Phantom Sound Sources Research Articles

Localization of real and tangent-law panned phantom sound sources in the frontal horizontal plane

Auditory source separations of as little as 1 degree are detectable. However, presenting auditory stimuli at small separations presents technical challenges, with loudspeaker separation limited by transducer diameter. An alternative procedure is to utilize phantom sources, with the perceived position of a single source determined by the relative output levels of two spatially separated loudspeakers. Therefore, it is important to determine whether real and phantom sources can be localized with the same precision. In the present experiment, listeners localized real (individual) sources and phantom sources computed using a tangent-law model giving the same nominal azimuthal angles as the real sources. Listeners used a laser pointer to indicate perceived source location. Infrared cameras detected pointer position with responses stored in terms of azimuth. Signals were broadband or narrowband (300-700 Hz and 3800–4200 Hz) noise, 100 or 500 ms in duration. Generally, phantom sources were localized with less precision than real sources, and high-frequency signals were localized with less precision than broadband or low-frequency signals, with no effect of duration. Results show that phantom sources are localized with sufficient accuracy and precision to be useful in assessing auditory spatial acuity, but they are not localized with the same precision as real sources.

Acoustic Measurements and Digital Image Processing Suggest a Link Between Sound Rituals and Sacred Sites in Northern Finland

In northern Finland, near the canyon lakes of Julma-Olkky, Somerjarvi, and Rotkojarvi, steep rock cliffs produce distinctive acoustic spaces. On these cliffs, prehistoric rock paintings (5200 to 1000 BC) as well as an ancient Sami offering site (circa 1100 to present) can be found. Ethnographic sources describe that the Sami used to sing and listen to echoes while making offerings there. This article presents the results of an archaeoacoustic research project that seeks to explore the role of sound in the development and use of these archaeological sites. The innovative set of methods includes multichannel impulse response recording, angle-of-arrival estimation of early reflections, spectrum analysis, digital image processing, and 3D laser scanning. On the basis of the analyses, it is concluded that the cliffs that have been painted or held as sacred are efficient sound reflectors. They create discrete echoes and, accordingly, phantom sound sources. Especially at the Varikallio cliff near Lake Somerjarvi, the sound appears to emanate directly from the painted figures. These results, together with previously unnoticed drumming figures in the Varikallio painting, provide a clue to the significance of the sound rituals at these sacred sites.

Acoustic measurements at the sacred sites in Finland

In Finland, near the canyon lakes of Julma-Ölkky, Somerjärvi and Taatsijärvi, steep rock cliffs produce distinctive acoustic spaces. On these cliffs, prehistoric rock paintings (5200—500 BC) as well as an ancient Sami offering site (cf. 1100—AD) can be found. Ethnographic sources describe that the Sami used to sing and listen to echoes while making offerings there. This paper presents the results of an archaeoacoustic project that seeks to explore the role of sound in the development and use of these archaeological sites. The applied methods include multichannel impulse response recording, angle of arrival estimation of early reflections, spectrum analysis, digital image processing, and 3D laser scanning. On the basis of the analyses, we have concluded that the cliffs that have been painted or held as sacred are efficient sound reflectors. They create discreet echoes and, accordingly, phantom sound sources. Especially at the Värikallio cliff, the sound appears to emanate directly from the painted figures. These results, together with previously unnoticed drumming figures in the Värikallio painting, provide a clue to the significance of the sound rituals at these sacred sites.

On the robustness of upper limits for circular auditory motion perception

We developed a new rendering system that consists of a image-source reflection model followed by a two-step strategy for modeling moving phantom sound sources (combining a physical propagation model and a spatialization method). This rendering system was used for simulations of pressure signals and acoustical localization cues, and for stimulus generation. We report three perceptual experiments measuring the upper limits, defined as auditory velocity thresholds beyond which listeners are no longer able to perceptually, resolve a smooth circular trajectory. These thresholds were measured in different reverberation conditions using white noise, band-limited white noise and band-limited white noise with a pure tone. Experiment 1 took place in a hemi-anechoic room and compared two conditions: dry and with simulated first-order reflections. In Experiment 2 we compare the thresholds measured in the hemi-anechoic room (dry and simulated second-order reflections) and in a reverberant room using two different loudspeaker configurations. Experiment 3 investigated the effect of audio source type in the dry condition at high velocities. No significant effects were observed among reverberation conditions, loudspeaker configurations and audio source type, suggesting that the upper limit is robust against reverberation.

Interactions between front-back confusion and visual capture in summing localization

Ambiguity in binaural timing and level information often causes front-back confusions in sound localization. This experiment investigated the extent to which front-back confusions are modulated by concurrent visual stimuli. 15-ms duration noise stimuli were presented over two loudspeakers positioned at ±45o in front or behind a listener with a delay between them of −1—1 ms in steps of 0.5 ms, thus evoking summing localization. These sound stimuli were presented with or without light-flash stimuli at three different frontal locations. In comparison to audio only trials, when visual stimuli were also presented front-back confusion decreased for sound stimuli presented in front and increased for rear-presented auditory stimuli. This effect was greater for phantom sound sources than single speaker controls. These results suggest that visual cues influence the perceived location of a sound source when it is outside the field of vision. Further, the phantom sound sources presented using “summing localization” may offer less robust localization cues than physical sound sources, resulting in greater front-back confusion and stronger visual capture. Acoustic analyses of binaural and spectral cues are conducted to identify potential causes of the differences in FBC rates between single speaker and phantom sound sources.

Reproduction of Phantom Sources Improves with Separation of Direct and Reflected Sounds

AbstractIn virtual acoustics or artificial reverberation, impulse responses can be split so that direct and reflected components of the sound field are reproduced via separate loudspeakers. The authors had investigated the perceptual effect of angular separation of those components in commonly used 5.0 and 7.0 multichannel systems, with one and three sound sources respectively (Kleczkowski et al., 2015, J. Audio Eng. Soc. 63, 428-443). In that work, each of the front channels of the 7.0 system was fed with only one sound source. In this work a similar experiment is reported, but with phantom sound sources between the front loud- speakers. The perceptual advantage of separation was found to be more consistent than in the condition of discrete sound sources. The results were analysed both for pooled listeners and in three groups, according to experience. The advantage of separation was the highest in the group of experienced listeners.

Phantom Materialization: A Novel Method to Enhance Stereo Audio Reproduction on Headphones

Loudspeaker reproduction systems are subject to a compromise between spatial realism and cost. By simulating loudspeaker reproduction on headphones, the resulting spatial realism is limited accordingly, despite the virtually unlimited spatial imaging capabilities of binaural audio rendering technology. More particularly, phantom imaging as often used for stereo audio material intended for loudspeaker reproduction is subject to various restrictions in terms of loudspeaker positioning in simulated space. As a consequence, phantom imaging should preferably be avoided when simulating virtual loudspeakers over headphones, especially if head tracking is incorporated or if a wide sound stage is desired. A novel method is described to extract phantom sound sources from stereo audio content and convert these to sound sources in a virtual listening environment.

Reduced order modeling of head related transfer functions for virtual acoustic displays

The purpose of this work is to improve the computational efficiency in acoustic virtual applications by creating and testing reduced order models of the head related transfer functions used in localizing sound sources. State space models of varying order were generated from zero-elevation Head Related Impulse Responses (HRIRs) using Kungs Single Value Decomposition (SVD) technique. The inputs to the models are the desired azimuths of the virtual sound sources (from minus 90 deg to plus 90 deg, in 10 deg increments) and the outputs are the left and right ear impulse responses. Trials were conducted in an anechoic chamber in which subjects were exposed to real sounds that were emitted by individual speakers across a numbered speaker array, phantom sources generated from the original HRIRs, and phantom sound sources generated with the different reduced order state space models. The error in the perceived direction of the phantom sources generated from the reduced order models was compared to errors in localization using the original HRIRs.

Cellular mechanisms for resolving phase ambiguity in the owl's inferior colliculus.

Both mammals and birds use the interaural time difference (ITD) for localization of sound in the horizontal plane. They may localize either real or phantom sound sources, when the signal consists of a narrow frequency band. This ambiguity does not occur with broadband signals. A plot of impulse rates or amplitude of excitatory postsynaptic potentials against ITDs (ITD curve) consists of peaks and troughs. In the external nucleus (ICX) of the owl's inferior colliculus, ITD curves show multiple peaks when the signal is narrow-band, such as tones. Of these peaks, one occurs at ITDi, which is independent of frequency, and others at ITDi +/- T, where T is the tonal period. The ITD curve of the same neuron shows a large peak (main peak) at ITDi and no or small peaks (side peaks) at ITDi +/- T, when the signal is broadband. ITD curves for postsynaptic potentials indicate that ICX neurons integrate the results of binaural cross-correlation in different frequency bands. However, the difference between the main and side peaks is small. ICX neurons further enhance this difference in the process of converting membrane potentials to impulse rates. Inhibition also appears to augment the difference between the main and side peaks.

Effects of Interaural Decorrelation on Neural and Behavioral Detection of Spatial Cues

The detection of interaural time differences (ITDs) for sound localization critically depends on the similarity between the left and right ear signals (interaural correlation). We show that, like humans, owls can localize phantom sound sources well until the correlation declines to a very low value, below which their performance rapidly deteriorates. Decreasing interaural correlation also causes the response of the owl’s tectal auditory neurons to decline nonlinearly, with a rapid drop followed by a more gradual reduction. A detection-theoretic analysis of the statistical properties of neuronal responses could account for the variance of behavioral responses as interaural correlation is decreased. Finally, cross-correlation analysis suggests that low interaural correlations cause misalignment of cross-correlation peaks across different frequencies, contributing heavily to the nonlinear decline in neural and ultimately behavioral performance.

Sound reproduction system having sonic image localization networks

In a sound reproduction system having two loudspeakers (1,1b) located in front of a listener, an input audio signal is divided into a plurality of channel signals which are respectively applied through voltage adjusting devices (3) to the front loudspeakers and also to additional speakers (1c, 1d) or localization networks (15) to generate additional real or phantom sound sources at ±90 degrees to the normal to the listener. The additional sound sources and the front loudspeakers are located symmetrically with respect to the listener. By manual operation of the adjusting devices a sonic image can be localized at any point in the front half plane. Continual operation of the adjusting devices causes the localized sonic image to be moved continuously within the localizable area.

Head-Related Two-Channel Stereophony with Loudspeaker Reproduction

Reproducing stereophonic signal pairs with two loudspeakers on the front left and right of a listener yields acoustical crosstalk resulting from sound diffraction at the head. Stereophonic phantom sound sources are therefore heard only between the two loudspeakers. However, signal pairs can be constructed from the original signals generating extremely lateral phantom sources. A filter for this purpose is constructed using concentrated elements (“90° filter”). The complex frequency response of the 90° filter is determined in a listening experiment with narrow-band noise. Two 90° filters are combined for a two-channel reproduction system. Signals received by a dummy head with a microphone in each ear are fed to the inputs of the 90° filters. This transmission system was tested for its directional fidelity with many test persons. Localization is nearly ideal for the entire horizontal plane including front-back discrimination and is partly correct even for the median plane.