Binaural unmasking in infants

Localization responses to a broadband noise signal presented against a broadband noise masker were obtained from 12-month-old infants and adults. Two loudspeakers, one to the left and one to the right of the listener, continuously presented identical broadband maskers. On a trial, a broadband signal was added to one of the loudspeakers. Subjects were required to identify the loudspeaker producing the signal. Noise signals were either coherent (from the same noise generator) or incoherent (from an independent noise generator). Both infants and adults found it easier to locate the incoherent signals even when the two types of signals were adjusted to produce equal increments in power. Since monaural performance, after this adjustment, should be equivalent for the two cases, superior performance for incoherent signals implies that binaural processing is involved. The same result was observed in control experiments in which coherent and incoherent signals were presented over earphones to adults. These results suggest that the mechanisms responsible for binaural unmasking are operative by 12 months of age.     

Binaural speech unmasking and localization in noise with bilateral cochlear implants using envelope and fine-timing based strategies

Four adult bilateral cochlear implant users, with good open-set sentence recognition, were tested with three different sound coding strategies for binaural speech unmasking and their ability to localize 100 and 500 Hz click trains in noise. Two of the strategies tested were envelope-based strategies that are clinically widely used. The third was a research strategy that additionally preserved fine-timing cues at low frequencies. Speech reception thresholds were determined in diotic noise for diotic and interaurally time-delayed speech using direct audio input to a bilateral research processor. Localization in noise was assessed in the free field. Overall results, for both speech and localization tests, were similar with all three strategies. None provided a binaural speech unmasking advantage due to the application of 700 micros interaural time delay to the speech signal, and localization results showed similar response patterns across strategies that were well accounted for by the use of broadband interaural level cues. The data from both experiments combined indicate that, in contrast to normal hearing, timing cues available from natural head-width delays do not offer binaural advantages with present methods of electrical stimulation, even when fine-timing cues are explicitly coded.     

Radioacoustic sounding method using an amplitude-modulated radio signal

Article submitted by author on diskette.     

Binaural unmasking with multiple adjacent masking electrodes in bilateral cochlear implant users

Bilateral cochlear implant (BiCI) users gain an advantage in noisy situations from a second implant, but their bilateral performance falls short of normal hearing listeners. Channel interactions due to overlapping electrical fields between electrodes can impair speech perception, but its role in limiting binaural hearing performance has not been well characterized. To address the issue, binaural masking level differences (BMLD) for a 125 Hz tone in narrowband noise were measured using a pair of pitch-matched electrodes while simultaneously presenting the same masking noise to adjacent electrodes, representing a more realistic stimulation condition compared to prior studies that used only a single electrode pair. For five subjects, BMLDs averaged 8.9 ± 1.0 dB (mean ± s.e.) in single electrode pairs but dropped to 2.1 ± 0.4 dB when presenting noise on adjacent masking electrodes, demonstrating a negative impact of the additional maskers. Removing the masking noise from only the pitch-matched electrode pair not only lowered thresholds but also resulted in smaller BMLDs. The degree of channel interaction estimated from auditory nerve evoked potentials in three subjects was significantly and negatively correlated with BMLD. The data suggest that if the amount of channel interactions can be reduced, BiCI users may experience some performance improvements related to binaural hearing.     

Observation of spin-wave envelope solitons in periodic magnetic film structures

The formation and propagation of light and dark microwave spin-wave envelope solitons in a periodic magnetic film structure have been observed. The periodic structure was manufactured on the basis of the single-crystal film of iron-yttrium garnet and a lattice of copper strips placed on the surface of the film perpendicularly to the propagation direction of carrying spin waves. The solitons are generated at frequencies corresponding to the band gap in the spectrum of the spin waves of the periodic structure, which is due to the first Bragg resonance.     

Discrimination of interaural envelope correlation and its relation to binaural unmasking at high frequencies.

Listeners' sensitivity to interaural correlation of the envelope of high-frequency waveforms and whether such sensitivity might account for detectability in a masking-level difference paradigm were assessed. Thresholds of interaural envelope decorrelation (from a reference correlation of 1.0) were measured for bands of noise centered at 4 kHz and bandwidths ranging from 50-1600 Hz. Decorrelation of the envelope was achieved by "mixing" two independent narrow-band noises. Separately, with the same listeners, NoSo and NoS pi detection thresholds were measured for maskers of the same center frequency and bandwidths. For bandwidths of noise up to about 400 Hz, listeners were similarly sensitive to interaural decorrelation in both types of task. However, for bandwidths greater than 400 Hz or so, while sensitivity in the discrimination task was unaffected, sensitivity was reduced in the NoS pi conditions. Additional data suggested that listeners were able to maintain their sensitivity independent of bandwidth in the discrimination task by focusing on binaural information within select spectral regions of the stimuli.     

Estimating the spectrum of a component of a digital signal containing an intense interference

A spectral analysis technique based on approximating a set of counts by a first-degree trigonometric polynomial with a varying frequency of its harmonics. With this technique, the parameters of a signal component that is two orders of magnitude weaker than an interference are estimated at a frequency ratio of 0.82.     

Stochastic resonance in an asymmetric bistable system with coloured noises and periodic rectangular signal

In this paper, we consider the phenomenon of stochastic resonance (SR) in an asymmetric bistable system with coloured noises and periodic rectangular signal. Expression of the signal-to-noise ratio (SNR) has been obtained under the adiabatic limit. We investigate the effect of any system parameter (such as p, q, r, τ₁, τ₂) on the SNR. The plot of SNR-τ₁ shows SR for some values of the additive noise self-correlation time τ₂, but not for the whole range of τ₂. The system bias r suppresses the SNR. When the intensity of additive noise q is increased, the SR phenomenon disappears in the plot of SNR-p, but the plot of SNR-q presents SR for almost all values of the multiplicative noise intensity p.     

Stochastic resonance induced by bounded noise and periodic signal in an asymmetric bistable system

Stochastic resonance (SR) induced by the bounded noise in an asymmetric bistable system is investigated. Based on stochastic simulation, the signal power amplification SPA$SPA$ is derived for the case of the additive modulated signal. The simulation results indicate that: (1) the SR phenomenon is weakened by the asymmetry of the system; (2) increasing the frequency ω$\omega$ restricts the SR of the system; (3) there is an optimum value of amplitude A$A$ matching the strongest SR phenomenon.     

Application of fast amplitude-modulated pulse trains for signal enhancement in static and magic-angle-spinning 47,49Ti-NMR spectra

It is demonstrated that the use of fast amplitude-modulated RF pulse trains with constant (FAM-I) and incremented pulse durations (SW-FAM) leads to considerable sensitivity enhancement for the central-transition signal (via spin population transfer from the satellite transitions) for solid-state NMR spectra of titanium, 47Ti (I = [Formula: see text] and 49Ti (I = [Formula: see text]. For the magic-angle spinning spectra of TiO2 and BaTiO3, the intensity of the 49Ti central-transition line was more than doubled compared to simple Hahn-echo acquisition, while for the static case, enhancement factors of 1.6 (TiO2) and 1.8 (BaTiO3) were obtained. No lineshape distortions are observed in either MAS or static spectra of both compounds. Employment of the FAM and SW-FAM sequences should be useful in the routine acquisition of 47,49Ti spectra, as the NMR signal can be detected much faster.     

Second harmonic generation: the solution for an amplitude-modulated initial pulse

We address the initial value problem for one-dimensional second harmonic generation starting from a purely amplitude-modulated fundamental wave. A general method to solve the problem in terms of a Schrödinger equation is presented, in which the initial pulse-shape is taken as a potential. Several examples with the complete solution given in analytical form are discussed. A much broader class of solutions can be found with the help of a single numerical integration. In particular, solutions with incident pulses approximating a sech -shape have been obtained.     

Hypersensitivity of a nonlinear system with multiplicative colored noise to an external periodic signal

A simple nonlinear stochastic system, an overdamped Kramers oscillator with multiplicative colored noise, is studied analytically and by numerical simulation. It is shown that in the region where on-off intermittency occurs, the system becomes hypersensitive to weak external periodic signals. Zh. éksp. Teor. Fiz. 116, 1484–1498 (October 1999)     

Effects of signal envelope on the pitch of short complex tones

Envelope-induced pitch shifts were measured for exponentially decaying complex tones consisting of two sinusoidal components with frequencies f1 = nf0 + 50 Hz and f2 = (n + 1) f0 + 50 Hz, where n equals 3, 4, or 5 and exponential decay rates were 0, 0.5, 1, and 2 dB/ms. Four subjects adjusted a sinusoidal comparison tone to match the virtual pitch of the (missing) fundamental and the pitches of the lower and upper partials f1 and f2. Pitch shifts for f1 are generally less, and pitch shifts for f2 always greater, than envelope-induced shifts observed in isolated sinusoidal tones of comparable frequency and envelope decay rate. Pitch-shift functions for virtual pitch are similar in magnitude and shape to average pitch-shift functions of the partials, which supports the idea that virtual pitch depends on spectral pitch.     

Effects of signal envelope on the pitch of short sinusoidal tones

The pitch of short sinusoidal tones with exponentially rising or decaying envelopes is judged higher than the pitch of a gated tone of the same frequency, duration, and energy. The upward pitch shift depends on the rise or decay rate, the intensity, and the frequency. The effect, which requires a nonlinearity in the auditory system, cannot be adequately explained by existing models of hearing. Control experiments on pitch matching for short tones of varying duration and varying intensity are described. These suggest that envelope-induced pitch effects are linked to changes in average intensity, so that they are essentially the same as intensity-induced pitch changes. A model based on these considerations is proposed.     

Binaural interference in the free field

In an anechoic chamber the minimum audible angle (MAA) was measured in seven normal-hearing adults for a narrow band of noise centered at 4000 Hz (target). In the absence of an interfering stimulus, the average MAA was 2.1°. When a low-frequency interferer (a narrow band of noise centered at 500 Hz) was pulsed on and off with the target from directly in front of the subject, the average MAA was significantly elevated (13.4°). However, if the interferer was continuously present, or if it consisted of two independent noises presented from ±90°, interference was much reduced. The interference effect was asymmetric: a high-frequency interferer did not result in elevation of MAA threshold for a low-frequency target. These results are similar to those that have been extensively reported for stimuli under headphones [Bernstein and Trahiotis (1995). J. Acoust. Soc. Am. 98, 155-163]. These data are consistent with the notion that interference from a spectrally remote low-frequency interferer occurs in the free field to the extent that the target and interferer are fused into a single perceptual object. If cues are provided that promote perceptual segregation (such as temporal onset differences or spatial location differences), the interference is reduced or eliminated.     

Improvement in underwater phase measurement of an amplitude-modulated laser beam by polarimetric techniques

The phase of the amplitude-modulated radiation reflected by a Lambertian target immersed in water was measured by using a linearly and circularly polarized sounding laser beam. Different values of the water extinction coefficient in the range of 0.06 - 2 m(-1) were realized by adding skim milk as a scattering element. It is shown that very efficient rejection of optical noise, resulting in reliable phase measurements, is accomplished with a cross-polarized and copolarized detection scheme for linear and circular polarization, respectively. The experiment demonstrates that phase measurements are very sensitive to optical noise suppression and that, as far as single scattering is the main involved mechanism, significant improvements can be achieved by adopting a polarization control on both the transmitter and the receiver stages of the apparatus.     

Discrimination of amplitude-modulated synthetic echo trains by an echolocating bottlenose dolphin

Bottlenose dolphins (Tursiops truncatus) have an acute ability to use target echoes to judge attributes such as size, shape, and material composition. Most target recognition studies have focused on features associated with individual echoes as opposed to information conveyed across echo sequences (feature envelope of the multi-echo train). One feature of aspect-dependent targets is an amplitude modulation (AM) across the return echoes in the echo train created by relative movement of the target and dolphin. The current study examined whether dolphins could discriminate targets with different AM envelopes. "Electronic echoes" triggered by a dolphin's outgoing echolocation clicks were manipulated to create sinusoidal envelopes with varying AM rate and depth. Echo trains were equated for energy, requiring the dolphin to extract and retain information from multiple echoes in order to detect and report the presence of AM. The dolphin discriminated amplitude-modulated echo trains from those that were not modulated. AM depth thresholds were approximately 0.8 dB, similar to other published amplitude limens. Decreasing the rate of modulation from approximately 16 to 2 cycles per second did not affect the dolphin's AM depth sensitivity. The results support multiple-echo processing in bottlenose dolphin echolocation. This capability provides additional theoretical justification for exploring synthetic aperture sonar concepts in models of animal echolocation that potentially support theories postulating formation of images as an ultimate means for target identification.     

Tight focus of an azimuthally polarized and amplitude-modulated annular multi-Gaussian beam

We investigate the focusing properties of an azimuthally polarized and amplitude modulated annular multi Gaussian beam by a high numerical aperture (NA) lens based on vector diffraction theory. We observe that our proposed system generates a subwavelength focal hole of 0.46λ having large uniform focal depth of 36λ without any annular obstruction. This kind of nondiffracting focal hole is called dark channel, which may have applications in atom optical experiments, such as with atomic lenses, atom traps, and atom switches.