William L. Martens

Range dependence of the response of a spherical head model

The head-related transfer function (HRTF) varies with range as well as with azimuth and elevation. To better understand its close-range behavior, a theoretical and experimental investigation of the HRTF for an ideal rigid sphere was performed. An algorithm was developed for computing the variation in sound pressure at the surface of the sphere as a function of direction and range to the sound source. The impulse response was also measured experimentally. The results may be summarized as follows. First, the experimental measurements were in close agreement with the theoretical solution. Second, the variation of low-frequency interaural level difference with range is significant for ranges smaller than about five times the sphere radius. Third, the impulse response reveals the source of the ripples observed in the magnitude response, and provides direct evidence that the interaural time difference is not a strong function of range. Fourth, the time delay is well approximated by well-known ray-tracing formul...

The effects of identification training on the identification and production of American English vowels by native speakers of Japanese

The effectiveness of a high variability identification training procedure to improve native Japanese identification and production of the American English (AE) mid and low vowels /ae/, / /, / /, / /, / / was investigated. Vowel identification and production performance for two groups of Japanese participants was measured before and after a 6-week identification training period. Recordings were made of both groups pre-/posttraining vowel productions of the five vowels, which were evaluated by a group of native AE listeners using a five-alternative, forced-choice identification task and by an acoustic analysis of the vowel productions. The overall results confirmed that the identification performance of the experimental (trained) participants improved after identification training with feedback and that the training also had a positive effect on their production of the target AE vowels.

Perceptual evaluation of tone mapping operators

High dynamic range (HDR) photography and physically based rendering produce images with full range of luminance data. The problem of mapping real world luminance into the limited luminance range of display devices has been addressed in the last decade through the development of many different tone mapping algorithms (see [Devlin et al. 2002] for a recent survey). Clearly, a sound methodology for comparing existing algorithms is needed to understand their strengths and weaknesses; this work takes a first exploratory step towards achieving this goal. We performed a series of psychophysical experiments in which human subjects assessed their perceptions associated with a set of 24 images, constructed by submitting four different scenes (both synthetic and photographic) to six popular tone mapping operators: Photographic Tone Reproduction (Reinhard et al. 2002), Uniform Scaling Quantization (Schlick 1994), Retinex (Frankle et al. 1983), Visual Adjustment (Ferwerda 1996), Revised Tumblin-Rushmeier (1993), and Histogram Adjustment (Ward et al. 1997; the human contrast sensitivity option was switched off). In this initial attempt, we chose an exploratory rather than confirmatory approach in which subjects first made global judgments regarding how perceptually similar or dissimilar the images were, without specifying the ways in which they might differ from one another. Global dissimilarity judgments were made for all pairwise comparisons of the six tone mapping operators separately for each of the four scenes by each of 11 subjects, and these data were submitted to INdividual DifferencesSCALing (INDSCAL) analysis. The primaryINDSCAL result of interest is a derivedStimulus Spacein which each stimulus is assigned coordinates on the dimensions describing the perceptual differences between the stimuli (see [Borg and Groenen 1997] for background on this analysis method). The spatial configuration of the six tone mappers on the two most salient dimensions is shown in Figure 1.

Range-dependence of the HRTF for a spherical head

This paper examines the range dependence of the head-related transfer function (HRTF) for a simple spherical model of the head in both the time-domain and the frequency domain. The variation of low-frequency interaural level difference (ILD) with range is shown to be significant for ranges smaller than five times the sphere radius. The impulse response explains the source of the ripples in the frequency response, and provides direct evidence that the interaural time delay (ITD) is not a strong function of range. Time-delay measurements confirm the Woodworth/Schlosberg formula. Numerical analysis indicates that the HRTF is minimum phase. Thus, except for the time delay, the impulse response can be reconstructed from a simple principle components analysis of the magnitude response.

Robotic spatial sound localization and its 3D sound human interface

We describe a robotic spatial sound localization system using an auditory interface with four microphones arranged on the surface of a spherical robot head. The time difference and intensity difference from a sound source to different microphones are analyzed by measuring HRTFs around the spherical head in an anechoic chamber. It was found while the time difference can be approximated by a simple equation, the intensity difference is more complicated for different azimuth, elevation and frequency. A time difference based sound localization method was proposed and tested by experiments. A sound interface for human listeners is also constructed by four loudspeakers with a similar arrangement to the microphone set. This interface can be used as a 3D sound human interface by passing the four channel audio signals from the microphone set directly to the loudspeakers. It can also be used to create 3D sound with arbitrary spatial position which is determined by a virtual sound image or the sound localization system.

Simulation of sparkling and depth effect in paints

This paper reports on our attempts to simulate light reflection from surfaces that exhibit sparkling and depth effects that are associated with paint coatings containing metallic flakes. The novelty of the approach is to explicitly model the sparkle geometry for rendering the surface of a graphic object. The light scattering within the system of metal flakes or particles creates the sparkling and glare effects with radial streaks of light around high intensity particles. The 3D geometry of the simulated flakes creates a view-dependent reflectance pattern that makes the surface appear differently in the two images rendered for each eyes view in a stereoscopic display. The results of 3D geometry-based rendering are then compared to the surfaces rendered using 2D random dot patterns that provide no cues to depth variation at the surface. Stereoscopic display of 3D objects with and without the 3D geometry-based surface rendering was used to validate the difference in perceived depth effects associated with the two cases. To confirm the applicability of the technique, we adopted a standard test in common use by paint designers in which the appearance of paints with different sparkle density is observed on silver plates. Our results showed typical variation in sparkling on plates with different statistical distributions of sparkles, which confirmed the robustness of the 3D sparkle modeling system. In a final application test, the technique was used to simulate the appearance of an expensive variety of Japanese lacquerware made using the nashiji technique.

Variation in Oral-Binaural Room Impulse Responses for Horizontal Rotations of a Head and Torso Simulator

Oral-binaural room impulse responses (OBRIRs) describe the room acoustical response from the mouth to the ears of a head or dummy head. In this study, we measured OBRIRs in ten rooms, ranging from small to large. In each room, a head and torso simulator (HATS) was rotated at 2 degree increments to sample the room response at the selected measurement position. In rotating the HATS, the radiation pattern of the mouth rotates with the reception pattern of ears. This paper characterises the variation in room gain and interaural response of the tested rooms, and in doing so, we consider how OBRIRs can be usefully understood in terms of acoustical parameters.

Evaluation of stage acoustics preference for a singer using oral-binaural room impulse responses

There are two objective measurement methods in current practice that can be used to evaluate the stage acoustic conditions for singers. One is the stage support metrics (STEarly and STLate, included in the standard ISO 3389-1), and the other is the voice support metrics proposed by Brunskog et. al. and revised by Pelegrin-Garcia (room gain (GRG) and voice support (STV)). All of these metrics use energy integration from impulse responses to derive the acoustic descriptors. This overlooks two potentially important features of the responses: the temporal distribution of the impulse response within the evaluation period, and the directional distribution for the spatial impulse response within the evaluation period. In this paper, a method to study the effect of these features is proposed and tested. This method is based on the auralisation of ones′ own voice in rooms using oral-binaural room impulse responses (OBRIRs). The OBRIRs used are created by combining synthesized early reflections with a recorded reverberant tail. Results of a pilot study indicate that a wide range of on-stage acoustic quality ratings can be observed for stimuli with a similar STEarly value due to variation in the temporal and spatial distribution of reflected energy.

Equal Reverberance Contours for Synthetic Room Impulse Responses Listened to Directly: Evaluation of Reverberance in Terms of Loudness Decay Parameters

This paper examines effects of listening level and reverberation time on the perceived decay rate of synthetic room impulse responses (RIRs). A listening test was conducted with synthetic RIRs having a range of listening levels and reverberation times: in the test, subjects adjusted a physical decay rate of the RIRs to match the perceived decay rate of reference stimuli. In this way, we constructed equal reverberance contours as a function of sound pressure level and reverberation time. The experiment results confirm that listening level and reverberation time both significantly affect reverberance. The study also supports our previous finding: that the loudness decay function can be used to predict reverberance better than the conventional reverberance predictors.

Predicting perceived sharpness of broadband noise from multiple moments of the specific loudness distribution

Equal sharpness contours for broadband noise were generated through matches made between a standard reference noise and comparison noise stimuli varying in spectral envelope. Based upon the results of the sharpness matching task, a set of 20 percussive broadband noise stimuli was constructed and presented to three subjects in a sharpness rating experiment using white noise as a standard stimulus. Predicting obtained sharpness ratings for these percussive broadband noise stimuli from first and second moments of the stimulus specific loudness distribution was more successful than conventional prediction based only upon the weighted first moment.