Comparisons of speech recognition in noise by mildly-to-moderately hearing-impaired children using hearing aids and FM systems

Four hearing aid arrangements (monaural-omnidirectional, monaural-directional, binaural-omnidirectional, binaural-directional) and a number of FM system-personal hearing aid combinations (including direct input, neck loop, and silhouette inductor--monaural and binaural--and environmental microphone on and off) were evaluated in a school classroom on nine children with mild-to-moderate sensorineural hearing losses. Two measures of speech recognition in noise were employed. First, the signal-to-noise ratio (S/N) yielding 50% identification of spondees was determined using a simple up-down adaptive procedure. Second, word recognition scores were obtained for three amplification arrangements at two different S/Ns (+6 and +15 dB). The average FM advantage over a personal hearing aid was equivalent to a 15-dB improvement in S/N. Activation of the hearing aid microphone caused most of the FM advantage to disappear. The benefit offered by the FM system decreased as the environmental S/N increased but remained significant even at +15 dB. Significant improvement also was found with the use of directional as compared to omnidirectional microphones, both in the hearing aids and FM teacher microphone.     

Impact of noise source configuration on directional hearing aid benefit and performance

OBJECTIVE: To evaluate the impact of the position of noise source(s) and reverberation on the directional benefit and performance of three commercially available directional hearing aids. DESIGN: Directional benefit and performance were measured for four different configurations of competing noise source(s) in two different reverberant rooms. Three pairs of hearing aids representing three commercial models were selected based on electroacoustic evaluation of directivity. Directional benefit and performance of 25 subjects with symmetrical, sloping, sensorineural hearing loss were measured in all test environments using a modified version of the Hearing in Noise Test. RESULTS: Both reverberation and configuration of the competing noise source(s) significantly affected directional benefit and performance. There was no significant correlation between directional benefit and directional performance. The order of benefit and performance across hearing aid brands (from best to worst) varied depending on the noise source configuration. CONCLUSIONS: Data revealed increasing reverberation significantly decreased directional benefit and performance. The absolute and relative (rank ordering) directional benefit and performance varied across hearing aid brand, with noise source configuration. These results suggest that data collected in traditional test environments (e.g., a single competing noise placed at 180 degrees azimuth) cannot be used to accurately predict directional benefit or performance in the majority of other test and real-world environments. The impact of reverberation and noise source configuration on directional benefit/performance can be explained fairly well by the interaction between the spatial properties of the noise source(s) and the polar directivity patterns of the hearing aids.     

Speech perception in noise: directional microphones versus frequency modulation (FM) systems

The major consequence of sensorineural hearing loss (SNHL) is communicative difficulty, especially with the addition of noise and/or reverberation. The purpose of this investigation was to compare two types of technologies that have been shown to improve the speech-perception performance of individuals with SNHL: directional microphones and frequency modulation (FM) systems. Forty-six adult subjects with slight to severe SNHL served as subjects. Speech perception was assessed using the Hearing in Noise Test (HINT) with correlated diffuse noise under five different listening conditions. Results revealed that speech perception was significantly better with the use of the FM system over that of any of the hearing aid conditions, even with the use of the directional microphone. Additionally, speech perception was significantly better with the use of two hearing aids used in conjunction with two FM receivers rather than with just one FM receiver. Directional microphone performance was significantly better than omnidirectional microphone performance. All aided listening conditions were significantly better than the unaided listening condition.     

Use of a digital hearing aid with directional microphones in school-aged children

The efficacy of a digital hearing aid with a directional microphone was examined in a school-aged population. Twenty children (9 with a mild-to-moderately-severe hearing loss and 11 with a moderate-to-severe hearing loss) between 7 1/2 and 13 2/3 years of age wore the study hearing aids binaurally for 30 days prior to the evaluation. The testing protocol included speech recognition tests using the CID W-22 word lists presented at 72 dB SPL, 65 dB SPL, and 52 dB SPL (at 0 degrees azimuth) in the presence of a 65 dB SPL party noise (180 degrees azimuth). Subjective rating of hearing aid efficacy in the classroom was examined using the Listening Inventory For Education (LIFE) questionnaire. Parental impression on hearing aid efficacy was also collected at the end of the study. The results showed improved speech recognition in noise with the digital directional hearing aid at all presentation levels. Preference for the digital directional hearing aids over the subjects' own omnidirectional analog hearing aids was also seen on the LIFE questionnaire and parental impression. The degree of hearing loss did not seem to have affected the benefits offered by the digital directional hearing aids. These results were compared to results from other studies on the use of directional microphones in hearing aids.     

Comparison of the electroacoustic characteristics of five hearing aids

This paper describes a comparison of the electroacoustic characteristics of five hearing aids: (1) a linear BTE aid of the type dispensed under the UK National Health Service (NHS), the BE19; (2) an analogue programmable BTE aid incorporating two-channel wide dynamic range compression, the ReSound BT2; (3) and (4) two digital BTE aids incorporating multi-channel wide dynamic range compression, the Phonak Claro and the Danavox Danalogic; (5) a disposable ITE hearing aid with single-channel compression, the Songbird. Measurements of frequency response using a 2 cc coupler showed that the NHS aid had a distinct undesired peak around 1 kHz. The response rolled off at low and high frequencies, and no gain was applied above about 4000 Hz. The BT2 and Claro both showed somewhat irregular responses with effective upper frequency limits of 5000-6000 Hz. The Danalogic had a reasonably smooth response and provided gain up to 6000 Hz. The Songbird had a smooth response and provided gain up to about 7000 Hz. All aids showed reasonably low harmonic and intermodulation distortion (probably below audible levels for hearing-impaired listeners), the Phonak Claro being the best in this respect. Measures of the effective input noise were obtained using two new methods. The NHS aid had the highest (worst) effective input noise, whereas the Songbird had the lowest, especially at low frequencies. The BT2 and the two digital aids had similar noise levels on one measure, but the BT2 was superior on the other measure. The compression circuits were characterized by measuring attack and release times and by using a method described by Stone and Moore (1992). The aids varied markedly in the extent to which they compressed amplitude modulation at the rates typically occurring in speech (2-10 Hz), the Claro providing the least compression and the Danalogic and Songbird aids providing the most. Overall, the results indicate that the NHS aid performed more poorly in several respects than the other aids. There were no great differences in electroacoustic characteristics between the remaining analogue aids and the digital aids, although the Songbird had a somewhat wider frequency range and lower effective input noise than the other aids.     

Impact of compression and hearing aid style on directional hearing aid benefit and performance.

OBJECTIVE: To evaluate the impact of low-threshold compression and hearing aid style (in-the-ear [ITE] versus behind-the-ear [BTE]) on the directional benefit and performance of commercially available directional hearing aids. DESIGN: Forty-seven adult listeners with mild-to-moderate sensorineural hearing loss were fit bilaterally with one BTE and four different ITE hearing aids. Speech recognition performance was measured through the Connected Speech Test (CST) and Hearing in Noise Test (HINT) for a simulated noisy restaurant environment. RESULTS: For both the HINT and CST, speech recognition performance was significantly greater for subjects fit with directional in comparison with omnidirectional microphone hearing aids. Performance was significantly poorer for the BTE instrument in comparison with the ITE hearing aids when using omnidirectional microphones. No differences were found for directional benefit between compression and linear fitting schemes. CONCLUSIONS: No systematic relationship was found between the relative directional benefit and hearing aid style; however, the speech recognition performance of the subjects was somewhat predictable based on Directivity Index measures of the individual hearing aid models. The fact that compression did not interact significantly with microphone type agrees well with previously reported electroacoustic data.     

Advantages of directional hearing aid microphones related to room acoustics.

In this study, two types of hearing aids were used. Both aids had the same frequency characteristics for frontal sound, but one employed an omnidirectional microphone and the other a directional microphone. The frequency characteristics of both hearing aids were measured for five azimuths on KEMAR and in situ in 12 normal-hearing subjects. For these subjects we also determined the speech reception threshold (SRT) with background noise in two rooms with different reverberation times. The direction of the speech stimuli was always frontal; the direction of the noise was varied. Additionally, directional hearing was measured with short noise bursts from eight loudspeakers surrounding the subject. In the less reverberant room, sounds coming from behind were less amplified by the hearing aid with the directional microphone than by the one with the omnidirectional microphone. In this room the monaural SRT values were largely determined by the level of the background noise. For the directional hearing aids there was an extra binaural advantage which depended on the direction of the background noise. Only for low-frequency noise bursts was directional hearing better with directional hearing aids. In the more reverberant room, no distinct differences between the frequency characteristics of the two hearing aid types were measured. However, a systematic difference between monaural SRT values measured through the two hearing aids was found. This difference was independent of noise azimuth. In conclusion, hearing aid(s) with a directional microphone showed no disadvantages and clear advantages under specific conditions.     

Making sense of directional microphone hearing aids

We have witnessed a large increase in the availability of directional microphone hearing aids over the past few years. Directional microphone technology is now available in analog, digitally controlled analog, and digital hearing aids, and has been implemented into both behind-the-ear and in-the-ear styles. This Short Course reviews basic design differences across directional microphone hearing aids. A number of different laboratory and clinical evaluation methods used for assessment of both electroacoustic and behavioral directivity are then reviewed. In addition, the potential impact of test conditions such as room reverberation and type and position of competing noise(s), on listener performance when fit with directional hearing aids are considered. Recommendations and suggestions relating to the clinical and laboratory assessment of directional hearing aids are provided.     

Comparison of hearing aids over the 20th century

OBJECTIVE: To measure hearing aid performance using circuitry representative of the major eras of technological advancement during the 20th century. DESIGN: Twenty subjects with audiometric profiles consistent with hearing aid candidacy were fit with each of seven hearing aids. No directional microphones were used and binaural benefit was not assessed. Each hearing aid was fit to the strategy or fitting scheme of the era, or that which was intended by the presenting manufacturer. Electroacoustic and/or real ear measures of gain, output, bandwidth, and distortion were obtained. Objective outcome measures assessing speech perception in backgrounds of noise were obtained. Subjective outcome measures of sound quality and ease of listening were obtained in the laboratory and in real life settings. RESULTS: Electroacoustic and real ear measures indicate that gain and bandwidth have increased, and output and distortion have decreased with current electronic aids. Speech perception ability across the different outcome measures showed significantly poorer performance with the body and linear hearing aids when input levels were high; when input levels were low, outcome measures with hearing aids using a dynamic range compression were not negatively affected. At the most adverse signal to noise ratios, none of the hearing aids was shown to be superior. Measured bandwidth did not correlate highly with speech perception ability for any of the objective outcome measures used. For the subjective measures of sound quality done in a blinded manner, no significant differences were found across different listening situations for current hearing aids. CONCLUSIONS: The two most important factors for aided speech perception appear to be the audibility and distortion of the signal. No current compression scheme proved superior with the outcome measures used in this investigation.     

Noise reduction hearing aids: release from masking and release from distortion.

Automatic frequency response (AFR) hearing aids usually reduce their low-frequency gain in the presence of noise; several investigators have reported improved recognition of high-frequency speech information in low-frequency band-limited noise with AFR versus non-AFR hearing aids. In this work, masking patterns (masked threshold for frequency-modulated probe tones as a function of probe frequency) were obtained for a narrowband low-frequency noise. Speech recognition threshold for a set of high-frequency loaded monosyllables also was obtained in the presence of the same noise. Aided speech and masking pattern data for one normal and two hearing-impaired subjects wearing a master hearing aid incorporating a commercially available AFR circuit showed modest AFR effects. Moreover, masking noise spectra measured in ear canals of subjects wearing the master hearing aid showed evidence of substantial hearing aid-generated distortion products in the AFR-off condition. Results obtained from the normal subject listening with a low-distortion laboratory simulation of an AFR hearing aid showed greater release from masking for the same low-frequency attenuation as provided by the hearing aid. Improvements of speech recognition in noise observed with AFR hearing aids may result from some combination of release from upward spread of masking and reduction of distortion products generated by the hearing aid in the non-AFR setting.     

Acceptable noise level as a measure of directional hearing aid benefit

An acceptable noise level (ANL) procedure for measuring hearing aid directional benefit was compared with masked speech reception threshold (SRT) and front-to-back ratio (FBR) procedures. ANL is the difference between the most comfortable listening level and the maximum accepted background noise level while listening to speech. Forty adult subjects wearing their own binaural hearing aids were evaluated in omnidirectional and directional modes. The subjects were fitted with a variety of hearing aids by clinical audiologists, independent of the study. For each procedure, speech and noise were presented through loudspeakers located at 0 degrees and 180 degrees azimuth, respectively. Mean ANL (3.5 dB), SRT (3.7 dB), and FBR (2.9 dB) directional benefits were not significantly different. The ANL and masked SRT benefits were significantly correlated. The ANL appears to be a quick, clinician/user friendly procedure for measuring hearing aid directional benefit.     

Full time directional versus user selectable microphone modes in hearing aids

OBJECTIVE: The purpose of this experiment was to systematically examine hearing aid benefit as measured by speech recognition and self-assessment methods across omnidirectional and directional hearing aid modes. These data were used to compare directional benefit as measured by speech recognition in the laboratory to hearing aid wearer's perceptions of benefit in everyday environments across full-time directional, full-time omnidirectional, and user selectable directional fittings. Identification of possible listening situations that resulted in different self reported hearing aid benefit as a function of microphone type was a secondary objective of this experiment. DESIGN: Fifteen adults with symmetrical, sloping sensorineural hearing loss were fitted bilaterally with in-the-ear (ITE) directional hearing aids. Measures of hearing aid benefit included the Profile of Hearing Aid Benefit (PHAB), the Connected Sentence Test (CST), the Hearing in Noise Test (HINT), and a daily use log. Additionally, two new subscales were developed for administration with the PHAB. These subscales were developed to specifically address situations in which directional hearing aids may provide different degrees of benefit than omnidirectional hearing aids. Participants completed these measures in three conditions: omnidirectional only (O), directional only with low-frequency gain compensation (D), and user-selectable directional/omnidirectional (DO). RESULTS: Results from the speech intelligibility in noise testing indicated significantly more hearing aid benefit in directional modes than omnidirectional. PHAB results indicated more benefit on the background noise subscale (BN) in the DO condition than in the O condition; however, this directional advantage was not present for the D condition. Although the reliability of the newly proposed subscales is as yet unknown, the data were interpreted as revealing a directional advantage in situations where the signal of interest was in front of the participant and a directional disadvantage in situations where the signal of interest was behind the listener or localization was required. CONCLUSIONS: Laboratory directional benefit is reflected in self-assessment measures that focus on listening in noise when the sound source of interest is in front of the listener. The use of a directional hearing aid mode; however, may have either a positive, a neutral, or a negative impact on hearing aid benefit measured in noisy situations, depending on the specific listening situation.     

Recognition performance for four combinations of FM system and hearing aid microphone signals in adverse listening conditions.

OBJECTIVE: Children with moderate to severe hearing loss routinely use personal frequency modulated (FM) systems in the classroom to improve the signal to noise ratio of teacher-directed speech with notable success. Attention is now being given to the ability of these children to hear other students via the hearing aid (HA) microphone while using an FM system. As a result, a variety of FM system and HA microphone combinations have been recommended for classroom use. To date, there are no studies regarding the efficacy of these FM/HA combinations. The purpose of this study was to evaluate recognition performance using four FM/HA combinations and to characterize that performance for stimuli received primarily through FM system and HA microphone transmission. DESIGN: Recognition performance for FM system and HA microphone signals was evaluated for two symmetrical and two asymmetrical FM/HA combinations using two commercially available FM systems (one conventional and one FM-precedence circuit). Eleven children (ages 9 to 12) with moderate to severe sensorineural hearing loss and eight children (ages 10 to 11) with normal hearing served as subjects. The two symmetrical FM/HA combinations included: 1) binaural FM system and HA microphone input using the conventional FM system, and 2) binaural FM and HA input using the FM-precedence circuit. The conventional FM system was used for the two asymmetrical combinations and included: 1) binaural FM input and monaural HA input, and 2) FM input to one ear and HA input to the other. Stimuli were 33 consonants presented in the form of nonsense syllables. The stimuli were presented through three loudspeakers representing a teacher and two fellow students in a classroom environment. Speech shaped noise was presented through two additional loudspeakers. RESULTS: In general, no statistically significant differences in recognition performance were found between any of the FM/HA combinations. Mean recognition scores for HA microphone transmission (55%) were significantly poorer than those for FM system transmission (75%). As expected, initial consonants were more easily recognized than final consonants via FM system and HA microphone transmission. However, voiceless consonants were more easily recognized than voiced consonants via HA microphone transmission, which was not predicted on the basis of previous research. CONCLUSIONS: These results suggest that a certain amount of flexibility is present when choosing an FM/HA combination. However, recognition performance via the HA microphones was consistently poorer than performance via FM transmission. Because relevant material also originates from fellow students (e.g., answering teacher-directed questions), input via the HAs is often as important as information originating from the teacher. The results suggest that attempts to improve performance for signals transmitted through the HA microphones in a classroom setting would benefit children with hearing loss.     

Comparison of two methods for estimating the sensation level of amplified speech.

Several methods have been proposed to estimate the sensation level (SL) at which children receive amplified speech from their hearing aids. The present study compared the SL estimates obtained with two such methods: (1) a sound field aided audiogram approach, and (2) an electroacoustic approach that incorporated the use of a probe tube microphone system (Seewald, Ross, & Stelmachowicz, 1987). Sound field aided thresholds were obtained for 13 hearing-impaired subjects at eight audiometric frequencies. For the electroacoustic approach, in situ thresholds were obtained using a button-type hearing aid receiver attached to a custom earmold. Real ear aided responses were measured using a 70 dB RMS speech-weighted composite noise signal (Frye, 1986). A comparison of the frequency-specific SL estimates derived from the two different methods revealed that the sound field aided audiogram approach yielded higher SL estimates for 74% of the individual comparisons. A detailed analysis of the findings obtained from two subjects suggested that when the results of the two methods did not agree, the differences were due to an interaction between signal level and the unique input/output characteristics of the subjects' hearing aids. A precautionary measure is suggested for those who wish to use sound field aided threshold data to estimate the SLs at which children receive amplified conversational speech.     

Predictability of speech-in-noise performance from real ear measures of directional hearing AIDS

OBJECTIVE: Inability to understand speech in noise has been cited repeatedly as the principal complaint of hearing aid users. While data exist documenting the benefit provided by hearing aids with directional microphones when listening to speech in noise, little work has been done to develop a standard clinical protocol for fitting these hearing aids. Our goal was to evaluate a clinical measure of the acoustic directivity of a directional hearing aid, including its association with a test of speech perception in noise. DESIGN: The performance of two commercially available directional behind-the-ear (BTE) hearing aids was evaluated using the Hearing in Noise Test (HINT) and the Real Ear Aided Response (REAR) on 24 adult participants with symmetric, mild to moderately severe, sensorineural hearing loss. The HINT was conducted with the speech signal presented from 0 degrees and the noise from 180 degrees and either 135 degrees or 225 degrees, depending on the ear tested. REAR was measured at the above three angles using swept pure tones, and these measures were used to compute in situ directivity for each subject and hearing aid. CONCLUSIONS: Directional benefit for the HINT was greatest when noise was presented from the azimuth of the published polar diagram null of a given hearing aid in its directional mode (180 or 135/225 degrees). The only significant correlation between HINT and REAR results, however, was found when the noise source was at 180 degrees. These results confirm the validity of using real ear measures as a way to assess directionality in situ, but also indicate the complexity of predicting perceptual benefit from them. These data suggest that factors beyond acoustic directionality may contribute to improvement in speech perception in noise when such improvements are found.     

Directivity quantification in hearing aids: fitting and measurement effects

OBJECTIVE: To evaluate the impact of venting, microphone port orientation, and compression on the electroacoustically measured directivity of directional and omnidirectional behind-the-ear hearing aids. In addition, the average directivity provided across three brands of directional and omnidirectional behind-the-ear hearing aids was compared with that provided by the open ear. DESIGN: Three groups of hearing aids (four instruments in each group) representing three commercial models (a total of 12) were selected for electroacoustic evaluation of directivity. Polar directivity patterns were measured and directivity index was calculated across four different venting configurations, and for five different microphone port angles. All measurements were made for instruments in directional and omnidirectional modes. Single source traditional, and two-source modified front-to-back ratios were also measured with the hearing aids in linear and compression modes. RESULTS: The directivity provided by the open (Knowles Electronics Manikin for Acoustic Research) ear was superior to that of the omnidirectional hearing aids in this study. Although the directivity measured for directional hearing aids was significantly better than that of omnidirectional models, significant variability was measured both within and across the tested models both on average and at specific test frequencies. Both venting and microphone port orientation affected the measured directivity. Although compression reduced the magnitude of traditionally measured front-to-back ratios, no difference from linear amplification was noted using a modified methodology. CONCLUSIONS: The variation in the measured directivity both within and across the directional microphone hearing aid brands suggests that manufacturer's specification of directivity may not provide an accurate index of the actual performance of all individual instruments. The significant impact of venting and microphone port orientation on directivity indicate that these variables must be addressed when fitting directional hearing aids on hearing-impaired listeners. Modified front-to-back ratio results suggest that compression does not affect the directivity of hearing aids, if it is assumed that the signal of interest from one azimuth, and the competing signal from a different azimuth, occur at the same time.     

Characterization of non-linear distortion in hearing aids using coherence analysis. A pilot study

Coherence is a frequency-domain measure of linear dependence between input and output of a system, e.g. a hearing aid, and describes the cumulative effect of different forms of signal corruption, e.g. noise and non-linear distortion. From the coherence function, a general frequency-dependent signal-to-noise ratio can be derived. In this investigation, the applicability of this measuring technique is demonstrated in connection with non-linear distortion in hearing aids. The influence of hearing aid gain and automatic gain control is illustrated, with speech-shaped noise as input signal. For the three hearing aids tested. The gain setting influences the signal-to-noise ratio heavily due to non-linear distortion, especially near maximum gain. The introduction of automatic gain control reduces the effect of non-linear distortion somewhat at high gain settings.     

Digital signal processing hearing aids, personal FM systems, and interference: is there a problem?

OBJECTIVES: To determine whether digital signal processing (DSP) hearing aids produce conducted radio frequency interference that can affect the use of personal FM systems, to quantify the nature of any such interference, and to discuss practical remedies. DESIGN: Sixteen DSP hearing aids were used. Measurements were made of the spectral characteristics of any conducted radio frequency interference produced by each aid with FM shoe and 40 cm direct audio input (DAI) lead when the DAI facility was enabled. Measurements were made with the aid, shoe, and lead inside an electrically screened chamber. The effect of DAI lead length was also investigated with one of the hearing aids. Finally, some subjective listening tests were carried out by using different FM systems coupled to a number of the aids. RESULTS: All but four of the DSP hearing aids tested produced readily measurable interference, with some much worse than others. Levels of interference were high enough with some hearing aids to be likely to significantly impede signal perception when the radio frequency of the interference coincided with the radio frequency of the FM system. This usually occurs intermittently as a result of the processor design of most DSP hearing aids. The listening tests suggested that when personal FM systems are in use with some DSP hearing aids, the interference would be audible, unpleasant, and detrimental to audio quality. CONCLUSIONS: DSP hearing aids without low electromagnetic interference processors should not be fitted to clients if personal FM systems are expected to be used. Manufacturers of DSP aids should be encouraged to use low electromagnetic interference processors in their DSP hearing aid design. Meanwhile, FM systems should be used with DSP hearing aids in such a way as to ensure high received radio signal levels, and FM receivers should be switched off when not in use.