Frequency-gain Characteristics Research Articles

A Follow-Up Clinical Trial Evaluating the Consumer-Decides Service Delivery Model.

ObjectivesThere were 2 main objectives. The primary objective was to replicate a prior clinical trial of a consumer-decides (CD) approach to selecting hearing aids in older adults as a potential model for over-the-counter (OTC) intervention using less front-end screening of participants and a wider range of frequency-gain characteristics in the devices. The 2nd objective, only feasible if participant choices allowed, was to evaluate the efficacy of the CD approach relative to a CD-based placebo device.DesignThe design of this study is a single-site, prospective, double-blind clinical trial. Outcome measures were obtained after a typical 4- to 5-week trial period. An optional follow-up of a 4-week audiology-based (AB) best practices trial was also included for replication and comparison purposes.SettingOlder adults from the general community were recruited via newspaper and community flyers to participate at a university research clinic.ParticipantsParticipants were adults, aged 54–78 years, with mild–moderate hearing loss. Forty-one participants enrolled as a volunteer sample; 40 completed the intervention.Intervention(s)All participants received the same high-end digital mini–behind-the-ear hearing aids fitted bilaterally. CD participants self-selected their own preprogrammed hearing aids via an OTC-type model. One of the 3 devices from which participants could choose was programmed to be a placebo device with no functional gain.Primary and Secondary Outcome MeasuresThe primary outcome measure is the 66-item self-report Profile of Hearing Aid Benefit (Cox & Alexander, 1990). The secondary outcome measure is the Connected Speech Test (Cox, Alexander, & Gilmore, 1987) benefit. Additional measures of hearing aid benefit and usage were also obtained.ResultsPer-protocol analyses based on the data from the 40 (of 41) participants who completed the study were performed. Hearing aid outcomes from this follow-up CD (CD2) cohort were positive and generally the same as for the original CD cohort. CD service delivery model was efficacious relative to CD-based placebo control, with medium effect sizes observed. Approximately half of the CD2 group was likely to purchase hearing aids after the trial, similar to findings for the original CD cohort. Outcomes improved significantly for the 32 CD2 participants who elected to complete the optional 4-week AB trial. For this largely unscreened sample, more individuals with healthy hearing sought amplification, and many of these individuals (35%) chose placebo devices for both ears.ConclusionsPrior positive outcomes for CD service delivery have been replicated in a less restrictive approach to participant recruitment. The CD approach was again found to be efficacious. Significantly better outcomes were observed after subsequent AB service delivery follow-up, also replicating prior findings. Efficacious OTC models, including those using similar CD approaches to hearing aid self-selection, may increase accessibility and affordability of hearing aids for millions of older adults. Front-end guidance to consumers regarding the best path to intervention, ranging from self-screening of hearing online to a full audiologic assessment, appears to be critical to optimize the success of OTC approaches.Trial RegistrationClinicaltrials.gov: NCT01788432; https://clinicaltrials.gov/ct2/show/NCT01788423Supplemental Materialhttps://doi.org/10.23641/asha.7728479

Design and Validation of Front-End Voltage Follower for Capacitive Electrocardiogram Measurement Using Bootstrapping Technique.

We addressed design of bootstrapped voltage follower (BVF) in analytical approach from a perspective of capacitive electrocardiogram (cECG) measurement. Theoretical formulas of transfer function, resonant frequency and damping ratio for BVF were derived and experimentally validated. The transfer function and resonant frequency were beneficial for predicting cutoff frequency and frequency-gain characteristics of attenuation band in the low-frequency region. The damping ratio was beneficial for foreseeing occurrences of resonance and distortion in cECG recording. Next challenge is to derive an index inferring settling time after contamination of movement artifact, and to evaluate it experimentally.

Application of Piezoelectric-Transformer-Based Resonant Circuits for AC LED Lighting-Driven Systems With Frequency-Tracking Techniques

This paper proposes an application of piezoelectric transformer (PT) based inverters for alternating current (ac) light-emitting diode (LED) lighting-driven systems with a new resonant topology and frequency-tracking technique. This proposed architecture comprises a full-bridge phase-shift inverter with a PT-based C-L-C series resonant circuit, piezoelectric ceramic transformers, and a microwinding resonant transformer to extend the range of operation frequency, hence achieving a two-terminal alternating voltage to drive ac LED arrays while stabilizing the resonant characteristics. In this system design, a frequency-tracking mechanism is also integrated into the field-programmable gate array controller to improve operation efficiency. Since only the secondary current of the resonant transformer is required for the controller, the circuit implementation is simple and additional sensor devices can be largely saved. Moreover, by considering that the frequency-gain characteristics of PTs may be affected due to temperature changes; the controller is designed to adjust the operation frequency swiftly in order to restrict the temperature effect. This proposed method has been verified on modular ac LED lighting systems. Test results confirm the feasibility of the method for lighting system applications.

The Effect of Low-Pass Filtering on Identification of Nonsense Syllables in Quiet by School-Age Children With and Without Cochlear Dead Regions

The presence of cochlear dead regions (DRs) can have a significant effect on speech perception. Some studies have reported that adults do not benefit from amplification of frequencies well inside an extensive DR. However, the importance of high-frequency amplification for hearing-impaired children has been emphasized by many researchers. This study investigates the benefit of high-frequency amplification for children with various degrees of high-frequency hearing impairment, with and without DRs. The children, aged 8 to 13 years, were divided into two groups according to the severity of their hearing impairment. Group MS had moderate to severe impairment (9 ears without DRs and 3 ears with restricted DRs). Group SP had severe to profound hearing impairment (7 ears with DRs and 1 ear without a DR). The vowel-consonant-vowel stimuli were subjected to the frequency-gain characteristics prescribed by the desired sensation level fitting method and presented via headphones broadband and under various low-pass filtering conditions. Group MS benefited from high-frequency amplification whether or not a restricted DR was present. In contrast, ears in group SP with continuous extensive DRs showed limited benefit from high-frequency amplification. For the latter, performance improved with increasing cutoff frequency up to approximately 1 octave above the edge frequency of the DR and generally stayed the same, or deteriorated, with further increases in bandwidth. In one case of severe to profound hearing impairment without evidence of DRs, performance increased with increasing cutoff frequency up to 2 kHz and remained almost constant with further increases in bandwidth. For children with severe to profound hearing impairment and continuous high-frequency DRs commencing from approximately 1 kHz, applying amplification only for frequencies up to approximately 1 octave above the edge frequency of the DR may be of benefit. Tests with more participants are needed to confirm this finding.

Evaluation of the Desired Sensation Level [Input/Output] Algorithm for Adults with Hearing Loss: The Acceptable Range for Amplified Conversational Speech

This study had two related purposes: first, to define the range of optimal ear canal levels of aided speech in both high frequency and low frequency regions for adults, using both subjective and objective definitions of optimal; and second, to determine whether a prescribed frequency response, such as that given by Desired Sensation Level [Input/Output], falls within the adult listener's optimal range. Twenty-three adult listeners with mild to moderately severe sensorineural hearing loss were selected from a pool of research volunteers. They were fitted in the laboratory with the Siemens Signia hearing instrument and tested with 20 nominally different frequency responses. All advanced processing options of the hearing instrument were disabled. Subjective ratings of loudness and quality and objective measures of consonant identification were obtained for every frequency response. These adult listeners had, on average, a 10 dB range of measured responses in both the low and the high frequencies that resulted in optimal performance on all the measurements. The range did not vary with degree or configuration of hearing loss, or previous hearing aid experience. Desired-Sensation-Level Input/Output targets were within the optimal range for the low frequencies, and 3 dB above the optimal range for the high frequencies. A range of aided ear canal frequency responses was determined within which adults with mild to moderately severe hearing loss performed optimally on both objective and subjective outcomes. Clinical implications of this finding include the following: prescriptive methods providing different targets may all result in optimal fittings; and a range of targets may be more appropriate than a single target when setting the frequency-gain characteristics of the hearing instrument.

An examination of the practicality of the simplex procedure.

The practical importance of the simplex procedure, a subjective technique used to refine the frequency gain characteristic (FGC) of a hearing aid according to listener preference, was determined for individual listeners by measuring hearing aid benefit using both laboratory studies and field studies. A digital research hearing aid with two memories was used as the test hearing aid. The modified simplex procedure was used to select the FGC judged to yield the best speech clarity in the presence of low-level vent noise and again in higher-level cafeteria noise by 10 experienced hearing aid users. The FGCs assessed by the listeners varied systematically from The National Acoustic Laboratories Revised (NAL-R) response in the amount of low-frequency or high-frequency amplification. The benefit obtained with these two simplex-selected settings was compared with that obtained using the NAL-R FGC. Measures of benefit included speech recognition testing in the laboratory and ratings of speech intelligibility in the field. In the first field study, the two simplex settings were compared. In the second field study, the simplex-selected setting for higher level noise and the NAL-R setting were compared. In the laboratory, the majority of listeners selected an increase in the low-frequency channel gain compared with the NAL-R. Desired high-frequency channel gain was correlated with degree of hearing loss and type of background noise. The benefit as measured using nonsense syllables did not differ significantly among the three fittings, but differences in benefit were measurable with the rating procedure. Five of eight participants noticed a significant difference in their speech understanding in the real world for the FGCs selected in different background noises. Two of seven participants reported significantly better speech intelligibility with a simplex-selected FGC compared with the NAL-R FGC in the real world. The remaining subjects reported similar speech understanding capabilities with both hearing aid settings. The majority of subjects included in this study selected an FGC with real ear insertion gain different than the NAL-R prescription to improve subjective speech understanding in the laboratory. A small number of these listeners rated the selected FGC as providing improved speech intelligibility over the NAL-R FGC in the real world. This finding indicates that the simplex procedure should be used selectively to modify the NAL-R prescription. A screening technique would be useful in selecting those who might benefit from a modified fitting. The simplex procedure may also prove to be useful in selecting listeners who would benefit from multiple memory hearing aids.

The time course of effects on intensity discrimination following monaural fitting of hearing aids

Systematic effects on intensity discrimination occur in listeners who have experienced long-term unilateral amplification [Robinson and Gatehouse, J. Acoust. Soc. Am. 97, 1183–1190 (1995)]. For a bandlimited tone complex centered at 3 kHz, the normally aided ear showed better discrimination performance than the unaided ear at sound-pressure levels greater than 85 dB, yet was worse than the unaided ear at lower levels. No effects were found at 0.25 kHz. In the present study, intensity discrimination was investigated prospectively for five listeners with bilateral, sensorineural, hearing impairment following provision of unilateral amplification. The stimuli were the same as those used previously. Weber functions for both ears were similar at both frequencies prior to fitting the aid, and 4 to 12 weeks postfitting. From 15 weeks, the normally aided ear showed better discrimination performance than the unaided ear at 3 kHz for a sound-pressure level of 95 dB. No effects were observed at 0.25 kHz, or for the unaided ear. Hence, changes in intensity discrimination occur in the normally aided ear following long-term exposure to the frequency-gain characteristics of unilateral amplification. Further research should examine the generality of these results to other listeners, and show whether it extends beyond intensity discrimination to other psychoacoustic capabilities.

Assistive listening devices and hearing aids: coupling considerations.

Assistive listening devices (ALDs) can be used to supplement hearing aid performance in difficult listening conditions, but caution must be used when coupling ALDs to personal hearing aids because the frequency-gain characteristics of the hearing aid may be altered. Data illustrating subject performance using ALDs coupled to personal hearing aids, ALDs with headsets, and ALDs coupled to hearing aids with programmable telecoils are presented to illustrate the problems associated with coupling ALDs and hearing aids and to provide possible solutions to these problems.

Derivation of frequency-gain characteristics for maximizing speech reception in noise.

Hearing aid gain-assignment schemes known as "prescriptions" were not designed for fitting hearing aids that modify their frequency responses to reduce background noise interference. Rather, prescriptions were developed for hearing aids having single, fixed frequency responses and aim to optimize speech reception in relatively quiet environments. Even though prescriptions do not apply to noisy conditions specifically, they embody the trade between maximizing speech audibility and maintaining loudness comfort that is critical to frequency-gain characteristic selection independent of whether noise is present or absent. The articulation index (AI) was used to examine the extent to which prescriptions' deference to loudness comfort causes them to fall short of maximizing speech spectrum audibility, thereby revealing (roughly) the magnitude of the loudness control built into prescriptions. AIs for speech amplified by an AI-maximizing rule (MAX AI) (Rankovic, Freyman, & Zurek, 1992) and according to several prescriptions were calculated as a function of hearing loss degree and configuration for quiet and noisy conditions. In quiet, AIs for prescriptions were similar to one another when presented with the same audiogram but were drastically smaller than MAX AIs, implying that prescriptions limit speech audibility to a large extent to prevent loudness discomfort. In noise, maximizing the AI required frequency-gain characteristics that were substantially different from prescription-assigned characteristics and that were unique to each noise/audiogram combination. A loudness constraint for the MAX AI scheme was developed to account for the gain discrepancy between prescription AIs and MAX AIs observed in the quiet condition, based on the highest comfortable loudness (HCL) equations presented by Cox (1989) in combination with a loudness model (von Paulus & Zwicker, 1972). The MAX AI scheme with the new loudness control was extended to specify frequency-gain characteristics expected to be optimal for several conditions containing noise, and examples are presented.

A model to evaluate and maximize hearing aid performance by integrating the articulation index across listening conditions.

A method for integrating the articulation index (AI) across listening conditions was developed and applied to a preliminary model for evaluating and optimizing prescriptions of hearing aid characteristics. The model takes hearing threshold, masking of noise, self-masking of speech, high level cochlear distortion, and the peak-clipping effects of a hearing aid into account. The integrated AI (IAI) across a range of listening conditions is used as a criterion for evaluating a specific hearing aid response characteristic and calculating an optimal frequency-gain characteristic that maximizes the IAI. For a high-frequency hearing loss, the frequency-gain characteristics and IAI's derived from an optimal IAI (OIAI), POGO, and NAL prescriptions are compared for two of listening situations: a quiet setting and a setting with a signal-to-noise ratio of -3 dB. The results predict that, in quiet, the OIAI prescription is not significantly different from the well-established prescriptive procedures such as the POGO and the NAL. For the noise condition, however, the optimal IAI model is predicted to be better in terms of speech intelligibility.

Potential benefits of adaptive frequency-gain characteristics for speech reception in noise

Some current single-microphone hearing aids employ techniques for adaptively varying the frequency-gain characteristics in an attempt to improve speech reception in noise. The potential benefit of this strategy depends on the spectral spread of masking and the degree to which it can be reduced by changing the frequency-gain characteristic. In this study these benefits were examined for subjects with normal hearing under static listening conditions. In the unprocessed condition, subjects were presented with nonsense syllables in an octave-band noise centered on 0.5, 1, or 2 kHz. The frequency-gain characteristic was then modified with the goal of reducing the intensity of the frequency region containing the octave-band noise. This processing resulted in increases as large as 60 percentage points in consonant-correct scores with the low- and mid-frequency octave noise bands, and a small increase with the high-frequency noise. Masking patterns produced by the octave noises were also measured and were related to the intelligibility results via an analysis based on Articulation Theory. The Articulation Index was also used to compare the effectiveness of three adaptive rules. A simple multiband volume control is expected to provide much of the benefit of more sophisticated systems without the need for separate estimation of input speech and noise spectra.

Potential benefits of varying the frequency-gain characteristic for speech reception in noise for hearing-impaired individuals.

Some commercially available hearing aids employ adaptive linear filtering in an attempt to improve speech reception in noise for hearing-impaired individuals. The potential benefit of this strategy depends on whether modifications to the frequency-gain characteristic can reduce the spectral spread of masking of the interfering noise. The purpose of our investigation is to address the finding that some hearing-impaired individuals benefit from this processing while others do not [e.g., J. N. Van Dijkhuizen, J. M. Festen, and R. Plomp, J. Acoust. Soc. Am. (in press)]. Hearing-impaired subjects were presented with nonsense syllables whose average spectrum is shaped to maximize audibility, together with a high-level octave-band noise centered on a low, mid, or high frequency. Frequency-gain characteristics are then imposed with the goal of reducing the overall level in the frequency band containing the noise. In addition to intelligibility scores, masking patterns are obtained for the noises presented in all listening conditions. Attempts will be made to account for individuals’ speech reception results in terms of their masking patterns via articulation theory. [Work supported by NIH.]

Studies with Digital Hearing Aids.

Digital hearing aids offer many advantages over conventional hearing aids. These include signal-processing capabilities that are superior to those of a conventional analog hearing aid, methods of signal-processing and control that are unique to digital systems and which cannot be implemented in conventional analog hearing aids, and innovative new techniques that are changing our way of thinking about hearing aids. An example of the first of these advantages is the extremely high precision with which the frequency-gain characteristic can be specified and the use of this capability to study the effects of frequency response irregularities commonly encountered with hearing aids. An example of the second advantage is the use of memory and logical operations in the implementation of multivariate adaptive paired-comparison techniques for more effective hearing-aid prescription. Another example is the use of powerful new signal processing techniques for noise reduction. The third advantage is the most important. The digital hearing aid can be viewed as a generalized hearing instrument which can be used for simulation, testing and prescription, as well as amplification. The use of the digital hearing aid as a simulator of other hearing aids is discussed and an illustrative example provided in which a new form of amplitude compression, orthogonal-polynomial compression, has been simulated using a digital master hearing aid.

Interaction between compression limiting and frequency gain characteristics

The interaction between frequency gain characteristics and compression limiting was investigated using an adaptive paired comparison strategy to estimate the optimum post-limiter frequency response for each of three pre-limiter frequency gain characteristics. Frequency-dependent compression limiting paralleling the subject's loudness discomfort level (LDL) curve was used. Stimulus intensity was adjusted to provide three levels of compression limiting. Results for both preference and intelligibility judgments with continuous discourse in quiet showed that most subjects tended to compensate for manipulations of the pre-compression filters with adjustments in the post-compression filters. The amount of compression had only a secondary effect on filter selection. [Research supported by NIDRR.]

An evaluation of three adaptive hearing aid selection strategies.

Paired-comparison judgments of intelligibility of speech in noise were obtained from eight hearing-impaired subjects on a large number of hearing aids simulated by a digital master hearing aid. The hearing aids which comprised a 5 X 5 matrix differed systematically in the amount of low-frequency and high-frequency gain provided. A comparison of three adaptive strategies for determining optimum hearing aid frequency-gain characteristics (an iterative round robin, a double elimination tournament, and a modified simplex procedure) revealed convergence on the same or similar hearing aids for most subjects. Analysis revealed that subjects for whom all three procedures converged on the same hearing aid showed a single pronounced peak in the response surface, while a broader peak was evident for the subjects for whom the three procedures identified similar hearing aids. The modified simplex procedure was found to be most efficient and the iterative round robin least efficient.

Amplification bandwidth and intelligibility of speech in quiet and noise for listeners with sensorineural hearing loss.

The intelligibility of speech mixed with noise (+6 dB S/N) and in quiet was measured as a function of its audible bandwidth for 7 listeners with moderate sensorineural hearing losses. Words were presented at three levels in the field (50, 60 and 70 dB SPL) and amplified with a Limiting Master Hearing Aid (LMHA) which allows independent control of gain and maximum power output in each of nine, half-octave channels. The frequency gain characteristics were chosen so the speech spectrum (60 dB SPL) at the input was shaped to approximate the individual listener's contour of most comfortable listening levels at the output. At each level and for the words in noise as well as quiet, the LMHA was set for four bandwidths (266-6 000, 375-4 242, 530-3 000 and 750-2 121 Hz). For these conditions, the listeners all obtained the highest mean score with the LMHA set for the widest bandwidth. The listener's ability to understand the words, after adjustment for individual differences, was well correlated (0.77-0.91) with the articulation index. Preliminary observations on the acceptability of amplified sounds, that is, loud speech, impact noise, an air-horn blast, and a party ratchet, are included.

Convergence characteristics of the kalman‐type equalizer in decision‐directed mode

AbstractIn general, in the adjustment of its tap weight, the automatic equalizer requires the reference signal in addition to the input to the equalizer. For the convergence to be correct, the reference signal should be the equalizer output after the convergence is completed correctly. Since this is impossible in practice, a decision‐directed operation occurs wherein the equalizer output during tap weight adjustment is given to the decision part and the output of the decision part is used as the reference signal. In such a case, the adjustment of the tap weight is not exactly correct.This paper shows first that the characteristics of the cascade system (which consists of the transfer characteristics of a Kalman‐type automatic equalizer after tap weight adjustment, operating by following the reference signal from the decision part) and the characteristics of the transmission channel can be determined directly from the signal type and the characteristics of the transmission channel and the characteristics of the decision part. In the above reasoning, the characteristics of the cascade system are obtained as the cross correlation between the transmitted signal and the output signal from the decision part. Then the mechanism to arrive at the correct convergence is described for the automatic equalizer, based on the knowledge obtained in the analysis of the cascade system. Finally, a Karhunen‐Loéve type decision‐assist fixed equalizer is proposed, which compensates the frequencygain characteristics of the transmission channel and assists the correct decision of the decision part.

Study of multichannel amplitude compression and linear amplification for persons with sensorineural hearing loss.

Experiments were conducted on five listeners with sensorineural hearing impairments using two 16-channel, computer-controlled, amplitude compression systems, and four linear systems. One of the compression systems was designed to restore normal equal loudness contours, the other employed reduced high-frequency emphasis and reduced compression ratios. The linear systems differed only in their frequency-gain characteristics (orthotelephonic plus three characteristics with high-frequency emphasis that were expected to produce better results than orthotelephonic). In the main experiment, all systems were compared for each listener using nonsense CVC monosyllables and sentence materials spoken by male and female talkers and presented in quiet/anechoic and noisy/reverberant environments at the most comfortable level for each listener. The linear systems with high-frequency emphasis performed substantially better than the orthotelephonic system. Performance with compression was generally slightly worse than with linear amplification. Compression was superior to linear amplification only when speech materials with significant item-to-item level variation were used in quiet with subjects with more severe losses and when reduced input speech levels were used. To the extent that these two conditions represent real-life communication conditions, these results suggest that compression is preferable to linear amplification in a wearable hearing aid.

Review of recent research on the selection of frequency-gain characteristics for hearing aids.

Following the Harvard master hearing aid study in 1947 there was little research on linear amplification. Recently, however, there have been a number of studies designed to determine the relationship between the frequency-gain characteristic of a hearing aid and speech intelligibility for persons with sensorineural hearing loss. These studies have demonstrated that a frequency-gain characteristic that rises at a rate of 6 dB/octave, as suggested by the Harvard study, is not optimal. They have also demonstrated that high-frequency emphasis of 10-40 dB above 500-1000 Hz is beneficial. Most importantly, they have demonstrated that hearing aids as they are presently being fit do not provide maximum speech intelligibility. Percent word correct scores obtained with the best frequency-gain characteristics tested in various studies have been found to be 9 to 19 percentage points higher than scores obtained with commercial aids owned by subjects. This increase in scores is equivalent to an increase in signal-to-noise ratio of 10 to 20 dB. This is a significant increase which could allow impaired listeners to communicate in many situations where they presently cannot. These results demonstrate the need for further research on linear amplification aimed at developing practical suggestions for fitting hearing aids.

Speaking clearly for the hard of hearing

It is obvious that the intelligibility of a given speech message depends strongly on how the message is spoken. Not only does intelligibility vary with the identity of the speaker, but also, for a given speaker, with the effort to speak clearly. Assuming that the variations in intelligibility associated with such factors are substantial, detailed knowledge of these factors could prove useful in the development of improved signal-processing schemes for hearing aids. Our initial goal is to estimate the magnitude of the intelligibility changes associated with a given speaker's attempt to speak clearly. Among the variables to be considered are presentation level of material, frequency-gain characteristic of amplification system, severity of hearing loss, identity of speaker, and degree of familiarity with speaker's voice. Preliminary results obtained using sentence material for a number of hearing-impaired listeners will be presented. [Work supported by NIH.]