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Frontiers in Audiology and Otology ·

Volume 4 - 2026 | https://doi.org/10.3389/fauot.2026.1850851

Reference threshold measures for a non-surgical skin-drive bone conduction hearing device

  • Eileen Webster1*corresponding author
  • Jonas Lochner2
  • Paula Folkeard1
  • Susan Scollie1,3
  • Hamidreza Mojallal2
  • Marlene Bagatto1
  1. 1National Centre for Audiology, Western University, London, ON, Canada
  2. 2MED-EL Medical Electronics, Innsbruck, Austria
  3. 3Bone and Joint Institute, Western University, London, ON, Canada
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Abstract

Skin-drive bone conduction devices (BCDs) are indicated for individuals with conductive or mixed hearing loss. For individuals who are not surgical candidates (i.e., young children), non-surgical skin-drive systems are available. Prescriptive targets for abutment-worn, percutaneous systems are established; however, device-specific reference equivalent threshold force levels (RETFLs) are not available for some contemporary skin-drive systems. Equivalent threshold force levels (ETFLs) may serve as preliminary reference data to support prescriptive development. Because standardized RETFLs require multi-site validation and are not yet available for these coupling conditions, the values derived in the present study are referred to as ETFLs. This terminology is used to distinguish these preliminary, single-site estimates from formally established RETFLs used for clinical calibration. This study derived ETFL values for the MED-EL ADHEAR when coupled via a soft headband and adhesive adapter. Thirty-six normal-hearing adults (18–25 years) underwent audiometric testing with a B-71 bone oscillator and in-situ threshold measurement using ADHEAR in the two coupling conditions. Force level output was measured on a skull simulator using the Audioscan ® Verifit 2. Dial force levels (DFLs) and ETFLs were calculated across 250–8000 Hz. Both ADHEAR coupling conditions produced significantly higher in-situ thresholds than the B-71 (p < 0.001). Threshold elevation was frequency-dependent, with a pronounced increase at 4,000 Hz. DFL patterns differed between the B-71 reference data (measured on the Artificial Mastoid) and ADHEAR measured on the Verifit Skull Simulator using a direct coupling adapter, particularly at higher frequencies. Derived ETFLs were higher for both ADHEAR coupling methods compared to the B-71 across most frequencies. These findings provide preliminary, device-specific ETFL data for the ADHEAR system. While not constituting standardized RETFL values, these results may inform future multi-site validation and prescriptive fitting approaches for these non-surgical skin-drive BCDs.

Keywords

1 Introduction

Conductive or mixed hearing loss in children and adults may be managed using bone-conduction devices (BCDs), which bypass the middle ear and stimulate the cochlea through vibration of the soft tissue and skull. Direct-drive BCDs include percutaneous systems, which implant an abutment in the mastoid that protrudes through the skin to allow for direct drive of vibration to the skull (Håkansson et al., 2019). Direct-drive and skin-drive transcutaneous systems are also a surgical option. Active transcutaneous systems implant the transducer under the skin with a magnet holding the audio processor over the intact mastoid skin to transmit vibrations directly to the skull without an abutment (Håkansson et al., 2019). Skin-drive transcutaneous systems have an external processor and transducer on top of the skin and a magnet implanted beneath the skin (Håkansson et al., 2019). Young children, typically under age 5 years, are not candidates for surgical BCDs of any configuration. They do not have the skull thickness required to support the current devices (Kim et al., 2020). As such, non-invasive skin-drive options are available and are coupled to the mastoid using a soft headband or adhesive adapter (Bagatto et al., 2022; Maier et al., 2022). These systems provide timely auditory access during critical stages of speech and language development for children with congenital permanent conductive or mixed hearing loss (Pittman, 2019; Skarzynski et al., 2019; Stelmachowicz et al., 2004; Walker, 2023). Clinical studies have reported favorable outcomes and user satisfaction in both pediatric and adult populations, particularly for children who are not yet candidates for surgically-implanted BCDs (Dahm et al., 2018; Liu et al., 2022; Neumann et al., 2019).

Coupling-related limitations of skin-drive BCDs are well documented (Dufour-Fournier et al., 2022; Gascon et al., 2022; Gordey and Bagatto, 2021). Finite-element modeling has demonstrated that skin-drive BCDs exhibit reduced high-frequency transmission relative to direct-drive percutaneous systems, largely due to the mechanical point impedance introduced by the soft-tissue interface (Chang and Stenfelt, 2019). These modeled results complement clinical findings, which show frequency-dependent attenuation in skin-drive BCDs. Percutaneous BCDs typically deliver more efficient high-frequency transmission than skin-drive systems, while the compliant soft-tissue interface of skin-drive couplings can introduce frequency-dependent attenuation, most notably at and above 4,000 Hz (Pittman, 2019). These differences highlight the importance of understanding how the coupling strategy affects device output.

A central component of bone-conduction calibration is the reference equivalent threshold force level (RETFL), which provides standardized force-level values corresponding to normal hearing thresholds for a specific transducer and coupling configuration. RETFLs exist for the clinical B-71 bone oscillator and form the basis for audiometric calibration and threshold interpretation (ANSI S3.6-2004 (R2018); Hodgetts and Scollie, 2017). However, comparable reference data are not yet available for some contemporary skin-drive BCDs. Recent work has begun to propose new approaches for defining output and reference metrics for modern BCDs, but robust, multi-site datasets are still needed to establish clinically valid RETFLs for these systems. Accurate programming of BCDs also depends on the availability of device-specific RETFL measurements. Prescriptive fitting algorithms such as DSL-BCD v2.0 translate hearing thresholds into target force levels (FL) using calibrated reference data (Scollie et al., 2025). While DSL-BCD prescriptive targets can be generated reliably for percutaneous devices with known RETFLs, less is known about prescriptive methods for non-invasive skin-drive systems. This leaves clinicians without validated programming methods for these devices, particularly in young children who rely on non-surgical bone-conduction systems for early auditory access (Zumach et al., 2011). Establishing device-specific RETFL values may inform accurate, evidence-based programming using modern fitting algorithms. The need for clearer FL characterization is underscored by recent evidence that nominal force readings do not reliably reflect actual force at the transducer–skin interface. Gascon et al. (2022) and Gascon et al. (2022) demonstrated significant discrepancies between dial levels and measured coupling force across different devices and headband systems, raising important questions about which force metric is most valid for clinical and research comparisons. Similarly, El-Naji et al. (2024) reported that FL output from skin-drive devices often deviates from prescribed values, particularly at higher frequencies. More recent findings from El-Naji et al. (2026) further emphasize variability in output and coupling-dependent performance in modern skin-drive systems, reinforcing the need for standardized reference metrics. These studies underscore the need for more accurate, device-specific measures to describe the characteristics of hearing thresholds for BCDs across coupling methods. The B-71 oscillator has long served as the clinical standard for bone-conduction audiometry, with well-established RETFLs [ANSI, 2004 (R2018)]. Normative threshold values, referenced to force level, for percutaneous bone conduction are also available (e.g., Carlsson and Håkansson, 1997). In contrast, less is known about how in-situ thresholds, dial-to-force levels (DFLs), and derived force-level metrics behave in newer adhesive and pressure-worn systems such as ADHEAR (El-Naji et al., 2026). While the present study did not directly measure coupling force at the skin interface, these findings provide important context for interpreting force-level metrics. The present study compared in-situ thresholds and estimated threshold force levels (ETFLs) across three coupling methods: the B-71 and two coupling configurations for the ADHEAR system (adhesive and soft headband). This study provides preliminary, device-specific ETFL values for the MED-EL ADHEAR system under two coupling conditions. These values are intended as an initial dataset to support future multi-site validation required for formal RETFL standardization.

2 Methods

2.1 Participants

Thirty-six adults (18–25 years of age) participated in this study. Participants were university students recruited from Western University and Fanshawe College and compensated $15 per hour. Eligibility was determined through a screening protocol to ensure normal hearing in the test ear. Screening included completion of a standardized hearing questionnaire based on Annex A of International Organization for Standardization, 2009 to confirm eligibility and normal-hearing status, as well as otoscopy, tympanometry, and pure-tone audiometry. Pure-tone thresholds were assessed at 15 dB HL from 250 to 8000 Hz using insert earphones in a double-walled, sound-treated booth (ANSI, 2004 (R2018); ISO 8253-1, 2010; American Speech-Language-Hearing Association, 1993). Only individuals who demonstrated normal otologic status and audiometric thresholds were included.

2.2 Ethics

All study procedures were reviewed and approved by the Western University Research Ethics Board. Informed consent was obtained from all participants prior to enrollment, and the study adhered to the principles outlined in the Declaration of Helsinki (World Medical Association, 2013). A letter of information (LOI) was provided to all participants informing them about the study's purpose, who was conducting it, and what participation would entail. The datasets generated and analyzed for this study are stored in a secure institutional repository. De-identified data will be made available by the authors upon reasonable request.

2.3 Apparatus and calibration

The ADHEAR is a non-invasive skin-driven BCD that can be coupled to the head using either a soft headband or an adhesive adapter. For the purposes of this study, a prototype ADHEAR device with a direct cable connection to the integrated transducer was developed to enable in-situ threshold measurements while minimizing the influence of onboard signal processing. This configuration minimized the influence of signal processing on signal transmission. To verify force level (FL) output, the ADHEAR was coupled to a skull simulator via a direct coupling adapter and measured using the Audioscan® Verifit 2 system (Audioscan, 2024). Calibration was completed with a 40 dB dial level input across 250–8,000 Hz. Verification checks were performed both prior to (see Supplementary Table 1) and following (see Supplementary Table 2) data collection to ensure consistency and reliability of device output. Two ADHEAR devices (Device 1: Blue; Device 2: Green) were used in this study to distribute testing across participants. Differences observed between pre- and post-measurements likely reflect variability in coupling conditions between the ADHEAR device and the skull simulator rather than true changes in device output. Small variations in positioning, alignment, and contact pressure can influence measured force levels, particularly at higher frequencies. No systematic drift was observed within devices across measurement sessions.

2.4 Procedure

Thresholds for the test ear were measured at frequencies between 250 and 8,000 Hz in a sound-treated booth. Three transducer conditions were evaluated:

  • B-71 bone oscillator
  • ADHEAR coupled to a soft headband (see Figure 1)
  • ADHEAR coupled to an adhesive adapter (see Figure 1)
Two coupling configurations for the MED-EL ADHEAR: **(a)** adhesive adapter; and **(b)** soft headband (Weiss et al., 2020).
Figure 1. Two coupling configurations for the MED-EL ADHEAR: (a) adhesive adapter; and (b) soft headband (Weiss et al., 2020).

For the soft headband condition, the device was fitted to allow two fingers to fit snugly beneath the headband, ensuring consistent application pressure (Hodgetts et al., 2006). For the adhesive adapter condition, an applicator tool was used to apply the adhesive adapter, which was held in place for a timed 30 s interval to ensure adequate and consistent adhesion. This procedure is recommended to minimize variability in placement and coupling strength across participants (MED-EL, 2020). Two ADHEAR devices were used in this study, marked either Device 1: Blue or Device 2: Green, to track their use across listeners.

2.5 Randomization

The assignment of test ear (left or right), ADHEAR device (Device 1: Blue or Device 2: Green), and transducer/coupling condition (B-71 bone oscillator, soft headband, adhesive adapter) was randomized across participants to reduce order effects and bias.

Statistical analyses were conducted using repeated-measures analysis of variance (ANOVA), with device type (3 levels) and frequency (10 levels) treated as within-subject factors. When significant main effects or interactions were observed, Bonferroni-adjusted pairwise comparisons were performed. Effect sizes are reported as partial eta squared (η2). Statistical significance was set at α = 0.05.

2.6 Dial force level

ETFLs were derived to express in-situ detection thresholds in units of force level (dB FL re: 1 μN), referenced to the skull simulator. ETFLs represent the estimated FL required to reach threshold in normal-hearing listeners for a specific transducer and coupling configuration. First, DFL values were calculated for the ADHEAR device at each frequency. DFL represents the difference between the measured force level output on the skull simulated and the input dial level:

DFL (dB) = measured force level (dB FL) - dial level (dB)

The FL output was verified using a 40 dB dial input across frequencies from 250 to 8,000 Hz. DFL values were calculated independently at each frequency for the ADHEAR system. Comparable direct skull simulator measurements were not obtained for the B-71 bone oscillator in the present study; therefore, B-71 force-level characteristics were estimated based on previously reported skull simulator data and published force-level trends (Hodgetts and Scollie, 2017). These values represent derived estimates rather than directly measured DFLs.

2.7 Equivalent threshold force level

ETFL values were derived by converting in-situ detection thresholds (measured in dB DL) to FL using the corresponding DFL value:

ETFL (dB FL) = In-situ threshold (dB DL) + DFL (dB)

This calculation was performed for each participant and frequency. Mean ETFL values were then calculated across participants for each device and coupling condition (B-71, adhesive adapter, soft headband). ETFL curves represent the force level required to achieve a 0 dB HL threshold for each coupling condition.

3 Results

3.1 Threshold comparisons across devices

A significant main effect of device was observed, F(2, 70) = 140.28, p < 0.001, η2 = 0.80, indicating a large effect of transducer type on threshold measurements. Frequency-specific threshold values and corresponding ANOVA results are summarized in Table 1. Significant differences between devices were observed at all frequencies except 8,000 Hz (p = 0.43). At 250–6,000 Hz, F-values ranged from 6.1 to 35.6, with corresponding p-values between 0.002 and 0.000001. The largest F-value was observed at 4,000 Hz (F(2, 70) = 35.6, p < 0.000001).

Table 1. Frequency-specific in-situ thresholds (dB HL) for each device and coupling condition.

Frequency (Hz)B-71 mean (SD)95% CIAdhesive mean (SD)95% CIHeadband mean (SD)95% CIF (2,70)p-value
250−1.25 (6.02)[−3.28, 0.78]5.69 (7.48)[3.16, 8.22]4.86 (6.38)[2.70, 7.02]22.5p < 0.001
500−3.19 (5.50)[−5.05, −1.33]9.44 (6.63)[7.20, 11.68]5.42 (6.69)[3.16, 7.68]28.3p < 0.001
750−1.53 (5.05)[−3.24, 0.18]4.86 (6.60)[2.63, 7.09]6.25 (6.59)[4.02, 8.48]15.7p < 0.001
1,000−2.78 (4.99)[−4.47, −1.09]5.83 (6.38)[3.67, 7.99]6.81 (6.23)[4.70, 8.92]18.9p < 0.001
1,500−1.67 (6.32)[−3.81, 0.47]7.22 (7.12)[4.81, 9.63]5.14 (6.81)[2.84, 7.44]20.4p < 0.001
2,000−0.14 (6.38)[−2.30, 2.02]5.97 (5.58)[4.08, 7.86]6.25 (6.48)[4.06, 8.44]16.8p < 0.001
3,000−4.58 (5.12)[−6.31, −2.85]0.83 (4.86)[−0.81, 2.47]4.31 (4.33)[2.85, 5.77]13.2p < 0.001
4,000−1.53 (6.30)[−3.66, 0.60]22.36 (7.51)[19.82, 24.90]15.83 (6.49)[13.63, 18.03]35.6p < 0.001
6,000−8.61 (2.83)[−9.57, −7.65]0.83 (6.49)[−1.37, 3.03]−5.56 (5.04)[−7.26, −3.86]6.1p = 0.002
8,000−8.33 (3.38)[−9.48, −7.18]−3.89 (5.87)[−5.88, −1.90]−8.61 (3.07)[−9.65, −7.57]0.9p = 0.43

Values are presented as mean ± standard deviation, with corresponding 95% confidence intervals. Results of repeated-measures ANOVA assessing the effect of device at each frequency are also shown.

Post-hoc Bonferroni comparisons indicated that both the adhesive and soft headband coupling methods had significantly higher in-situ thresholds than the B-71. The adhesive coupling differed from the B-71 by 9.2 dB (p < 0.001), and the soft headband differed from the B-71 by 7.43 dB (p < 0.001). The comparison between the adhesive and soft headband methods yielded a mean difference of 1.84 dB, which was not statistically significant (p = 0.21). Frequency-specific Bonferroni-adjusted pairwise comparisons are provided in Supplementary Table 3.

3.2 Frequency-specific effects

Table 1 shows that across all devices, threshold levels varied systematically with frequency (see Supplementary Figure 1 for graphical representation). Mean in-situ threshold values for each of the ADHEAR coupling conditions demonstrate higher thresholds than the B-71 across most frequencies. For the B-71, mean thresholds remained close to 0 dB HL from 250–4,000 Hz (range: −4.58 to −0.14 dB HL) with relatively small variability (SDs 4.9–6.4 dB). At higher frequencies, the B-71 demonstrated noticeably lower thresholds, reaching −8.61 dB HL at 6,000 Hz and −8.33 dB HL at 8,000 Hz. The adhesive device showed overall elevated thresholds compared with the B-71, particularly at low and mid frequencies (250–2,000 Hz), where mean values ranged from 4.86 to 9.44 dB HL. Thresholds decreased around 3,000 and 6,000 Hz (0.83 dB HL at both frequencies) but increased sharply at 4,000 Hz (22.36 dB HL), producing the highest value observed among all conditions. Variability for the adhesive device was moderately larger (SDs 4.9–7.5 dB).

The headband coupling condition produced thresholds similar to those of the adhesive device at most frequencies, with mean values ranging from 4.31 to 6.81 dB HL from 250 to 3,000 Hz. A pronounced increase occurred at 4,000 Hz (15.83 dB HL), followed by substantially lower thresholds at the highest frequencies (−5.56 dB HL at 6,000 Hz and −8.61 dB HL at 8,000 Hz). SD values were comparable to the adhesive device across frequencies.

Mean threshold differences and corresponding 95% confidence intervals are shown in Supplementary Table 3. Thresholds obtained with both adhesive and soft headband coupling were significantly elevated relative to the B-71 at all frequencies except 8,000 Hz for the headband condition. The largest separation was observed at 4,000 Hz (Adhesive: +23.89 dB, 95% CI [20.57, 27.21]; Headband: +17.38 dB, 95% CI [14.32, 20.45]). Differences between adhesive and soft-headband coupling were small and frequency-dependent. Confidence intervals crossed zero at several frequencies, and although statistically significant differences were observed at 4,000 Hz and 6,000 Hz, the magnitude of separation was modest ( ≤ 6.5 dB).

3.3 Dial force level per device

Figure 2 presents the DFL for the MED-EL ADHEAR (solid line) and the B-71 bone oscillator (dashed line). DFL values for the B-71 bone oscillator were based on previously reported data obtained using measurements on an artificial mastoid (Hodgetts and Scollie, 2017). For the ADHEAR, DFLs were calculated by measuring the force levels per audiometric test frequency on a skull simulator fixed at 40 dB dial level and subtracting the 40 dB dial level. This was collapsed across the two ADHEAR devices used in this study, prior to study data collection (Supplementary Tables 1, 2). Across frequency, the two devices showed different DFL patterns. The B-71 bone oscillator (dashed line) demonstrated a steady decrease in DFL from 250 Hz through ~1,500 Hz, followed by smaller variations between 1,500 and 4,000 Hz, and a slight rise toward 8,000 Hz. The ADHEAR device (solid line) also showed a decline from 250 to 1,500 Hz, with values closely matching the B-71 near 1,500–2,000 Hz. Above 2,000 Hz, ADHEAR DFLs remained relatively stable until 4,000 Hz, at which point they increased, reaching the highest value in the dataset at 8,000 Hz. Across the entire frequency range, the ADHEAR and B-71 curves were closest between 1,500 and 2,500 Hz and diverged most at the highest frequencies.

Dial Force Level (DFL), measured in the Verifit 2 test box with a skull simulator. FL output was verified using a 40 dB dial input from 250-8000 Hz. DFL values were calculated separately for each frequency. Measured for the ADHEAR (solid, purple line), and reference data for the B-71 transducer (dashed, green line).
Figure 2. Dial Force Level (DFL), measured in the Verifit 2 test box with a skull simulator. FL output was verified using a 40 dB dial input from 250-8000 Hz. DFL values were calculated separately for each frequency. Measured for the ADHEAR (solid, purple line), and reference data for the B-71 transducer (dashed, green line).

3.4 Equivalent threshold force level

Mean ETFL values for each device across frequency are presented in Table 2, providing a reference comparison across coupling methods (see Supplementary Figure 2 for graphical representation). Across devices, ETFL values decreased from 250 Hz to ~1,500 Hz. The B-71 (small-dashed line) showed the lowest ETFL values at all frequencies, with a gradual decline from 250 to 1,500 Hz and comparatively smaller changes above 3,000 Hz. Both the adhesive transducer (long-dashed line) and the soft headband (solid line) followed similar trajectories across frequency, showing higher ETFL values than the B-71 throughout the measured range. For both devices, ETFL values decreased from 250 to ~1,500–2,000 Hz, after which they increased steadily through 8,000 Hz. The adhesive and soft headband conditions showed similar ETFL values at several frequencies, particularly between 500 and 2,000 Hz, where differences were generally small (~0–3 dB). At higher frequencies, the conditions diverged slightly, with the adhesive condition demonstrating larger increases, particularly at 4,000 Hz (by ~6.5 dB) and 6,000 Hz (by ~6.4 dB). Across the frequency range, the B-71 consistently demonstrated lower ETFL values than both ADHEAR coupling methods, with differences typically ranging from ~5 to 24 dB.

Table 2. Frequency-specific equivalent threshold force levels (ETFL; dB FL re: 1 μN) for each device and coupling condition.

Frequency (Hz)B-71 mean (SD)95% CIAdhesive mean (SD)95% CIHeadband mean (SD)95% CI
25065.75 (6.02)[63.72, 67.78]61.69 (7.48)[59.16, 64.22]60.86 (6.38)[58.70, 63.02]
50054.81 (5.50)[52.95, 56.67]57.44 (6.63)[55.20, 59.68]53.42 (6.69)[51.16, 55.68]
75046.97 (5.05)[45.26, 48.68]57.36 (6.60)[55.13, 59.59]58.75 (6.59)[56.52, 60.98]
1,00040.97 (4.99)[39.28, 42.66]55.00 (6.38)[52.84, 57.16]55.81 (6.23)[53.70, 57.92]
1,50034.83 (6.32)[32.69, 36.97]40.72 (7.12)[38.31, 43.13]38.64 (6.81)[36.34, 40.94]
2,00030.86 (6.38)[28.70, 33.02]40.47 (5.58)[38.58, 42.36]40.75 (6.48)[38.56, 42.94]
3,00025.42 (5.12)[23.69, 27.15]36.33 (4.86)[34.69, 37.97]39.81 (4.33)[38.35, 41.27]
4,00033.97 (6.30)[31.84, 36.10]49.86 (7.51)[47.32, 52.40]43.33 (6.49)[41.13, 45.53]
6,00031.39 (2.83)[30.43, 32.35]54.33 (6.49)[52.13, 56.53]47.94 (5.04)[46.24, 49.64]
8,00031.67 (3.38)[30.52, 32.82]57.61 (5.87)[55.62, 59.60]52.89 (3.07)[51.85, 53.93]

Values are presented as mean ± standard deviation, with corresponding 95% confidence intervals.

4 Discussion

The present study evaluated in-situ thresholds and ETFLs across three coupling methods, one for the B-71 bone oscillator and two coupling configurations for the ADHEAR system (adhesive and soft headband). These measures were used to characterize how the coupling strategy influences the mechanical and perceptual output of bone-conduction stimulation across frequency. The observed differences provide new insight into the relative efficiency of these coupling approaches and complement emerging evidence regarding force delivery and transmission behavior in skin-drive BCDs. The patterns observed across devices are consistent with known effects of coupling force, mechanical impedance, skin-mediated vibration transmission, and expand on recent work examining similar effects in clinical bone-conduction systems (El-Naji et al., 2024, 2026; Gascon et al., 2022).

4.1 Device effects and coupling efficiency

The overall ANOVA demonstrated a robust effect of device on hearing thresholds, with both the adhesive and soft headband couplings producing higher thresholds than the B-71, and these differences were consistent across frequencies. ETFL results further clarified the magnitude and nature of these differences. Across all frequencies, the B-71 exhibited the lowest ETFLs, reinforcing its strong mechanical performance. The adhesive and soft headband systems displayed similar ETFL trajectories, particularly at mid frequencies, consistent with earlier reports showing that non-rigid couplers often share similar vibratory transmission limitations (Hodgetts et al., 2006; Liu et al., 2022; Skarzynski et al., 2019). The rise in ETFLs above 2000 Hz for both devices reflects well-known frequency-dependent reductions in skin-coupler efficiency and parallels recent findings showing similar high-frequency limitations in skin-drive devices (El-Naji et al., 2024, 2026; Pittman, 2019). The International Consensus on Bone Conduction Devices (2022) reinforces this interpretation, noting that output and maximum power are directly affected by coupling efficiency, particularly in skin-driven systems where attachment type, vibrator position, and soft-tissue properties introduce natural variability. Together, these findings support the expectation that while both non-rigid coupling options yield higher thresholds than rigid stimulation, the differences between adhesive and soft headband conditions are minimal and not clinically meaningful (Hodgetts et al., 2006).

4.2 Frequency-dependent threshold patterns

The frequency-specific data revealed elevated thresholds for both adhesive and soft headband coupling conditions relative to the B-71 bone oscillator, with differences observed across low and mid frequencies and a pronounced peak at 4,000 Hz. These differences likely reflect reduced transmission efficiency associated with skin-driven coupling, where vibratory energy must pass through the soft-tissue interface rather than being delivered directly to the skull. High-frequency reductions in transmission efficiency are well documented in skin-driven bone conduction systems, where the viscoelastic and mass–stiffness properties of the soft-tissue interface alter vibratory energy transfer (Eeg-Olofsson et al., 2008). Finite-element modeling and skull vibration analyses have demonstrated similar frequency-dependent transmission irregularities in mid-to-high frequency regions (Chang and Stenfelt, 2019). Recent in-situ measurements obtained in simulated conductive hearing loss further support this pattern, showing reduced high-frequency output in skin-drive systems despite nominal device targets (El-Naji et al., 2024). Although mechanical impedance was not directly measured in the present study, the consistent 4,000 Hz elevation observed across both non-rigid coupling methods suggests that this effect reflects a coupling-related transmission phenomenon rather than device-specific signal processing differences.

4.3 Dial force level characteristics and coupling force considerations

DFLs also differed systematically across devices. The B-71 demonstrated a smooth decline in DFLs across frequency, consistent with established characteristics of rigidly coupled bone oscillators (Hodgetts and Scollie, 2017). In contrast, the ADHEAR displayed elevated DFLs at low frequencies and a marked increase above 4,000 Hz, suggesting reduced transfer efficiency at these frequencies (Skarzynski et al., 2019). A critical consideration, however, is that dial-indicated force does not always reflect true applied force. Recent work by Gascon et al. (2022) showed substantial discrepancies between nominal (dial) force levels and the actual coupling forces delivered to the skin across different devices and headband configurations. Their findings demonstrate that the force values presented by the device may not fully represent the mechanical input at the transducer–skin interface. This uncertainty complicates direct comparisons between devices and supports the need for more precise measurement approaches. The present findings align with this emerging evidence and illustrate why force-level interpretation must be approached cautiously.

4.5 Implications for clinical use

These findings underscore the importance of coupling method and force characteristics when fitting non-surgical skin-drive BCDs. The strong performance of the B-71 aligns with its use as the clinical standard for threshold measurement (Hodgetts and Scollie, 2017). Adhesive and soft headband couplings remain clinically valuable alternatives, particularly for patients who are not old enough for the surgical procedures involved in percutaneous and active transcutaneous devices; however, the elevated thresholds and high-frequency force requirements observed here suggest that additional gain or targeted programming adjustments may be needed. The present results also reinforce the emerging evidence that actual coupling force is less predictable than dial indicators suggest, emphasizing the need for caution when interpreting force-level metrics clinically (Gascon et al., 2022).

4.6 Limitations and future directions

A limitation of the present study is that DFL values were derived using different approaches for the ADHEAR and B-71 devices. DFLs for the ADHEAR were calculated directly from skull simulator measurements obtained at a fixed 40 dB input, whereas B-71 values were based on previously published force-level data (Hodgetts and Scollie, 2017). Notably, Hodgetts and Scollie (2017) did not report direct DFL values for the B-71 bone oscillator; rather, their work described force-level output and threshold relationships using skull simulator measurements in combination with real-head correction transforms. Equivalent direct measurements were not obtained for the B-71 because the calibration setup used in the present study was designed for the ADHEAR direct coupling adapter and was not compatible with the B-71 bone oscillator, which requires a different coupler/interface for force-level verification. Accordingly, B-71 force-level characteristics in the present study were estimated from these reported trends rather than derived from direct fixed-input skull simulator measurements. As a result, absolute equivalence between devices cannot be assumed, and comparisons should be interpreted in terms of relative trends across frequency rather than exact force-level differences.

Future work should aim to obtain standardized skull simulator measurements for audiometric bone oscillators using contemporary verification methods to enable more direct comparisons. Despite this limitation, the use of the B-71 as a clinical reference standard provides a meaningful benchmark for contextualizing the performance of newer skin-driven systems.

Further, although force output was verified using a skull simulator, variability in coupling conditions between measurement sessions may have contributed to differences in absolute force-level values. Small variations in oscillator positioning, alignment, and coupling force are known to influence bone-conduction thresholds, with measurable differences observed across placement locations, coupling methods, and applied static force (Small et al., 2007; Han and Yu, 2012; Margolis et al., 2022). Accordingly, absolute values should be interpreted with caution, while relative patterns across frequency and between devices, particularly the consistency observed between the two ADHEAR devices, are considered more robust.

An additional limitation is that the present study provides ETFLs obtained in a single laboratory. To establish RETFLs suitable for standardization or calibration purposes, validation across multiple laboratories would be required. As this is the first study to derive ETFLs for these specific coupling methods, these findings represent an initial dataset that will benefit from multi-site replication. Further, while force output was verified using a skull simulator, this approach may not fully replicate in-vivo skin-drive transmission characteristics. Objective mechanical measures, such as skull-surface accelerometry, would provide further insight into the physical mechanisms underlying device behavior (Eeg-Olofsson et al., 2008). Future research should also explore how skin properties, adhesive performance, and placement stability contribute to coupling variability, particularly in light of recent evidence showing substantial individual differences in true applied force (Eeg-Olofsson et al., 2011; El-Naji et al., 2024; Gascon et al., 2022). This study was conducted in normal-hearing adults, which may limit generalizability to pediatric populations or individuals with conductive or mixed hearing loss. Although headband tension was standardized using a two-finger method, small variations in contact force cannot be completely ruled out. Previous work has shown that variability in headband coupling force is generally small within clinical fittings and is therefore unlikely to have meaningfully influenced the present findings (Hodgetts et al., 2006).

5 Conclusion

This study characterized in-situ thresholds and ETFLs for the MED-EL ADHEAR adhesive and the soft headband coupling methods. Across measurements, the B-71 demonstrated lower thresholds and lower ETFLs than both ADHEAR coupling conditions, particularly at low and mid frequencies. The adhesive and soft headband conditions produced similar threshold patterns across most frequencies, with both showing elevated values relative to the B-71 and marked increases at 4,000 Hz. DFL measurements further illustrated device-specific and frequency-dependent differences in force output between the ADHEAR and B-71 systems.

Together, these results provide foundational reference data describing how different coupling methods influence force-level behavior and threshold performance in skin-drive BCDs. These findings contribute preliminary ETFL values for adhesive and soft headband configurations and highlight areas where further multi-site work is needed to establish device-specific RETFLs suitable for clinical calibration and prescriptive fitting.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fauot.2026.1850851/full#supplementary-material

Pre-test device equivalence evaluation. Frequency-specific output levels (dB FL) measured in response to a 40 dB input signal for Device 1 (Blue) and Device 2 (Green). Differences represent Device 1 minus Device 2 values. Supplementary material at the source

Post-test device equivalence evaluation. Frequency-specific output levels (dB FL) measured in response to a 40 dB input signal for Device 1 (Blue) and Device 2 (Green). Differences represent Device 1 minus Device 2 values. Supplementary material at the source

Bonferroni-adjusted pairwise comparisons of in-situ thresholds across device and coupling conditions at each frequency. Mean differences are reported in dB HL with corresponding 95% confidence intervals. Supplementary material at the source

Frequency-specific mean in-situ thresholds (dB HL) measured with the B-71 bone oscillator (dashed green), ADHEAR adhesive adapter (dashed purple), and soft headband (solid purple). Both non-rigid coupling conditions demonstrated elevated thresholds relative to the B-71, particularly at 4000 Hz, reflecting reduced vibratory transmission efficiency at higher frequencies. Supplementary material at the source

Supplementary material Supplementary material at the source

Footnotes

Edited by: Achintya K. Bhowmik, Stanford University, United States

Reviewed by: Hantai Kim, Pamela Youde Nethersole Eastern Hospital, China

Michael Dalè, Ospedale PA Micone Sestri Ponente, Italy

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This study was funded by MED-EL Corporation, Innsbruck, Austria. The sponsor provided financial support for equipment and study-related expenses but had no role in data collection, data analysis, interpretation of results, or the writing of the manuscript. The authors maintained full control over study design, analysis, and reporting.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethics approval

The studies involving humans were approved by Western University Research Ethics Board (WREM). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

Author contributions

EW: Writing – original draft, Methodology, Data curation, Investigation, Project administration, Formal analysis. JL: Resources, Writing – review & editing. PF: Methodology, Supervision, Writing – review & editing, Conceptualization. SS: Resources, Conceptualization, Writing – review & editing, Methodology. HM: Methodology, Writing – review & editing, Resources. MB: Conceptualization, Resources, Methodology, Project administration, Writing – review & editing, Investigation, Funding acquisition, Supervision.

Acknowledgements

We thank the participants for their time and cooperation. The authors also acknowledge MED-EL Corporation (Innsbruck, Austria) for providing equipment and financial support for this project. We extend our appreciation to the laboratory staff and student research assistants who supported data collection and logistics.

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Citation

Webster, E., et al. (2026). Reference threshold measures for a non-surgical skin-drive bone conduction hearing device. Frontiers in Audiology and Otology, Volume 4. https://doi.org/
Received
Accepted
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Volume
4 - 2026

Copyright

Copyright © 2026 Webster, Lochner, Folkeard, Scollie, Mojallal and Bagatto.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Eileen Webster

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