Acoustic radiation from bone vibrators

Extended high-frequency bone conduction audiometry Calibration of bone conductor transducers in the conventional and extended high-frequency range

OBJECTIVE

To monitor ototoxicity, air conduction (AC) extended high frequency (EHF) thresholds can be measured up to 16 kHz. However, conductive hearing loss might influence these results. This is unfortunate because the EHF thresholds are important to follow the impact of ototoxic medication during therapy. Therefore a suitable bone conduction (BC) transducer and norm values for EHF BC measurements are needed.

DESIGN

In this study three different BC transducers were used: the B71 (Radioear), the KH70 (Präcitronic), and the KLH96 (Westra). Hearing thresholds were measured from 0.125 to 16 kHz using AC transducers (Telephonics TDH39, Sennheiser HDA200), and BC thresholds from 0.25 to 8 kHz with the B71, and from 0.25 to 16 kHz with the KLH96 and KH70.

STUDY SAMPLE

60 ears of 30 normal hearing subjects were measured.

RESULTS

The KLH96 showed the highest output for the high frequencies, and distortion measurements were similar to the KH70. The results show that EHF measurements are possible using the KLH96 and KH70 bone conductors.

CONCLUSION

EHF BC measurements are reliable when using the KLH96 and KH70 bone conductors. The extended force sensitivity of the used artificial mastoid should be determined for a proper EHF BC calibration.

Effects of Stimulation Position and Frequency Band on Auditory Spatial Perception with Bilateral Bone Conduction

Virtual sound localization tests were conducted to examine the effects of stimulation position (mastoid, condyle, supra-auricular, temple, and bone-anchored hearing aid implant position) and frequency band (low frequency, high frequency, and broadband) on bone-conduction (BC) horizontal localization. Non-individualized head-related transfer functions were used to reproduce virtual sound through bilateral BC transducers. Subjective experiments showed that stimulation at the mastoid gave the best performance while the temple gave the worst performance in localization. Stimulation at the mastoid and condyle did not differ significantly from that using air-conduction (AC) headphones in localization accuracy. However, binaural reproduction at all BC stimulation positions led to similar levels of front-back confusion (FBC), which were also comparable to that with AC headphones. Binaural BC reproduction with high-frequency stimulation led to significantly higher localization accuracy than with low-frequency stimulation. When transcranial attenuation (TA) was measured, the attenuation became larger at the condyle and mastoid, and increased at high frequencies. The experiments imply that larger TAs may improve localization accuracy but do not improve FBC. The present study indicates that the BC stimulation at the mastoid and condyle can effectively convey spatial information, especially with high-frequency stimulation.

Influence of transducer types on bone conduction hearing thresholds

Objective

Different types of bone conduction transducers with different physical and electro-acoustic properties are available for audiometric hearing threshold measurements. The reference equivalent threshold vibratory force levels (RETVFL) specified in ISO 389–3 are based on measurements conducted with the B71 and KH70 transducers but apply to all types of transducers available for bone conduction audiometry. The objective of this study was to compare bone conduction hearing thresholds measured by different transducers.

Design

In a prospective study the hearing thresholds were measured psychometrically between 125 Hz and 8000 Hz using the Radioear B71, B81 and Präcitronic KH70 transducers.

Study sample

Twenty-one normal hearing participants and fifteen hearing impaired participants.

Results

In both groups significant differences were found between the thresholds measured with the different transducers at the low frequencies 125 Hz and 250 Hz and the high frequencies 3000 Hz, 4000 Hz, 6000 Hz and 8000 Hz. In the normal hearing group, deviations from the reference threshold 0 dB HL towards lower thresholds were observed for the B71 and B81 at 125 Hz and at the high frequencies 3000 Hz, 4000 Hz, 6000 Hz and 8000 Hz.

Conclusions

RETVFL-values should be reassessed and provided specifically for the different transducers.

Is it necessary to occlude the ear in bone-conduction testing at 4 kHz, in order to prevent air-borne radiation affecting the results?

OBJECTIVE

To re-evaluate the current BSA recommendation that the test ear should be occluded during the bone-conduction procedure at frequencies above 2 kHz to prevent audible air-borne radiation.

DESIGN

Pure-tone audiometry was undertaken during routine hearing tests. The audiograms of fifty-two ears met the criteria for the study and were included. Bone conduction at 4 kHz was tested in three different conditions: test ear open/occluded by earplug and occluded by circumaural earphone.

STUDY SAMPLE

Forty-four adults aged 41-77 years with average hearing levels from normal to severe loss. All complied fully with the test procedure. No audiogram had a significant conductive element.

RESULTS

There was no significant difference in each of the three test situations. Only two audiograms showed any (5 dB) difference at 4 kHz when bone conduction was retested with the ear occluded.

CONCLUSIONS

The errors that result in a false air-bone gap at 4 kHz would not appear to be due to air-borne radiation. Failure to occlude the ear canal at 4 kHz, where air-borne radiation is greatest, makes no significant difference to the audiometric results. It is therefore suggested that it is unnecessary to block the test ear during routine pure-tone bone-conduction testing to prevent audible air-borne radiation, and that this should no longer form part of normal clinical practice.

Aging and the 4-kHz air-bone gap

PURPOSE

In this study, the authors assessed age- and sex-related patterns in the prevalence and 10-year incidence of 4-kHz air-bone gaps and associated factors.

METHOD

Data were obtained as part of the longitudinal, population-based Epidemiology of Hearing Loss Study (Cruickshanks et al., 1998). An air-bone gap at 4 kHz was defined as an air-conduction threshold ≥ 15 dB higher than the bone-conduction threshold in the right ear.

RESULTS

Among 3,553 participants ages 48-92 years at baseline (1993-1995), 3.4% had a 4-kHz air-bone gap in the right ear. The prevalence increased with age. Among the 120 participants with an air-bone gap, 60.0% did not have a flat tympanogram or an air-bone gap at 0.5 kHz. Ten years later, the authors assessed 2,093 participants who did not have a 4-kHz air-bone gap at baseline; 9.2% had developed a 4-kHz air-bone gap in the right ear. The incidence increased with age. Among the 192 participants who had developed an air-bone gap, 60.9% did not have a flat tympanogram or air-bone gaps at other frequencies.

CONCLUSION

These results suggest that a finding of a 4-kHz air-bone gap may reflect a combination of aging and other factors and not necessarily exclusively abnormal middle-ear function.

Acoustic method for calibration of audiometric bone vibrators

The standard method for the calibration of audiometric bone vibrators requires the use of an artificial mastoid, a device that converts vibratory energy to an electrical analog. The mechanical input impedance of the device is designed to represent the average mechanical impedance of the human head. For calibration purposes, it is not necessary that the coupling device represent the impedance of the head. It is only necessary that it provides a repeatable measurement of the output of the vibrator that can be related to the normal threshold of hearing at each test frequency. In addition to the mechanical output that serves as the stimulus for the hearing test, bone vibrators produce an acoustic signal that is proportional to the mechanical force delivered to the head. By determining the transfer function relating the acoustic sound pressure to the mechanical force, the acoustic signal can serve as a proxy for the vibratory stimulus. This article describes the design and validation of an acoustic coupler for the calibration of audiometric bone vibrators.

AMTAS: automated method for testing auditory sensitivity: validation studies

Three studies are reported assessing the validity of AMTAS, an automated method for obtaining an audiogram, including air- and bone-conduction thresholds (stimuli delivered by a forehead-placed transducer) with masking noise presented to the non-test ear. In Study 1, six subjects at each of three sites were tested using manual audiometry by two audiologists at each site. The mean differences between the audiograms for the paired audiologists provided a measure of the reliability of traditional audiometry. In Study 2, thirty subjects (5 normal hearing, 25 hearing impaired) were tested using AMTAS and manual audiometry. For air-conduction thresholds, AMTAS-manual differences were similar to inter-tester differences in Study 1, but for bone-conduction thresholds, the former were larger. Two possible sources of the greater differences were identified, (1) incorrect reference-equivalent threshold force levels for forehead bone conduction, and (2) a differential effect of middle-ear disease on forehead and mastoid bone-conduction thresholds. In Study 3, intersubject variability was studied for forehead and mastoid bone-conduction thresholds. The results indicate similar variability for the two placement sites.

Elevated bone conduction thresholds associated with middle ear fluid in adults

Longitudinal observations of adult patients with documented cases of otitis media revealed fluctuations in bone conduction thresholds as well as air conduction thresholds. Previous investigations in this area presented conflicting information regarding temporary and permanent effects of serous otitis media on sensori-neural function. We conducted a detailed study, including complete otologic, audiologic, and tympanometric evaluation, of 30 adult patients exhibiting serous otitis media. Myringotomies were performed on all patients after appropriate medical management failed to clear the middle ear fluid and subsequent hearing loss. Pre- and postmyringotomy audiograms support our conclusion that middle ear fluid can produce artifactual shifts in bone conduction thresholds. Although the data presented was collected from a cohort of adults, the clinical implications are applicable to the pediatric population. We have observed a similar shift in bone conduction thresholds in children exhibiting serous otitis media, and we have observed improvement in the thresholds with removal of the fluid either by appropriate medical management or by myringotomy with fluid aspiration.