Effects of relative and absolute frequency in the spectral weighting of loudness

Enhanced salience of edge frequencies in auditory pattern recognition

Within musical scenes or textures, sounds from certain instruments capture attention more prominently than others, hinting at biases in the perception of multisource mixtures. Besides musical factors, these effects might be related to frequency biases in auditory perception. Using an auditory pattern-recognition task, we studied the existence of such frequency biases. Mixtures of pure tone melodies were presented in six frequency bands. Listeners were instructed to assess whether the target melody was part of the mixture or not, with the target melody presented either before or after the mixture. In Experiment 1, the mixture always contained melodies in five out of the six bands. In Experiment 2, the mixture contained three bands that stemmed from the lower or the higher part of the range. As expected, Experiments 1 and 2 both highlighted strong effects of presentation order, with higher accuracies for the target presented before the mixture. Notably, Experiment 1 showed that edge frequencies yielded superior accuracies compared with center frequencies. Experiment 2 corroborated this finding by yielding enhanced accuracies for edge frequencies irrespective of the absolute frequency region. Our results highlight the salience of sound elements located at spectral edges within complex musical scenes. Overall, this implies that neither the high voice superiority effect nor the insensitivity to bass instruments observed by previous research can be explained by absolute frequency biases in auditory perception.

Cross-frequency weights in normal and impaired hearing: Stimulus factors, stimulus dimensions, and associations with speech recognition

Previous studies of level discrimination reported that listeners with high-frequency sensorineural hearing loss (SNHL) place greater weight on high frequencies than normal-hearing (NH) listeners. It is not clear whether these results are influenced by stimulus factors (e.g., group differences in presentation levels, cross-frequency discriminability of level differences used to measure weights) and whether such weights generalize to other tasks. Here, NH and SNHL weights were measured for level, duration, and frequency discrimination of two-tone complexes after measuring discriminability just-noticeable differences for each frequency and stimulus dimension. Stimuli were presented at equal sensation level (SL) or equal sound pressure level (SPL). Results showed that weights could change depending on which frequency contained the more discriminable level difference with uncontrolled cross-frequency discriminability. When cross-frequency discriminability was controlled, weights were consistent for level and duration discrimination, but not for frequency discrimination. Comparing equal SL and equal SPL weights indicated greater weight on the higher-level tone for level and duration discrimination. Weights were unrelated to improvements in recognition of low-pass-filtered speech with increasing cutoff frequency. These results suggest that cross-frequency weights and NH and SNHL weighting differences are influenced by stimulus factors and may not generalize to the use of speech cues in specific frequency regions.

Uni- and bilateral spectral loudness summation and binaural loudness summation with loudness matching and categorical loudness scaling

OBJECTIVE

Current hearing aid prescription rules assume that spectral loudness summation decreases with hearing impairment and that binaural loudness summation is independent of hearing loss and signal bandwidth. Previous studies have shown that these assumptions might be incorrect. Spectral loudness summation was measured and compared for loudness scaling and loudness matching.

DESIGN

In this study, the effect of bandwidth on binaural summation was investigated by comparing loudness perception of low-pass filtered, high-pass filtered, and broadband pink noise at 35 Categorical Units for both unilateral and bilateral presentation.

STUDY SAMPLE

Sixteen hearing-impaired listeners.

RESULTS

The results show that loudness differences between the three signals are different for bilateral presentation than for unilateral presentation. In specific, binaural loudness summation is larger for the low-pass filtered pink noise than for the high-pass filtered pink noise. Finally, individual variability in loudness perception near loudness discomfort level was found to be very large.

CONCLUSIONS

Loudness matching is offered as a fast and reliable method to measure individual loudness perception. As discomfort with loud sounds is one of the major problems encountered by hearing aid users, measurement of individual loudness perception could improve hearing aid fitting substantially.

Perceptual Weighting of Binaural Lateralization Cues across Frequency Bands

The auditory system uses interaural time and level differences (ITD and ILD) as cues to localize and lateralize sounds. The availability of ITDs and ILDs in the auditory system is limited by neural phase-locking and by the head size, respectively. Although the frequency-specific limitations are well known, the relative contribution of ITDs and ILDs in individual frequency bands in broadband stimuli is unknown. To determine these relative contributions, or spectral weights, listeners were asked to lateralize stimuli consisting of eleven simultaneously presented 1-ERB-wide noise bands centered between 442 and 5544 Hz and separated by 1-ERB-wide gaps. Either ITDs or ILDs were varied independently across each noise band, while fixing the other interaural disparity to either 0 dB or 0 μs. The weights were obtained using a multiple linear regression analysis. In a second experiment, the effect of auditory enhancement on the spectral weights was investigated. The enhancement of single noise bands was realized by presenting ten of the noise bands as preceding and following sounds (pre- and post-cursors, respectively). Listeners were asked to lateralize the stimuli as in the first experiment. Results show that in the absence of pre- and post-cursors, only the lowest or highest frequency band received highest weight for ITD and ILD, respectively. Auditory enhancement led to significantly enhanced weights given to the band without the pre- and post-cursor. The weight enhancement could only be observed at low frequencies, when determined with ITD cues and for low and high frequencies for ILDs. Hence, the auditory system seems to be able to change the spectral weighting of binaural information depending on the information content.

Contribution of frequency bands to the loudness of broadband sounds: Tonal and noise stimuli

Contributions of individual frequency bands to judgments of total loudness can be assessed by varying the level of each band independently from one presentation to the next and determining the relation between the change in level of each band and the loudness judgment. In a previous study, measures of perceptual weight obtained in this way for noise stimuli consisting of 15 bands showed greater weight associated with the highest and lowest bands than loudness models would predict. This was true even for noise with the long-term average speech spectrum, where the highest band contained little energy. One explanation is that listeners were basing decisions on some attribute other than loudness. The current study replicated earlier results for noise stimuli and included conditions using 15 tones located at the center frequencies of the noise bands. Although the two types of stimuli sound very different, the patterns of perceptual weight were nearly identical, suggesting that both sets of results are based on loudness judgments and that the edge bands play an important role in those judgments. The importance of the highest band was confirmed in a loudness-matching task involving all combinations of noise and tonal stimuli.

How does the perceptual organization of a multi-tone mixture interact with partial and global loudness judgments?

Two experiments were conducted to investigate how the perceptual organization of a multi-tone mixture interacts with global and partial loudness judgments. Grouping (single-object) and segregating (two-object) conditions were created using frequency modulation by applying the same or different modulation frequencies to the odd- and even-rank harmonics. While in Experiment 1 (Exp. 1) the two objects had the same loudness, in Experiment 2 (Exp. 2), loudness level differences (LLD) were introduced (LLD = 6, 12, 18, or 24 phons). In the two-object condition, the loudness of each object was not affected by the mixture when LLD = 0 (Exp. 1), otherwise (Exp. 2), the loudness of the softest object was modulated by LLD, and the loudness of the loudest object was the same regardless of whether it was presented in or out of the mixture. In the single- and the two-object conditions, the global loudness of the mixture was close to the loudness of the loudest object. Taken together, these results suggest that while partial loudness judgments are dependent on the perceptual organization of the scene, global loudness is not. Yet, both partial and global loudness computations are governed by relative "saliences" between different auditory objects (in the segregating condition) or within a single object (in the grouping condition).

Relative contributions of specific frequency bands to the loudness of broadband sounds

Listeners with normal hearing (NH) and sensorineural hearing loss (SNHL) were asked to compare pairs of noise stimuli and choose the louder noise in each pair. Each noise was made up of 15, two-ERB_N (equivalent rectangular bandwidth) wide frequency bands that varied independently over a 12-dB range from one presentation to the next. Mean levels of the bands followed the long-term average speech spectrum (LTASS) or were set to 43, 51, or 59 dB sound pressure level (SPL). The relative contribution of each band to the total loudness of the noise was determined by computing the correlation between the difference in levels for a given band on every trial and the listener's decision on that trial. Weights for SNHL listeners were governed by audibility and the spectrum of the noise stimuli, with bands near the spectral peak of the LTASS noise receiving greatest weight. NH listeners assigned greater weight to the lowest and highest bands, an effect that increased with overall level, but did not assign greater weight to bands near the LTASS peak. Additional loudness-matching and paired-comparison studies using stimuli missing one of the 15 bands showed a significant contribution by the highest band, but properties other than loudness may have contributed to the decisions.