Masked Thresholds Research Articles

Release from informational masking in children: Effect of multiple signal bursts

This study examined the degree to which increasing the number of signal presentations provides children with a release from informational masking. Listeners were younger children (5-7 years), older children (8-10 years), and adults. Detection thresholds were measured for a sequence of repeating 50-ms bursts of a 1000-Hz pure-tone signal embedded in a sequence of 10- and 50-ms bursts of a random-frequency, two-tone masker. Masker bursts were played at an overall level of 60-dB sound pressure level in each interval of a two-interval, forced choice adaptive procedure. Performance was examined for conditions with two, four, five, and six signal bursts. Regardless of the number of signal bursts, thresholds for most children were higher than thresholds for most adults. Despite developmental effects in informational masking, however, masked threshold decreased with additional signal bursts by a similar amount for younger children, older children, and adults. The magnitude of masking release for both groups of children and for adults was inconsistent with absolute energy detection. Instead, increasing the number of signal bursts appears to aid children in the perceptual segregation of the fixed-frequency signal from the random-frequency masker as has been previously reported for adults [Kidd, G., et al. (2003). J. Acoust. Soc. Am. 114, 2835-2845].

A joint encoder–decoder framework for supporting energy efficient audio decoding

In comparison with the relatively slow progress of battery technology, semiconductor memory has improved much more rapidly, making storage a less critical limiting factor in designing low power embedded systems such as PDAs. To exploit such technology trends, we present a novel framework, a joint encoder–decoder framework (JEDF), which allows the decoder to tradeoff energy and memory consumption without sacrificing playback quality. We employ sum-of-powers-of-two (SOPOT) technique, an approximate signal processing (ASP) technique, in an MPEG AAC decoder to reduce the computational workload. The SOPOT introduces additional ASP noise (in the decoder) on top of the quantization noise introduced in the process of lossy compression (in the encoder). The sum of these two kinds of noise may become audible when it exceeds the masking threshold. We tackle this problem from a new perspective: the proposed JEDF allows the ASP and quantization noises to be shaped jointly to match the masking threshold. In the case that the perceptual room between the masking threshold and the quantization noise is insufficient for the ASP noise, the JEDF can reduce the quantization noise level which results in an increase in bitrate. To implement the proposed scheme, we have developed two new techniques: (1) SOPOT truncation noise shaping; (2) truncation noise allocation based on a perceptual model. Experimental results show the effectiveness of our approach.

Binaural sluggishness precludes temporal pitch processing based on envelope cues in conditions of binaural unmasking

Binaural sluggishness refers to the binaural system's inability to follow fast changes in the interaural configuration of the incoming sound stream. Several studies have measured binaural sluggishness by measuring signal detection in conditions of binaural unmasking when the interaural configuration of the masker is changed over time. However, it has been shown that, in conditions of binaural unmasking, binaural sluggishness also affects the perception of temporal changes in the properties of the signal (i.e., its frequency or level) and not just in the interaural configuration of the masker. By measuring the temporal modulation transfer function for sinusoidally modulated noise presented in conditions of binaural unmasking, the first experiment of the current study showed that, due to binaural sluggishness, the internal representation of binaurally unmasked sounds conveys little or no information about envelope fluctuations with rates within the pitch range (i.e., above 30 Hz). The second experiment measured the masked detection threshold for musical interval recognition in binaurally unmasked harmonic tones and showed that, in conditions of binaural unmasking, pitch wanes when the harmonics become unresolved by the cochlear filters. These results suggest that binaural sluggishness precludes temporal pitch processing based on envelope cues in binaurally unmasked sounds.

Frequency Tuning Curves Derived from Auditory Steady State Evoked Potentials: A Proof-of-Concept Study

Assess the feasibility of drawing tuning curves from the masking function of steady state potentials. Develop a noninvasive tool for research applications on cochlear frequency selectivity in sedated animals. Obtain pilot human data validating auditory steady state evoked potential-derived (ASSEP) tuning curves against psychophysical data. ASSEP tuning curves were drawn in 10 Beagle puppies and six human adults using amplitude-modulated probes. Two probe frequencies (1 and 2 kHz) were used in dogs and only one (2 kHz) in humans. The modulation rates of the two probes were set to 81 and 88 Hz, respectively. Psychophysical tuning curves were obtained in 12 normal human subjects using the same maskers and either a pure-tone or an amplitude-modulated probe to verify if the latter had a specific effect on tuning curve parameters. Six of these 12 subjects participated in the electrophysiologic measurements. For each tuning curve, the intensity of the narrowband masker required just to mask the fixed probe was plotted for different masker center frequencies. Masker center frequencies extended to about half an octave above and an octave below the probe frequencies in 100-Hz steps. Tuning curve width (Q10 dB values), high- and low-frequency slopes (in dB/octave) and the masker frequency yielding the lowest masking threshold (maximal masker frequency) were computed. Canine Q10 dB values obtained were compared with those published for several species with other techniques. For humans, ASSEP and psychophysical tuning curves were directly compared in the same subjects and with published data. In dogs, the ASSEP method yielded reproducible tuning curves with qualitative and quantitative parameters similar to other physiologic measures of tuning obtained in various animals. Q10 dB values were greater at 2 than at 1 kHz, reflecting the well-known correlation between sharpness of tuning and central frequency. In humans, ASSEP Q10 dB values were slightly smaller than the psychophysical ones, but were greater by a factor of 2 than those obtained with previously published electrophysiologic procedures. In both species, detuning-a shift of the tip of the curve away from the probe frequency-was frequently observed as upward shifts with a maximal value of 200 Hz. Human psychophysical tuning curves also showed a certain amount of upward detuning. The intraindividual comparison of the two types of probes performed on human subjects with the psychophysical method did not indicate a specific effect of the amplitude-modulated probe on the curve parameters. Neither did the intraindividual comparisons indicate that an amplitude-modulated probe per se promoted detuning. Detuning has been observed with several other techniques and is usually attributed to nonlinear interactions between masker and probe in simultaneous masking. The results demonstrate the feasibility of measuring realistic ASSEP tuning curves in sedated dogs and in sleeping human adults. The ASSEP tuning curves exhibit a series of classical features similar to those obtained with time-honored methods. These results pave the way for the development of a noninvasive electrophysiologic method for tuning curve recording and its applications in noncooperative experimental animals or clinical subjects.

Effect of presentation level on diagnosis of dead regions using the threshold equalizing noise test

The effect of the level of threshold equalizing noise (TEN) on the diagnosis of dead regions (DRs) was investigated. Participants comprised 23 adults with sensorineural hearing impairment. Masked thresholds were measured monaurally with TEN at 60, 70, 80, and 90 dB HL/ERBN. Absolute and masked thresholds (with TEN at 80 dB HL/ERBN) were retested. The diagnosis was unaffected by TEN level at any frequency for eight of the 13 participants who met the criteria for a DR. For four of the latter, increasing the TEN level changed the diagnosis from DR to no DR, mainly at 1.5 kHz, corresponding to the edge frequency, fe, of the DR. For one participant with a low-frequency DR, increasing the TEN level changed the diagnosis from no DR to DR, only at 1 kHz. The changes with level were too large to be explained in terms of test repeatability. Overall, the results of the TEN test are usually not affected by TEN level, except for test frequencies close to fe when absolute thresholds are near-normal for frequencies adjacent to fe.

A Perceptual Approach to Reduce Musical Noise Using Critical Bands Tonality Coefficients and Masking Thresholds

Traditional noise reduction techniques have the drawback of generating an annoying musical noise. A new scheme for speech enhancement in high noise environment is developed by considering human auditory system masking characteristics. The new scheme considers the masking threshold of both noisy speech and the denoised one, to detect musical noise components. To make them inaudible, they are set under the noise masking threshold. The improved signal is subjected to extensive subjective and objective tests. It is ob-served that the musical noise is appreciably reduced even at very low signal to noise ratios.

A bit reduction algorithm for Spectral Band Replication based on human auditory characteristics

Spectral band replication (SBR) is an effective tool for low bitrate audio codecs such as HE-AAC and enhanced aacPlus. We propose a new algorithm for the bitrate reduction of SBR, which modifies the envelope data coding of SBR considering the threshold in quiet and the masking threshold. Experimental results show that the proposed algorithm significantly reduces the bitrate compared to the conventional SBR with no degradation in perceived sound quality.

Acoustic, psychophysical, and neuroimaging measurements of the effectiveness of active cancellation during auditory functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) is one of the principal neuroimaging techniques for studying human audition, but it generates an intense background sound which hinders listening performance and confounds measures of the auditory response. This paper reports the perceptual effects of an active noise control (ANC) system that operates in the electromagnetically hostile and physically compact neuroimaging environment to provide significant noise reduction, without interfering with image quality. Cancellation was first evaluated at 600 Hz, corresponding to the dominant peak in the power spectrum of the background sound and at which cancellation is maximally effective. Microphone measurements at the ear demonstrated 35 dB of acoustic attenuation [from 93 to 58 dB sound pressure level (SPL)], while masked detection thresholds improved by 20 dB (from 74 to 54 dB SPL). Considerable perceptual benefits were also obtained across other frequencies, including those corresponding to dips in the spectrum of the background sound. Cancellation also improved the statistical detection of sound-related cortical activation, especially for sounds presented at low intensities. These results confirm that ANC offers substantial benefits for fMRI research.

Method and apparatus for introducing information into a data stream and method and apparatus for encoding an audio signal

An inventive method for introducing information into a data stream including data about spectral values representing a short-term spectrum of an audio signal first performs a processing of the data stream to obtain the spectral values of the short-term spectrum of the audio signal. Apart from that, the information to be introduced are combined with a spread sequence to obtain a spread information signal, whereupon a spectral representation of the spread information is generated which will then be weighted with an established psychoacoustic maskable noise energy to generate a weighted information signal, wherein the energy of the introduced information is substantially equal to or below the psychoacoustic masking threshold. The weighted information signal and the spectral values of the short-term spectrum of the audio signal will then be summed and afterwards processed again to obtain a processed data stream including both audio information and information to be introduced. By the fact that the information to be introduced are introduced into the data stream without changing to the time domain, the block rastering underlying the short-term spectrum will not be touched, so that introducing a watermark will not lead to tandem encoding effects.

Critical ratios of beluga whales (Delphinapterus leucas) and masked signal duration

This article examines the masking of a complex beluga vocalization by natural and anthropogenic noise. The call consisted of six 150 ms pulses exhibiting spectral peaks between 800 Hz and 8 kHz. Comparing the spectra and spectrograms of the call and noises at detection threshold showed that the animal did not hear the entire call at threshold. It only heard parts of the call in frequency and time. From the masked hearing thresholds in broadband continuous noises, critical ratios were computed. Fletcher critical bands were narrower than either 15 or 111 of an octave at the low frequencies of the call (<2 kHz), depending on which frequency the animal cued on. From the masked hearing thresholds in intermittent noises, the audible signal duration at detection threshold was computed. The intermittent noises differed in gap length, gap number, and masking, but the total audible signal duration at threshold was the same: 660 ms. This observation supports a multiple-looks model. The two amplitude modulated noises exhibited weaker masking than the unmodulated noises hinting at a comodulation masking release.

Masking Property Based Residual Acoustic Echo Cancellation for Hands-Free Communication in Automobile Environment

A residual acoustic echo cancellation method that employs the masking property is proposed to enhance the speech quality of hands-free communication devices in an automobile environment. The conventional masking property is employed for speech enhancement using the masking threshold of the desired clean speech signal. In this Letter, either the near-end speech or residual noise is selected as the desired signal according to the double-talk detector. Then, the residual echo signal is masked by the desired signal (masker). Experiments confirm the effectiveness of the proposed method by deriving the echo return loss enhancement and by examining speech waveforms and spectrograms.

Issues with threshold masking in voxel-based morphometry of atrophied brains

There is great interest in using automatic computational neuroanatomy tools to study ageing and neurodegenerative disease. Voxel-based morphometry (VBM) is one of the most widely used of such techniques. VBM performs voxel-wise statistical analysis of smoothed spatially normalised segmented Magnetic Resonance Images. There are several reasons why the analysis should include only voxels within a certain mask. We show that one of the most commonly used strategies for defining this mask runs a major risk of excluding from the analysis precisely those voxels where the subjects' brains were most vulnerable to atrophy. We investigate the issues related to mask construction, and recommend the use of alternative strategies which greatly decrease this danger of false negatives.

A preliminary study on spatial unmasking of virtual separated sources

An experimental method with headphone virtual reproduction is proposed and a series of experiments to study forward masking effect when the masker and the masked signal are spatially separated in azimuth are conducted. Then, the masking thresholds are compared with those when the masker and the masked signal source are at the same place. The results show that, although both the thresholds of 0° and ±30° sound images increase with the sound pressure level (SPL) of the masker, spatial unmasking may be really observed. The maximum unmasking is as large as 15 dB. This spatial unmasking effect is mainly attributed to better-ear contribution.

Psychophysical assessment of the level-dependent representation of high-frequency spectral notches in the peripheral auditory system

To discriminate between broadband noises with and without a high-frequency spectral notch is more difficult at 70-80 dB sound pressure level than at lower or higher levels [Alves-Pinto, A. and Lopez-Poveda, E. A. (2005). "Detection of high-frequency spectral notches as a function of level," J. Acoust. Soc. Am. 118, 2458-2469]. One possible explanation is that the notch is less clearly represented internally at 70-80 dB SPL than at any other level. To test this hypothesis, forward-masking patterns were measured for flat-spectrum and notched noise maskers for masker levels of 50, 70, 80, and 90 dB SPL. Masking patterns were measured in two conditions: (1) fixing the masker-probe time interval at 2 ms and (2) varying the interval to achieve similar masked thresholds for different masker levels. The depth of the spectral notch remained approximately constant in the fixed-interval masking patterns and gradually decreased with increasing masker level in the variable-interval masking patterns. This difference probably reflects the effects of peripheral compression. These results are inconsistent with the nonmonotonic level-dependent performance in spectral discrimination. Assuming that a forward-masking pattern is a reasonable psychoacoustical correlate of the auditory-nerve rate-profile representation of the stimulus spectrum, these results undermine the common view that high-frequency spectral notches must be encoded in the rate-profile of auditory-nerve fibers.

Evaluation of an Aided TEN Test for Diagnosis of Dead Regions in the Cochlea

To compare results for the original version of the threshold equalizing noise [TEN(HL)] test for diagnosis of dead regions (DRs) in the cochlea, using stimuli presented via headphones, with results for an aided version of the test, the ATEN test, in which subjects listened to stimuli presented in free field using their own hearing aids. The test tones were warble tones for both the TEN(HL) and the ATEN test. Twenty-five subjects (12 males and 13 females), aged between 12 and 19 yr, with severe or profound sensorineural hearing loss were tested. For each test, two levels of the TEN were used, chosen to fall within the comfortable range of levels for the individual subject. A DR was considered to be present when the TEN(HL) produced at least 10 dB of masking and when the masked threshold was at least 10 dB above the nominal TEN(HL) level. Measurements of the outputs of the hearing aids in response to the TEN(HL) plus the test tones were obtained using a KEMAR acoustic manikin to assess the extent to which distortion or compression might have influenced the outcomes. For the TEN(HL) test, the results were often inconclusive, because the TEN(HL) could not be made sufficiently intense to give at least 10 dB of masking. The incidence of these inconclusive cases was markedly reduced for the ATEN test. There were more positive diagnoses of DRs for the ATEN test than for the TEN(HL) test. The KEMAR measurements indicated that distortion, compression, and/or feedback cancellation probably influenced the outcomes in some cases, leading to a moderate incidence of false positives for the ATEN test, and also some "missed" cases. The ATEN test leads to a lower incidence of inconclusive results than the TEN(HL) test in the diagnosis of DRs in people with severe to profound hearing loss. However, for some hearing aids the gain changed rapidly as a function of frequency, which undermined the validity of the ATEN test. Also, some hearing aids introduced distortion that probably affected the outcome of the test and gave misleading results. Hence, the ATEN test cannot be recommended for use in the clinic.

The impacts of age-related hearing impairments on the L2 classroom

Older students (aged 65+) in the language classroom may be presenting hearing deficits not experienced by their traditional university classmates (aged 18–24). L2 teachers need to be aware of the main processes involved in hearing, as well as such important factors as the temporary threshold effect and masking effects, in order to encourage language acquisition in all their students. Familiarity with the factors involved in hearing impairments will enable L2 teachers to make the most appropriate pedagogical choices, regarding selection of activities, timing and modification of instruction techniques. Hearing impairment will affect both language acquisition and assessment. Losses in certain frequencies potentially affect the perception and acquisition of new L2 phonemes, as well as the interpretation and production of new L2 words. Paralinguistic and visual information assume greater importance. Furthermore, hearing impairments will most likely negatively affect reaction time, with consequences for both instruction and testing. Lastly, the psycho–social impacts of hearing loss will influence not only the individual but also the student group and probably the teacher; hence, teachers need to be prepared to facilitate the resultant social dynamics.

Comodulation masking release in bottlenose dolphins (Tursiops truncatus)

The acoustic environment of the bottlenose dolphin often consists of noise where energy across frequency regions is coherently modulated in time (e.g., ambient noise from snapping shrimp). However, most masking studies with dolphins have employed random Gaussian noise for estimating patterns of masked thresholds. The current study demonstrates a pattern of masking where temporally fluctuating comodulated noise produces lower masked thresholds (up to a 17 dB difference) compared to Gaussian noise of the same spectral density level. Noise possessing wide bandwidths, low temporal modulation rates, and across-frequency temporal envelope coherency resulted in lower masked thresholds, a phenomenon known as comodulation masking release. The results are consistent with a model where dolphins compare temporal envelope information across auditory filters to aid in signal detection. Furthermore, results suggest conventional models of masking derived from experiments using random Gaussian noise may not generalize well to environmental noise that dolphins actually encounter.

Auditory brainstem responses in birds: How well do they compare to behavioral techniques for assessing hearing

Behavioral methods are the gold standard for assessing hearing sensitivity in birds. In many cases, however, behavioral techniques are simply impractical. The non-invasive auditory brainstem response (ABR) represents perhaps the next best approach, and researchers are increasingly turning to the ABR to obtain estimates of hearing sensitivity, auditory system function and development. Our lab has evaluated hearing sensitivity in over 11 species of birds using the ABR, and for six of these species, we have also obtained auditory thresholds by behavioral conditioning methods. These data, along with other results from the literature on bird hearing, now allow general conclusions about the use of ABR as a tool for measuring auditory sensitivity in birds. Waveform morphology is highly conserved across orders of birds. Regardless of how threshold is defined, the ABR audiogram reliably reflects the frequency range of hearing and the shape of the behavioral audiogram. However, ABR thresholds are higher than behavioral thresholds by 20-30 dB but with some variation across species. Aside from audiograms based on simple detection thresholds for tone bursts, ABR methods in birds have also been useful in assessing auditory development, hearing impairment, recovery of function with hair cell regeneration, masked thresholds and aspects of spectral and temporal processing.

Fusion and masking threshold of a tone in narrowband noise

When comparing different pitch phenomena, it is not clear whether the expression pitch strength points towards a unique perceptive phenomenon or object. As many studies concentrate on one special phenomenon, the question, what is actually judged, seems only implicitly answered. Kubovy and Van Valkenburg (2001) define a perceptual object as "that what is susceptible to figure-ground segregation". Pitch plays a major role in these grouping processes. In this context the judgment of pitch strength refers to two perceptual cases. The first one is the pitch strength of a tone-in-noise, i.e. the case where the pitch is linked to a separate contour within the narrowband noise (NBN), while the second is the pitch strength of tonal noise. Here, the noise evokes a pitch sensation that is not related to a separate object. In order to explore the transition from case /1 tone-in-noise to case /2 tonal noise, the identification threshold hearing a tone in NBN centered on the tone is determined in a lab experiment. This segregation or fusion threshold is compared to the measured masking threshold for center frequencies 250-4000 Hz octave-wise at 60 dB SPL noise level. The bandwith of the NBN is varied from 50-250 Hz accordingly.

Masking release based on fine‐grain temporal pitch cues in speech‐like stimuli ‐ mechanisms and implications for the cocktail‐party phenomenon

One of the most important cues for segregating wanted from unwanted sounds when listening in noisy environments is temporal regularity (or harmonicity), which gives rise to the perception of pitch. This study measured masked signal detection thresholds when both the signal and the masker are complex tonal stimuli similar to speech. Detection was considerably easier when the pitch of the masker differed from that of the signal. A control experiment involving non‐simultaneous masking revealed that this pitch‐based release from masking was not due to spectral differences between the signal and the masker. A second experiment investigated the effect of a pitch difference between the signal and the masker on the audibility of certain perceptual features of the signal when the signal is presented at a supra‐threshold level. Two features were used that are known to be particularly important for speech perception: amplitude modulation and spectral profile related to formant frequencies. The supra‐threshold tasks enabled to quantify the unmasking effect of pitch on the processing of these features over and above the effect on the signal detection threshold. The results provide new insights into the low‐level processes involved in the cocktail‐party phenomenon. Acknowledgement: Work supported by the Medical Research Council.