Effect Of Signal Level Research Articles

Effects of Signal Level and Spectral Contrast on Vowel Formant Discrimination for Normal-Hearing and Hearing-Impaired Listeners

The aim of this study was to determine whether increasing the overall speech level or the individual spectral contrasts of vowel sounds can improve vowel formant discrimination for listeners both with and without normal hearing. Thresholds of vowel formant discrimination were examined for the F2 frequencies of 3 American English vowels for listeners with and without normal hearing. Spectral contrasts of the F2 were enhanced by 3, 6, and 9 dB. Vowel stimuli were presented at 70 and 90 dB SPL. The thresholds of listeners with hearing impairment were reduced significantly after spectral enhancement was implemented, especially at 90 dB SPL, whereas normal-hearing listeners did not benefit from spectral enhancement. These results indicate that a combination of spectral enhancement of F2 and high speech level is most beneficial to improve vowel formant discrimination for listeners with hearing impairment.

Effects of Signal Level and Background Noise on Spectral Representations in the Auditory Nerve of the Domestic Cat

Background noise poses a significant obstacle for auditory perception, especially among individuals with hearing loss. To better understand the physiological basis of this perceptual impediment, the present study evaluated the effects of background noise on the auditory nerve representation of head-related transfer functions (HRTFs). These complex spectral shapes describe the directional filtering effects of the head and torso. When a broadband sound passes through the outer ear en route to the tympanic membrane, the HRTF alters its spectrum in a manner that establishes the perceived location of the sound source. HRTF-shaped noise shares many of the acoustic features of human speech, while communicating biologically relevant localization cues that are generalized across mammalian species. Previous studies have used parametric manipulations of random spectral shapes to elucidate HRTF coding principles at various stages of the cat's auditory system. This study extended that body of work by examining the effects of sound level and background noise on the quality of spectral coding in the auditory nerve. When fibers were classified by their spontaneous rates, the coding properties of the more numerous low-threshold, high-spontaneous rate fibers were found to degrade at high presentation levels and in low signal-to-noise ratios. Because cats are known to maintain accurate directional hearing under these challenging listening conditions, behavioral performance may be disproportionally based on the enhanced dynamic range of the less common high-threshold, low-spontaneous rate fibers.

Psychometric functions for detection of tones following on- and off-frequency forward maskers

Recent data obtained in our laboratory suggest that the slope of the psychometric function in forward masking can vary widely across conditions, with shallower slopes occurring in conditions with higher signal levels at threshold. The goal of the present study was to separate possible effects of masker frequency, masker level, and signal delay from the effect of signal level per se. Thresholds and psychometric functions were obtained for three normal-hearing listeners using a 2IFC adaptive task. The signal was a 4-kHz, 10-ms tone. The on-frequency masker was a 200-ms tone presented at 30- to 90-dB SPL in 10-dB steps. The off-frequency masker was a 2.4-kHz, 200-ms tone presented at 60- to 90-dB SPL in 10-dB steps. Signal delays were 0, 10, or 30 ms. As expected, forward masking grew more rapidly with increases in masker level in the off-frequency than the on-frequency conditions. Signal delay had little independent effect on the slopes of the psychometric functions. The effects of masker level and signal level were similar for on- and off-frequency masker conditions in most cases. The results will be discussed in the context of a model that incorporates peripheral nonlinearity and internal noise. [Work supported by NIH.]

Discriminability of unresolved two‐component Complex tones: Effects of signal level and duration

In the study to be reported, the discriminability of unresolved two‐component complex (UTCC) tones has been investigated as a function of signal level and duration. These signals were composed of two sinusoidal components separated in frequency by a small ΔF (5, 10, 20, 50, and 100 Hz), and in amplitude by a small ΔI (0, 1, or 3 dB). The complement of each two‐component combination was a signal with the same component frequencies but opposite amplitude ratio. In experiment one, comparisons were made between pure‐tone frequency DL's and the discriminability of members of complementary pairs. In experiment two, discriminability of each member of an UTCC pair from a third UTCC signal having equal component amplitudes was investigated. Changes in the DL were examined as signal level and duration were independently manipulated. Signal durations ranged from 500 to 20 ms. Signal levels ranged from 50 to 10 dB HL. Preliminary analysis of the results of experiment one shows that the DL for pure tones and the discriminability of complementary‐UTCC tones co‐vary with changes in presentation level and duration. For experiment two, the preliminary analysis shows that members of complementary pairs of UTCC tones are more discriminable from one another than from an equal amplitude‐UTCC. Further analysis should show the asymmetries in discriminability evident in the pitch matching of the equal amplitude UTCC tone to each member of the complementary pair [Feth, O'Malley, and Ramsey, J. Acoust. Soc: Am. (in press)]. [Work supported by grants from NINCDS.]