How to assess outcome of hearing aid fitting in children.

Assessment of outcome of hearing aid fitting in children should contain several dimensions: audibility, speech recognition, subjective benefit and speech production. Audibility may be determined by means of aided hearing thresholds or real-ear measurements. For determining speech recognition, methods different from those used for adult patients must be used, especially for children with congenital hearing loss. In these children the development of the spoken language and vocabulary has to be considered, especially when testing speech recognition but also with regard to speech production. Subjective assessment of benefit to a large extent has to rely on the assessment by parents and teachers for children younger than school age. However, several studies have shown that children from the age of around 7 years can usually produce reliable responses in this respect. Speech production has to be assessed in terms of intelligibility by others, who may or may not be used to the individual child's speech. Ideally, the outcome should be assessed repeatedly at suitable intervals in order to be able to follow the child's communicative development.

Clinical assessment of modern hearing aids.

The increasing complexity of the non-linear signal processing of modern hearing aids makes the use of clinical assessment of hearing aid benefit more and more important. The paper discusses the general structure of such studies with regard to choice of test subjects, reference hearing aids and the advantage of double-blind and cross-over design. Assessment should contain at least the three dimensions: perceived hearing aid benefit (preferably including perceived sound quality and preference between test and reference aids), speech recognition in noise, and electroacoustic verification by means of real-ear measurements.

Hearing thresholds for speech using insert earphones versus supra-aural earphones.

Speech recognition thresholds (SRT) have been determined in two groups of subjects, one with normal hearing and one with various types and degrees of hearing loss. One ear was tested on each subject, using spondees as test-words, with Telephonics TDH-39 supraural earphones and EARTONE 3-A insert earphones. The mean difference in SRT using the two transducers was 1 dB and not statistically significant. No difference was seen between the two groups of subjects. We conclude that the two transducers, in spite of measurable differences in frequency response, are interchangeable. If an exact calibration of the insert earphone is to be made, the equipment should be adjusted to produce 1 dB lower test levels for the insert earphone than for the TDH-39 in standardized calibration set-ups.

Masking of speech by amplitude-modulated noise.

The masking of speech by amplitude-modulated and unmodulated speech-spectrum noise has been evaluated by the measurement of monaural speech recognition in such noise on young and elderly subjects with normal-hearing and elderly hearing-impaired subjects with and without a hearing aid. Sinusoidal modulation with frequencies covering the range 2-100 Hz, as well as an irregular modulation generated by the sum of four sinusoids in random phase relation, was used. Modulation degrees were 100%, +/- 6 dB, and +/- 12 dB. Root mean-square sound pressure level was equal for modulated and unmodulated maskers. For the normal-hearing subjects, essentially all types of modulated noise provided some release of speech masking as compared to unmodulated noise. Sinusoidal modulation provided more release of masking than the irregular modulation. The release of masking increased with modulation depth. It is proposed that the number and duration of low-level intervals are essential factors for the degree of masking. The release of masking was found to reach a maximum at a modulation frequency between 10 and 20 Hz for sinusoidal modulation. For elderly hearing-impaired subjects, the release of masking obtained from amplitude modulation was consistently smaller than in the normal-hearing groups, presumably related to changes in auditory temporal resolution caused by the hearing loss. The average speech-to-noise ratio required for 30% correct speech recognition varied greatly between the groups: For young normal-hearing subjects it was -15 dB, for elderly normal-hearing it was -9 dB, for elderly hearing-impaired subjects in the unaided listening condition it was +2 dB and in the aided condition it was +3 dB. The results support the conclusion that within the methodological context of the study, age as well as sensorineural hearing loss, as such, influence speech recognition in noise more than what can be explained by the loss of audibility, according to the audiogram and the masking noise spectrum.

Speech recognition in noise before and after a work-day's noise exposure.

A common complaint after a work-shift in high noise levels is fatigue, in the sense that listening to other people's spoken messages requires more effort than when well rested. The purpose of this study was to examine if this could be verified as reduced speech recognition in noise due to auditory fatigue. Speech recognition in noise, using low-redundancy sentence material, as well as pure-tone hearing thresholds were tested on 13 subjects at the beginning and end of a 7-8 hours work-shift with equivalent noise levels in the range 78-90 dBA as measured by personal noise dose meters. Seven of the test subjects wore hearing protectors during work while six did not. No significant shift in mean values from the beginning of the work-shift to its end could be found, neither in speech recognition in noise nor in hearing thresholds for pure tones and neither for those wearing hearing protectors nor for those without.

Normal hearing threshold levels in the low-frequency range determined by an insert earphone.

Hearing threshold levels have been determined in the low-frequency range (20-500 Hz) on a group of 30 young normal-hearing subjects using monaural stimulus presentation through an insert earphone (Etymotic Research ER-3A). A retest was performed on half of the group to provide data on test-retest reliability. The mean hearing threshold levels obtained agree closely with the Minimum Audible Field data of ISO 226, however, with some deviation at the very lowest frequencies below 40 Hz. The test-retest difference results yielded mean values that averaged 1.15 dB with an average standard deviation across test frequencies of 3.9 dB. The results show that low-frequency hearing thresholds for pure tones of frequencies from 40 Hz and up can be determined with acceptable validity and reliability by the use of this type of insert earphone.

Reliability in warble-tone sound field audiometry.

Test-retest reliability of sound field audiometry has been evaluated by means of repeated determinations of hearing threshold levels for frequency-modulated tones on normal-hearing and hearing-impaired subjects. Two modulation frequencies, 5 and 20 Hz, and two frequency deviations, 4 and 25%, were used, with centre frequencies in the range from 250 to 8,000 Hz. No significant influence on the standard deviations for test-retest differences was found for any of the three parameters studied: modulation frequency, frequency deviation, and normal versus impaired hearing in the listeners. The standard deviation values obtained were smaller than those previously obtained for regular pure-tone threshold audiometry using headphones. They indicate a test-retest reliability allowing functional gain measurements of hearing aids on the real ear to be performed at least as accurately as insertion gain measurements at frequencies above 2 kHz. In the hearing-impaired listeners, some influence was found from modulation frequency and frequency deviation on the sound field hearing threshold levels obtained.

Audiological and vestibulo-oculomotor findings in workers exposed to solvents and jet fuel.

Three groups of subjects with long-term (5-41 years) occupational exposure to industrial solvents have been evaluated with extensive audiological and vestibular test batteries. Group A comprised 16 subjects with a confirmed diagnosis of psycho-organic syndrome (POS), while group B consisted of 7 subjects with suspected POS. Both groups had been exposed to mixtures of aliphatic and aromatic solvents. Eight subjects with long-term exposure to jet fuel constituted group C. In the audiological test battery, discrimination of interrupted speech and evoked cortical potentials in response to frequency glides were the tests that yielded significantly abnormal results. In the vestibular test battery, considerable pathology was seen in electronystagmography, and in addition, visual suppression test and saccade test indicated CNS disturbance. In general, when a test yielded pathological results, the incidence of pathology was highest in group A and lowest in group C.

Sound attenuation of TDH-39 earphones in a diffuse field of narrow-band noise.

The sound attenuation characteristics of the audiometric earphone Telephonics TDH-39 with cushion MX-41/AR have been tested, using the hearing threshold method according to the International Standard ISO 4869-1981 ("Acoustics--Measurement of sound attenuation of hearing protectors--Subjective method"). This method specifies 1/3-oct bands of noise as test signals and a diffuse sound field. This is in contrast to the use of pure tones in a frontally incident free sound field, by means of which most earlier attenuation data have been obtained. The mean sound attenuation values obtained from a group of 20 normal-hearing subjects were up to 3-5 dB lower than those previously reported for pure tones in a free field.

Auditory evoked cortical responses to frequency glides in subjects with retrocochlear hearing impairment.

Auditory cortical responses evoked by stimulation with frequency glides of a continuous tone have been recorded from 44 subjects with intracranial tumours affecting auditory function. Response latencies obtained when stimulating the ear on the side of the tumour were different from the non-tumour ear as well as from groups with cochlear impairment and normal hearing. Tumours caused a reduction of a specific sensitivity to the frequency glide stimulation, affecting further processing of the dynamic change in tone frequency. The test appears to have diagnostic potential.

Preoperative bone-conducted electrocochleography in otosclerosis.

Prior to surgery, pure-tone audiometry and bone-conducted (BC) electrocochleography (ECOG) were performed on 17 patients with preoperatively diagnosed otosclerosis. Short tone bursts, approximately 5 ms in duration and presented by means of a bone conductor on the exposed surface of the mastoid, were used as stimuli in BC ECOG. The thresholds thus obtained were compared by regression analysis with conventional psychoacoustic thresholds. Thresholds recorded at BC ECOG showed a close correlation to preoperative pure-tone BC thresholds, and there was no difference in predictability of postoperative hearing thresholds between preoperative BC ECOG and conventional preoperative pure-tone BC thresholds. The results lend support to the conclusion that BC ECOG, offering a monaural, objective estimation of cochlear function, is a valuable complement to conventional audiometry in the preoperative evaluation of hearing in otosclerosis.

Hearing gain calculations after stapedectomy.

A comparison of the postoperative air-bone gaps to pre- and postoperative bone condition (BC) was made in two groups of patients with otosclerosis, 18 cases with poor preoperative BC thresholds and 15 cases with good preop, thresholds. The gaps to the postoperative BC showed less variation than the gaps to the preop. BC threshold, the postop. BC may therefore serve as a more stable and natural reference when calculating a postoperative air--bone gap.

Comparison of ascending and bracketing methods in pure tone audiometry. A multi-laboratory study.

Four laboratories participated in this study to compare ascending and bracketing methods in manual determination of hearing thresholds using attenuators with 5 dB step. In all, 214 ears were measured. No significant difference was found between mean thresholds obtained with the two methods in the frequency range 0.5--6 kHz; the four-laboratory, six-frequency grand mean difference was -0.31 dB. The variability among subjects also remained essentially the same for the two methods. However, the bracketing method required a significantly longer test time than the ascending method.

Skull distortion of bone conducted signals.

A previously essentially unknown type of distortion of bone conduction (BC) signal has been studied on the skulls of four human cadavers. The method was based on a miniature accelerometer, rigidly attached to the cranial bone, converting the skull vibration close to the cochlea into an electrical signal which was analysed with regard to harmonic distortion. The BC signals, pure tones, were presented by means of a high-quality vibrator. The distortion was found to be limited to the lower audiometric frequencies, with a maximum around 500 Hz, and of such a degree as to be able to significantly influence the results of BC audiometry. The distortion is probably caused by nonlinear mechanical properties of the human skull.

Thresholds for linear amplitude change of a continuous pure tone.

The human auditory sensitivity in detecting linear amplitude change of a continuous pure tone has been studied in normal-hearing subjects. It is shown that for short glide durations (less than 100 ms) the duration of the following plateau exerts a significant influence on the DLI. The average DLI at 1 kHz and 60 dB HL was found to be of the order of 0.8 dB when the intensity glide had a duration of 10 ms and was followed by a much longer plateau. For longer glide durations (greater than or equal to 200 ms) the DLI increased significantly as compared with shorter durations. There was no significant difference between increasing and decreasing intensity change. Significantly larger DLIs were found at 250 and 500 Hz than at 1, 2 and 4 kHz. The sound level was found to have a significant influence on the DLI. At low levels of 40 dB HL, and lower, the increase in DLI for detecting sound levels is highly significant. A falling exponential function offers a mathematical description of the relationship with good fit. It is concluded that an integrating mechanism with an integration time of approx. 200 ms could explain the auditory ability to detect linear amplitude glides of a continuous tone. The results are discussed in relation to previous intensity discrimination data, where pulse pairs, continuous intensity modulation or intensity glides were used as stimuli.

Bone-conducted stimulation in electrocochleography.

The mechanical vibration patterns close to the cochlea in intact skulls of human cadavers have been studied by means of a miniature accelerometer. A Radioear B70A vibrator and a Brüel & Kjaer Mini Shaker have been used, fed with filtered clicks and with short tone bursts. The tone bursts were found to be superior to the clicks with regard to the vibration spectrum. At 500 Hz a considerable distortion was observed in the accelerometer signal, also when using tone bursts. This distortion was presumably due to resonant vibrations in the skull itself, and may be a source of error not only when using stimuli of short duration as in bone-conduction ECoG but also in conventional bone-conduction audiometry. When the vibrations were applied to the exposed bone surface of the mastoid, vibration levels increased by 10-25 dB compared with when soft tissues covered the point of application. This could be of advantage in bone-conduction ECoG performed at ear surgery.

Peroperative temporary threshold shift in ear surgery. An electrocochleographic study.

TTS resulting from drilling in the temporal bone was detected by means of pre- and post-exposure ECoG recordings taken during ear surgery for chronic otitis media. The TTS obtained varied between 5 and 40 dB at 4 and 8 kHz. The correlation between duration of noise exposure and magnitude of TTS was statistically significant. The results indicate that drill-induced noise in ear surgery may result in postoperative high-tone sensorineural hearing loss, and support the view that manipulation of the ossicular chain during ear surgery causes mainly a threshold shift at the lower frequencies.

Electrocochleography used as a clinical hearing test in difficult-to-test children.

Thirty difficult-to-test children have been tested with transtympanic ECoG. When possible, informal hearing tests and/or free-field testing were performed. Children in whom no action potential (AP) could be recorded were submitted to conventional radiography of the inner ear, and vestibular tests. The correlation between free-field test thresholds and AP "thresholds" was good, especially in subjects with relatively good hearing. Response amplitude increased, and latency decreased, with increasing frequency of the stimulus implying that different parts of the basilar membrane are stimulated according to the frequency of the stimulus. Input-output curves of response amplitude and latency were plotted, and three different types were distinguished. ECoG can contribute to the evaluation of peripheral hearing in difficult-to-test children, and vestibular tests should always be performed on a child with suspected deafness or sensorineural hearing loss. Conventional radiography of the inner ear, however, seems to be of little value.

Slow evoked cortical responses to linear frequency ramps in subjects with cochlear lesions.

Slow evoked cortical potentials in response to linear frequency ramps of a continuous pure tone with a 1 kHz base frequency have been recorded from ten relatively young subjects with hearing loss of cochlear origin. At small frequency ramps, the N1-latency of their responses to the three ramp durations studied (20, 100, 500 msec) was significantly longer than those of a group with normal hearing. As the ramps are made larger, the difference between the latencies of the pathological group and the normal group becomes smaller; above a certain rate of frequency change, the latency of N1 becomes smaller in the pathological cases than in the normal group. This crossover occurs at frequency change rates around 1-3 kHz/sec. It is concluded that recording of evoked cortical responses to frequency ramps may provide an additional tool in the differential diagnosis of hearing disorders.