CA2707284A1 - Method and system for driving bilateral bone anchored hearing aids - Google Patents

Abstract

Current bilateral BAHA configurations, for sounds directly facing a listener, apply forces to the skull that are in-phase with each other and directed towards the center of the head.
The head's response approximates that of a rigid body at frequencies < 1000Hz, thus it is preferable to drive bilateral BAHAs such that when one pushes, the other pulls. Adjusting the relative phase offset of BAHAs, achieves this, resulting in greater vibration and improved hearing.

Description

METHOD AND SYSTEM FOR DRIVING BILATERAL BONE ANCHORED
HEARING AIDS

FIELD
[0001] The present disclosure relates generally to bone anchored hearing aids (BAHAs). More particularly, the present disclosure relates to a method and system for driving a BAHA.

BACKGROUND

[0002] The bone anchored hearing aid (BAHA) is an effective implantable prosthesis for patients with conductive hearing loss, who, for various reasons (i.e. chronic infections in ear canal), are unlikely to benefit from traditional air conduction hearing aids (Tjellestrom et al. 2001). The BAHA has traditionally been indicated for moderate to severe conductive hearing loss, but may also be used in cases of moderate sensorineural loss or mixed loss. More recently it has been used for contralateral routing of sound (CROS) applications for unilaterally deaf patients (Niparko et al. 2003;
Stenfelt 2005).

[0003] Typically, the BAHA consists of a microphone, amplification and signal processing electronics and an electromagnetic vibration actuator that is attached through a snap coupling to an implanted titanium fixture called an abutment. The abutment is screwed 4 mm into the skull, where it osseointegrates into the temporal bone, providing a stiff mechanical coupling to the bone.

[0004] Bone vibrations are transformed into sound by a number of processes, most notably the inertial movement of the inner ear ossicles and inner ear fluids which dominates at low frequencies, compression of the cochlear shell, and sound radiation from the vibrations in the skull to the external and middle ear spaces (Tonndorf, 1966, Stenfelt & Goode 2005). From the point of view of perceived hearing, cochlear vibrations caused by bone vibration are entirely equivalent to those caused by air-conducted sound.
In fact, bone-borne and air-borne vibrations can be made to cancel each other in the cochlea (von Bekesy 1932; Stenfelt, 2007).

[0005] The BAHA generates bone vibration with an electromagnetic motor that pushes a counterweight mass within its housing. This generates a reactive force into the abutment that drives vibration of the skull. At low frequencies below 1000 Hz, these vibrations act to move the entire head in phase (H6kansson et al. 1993;
Stenfelt & Goode 2005), and it is useful to think of the head as being a single rigid mass.
Above 1000 Hz, the time taken for acoustic energy to propagate across the skull becomes comparable to the period of the oscillation. Consequently different parts of the skull become mutually out of phase, and the skull can no longer be thought of as a single rigid body. At all frequencies the BAHA delivers sound to both cochleae. Although there is some evidence for attenuation of sound in crossing the skull at the highest frequencies (Nolan & Lyons 1981; Stenfelt & Goode 2005), this attenuation is generally considered to be less than 10dB.

[0006] Given that a single BAHA transmits sound to both cochleae, and that fitting a second device is associated with additional financial cost along with the risks associated with additional surgical procedures, it might be thought that there is no benefit to fitting a second BAHA, particularly since a second device yields only a modest improvement in audiological thresholds (Priwin et al. 2004). However, bilateral BAHAs are associated with greater quality of life (Ho et al. 2009) and improved performance in a number of listening situations.

[0007] Investigations into the efficacy of bilateral BAHAs relative to unilateral BAHAs have shown, fairly consistently, that localization ability improves (Van der Pouw et al. 1998; Bosman et al. 2001; Priwin et al. 2004). Azimuthal localization improvements are presumably due to the different azimuthal dependence of the signals received by the two microphones in the presence of head shadow and interaural delay effects (Rayleigh, 1907). In addition to superior localization ability, different measures of speech perception show improvements with bilateral BAHAs relative to unilateral BAHAs (Van der Pouw et al. 1998; Bosman et al. 2001; Duff et al. 2002; Priwin et al. 2004). It has been shown, fairly consistently, that speech perception in quiet, measured by the level needed for accurate speech comprehension, improves with bilateral BAHAs relative to unilateral BAHAs (Van der Pouw et al. 1998; Bosman et al. 2001; Priwin et al. 2004).
Bilateral BAHAs yield some improvement for speech in noise, relative to unilateral BAHAs, although these improvements are smaller (Van der Pouw et al. 1998; Bosman et at. 2001;
Priwin at al. 2004). Therefore fitting bilateral BAHAs may result in additional benefit that goes beyond the simple doubling in power that results from two amplifiers and transducers.

[0008] Given the potential benefit of bilateral BAHAs relative to unilateral BAHAs, it seems pertinent to attempt to establish optimal configuration for two devices that work in tandem on the same skull. Currently, bilateral BAHAs are not differentiated into left and right versions, except in some models where the microphone placement is changed to make them cosmetically symmetric. This means that in response to a positive pressure at the microphone, both bilateral BAHAs will generate a force directed inward towards each other or outwards away from each other as shown in Figure 1a. We say that the two BAHAs act in phase. The result is that to the extent that the skull can be thought of as a rigid body, particularly at low frequencies, the forces from the two BAHAs substantially cancel each other.

[0009] It is, therefore, desirable to provide an improved method and system for driving bilateral BAHAs that improves their efficiency, particularly at low frequencies.
BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.

[0011] Figure 1: Illustration of forces applied to the skull for low frequency sounds when bilateral BAHAs operate (a) in phase and (b) out of phase. These figures illustrate that at frequencies where the head moves as a solid mass (i.e. below 1000Hz), larger motion is experienced when BAHAs are out of phase.

[0012] Figure 2: Change in threshold for bilateral BAHAs relative to the unilateral BAHA. Data is displayed for each frequency tested, for in phase (triangles) and out of phase (squares) bilateral BAHA configurations. Error bars reflect SEM.
Negative values reflect an improvement relative to unilateral thresholds.

[0013] Figure 3: Measurements of the velocity of the cochlear promontory on the cadaver head for in- phase and out-of-phase bilateral BAHAs. The four graphs give the magnitude of the velocity and the components in the vertical (x), Frankfurt line (y) and interaural (z) directions. Measurements are given relative to the velocity measured with a single BAHA driving the head.

[0014] Figure 4: Velocity of the cochlear promontory under various driving conditions. From left to right, the graphs give the velocity under bilateral in-phase drive, bilateral out-of-phase-drive, and unilateral drive. The measured vibration levels of hundreds of pm/s to 1 mm/s are typical of the BAHA Divino's maximum output.
DETAILED DESCRIPTION

[0015] Generally, the present disclosure provides a method and system for driving bilateral BAHAs. The BAHAs are driven out of phase, such that in response to positive sound pressure one BAHA applies a force directed into the head and the other applies a force directed out of the head as shown in Figure 1 b. Such a "push-pull"
configuration is more efficient at moving the skull and hence at creating bone-conducted hearing.

[0016] Changing from in-phase to out-of-phase driving can be implemented in many ways. Modifications to the driving circuitry or digital signal processing can be easily effected. It could, for instance, be achieved by swapping the electrical leads attached to either the microphone or the electromagnetic motor. In newer BAHAs that incorporate a digital signal processing unit, the phase flip could be achieved by changing the signal processor firmware.

[0017] The effect of driving two BAHAs out of phase has been examined, and results in lower thresholds of hearing for the same electrical power draw. We compare performance of bilateral BAHAs driven in this configuration to the standard configuration.
In twelve normal participants we show significant improvements in low-frequency ( <1000Hz) hearing thresholds using out-of-phase BAHAs. This is further supported by velocimetric measurements taken at the cochlear promontory in a cadaveric head.
Comparing vibration arising from each configuration confirms that out-of-phase driving results in greater vibration. Our first experiment compares audiometric hearing thresholds for unilateral BAHAs and bilateral BAHAs where phases are equal and where phases are opposite. This is tested in participants who are audiologically normal. In the second study, the 3D velocity at the cochlear promontory of a cadaver head is measured.
Vibration level arising from in phase and out of phase bilateral BAHAs are compared across a range of frequencies.

METHODS: AUDIOLOGICAL MEASUREMENTS

[0018] Subjects. Twelve normal hearing participants were tested in this study (8 males). Ages ranged from 27 to 48 years old, with a mean of 35.67 years old (S.D 7.808).
All had bone thresholds better than 25 dB in the frequency range 250-4000Hz with the exception of one participant, who had a mild high frequency hearing loss at 4000Hz.
Informed consent was obtained from each participant prior to their participation in the study.
Instrumentation. All testing was completed in a sound isolated double walled sound booth (Industrial Acoustics Inc.), with the subject seated in the booth and the tester outside. The ambient noise level of the booth met ANSI specifications for threshold testing. Two electromagnetic motors removed from BAHA Intenso (Cochlear Corp.) devices were used as the bone actuator. These were driven directly by a Grason-Stadler 61 Clinical Audiometer calibrated to ANSI specifications (S3.6-1996). Phase was controlled using a phase inverter which ran between the motor and audiometer and acted to reverse the polarity of the electrical signal applied to the motor. Prior to experimental testing it was confirmed that the phase inverter did not affect audiological thresholds, and the output as measured by an accelerometer was the same for both BAHAs with, and without the phase inverter.

[0019] Procedure. Bone conduction thresholds were collected for 250, 500, 750, 1000, 1500, 2000, 3000 and 4000 Hz, using unilateral, bilateral in-phase (opposing forces), or bilateral out-of-phase (co-directed forces) BAHAs.

[0020] The BAHA motor was placed on the participant's head, held in place by a metal headband, and placed on the usual BAHA site of 50-55mm from the ear canal at either the 10 o'clock or the 2 o'clock position, for the right and left sides respectively.
Three experimental conditions were run. In the single BAHA condition, one motor was placed on either the left or right side of the head. Placement side was randomized. In the "in-phase" condition, two BAHAs were placed on the left and right BAHA sites and were driven from the same electrical signal. In the "out-of-phase" condition, one of the BAHAs was driven through the phase inverter. To eliminate any experimental bias, both the participant and the clinician measuring thresholds were blind to the experimental condition being tested. To minimize the chance that small differences in force or device placement (e.g. Dirks 1964) could potentially contaminate our results, thresholds were obtained three times for each condition, spread across three experimental sessions. Each individual session consisted of the three experimental conditions presented in a random order. Data were only accepted for participants with good test-retest reliability, as measured by reliability coefficients. Threshold was taken to be the mean HL
obtained across three trials.

RESULTS: AUDIOLOGICAL MEASUREMENTS
Test-retest reliability [0021] Test retest reliability was high. For each participant, reliability coefficients were calculated. Reliability coefficients (r) for the three tests in each condition and for each participant exceeded 0.7. Therefore, threshold for each frequency was taken as the mean of the three thresholds obtained within each condition.
Differences in thresholds [0022] To compare performance of bilateral in phase and out of phase BAHAs, the difference from baseline (single BAHA) was calculated for each of the frequencies tested. These data are plotted in Figure 2, with the change in threshold (y-axis) plotted against the frequencies tested (x-axis) for the out of phase condition (squares) and the in phase condition (triangles). Negative values reflect an improvement in threshold. Error bars reflect standard error of the mean.

[0023] Figure 2 shows a small (3-6dB HL) improvement in audiometric thresholds for bilateral out-of-phase BAHAs relative to bilateral in-phase BAHAs at low test frequencies (below 1000Hz). A repeated measures two-way ANOVA revealed significant main effects of phase (f(1,11)= 16.02, p<0.01) and of frequency (f(7,77)=3.36, p<0.05).
The analysis also revealed a significant interaction between phase and frequency (f(7,77)=3.56, p<0.01). To establish at which frequencies phase has a significant effect, repeated measures one-way ANOVAs were carried out at each frequency. These analyses showed significantly lower thresholds in the out-of-phase condition than in the in-phase condition at 250 (f(1,11)=16.20, p<0.01), 500 (f(1,11)=29.36, p=<0.01) and 750 Hz (f(1,11)=4.90, p<0.05). There was no significant difference at any other frequency. We conclude that at frequencies below 1000Hz, bilateral out of phase BAHAs yield significantly lower thresholds than bilateral in phase BAHAs and that at no frequency do they significantly increase thresholds.

METHODS: LASER DOPPLER VELOCIMETRY

[0024] Out-of-phase and in phase driving was also studied on an embalmed head where, unlike in living subjects, detailed velocimetry of the motion of the cochlear promontory was possible. The head was from a female, aged between 60 and 70 years that had been deceased approximately 6 months at the time of the experiment.
The embalming procedure consisted of an injection of 40-60 L of embalming fluid through the femoral artery, followed by another 20 L of hypodermic injection at various sites. At the time of testing the head weighed 3730 grams.

[0025] Measurements were performed with a Polytech CSV-3D (Polytech GmbH, Waldbronn, Germany) 3D laser Doppler vibrometer which was capable of simultaneously measuring the magnitude and direction of the velocity of a 150 m diameter area on a surface. In order to have the laser beams reach the cochlea, the ear canal was widened to 2 cm in diameter and the tympanic membrane and ossicular chain removed. The lasers shone directly on the cochlear promontory.

[0026] Two BAHA abutments were implanted into the embalmed head using methods similar to standard surgical implantation. The abutments were positioned 55 mm behind the ear canal at either the 10 o'clock or the 2 o'clock position. A
pilot hole was drilled and the self-tapping abutment screwed 4 mm into the skull using a wrench designed for abutment implantation.

[0027] Two motors removed from BAHA Divinos (Cochlear Corp, Australia) were used to drive the abutments. Both BAHAs were driven through an 8-ohm Crown audio amplifier, and the phase of one was reversed by swapping the wires driving the motor.

t0028] The .CLV-3D produced an..autput voltage proportional to the:valocity along three axes, which were carefully aligned, to the interaural, Frankfurt line (connecting. the inferior, orbital ridge to the center of.the aperture of the, external auditory mpatus),,and.
vertical directions. The BAHAs were harmonically excited by a 1VRMS sine wave that was stepped through 200 frequencies from 1b0 610,000 Hz. 0.5 seconds-were allowed for any, transient response of the skull tostablkze, folio pd by. 1second aGgwsiton at each frequency. The output voltage of the-LDV'system was acquired Vith a'44channel' National Instruments (Austin,TX) PCI-4452 data acquisition card controlled by a custom Labview interface.

RESULTS

[0029] Measurements were obtained for bilateral in-phase, bilateral out-of-phase and unilateral conditions. Figure 3 shows the results. In order to facilitate comparison with the audiological data, the velocities for the two bilateral conditions were referenced to the unilateral condition and the y-axis of the plots reversed so that larger velocities relative to the unilateral condition are lower on the y-axis. In order to give the reader a sense of the absolute size of the three velocity components, all three components are plotted for the three conditions separately in Figure 4. At low frequencies the velocity is primarily directed along the interaural.direction z because the force of the BAHAs is primarily directed along this direction. The fact that the force and response of the head are parallel is indicative of rigid body motion. At higher frequencies this is no longer true. Averaged over frequencies, the three directions have roughly similar levels above 1000 Hz, although at any given frequency one or the other direction may dominate. This behavior is indicative of non-rigid modal vibration.
[0030] Several qualitative features are strikingly similar between the audiological and velocimetric data. In both cases there is a clear separation into high frequency and low frequency regimes. In the high frequency regime there is no clear advantage to in phase versus out of phase driving, whereas in the low frequency regime the out-of-phase condition results in significantly higher velocities than the in-phase condition. The cadaveric data shows that the increase in velocity is strongly directionally-dependent, with the interaural direction showing a difference between the in-phase and out-of-phase driving conditions as large as 30 dB. In the Frankfurt-line direction, the in phase condition produces a 15 dB higher velocity than the out of phase component, although the absolute level of motion in this direction is much smaller than the motion in the interaural direction.
Relative to the unilateral condition, the vector sum of the three velocity components shows an increase of between 5 and 8 dB for the out of phase condition and between -2 and 4 dB for the in phase condition. These changes correspond very well with the observed change in audiological hearing level in the test subjects. It has been asserted by others (Stenfelt & Goode 2005) that velocity magnitude appears to be more strongly correlated with hearing level than motion along any particular direction. Our measurements support this assertion.

DISCUSSION
[0031] The two experiments described in this paper strongly suggest that clinical gains obtained from bilateral BAHAs can be optimized by offsetting signal phase of one device relative to another. Data obtained at the level of the promontory in a cadaveric head supports audiological data obtained from humans to provide a compelling argument for offsetting the phase of bilateral BAHAs. Both datasets show that improvements associated with out-of-phase bilateral BAHAs are significant at frequencies below 1000Hz, while frequencies above this are not significantly affected by the phase offset either positively or negatively. The improvements we have observed in hearing thresholds for bilaterally implanted BAHA patients can be implemented with a very small change in the way that BAHAs are currently manufactured and fitted. Particularly in new programmable BAHA designs, programming the relative phase between the microphone signal and force output should be extremely easy, allowing even a frequency-dependent phase to be programmed in. Our results suggest that making the phase offset a standard part of the BAHA fitting procedure will result in improved low frequency hearing.
[0032] We know of no disadvantage to applying our technique. In particular there is no obvious reason to think that applying different phases at the two BAHAs should reduce the improvements in speech comprehension and localization ability observed for bilateral BAHAs. On the contrary, given that a common complaint of BAHA users is that they sound "tinny" (Stephens et al. 1996), with an over-emphasis on the high frequencies, an improvement in low frequency loudness can be expected to have a beneficial effect on perceived sound quality, and possibly on speech comprehension.
[0033] A few points of experimental methodology deserve further discussion.
The decision not to simulate a conductive loss on the normal-hearing participants was motivated by the concern that plugging the ears would result in occlusion effects (Goldstein & Hayes 1965). The occlusion effect was expected to improve low frequency thresholds by as much as 20 dB HL below 1000 Hz. As these are the frequencies of primary interest in this study, it was deemed that plugging, by introducing an effect considerably larger than the one being measured, could complicate the experiment.
Because participants' ears were left open, it could be argued that thresholds arise not because of bone-conduction, but rather by air-conduction that arises from radiation from the BAHA. We argue that this is, however, extremely unlikely for a number of reasons:
Firstly, the amount of radiation caused by the device is negligible, especially at the threshold levels at which we are presenting the stimuli. Furthermore, if thresholds arose from air conduction it is unclear why there would be a differential effect of phase across conditions. This effect is consistent across participants, and is consistent with measurements obtained from the promontory of cadaveric heads which were completely insensitive to air-borne sound. We are therefore confident that the effects measured are due to bone conduction and not air conduction.
[0034] Laser Doppler vibrometry on a cadaver head showed that out-of-phase bilateral BAHAs create a greater level of motion than either bilateral in phase BAHAs or a unilateral BAHA, with the level of improvement typically 5-8 dB relative to a unilateral BAHA. This agrees quite well with our audiological findings showing a 4-6 dB
improvement at the frequencies measured.
[0035] Though small, the observed improvements in audiological thresholds are of obvious clinical benefit, particularly as the low frequencies (500 and 750Hz) are involved in speech perception (French & Steinberg 1947). Improved ability to perceive speech is the primary motivator for patients to seek treatment (Crowley &
Nabelek 1996;
Garstecki & Erler 1998) and impaired speech perception is regularly reported by people with hearing impairments (Gatehouse & Noble 2004).
[0036] A number of studies have shown that traditional in-phase bilateral BAHA
configurations yield improvements in speech comprehension both in quiet, and in noise. It is important to establish whether the improvement offered by out-of-phase BAHAs translates to improved speech comprehension. Of equal importance is establishing how the out of phase bilateral BAHA configuration affects sound localization.
Improving localization performance is a key factor in fitting bilateral BAHAs as, in addition to avoiding environmental hazards, aids people listening to speech in noise (Freyman et al.
1999). This low-frequency improvement in performance will only be useful if it does not interfere with speech and localization capabilities. These are situations that our laboratory intends to investigate.
[0037] This study shows that by offsetting the phase of one BAHA in a bilateral BAHA pair, improvements of up to 6dB (equivalent to a doubling of loudness) are seen in the low frequencies, relative to regular bilateral configurations. This improvement can be achieved with very simple change in BAHA design or fitting, and can be used either to increase perceived loudness or to reduce power consumption at a given loudness. We conclude that when designing BAHAs intended for bilateral use, manufacturers should consider offsetting the phase of one device relative to another.
[0038] In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments.
However, it will be apparent to one skilled in the art that these specific details are not required. In other instances, well-known electrical structures and circuits are shown in block diagram form in order not to obscure the understanding. For example, specific details are not provided as to whether the embodiments described herein are implemented as a software routine, hardware circuit, firmware, or a combination thereof.
[0039] The above-described embodiments are intended to be examples only.
Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope, which is defined solely by the claims appended hereto.

REFERENCES

The following references are incorporated herein by reference in their entirety:
Bosman, A.J., Snik, A.F.., van der Pouw, C.T., Mylanus, E.A., Cremers, C.W.
(2001).
Audiometric evaluation of bilaterally fitted bone anchored hearing aids. Int J
Audiol, 40, 158-167.
Crowley, H.J., Nabelek, I.V. (1996). Estimation of client-assessed hearing aid performance based upon unaided variables. J Speech Hear Res, 39, 19-27.
Dirks, D. (1964). Factors related to bone conduction reliability. Arch Otolaryngol, 79, 551-558.
Dutt, S.N., McDermott, A.L., Burrell, S.P., Cooper, H.R., Reid, A.P., Proops, D.W. (2002).
Speech intelligibility with bilateral bone-anchored hearing aids: the Birmingham experience. J Laryngo! Oto, 116, 47-51.
French, N.R., Steinberg, J.C. (1947). Factors governing the intelligibility of speech sounds. The J Acoust Soc Am, 19, 90-119.
Freyman, R.L., Helfer, K.S., McCall, D.D., Clifton, R.K. (1999). The role of perceived spatial separation in the unmasking of speech. J Acoust Soc Am, 106, 3578-3588.
Garstecki, D.C., Eder, S.F. (1998). Hearing loss, control, and demographic factors influencing hearing aid use among older adults. J Speech Lang Hear Res, 41, 527-537.
Gatehouse, S., Noble, W. (2004). The Speech, Spatial and Qualities of hearing Scale (SSQ). Int J Audiol, 43, 85-99.
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Hakansson, B., Brandt, A., Carlsson, P., Tjellstrom, A. (1993). Resonance frequencies of the human skull in vivo. J Acoust Soc Am, 95, 1474-1481.
Ho, E.C., Monksfield, P., Egan, E., Reid, A., Proops, D. (2009). Bilateral bone-anchored hearing aid: impact on quality of life measured with the Glasgow benefit inventory.
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Niparko, J.K., Cox, K.M., Lustig, L.R. (2003). Comparison of the bone anchored hearing aid implantable hearing device with contralateral routing of offside signal amplification in the rehabilitation of unilateral deafness. Oto! Neurotol, 24, 73-78.
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Priwin, C., Stenfelt, S., Granstrbm, G., Tjellstrom, A., Hakansson, B. (2004).
Bilateral bone-anchored hearing aids (BAHAs): An audiometric evaluation. Laryngoscope, 114, 77-84.
Rayleigh, L. (1907). On our perception of sound direction. Phil Mag, 13, 214-232.
Stenfelt, S. (2005). Bilateral fitting of BAHAs and BAHA fitted in unilateral deaf persons:
Acoustical aspects. Int J Audiol, 44 178-189.
Stenfelt, S, Goode, R.L. (2005). Transmission properties of bone conducted sound:
Measurements in cadaver heads. J Acoust Soc Am, 118, 2372-2391.
Stenfelt, S. (2007). Simultaneous cancellation of air and bone conduction tones at two frequencies: Extension of the famous experiment by von Bekesy. Hear Res, 225, 105-116.
Stephens, D., Board, T., Hobson, J., Cooper, H. (1996). Reported benefits and problems experienced with bone-anchored hearing aids. Br J Audiol, 30, 215-220.
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Claims (10)

1. A method of driving bilateral bone anchored hearing aids, comprising:
sensing sound waves at a microphone;
driving the bilateral bone anchored hearing aids out of phase in response to low frequency sound waves, such that one of the bilateral bone anchored hearing aids pushes towards a wearer's skull, while the other of the bilateral bone anchored hearing aids pulls away from the skull, thereby generating bone vibration to excite the movement of cochlear fluids.

2. The method of claim 1, wherein driving the bilateral bone anchored hearing aids comprises driving the bilateral bone anchored hearing aids 180° out of phase.

3. The method of claim 1, wherein driving the bilateral bone anchored hearing aids comprises driving the bilateral bone anchored hearing aids at frequencies below 1000Hz.

4. A bilateral bone anchored hearing aid (BAHA) system, comprising:
bilateral abutments anchored to a wearer's skull;
an vibration actuator attached to each abutment;
driving circuitry for driving each electromagnetic vibration actuator in response to sound waves sensed by a microphone associated with each electromagnetic vibration actuator, the driving circuitry being configured to drive the electromagnetic vibration actuators out of phase with respect to each other, such that as force is applied towards the wearer's skull on one side, and away from the wearer's skull on the opposing side, thereby generating bone vibration to excite the movement of cochlear fluids.

5. The BAHA system of claim 4, wherein the driving circuitry comprises digital signal processing means.

6. The BAHA system of claim 4, wherein the vibration actuators are attached to the abutments through a snap coupling.

7. The BAHA system of claim 4, wherein the abutments are implanted titanium fixtures.

8. The BAHA system of claim 4, wherein the vibration actuators comprise electromagnetic vibration actuators.

9. The BAHA system of claim 8, wherein the electromagnetic vibration actuators comprise an electromagnetic motor driving a counterweight.

10. The BAHA system of claim 4, wherein the vibration actuators comprise piezoelectric vibration actuators.