical experiments using edge frequencies of 100 Hz and below find that LP-noise pitches exist but HP-noise pitches do not. The result is consistent with a temporal analysis in tonotopic regions outside the noise band. LP and HP experiments with high-frequency edges find that pitch tends to disappear as the edge frequency approaches 5000 Hz, as expected from a timing theory, though exceptional listeners can go an octave higher.

Localization of sounds in the horizontal plane depends on interaural differences established by the sound source location, environment, and anatomy of the listener. Important interaural differences are the interaural level difference (ILD) and the interaural time difference (ITD). All the anatomical effects responsible for the interaural differences in humans are captured, with varying accuracy, by the spherical-head model. The cones of confusion of the spherical-head model approximately apply to real-world human sound localization, and they are best represented by the lateral-polar system of angular coordinates which can be related to the spherical-polar system. The sensitivity of human listeners to interaural differences can be studied directly using headphone presentation or practically using spatially distributed real sound sources. The difference between these two experimental modes corresponds to the perceptual difference between lateralization (heard within the head) and localization (heard as external). Both lateralization and localization experiments indicate that the effective ITD representation is the ITD itself and not the interaural phase difference. According to the duplex model, ITDs are the dominant cues for localization of low-frequency sounds, whereas ILDs are dominant for high. ITDs themselves appear to be encoded according to a place process (Jeffress model) at the high end of the ITD frequency range and by an opponency process at the low end, although this distinction is an area of current research. Models such as the position variable model attempt to account for the combined effects of the ITD and ILD. The central nervous system preferentially weights interaural differences according to arrival time. The precedence effect is an important example of this preferential weighting.

Auditory research has a rich history of combining experimental evidence with computational simulations of auditory processing in order to deepen our theoretical understanding of how sound is processed in the ears and in the brain. Despite significant progress in the amount of detail and breadth covered by auditory models, for many components of the auditory pathway there are still different model approaches that are often not equivalent but rather in conflict with each other. Similarly, some experimental studies yield conflicting results which has led to controversies. This can be best resolved by a systematic comparison of multiple experimental data sets and model approaches. Binaural processing is a prominent example of how the development of quantitative theories can advance our understanding of the phenomena, but there remain several unresolved questions for which competing model approaches exist. This article discusses a number of current unresolved or disputed issues in binaural modelling, as well as some of the significant challenges in comparing binaural models with each other and with the experimental data. We introduce an auditory model framework, which we believe can become a useful infrastructure for resolving some of the current controversies. It operates models over the same paradigms that are used experimentally. The core of the proposed framework is an interface that connects three components irrespective of their underlying programming language: The experiment software, an auditory pathway model, and task-dependent decision stages called artificial observers that provide the same output format as the test subject.

Previous headphone experiments have shown that listeners can lateralize high-frequency sine-wave amplitude-modulated (SAM) tones based on interaural time differences in the envelope. However, when SAM tones are presented to listeners in free field or in a room, diffraction by the head or reflections from room surfaces alter the modulation percentages and change the shapes of the envelopes, potentially degrading the envelope cue. Amplitude modulation is transformed into mixed modulation. This article presents a mathematical transformation between the six spectral parameters for a modulated tone and six mixed-modulation parameters for each ear. The transformation was used to characterize the stimuli in the ear canals of listeners in free-field localization experiments. The mixed modulation parameters were compared with the perceived changes in localization attributable to the modulation for five different listeners, who benefited from the modulation to different extents. It is concluded that individual differences in the response to added modulation were not systematically related to the physical modulation parameters themselves. Instead, they were likely caused by individual differences in processing of envelope interaural time differences.

Transaural synthesis using loudspeaker signals determined through contemporaneous ear canal calibration is proposed as an alternative to headphone presentation for critical psychoacoustical experiments. The proposed technique can afford greater accuracy, improved reproducibility, and continuous signal monitoring. It allows the experimenter to compare listener responses to real and virtual presentations. In this article, the advantages of transaural (three or four loudspeakers) compared to crosstalk cancellation (two loudspeakers) are shown through computer modeling and manikin measurements in a moderately reverberant room. Measurements employ binaurally challenging signals and speech from a distant source. Transaural synthesis is shown to be a better solution to the essential inverse problem resulting in reduced average synthesis amplitudes, fewer large-amplitude outliers, improved amplitude and phase accuracy for real and imagined sources, and improved noise immunity. Immunity to inadvertent listener head rotation depends sensitively on loudspeaker placement and is not an advantage in general. Appendixes review the relevant mathematical foundation and extend it to the relationship between ear canal signals and eardrum signals.

Forty high-resolution (3480 × 2160) video presentations were created in an attempt to replace in-person demonstrations for teaching elementary musical acoustics. Demonstrations were selected to accompany the author's textbook, Principles of Musical Acoustics (W. M. Hartmann, Springer, NY, 2013). Mean duration of the demonstrations was 4.1 min, though one was as long as 17 min while another lasted only 30 s. Demonstrations were devoted to physical principles of vibrations and waves (13), musical instruments and other sound making devices (20), and signal processing (3). Electronic instrumentation appeared somehow in almost all the demonstrations with (4) devoted specifically to instrumentation itself. Attempts were made to evaluate the effectiveness of the demonstrations through questions on the weekly quizzes. In any case, the demonstrations provided a welcome relief from a straightforward power-point lecture. A second form of relief presented some topics, especially mathematical topics, using pen and paper together with a document camera. Evaluations of the methods will be complete before the June meeting.

The roles of interaural time difference (ITD) and interaural level difference (ILD) were studied in free-field source localization experiments for sine tones of low frequency (250-750 Hz). Experiments combined real-source trials with virtual trials created through transaural synthesis based on real-time ear canal measurements. Experiments showed the following: (1) The naturally occurring ILD is physically large enough to exert an influence on sound localization well below 1000 Hz. (2) An ILD having the same sign as the ITD modestly enhances the perceived azimuth of tones for all values of the ITD, and it eliminates left-right confusions that otherwise occur when the interaural phase difference (IPD) passes 180°. (3) Increasing the ILD to large, implausible values can decrease the perceived laterality while also increasing front-back confusions. (4) Tone localization is more directly related to the ITD than to the IPD. (5) An ILD having a sign opposite to the ITD promotes a slipped-cycle ITD, sometimes with dramatic effects on localization. Because the role of the ITD itself is altered by the ILD, the duplex processing of ITD and ILD reflects more than mere trading; the effect of the ITD can be reversed in sign.

The R. Bruce Lindsay Award (formerly the Biennial Award) is presented in the Spring to a member of the Society who is under 35 years of age on 1 January of the year of the Award and who, during a period of two or more years immediately preceding the award, has been active in the affairs of the Society and has contributed substantially, through published papers, to the advancement of theoretical or applied acoustics, or both. The award was presented biennially until 1986. It is now an annual award.

The Woodworth model and formula for interaural time difference is frequently used as a standard in physiological and psychoacoustical studies of binaural hearing for humans and other animals. It is a frequency-independent, ray-tracing model of a rigid spherical head that is expected to agree with the high-frequency limit of an exact diffraction model. The predictions by the Woodworth model for antipodal ears and for incident plane waves are here compared with the predictions of the exact model as a function of frequency to quantify the discrepancy when the frequency is not high. In a second calculation, the Woodworth model is extended to arbitrary ear angles, both for plane-wave incidence and for finite point-source distance. The extended Woodworth model leads to different formulas in six different regions defined by ear angle and source distance. It is noted that the characteristic cusp in Woodworth's well-known function comes from ignoring the longer of the two paths around the head in circumstances when the longer path is actually important. This error can be readily corrected.

Noise with a sharp spectral edge produces a definite, perceived pitch. The pitch of lowpass noise lies slightly below the edge and the pitch of highpass noise slightly above it. The pitch shifts away from the edge, expressed in semitones, increase with the decreasing edge frequency. A neural timing model based on positive peaks of the autocorrelation function accounts for the broad features of the shifts. So, does a place model based on the peak of an excitation pattern as enhanced by lateral inhibition. As the edge frequency decreases below 150 Hz, the pitch persists for a lowpass noise but disappears for highpass noise. This result is consistent with the timing model but not with the place model. For high edge frequencies, the pitch is stronger for highpass noise than for lowpass noise—consistent with both timing and place models. As the edge frequency approaches 5000 Hz, the pitch disappears for most listeners, but for some listeners, a pitch persists for edge frequencies beyond 8000 Hz. The latter res...

A framework aimed at improving the testability and comparability of binaural models will be presented. The framework consists of two key elements: (1) a repository of testing software that evaluates the models against published data and (2) a model repository. While the framework is also intended for physiological data, the planned initial contribution will be psychoacoustical data together with their psychoacoustical testing protocols, as well as existing binaural models from available auditory toolboxes. Researchers will be invited to provide their established as well as newly developed models in whatever programming language they prefer, given the models are compatibility with the proposed interface to the testing software. This entails that the models act as artificial observers, testable with exactly the same procedure as the human subjects. A simple communication protocol based on wav and txt-files is proposed because these are supported by every programming environment, and are able connect models and testing software of any programming language. Examples will illustrate the principle of testing models with unaltered signal processing stages on various seminal data sets such as tone detection in so-called double-delayed masking noise, or lateralization of ¾-period delayed noise and sounds with temporally asymmetric envelopes.

After finishing his Ph.D. work with Estes at Stanford, David Wessel came to Michigan State as an assistant professor of psychology. Dave arrived with a strong background in mathematical psychology and with an intense passion for making music with computers. In the 1970s, Dave and I presented two electronic music concerts to standing-room-only audiences. The division of our labors was straightforward. I provided the venue and audio system. Dave provided all the music, which he was able to do because of his many connections with electronic musicians around the world. Dave also organized the first computer music conference, held in East Lansing during one of the worst snow storms of the year. This talk will also describe Dave's initial studies of musical timbre and his early years at IRCAM, as I observed them from East Lansing and in Paris.

There are three standard methods for generating two channels of partially correlated noise: the two-generator method, the three-generator method, and the symmetric-generator method. These methods allow an experimenter to specify a target cross correlation between the two channels, but actual generated noises show statistical variability around the target value. Numerical experiments were done to compare the variability for those methods as a function of the number of degrees of freedom. The results of the experiments quantify the stimulus uncertainty in diverse binaural psychoacoustical experiments: incoherence detection, perceived auditory source width, envelopment, noise localization∕lateralization, and the masking level difference. The numerical experiments found that when the elemental generators have unequal powers, the different methods all have similar variability. When the powers are constrained to be equal, the symmetric-generator method has much smaller variability than the other two.

Binaural recordings of noise in rooms were used to determine the relationship between binaural coherence and the effectiveness of the interaural time difference (ITD) as a cue for human sound localization. Experiments showed a strong, monotonic relationship between the coherence and a listener's ability to discriminate values of ITD. The relationship was found to be independent of other, widely varying acoustical properties of the rooms. However, the relationship varied dramatically with noise band center frequency. The ability to discriminate small ITD changes was greatest for a mid-frequency band. To achieve sensitivity comparable to mid-band, the binaural coherence had to be much larger at high frequency, where waveform ITD cues are imperceptible, and also at low frequency, where the binaural coherence in a room is necessarily large. Rivalry experiments with opposing interaural level differences (ILDs) found that the trading ratio between ITD and ILD increasingly favored the ILD as coherence decreased, suggesting that the perceptual weight of the ITD is decreased by increased reflections in rooms.

Transaural synthesis using loudspeakers is a powerful technique for conducting binaural hearing experiments while avoiding myriad problems caused by headphones. It is the most promising method to test binaural hearing for listeners using cochlear implants. Transaural synthesis is typically implemented using crosstalk cancellation, a technique whereby the left-channel signal and right-channel signal are adjusted so that reproduction by two synthesis loudspeakers results in near perfect transmission of the target left and right signals to the left and right ears (or processor microphones). Crosstalk cancellation has enabled precise experimental control over the stimulus delivered to each ear. Unfortunately, the 2×2 matrix inversion required for crosstalk cancellation can occasionally introduce spuriously large amplitudes for specific frequencies into the adjusted signals. These large amplitudes lead to perceptually salient tones. We demonstrate through simulation and experiment in a real room that adding a third loudspeaker and solving the resulting 2×3 problem using the Moore-Penrose pseudoinverse matrix yields dramatically fewer large amplitudes in the resulting adjusted waveforms. We have also conducted experiments to investigate the 2×3 system's robustness to inadvertent listener motions.Transaural synthesis using loudspeakers is a powerful technique for conducting binaural hearing experiments while avoiding myriad problems caused by headphones. It is the most promising method to test binaural hearing for listeners using cochlear implants. Transaural synthesis is typically implemented using crosstalk cancellation, a technique whereby the left-channel signal and right-channel signal are adjusted so that reproduction by two synthesis loudspeakers results in near perfect transmission of the target left and right signals to the left and right ears (or processor microphones). Crosstalk cancellation has enabled precise experimental control over the stimulus delivered to each ear. Unfortunately, the 2×2 matrix inversion required for crosstalk cancellation can occasionally introduce spuriously large amplitudes for specific frequencies into the adjusted signals. These large amplitudes lead to perceptually salient tones. We demonstrate through simulation and experiment in a real room that adding a ...

This paper reports the results of experiments performed in an effort to find a formulaic relationship between the interaural waveform coherence of a band of noise gamma(W) and the interaural envelope coherence of the noise band gamma(E). An interdependence described by gamma(E)=pi/4+(1-pi/4)(gamma(W))(2.1) is found. This relationship holds true both in a computer experiment and for binaural measurements made in two rooms using a KEMAR manikin. Room measurements are used to derive a measure of reliability for the formula. Ultimately, a user who knows the waveform coherence can predict the envelope coherence with a small degree of uncertainty.

In order to determine whether a sound source is in front or in back, listeners can use location-dependent spectral cues caused by diffraction from their anatomy. This capability was studied using a precise virtual reality technique (VRX) based on a transaural technology. Presented with a virtual baseline simulation accurate up to 16 kHz, listeners could not distinguish between the simulation and a real source. Experiments requiring listeners to discriminate between front and back locations were performed using controlled modifications of the baseline simulation to test hypotheses about the important spectral cues. The experiments concluded: (1) Front/back cues were not confined to any particular 1/3rd or 2/3rd octave frequency region. Often adequate cues were available in any of several disjoint frequency regions. (2) Spectral dips were more important than spectral peaks. (3) Neither monaural cues nor interaural spectral level difference cues were adequate. (4) Replacing baseline spectra by sharpened spectra had minimal effect on discrimination performance. (5) When presented with an interaural time difference less than 200 micros, which pulled the image to the side, listeners still successfully discriminated between front and back, suggesting that front/back discrimination is independent of azimuthal localization within certain limits.

Summary Reasons for performing study: Laryngoplasty is the technique of choice for treatment of laryngeal hemiplegia, with the aim of improving airway function and/or eliminating respiratory noise. However, there are no quantitative data in the literature describing the effect of laryngoplasty on upper airway noise or its relationship to upper airway mechanics in horses with laryngeal hemiplegia. Objectives: To determine whether laryngoplasty reduces respiratory noise in exercising horses with laryngeal hemiplegia; and to establish whether the degree of upper airway obstruction can be predicted by upper airway noise, or the degree of arytenoid abduction correlated with airway obstruction and noise production. Methods: Six Standardbred horses with normal upper airways during maximal exercise were used. Respiratory sounds and inspiratory transupper airway pressure (Pui) were measured in all horses before and after induction of laryngeal hemiplegia and 30, 60 and 90 days after laryngoplasty. Inspiratory sound level (SL) and the sound intensity of the 3 inspiratory formants (F1, F2 and F3, respectively) were measured using a computer-based sound analysis programme. The degree of abduction was graded by endoscopic visualisation 1, 30, 60 and 90 days post operatively. Linear regression analysis was used to determine correlations between Pui, sound indices and grades of arytenoid abduction. Results: In laryngeal hemiplegia-affected horses, Pui, inspiratory SL and the sound intensity of F1, F2 and F3 were significantly increased. At 30 days following laryngoplasty, the sound intensity of F1 and Pui returned to baseline values. The sound intensities of F2, F3 and SL were significantly improved from laryngeal hemiplegia values at 30 days post operatively, but did not return to baseline at any measurement period. Sound level, F2 and F3 were significantly correlated with Pui (P

When a periodic waveform with a discrete-harmonic spectrum is temporally windowed to make a signal, its spectrum becomes a continuous function of frequency. However, there are discrete-frequency representations for windowed signals such as the Fourier series representation of a periodically extended signal. This article introduces the concept of matching between the temporal window and the periodic waveform. Matching leads to a discrete-frequency representation in which the Fourier transform of the windowed signal preserves the amplitudes and phases of the waveform on the set of original waveform frequencies. Generating signals with matched window and waveform leads to important control of experiments.

Informational masking of a target female talker by female distracters was measured with target and distracters presented from directly in front of the listener as a baseline condition. Next, it was found that if the distracters were also presented from directly in back of the listener, advanced or delayed by a few milliseconds with respect to the distracters in front, release from informational masking occurred. Release from informational masking was found for all delays within the Haas region of +/-50 ms, with peak release of about 3.5 dB. This peak occurred for a delay of +/-2 ms and it was shown to be the result of delay-and-add filtering. Release from energetic masking was also found, but only for delays of +/-0.5 ms or less.

The Huggins pitch is a sensation of pitch generated from a broadband noise having a narrowband boundary region where the interaural phase difference varies rapidly as a function of frequency. Models of binaural hearing predict that the pitch image should be well lateralized. A direct psychophysical experimental method was used to estimate the lateral positions of Huggins pitch images with two different forms of phase boundaries, linear phase and stepped phase. A third experiment measured the lateral positions of sine tones with controlled interaural phase differences. The results showed that the lateralization of Huggins pitch stimuli was similar to that of the corresponding sine tones and that the lateralizations of the two forms of Huggins pitch phase boundaries were even more similar to one another. Both Huggins pitches and sine tones revealed strong laterality compression (exponent approximately 0.5). Ambiguous stimuli, with an interaural phase difference of 180 degrees , were consistently lateralized on one side or the other according to individual asymmetries-an effect called "earedness." An appendix to this article develops a new first-order lateralization model, the salient phase density model, which combines attributes of previous models of dichotic pitch lateralization.

The Distinguished Service Citation is awarded to a present or former member of the Society in recognition of outstanding service to the Society.

Acoustical interaural differences were computed for a succession of idealized shapes approximating the human head-related anatomy: sphere, ellipsoid, and ellipsoid with neck and torso. Calculations were done as a function of frequency (100–2500 Hz) and for source azimuths from 10 to 90 degrees using finite element models. The computations were compared to free-field measurements made with a manikin. Compared to a spherical head, the ellipsoid produced greater large-scale variation with frequency in both interaural time differences and interaural level differences, resulting in better agreement with the measurements. Adding a torso, represented either as a large plate or as a rectangular box below the neck, further improved the agreement by adding smaller-scale frequency variation. The comparisons permitted conjectures about the relationship between details of interaural differences and gross features of the human anatomy, such as the height of the head, and length of the neck.

Objective— To report the effect of unilateral laser vocal cordectomy on respiratory noise and airway function in horses with experimentally induced laryngeal hemiplegia (LH). Study Design— Experimental study. Animals— Six Standardbred horses without upper airway abnormities at rest or during high-speed treadmill exercise. Methods— Respiratory sounds and inspiratory trans-upper airway pressure (PUi) were measured before (baseline) and 14 days after induction of LH by left recurrent laryngeal neurectomy, and again 30, 60, 90, and 120 days after endoscopically assisted laser cordectomy of the left vocal cord. Data were collected with the horses exercising on a treadmill at a speed producing maximum heart rate (HRmax). Results— In horses exercising at HRmax, induction of LH caused a significant increase in PUi, sound level (SL), and the sound intensity of formant 2 (F2) and 3 (F3). The sound intensity of formant 1 (F1) was unaffected by induction of LH. Laser vocal cordectomy had no effect on SL, or on the sound intensity of F1 and F3. At 30, 60, 90, and 120 days after surgery, PUi and the sound intensity of F2 were significantly reduced, but these variables remained significantly different from baseline values. Conclusions— Unilateral laser vocal cordectomy did not effectively improve upper airway noise in horses with LH. The procedure decreased upper airway obstruction to the same degree as bilateral ventriculocordectomy. Clinical Relevance— Currently, laser vocal cordectomy cannot be recommended for the treatment of upper airway noise in horses with LH.

The interaural level difference (ILD) plays a significant role in sound localization. However, the definition of ILD for noise is open to some interpretation because it is not obvious how to deal with the inevitable level fluctuations. In this article, the ILD is interpreted as an energylike (time-integrated) measure of stimulus level, independent of other stimulus details-particularly interaural correlation. This concept is called the "level-meter model." The model was tested by measuring human ILD thresholds for noise stimuli that were interaurally correlated, or anticorrelated, or uncorrelated. An additional test (not involving lateralization) measured the threshold for level discrimination based on loudness. According to the level-meter model, all four thresholds should be the same. The experimental results showed that the predictions of the level-meter model held good to within about half a dB, although thresholds for level discrimination were systematically higher than ILDs. Among the ILDs themselves, thresholds were slightly higher for uncorrelated noise. The latter result could be explained by replacing the level-meter model with a loudness-meter model, incorporating temporal integration. The same model accounted for the bandwidth dependence of the threshold.

Listeners reported the perceived azimuths of 36 sine-tone sources in a room. For some trials, listener motion was forbidden and for others unstructured head and torso motion was encouraged with the listener remaining seated. Tone frequencies were 500, 1000, 2000, and 4000 Hz. Onsets and offsets were masked, leaving a steady-state duration of 7.6 s for exploration. It was found that the benefits of motion on source localization accuracy were clearly frequency dependent. Dynamic interaural cues were obtained from probe-microphone ear-canal recordings. Interaural cues were combined with head-tracker orientation data to predict perceived azimuths according to diverse hypotheses. Responses and interaural cues for trials without motion were used to predict a dynamic inferred location within the room for runs with motion and to identify stable inferences. Analysis of front/back confusions was conducted based on the regression between changes in interaural level differences and changes in head angle sampled at 76...

Difference limens for the interaural time difference (ITD) can be measured with a 2-interval, 2-alternative forced-choice staircase using the ITD as the staircase variable. Experimental results can be predicted by a computational model that simulates the experiment protocol in every important detail, as applied to human listeners, but the computer can tolerate hundreds of times more runs. At the core of the simulation is a decision process based on an opponency model for the two medial superior olives (MSO). MSO firing rates as functions of ITD are initially determined by a stochastically driven Hodgkin-Huxley cell model and represented in the simulation by a four-parameter fitted function. A corresponding noise function is estimated from multiple runs of the cell model. Left-right symmetry in both the model and the experiment protocol simplifies the calculations. Simulations have practical value in relating staircase thresholds to the underlying parameterized firing rate functions for given staircase var...

The technique of crosstalk cancellation uses two synthesis loudspeakers in an attempt to produce well separated signals in a listener's ear canals. A variation on the technique can be applied in precise psychoacoustical experiments by using probe tubes in the ear canals throughout the presentation. The probe-tube application exhibits remarkable immunity to amplitude and phase variations in the responses of all transducers because the same system is used for presentation and for initial calibration. The self-correcting nature of the method even confers immunity to variations in the depth of the probe tubes within the ear canals. Because the solution to the 2-ear-2-speaker problem involves the inverse of a 2x2 matrix, the solution can become unstable, leading to pathologically large amplitudes. This problem can be almost entirely alleviated by using more than two synthesis speakers, and solving the resulting underdetermined inverse problem through the Moore-Penrose pseudoinverse. Random perturbation calculations show that using three or four synthesis speakers also reduces the ear-canal amplitude and phase sensitivity to inadvertent listener movements. However, more realistic calculations for inadvertent head-rotations indicate an additional important role for synthesis speaker location. [Work supported by the AFOSR.]

Signal processing refers to the acquisition, storage, display, and generation of signals – also to the extraction of information from signals and the re-encoding of information. As such, signal processing in some form is an essential element in the practice of all aspects of acoustics. Signal processing algorithms enable acousticians to separate signals from noise, to perform automatic speech recognition, or to compress information for more efficient storage or transmission. Signal processing concepts are the building blocks used to construct models of speech and hearing. Now, in the 21st century, all signal processing is effectively digital signal processing. Widespread access to high-speed processing, massive memory, and inexpensive software make signal processing procedures of enormous sophistication and power available to anyone who wants to use them. Because advanced signal processing is now accessible to everybody, there is a need for primers that introduce basic mathematical concepts that underlie the digital algorithms. The present handbook chapter is intended to serve such a purpose.

Human listeners, and other animals too, use interaural time differences (ITD)to localize sounds. If the sounds are pure tones, a simple frequency factorrelates the ITD to the interaural phase difference (IPD), for which there areknown iso-IPD boundaries, 90$^circ$, 180$^circ$~ldots defining regions ofspatial perception. In this article, iso-IPD boundaries for humans aretranslated into azimuths using a spherical head model, and the calculationsare checked by free-field measurements. The translated boundaries providequantitative tests of an ecological interpretation for the dramatic onset ofITD insensitivity at high frequencies. According to this interpretation, theinsensitivity serves as a defense against misinformation and can be attributedto limits on binaural processing in the brainstem. Calculations show that theecological explanation passes the tests only if the binaural brainstemproperties evolved or developed consistent with heads that are 50% smallerthan current adult heads. Measurements on more realistic head shapes relaxthat requirement only slightly. The problem posed by the discrepancy betweenthe current head size and a smaller, ideal head size was apparently solved bythe evolution or development of central processes that discount large IPDs infavor of interaural level differences. The latter become more important withincreasing head size.

Objective—To record respiratory sounds in exercising horses and determine whether spectrum analysis could be use to identify sounds specific for laryngeal hemiplegia (LH) and dorsal displacement of the soft palate (DDSP). Animals—5 Standardbred horses. Procedure—Respiratory sounds were recorded and pharyngeal pressure and stride frequency were measured while horses exercised at speeds corresponding to maximum heart rate, before and after induction of LH and DDSP. Results—When airway function was normal, expiratory sounds predominated and lasted throughout exhalation. After induction of LH, expiratory sounds were unaffected; however, all horses produced inspiratory sounds characterized by 3 frequency bands centered at approximately 0.3, 1.6, and 3.8 kHz. After induction of DDSP, inspiratory sounds were unaffected, but a broad-frequency expiratory sound, characterized by rapid periodicity (rattling) was heard throughout expiration. This sound was not consistently detected in all horses. Conclusions and Clinical Relevance—The technique used to record respiratory sounds was well tolerated by the horses, easy, and inexpensive. Spectrum analysis of respiratory sounds from exercising horses after experimental induction of LH or DDSP revealed unique sound patterns. If other conditions causing airway obstruction are also associated with unique sound patterns, spectrum analysis of respiratory sounds may prove to be useful in the diagnosis of airway abnormalities in horses. (Am J Vet Res 2001;62:659–664)

The binaural coherence edge pitch (BICEP) is a dichotic broadband noise pitch effect similar to the binaural edge pitch (BEP). The BICEP stimulus is made by summing spectrally dense sine wave components with random phases. The interaural phase angle is a constant (0 or pi) for components with frequencies below (or above) a chosen edge frequency, and it is a random variable for the remaining components. The chosen edge frequency is a coherence edge because the noises to the two ears are mutually coherent within any band of frequencies on one side of the edge and they are mutually incoherent in any band on the other side. Pitch-matching experiments show that the BICEP exists for coherence edge frequencies between about 300 and 1000 Hz. It is matched by a pure-tone frequency that differs from the edge frequency by 5% to 10%. The matching frequency lies on the incoherent side of the edge, an important result that is consistent with the way that the equalization-cancellation model has been applied to binaural pitch effects, especially the BEP. The results of BICEP experiments depend upon whether the coherent components are presented in 0 or pi interaural phase for some listeners but not for all. The BICEP persists if the noise to one of the ears is delayed, but it becomes weaker and less well matched as the delay increases beyond 2 ms. The BICEP does not depend on whether the component amplitudes are all created equal or are given a Rayleigh distribution. Some reliable pitch sensation exists even when the component amplitudes are entirely independent in the two ears, so long as the phase coherence conditions of the BICEP stimulus are maintained. The existence of the BICEP is a challenge for current models of dichotic pitch because none of them predicts all its features.

For as long as we humans have lived on Earth, we have been able to use our ears to localize the sources of sounds. Our ability to localize warns us of danger and helps us sort out individual sounds from the usual cacophony of our acoustical world. Characterizing this ability in humans and other animals makes an intriguing physical, physiological, and psychological study (see figure 1).

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.

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.

This study reports a role of temporal regularity on the perception of auditory streams. Listeners were presented with two-tone sequences in an A-B-A-B rhythm that was either regular or had a controlled amount of temporal jitter added independently to each of the B tones. Subjects were asked to report whether they perceived one or two streams. The percentage of trials in which two streams were reported substantially and significantly increased with increasing amounts of temporal jitter. This suggests that temporal predictability may serve as a binding cue during auditory scene analysis.

Different models of the binaural system make different predictions for the just-detectable interaural time difference (ITD) for sine tones. To test these models, ITD thresholds were measured for human listeners focusing on high- and low-frequency regions. The measured thresholds exhibited a minimum between 700 and 1,000 Hz. As the frequency increased above 1,000 Hz, thresholds rose faster than exponentially. Although finite thresholds could be measured at 1,400 Hz, experiments did not converge at 1,450 Hz and higher. A centroid computation along the interaural delay axis, within the context of the Jeffress model, can successfully simulate the minimum and the high-frequency dependence. In the limit of medium-low frequencies (f), where f . ITD1, mathematical approximations predict low-­ frequency slopes for the centroid model and for a rate-code model. It was found that measured thresholds were approximately inversely proportional to the frequency (slope = –1) in agreement with a rate-code model. However, the centroid model is capable of a wide range of predictions (slopes from 0 to –2).

Sound is all around us. It warns us of danger, enables us to communicate with others, annoys us with its noise, entertains us by radio and iPods, and captivates us in music. It is an important part of the daily lives of hearing people.