Peder Carlsson

Resonance frequencies of the human skull in vivo

Patients with skin penetrating titanium implants in the temporal bone, for attachment of bone-anchored hearing aids, have made it possible to investigate the free-damped natural frequencies (resonance frequencies) of the human skull in vivo. The resonance frequencies of the skull of six subjects were investigated. Teh resonance frequencies were extracted from two frequency response functions (acceleration/force) measured on each subject: One point measurement where the force and acceleration were both measured at the same point, and one transcranial measurement where the acceleration was measured contralaterally. Between 14 and 19 resonance frequencies were identified for each subject in the frequency range 500 Hz to 7.5 kHz. The two lowest resonance frequencies were found to be on the average 972 (range 828-1164) and 1230 (range 981-1417) Hz. The relative damping coefficients of all resonances were found to be between 2.6 and 8.9%. Due to the relatively high damping coefficients, it is assumed that the resonance frequencies do not significantly affect bone conducted sound. In the transcranial measurements, however, a few large antiresonances were found which may affect bone-conducted sound. Intersubject variations were large, probably due to individual variations in skull geometry and in mechanical parameters. The results were shown to be consistent with previous results obtained on dry skulls. No obvious correlation between lowest resonance frequency and skull size was found.

Arrangement in a hearing aid device

The present invention relates to an arrangement in a hearing aid comprising an oscillator apparatus, which via a coupling portion can be connected and anchored to the skull bone of a person with impaired hearing ability, said apparatus serving the purpose of an emitter for mechanical transmission of sound information to the skull bone.

Skull Simulator for Direct Bone Conduction Hearing Devices

The Bone-Anchored Hearing Aid (BAHA) is a direct bone conduction hearing device which has given patients with various middle ear disorders a significantly improved quality of life. As the BAHA has gained acceptance as a valuable contribution to the Swedish hearing aid rehabilitation program, the need for equipment which can perform objective frequency response measurements has grown. Such equipment is indispensable for carrying out quality assurance, service, and fitting evaluation. To meet the above-mentioned demands, the skull simulator TU-1000 has been developed. The dynamic behaviour of the skull simulator TU-1000 can be characterized as that of a rigid mass body with a weight significantly exceeding the weight corresponding to the dynamic mass of the transducer incorporated in the BAHA. The motions of the mass body are measured by an accelerometer the output signal of which is amplified by a precalibrated amplifier. The output signal is proportional to the output force level from the BAHA. The skull simulator TU-1000 is capable of measuring the output force level from the BAHA with high reliability for frequencies ranging from 100 Hz to 10 kHz.

The mechanical point impedance of the human head, with and without skin penetration

The fact that a titanium screw can be implanted into the mastoid portion of the human skull, at the same time establishing a permanent, reaction-free skin penetration, has made it possible to attach a new bone conduction hearing aid directly to the skull. To understand and improve this new method of bone stimulation, the mechanical point impedance of the titanium screw-skull system was measured. The conventional point impedance of the skin-covered mastoid portion of the temporal bone was also measured and the difference in magnitude between the two impedances was calculated. An impedance head (Brüel & Kjaer 8001) and an FFT analyzer (Hewlett-Packard 5423) were used for mechanical point impedance measurements. Seven patients have been investigated. The magnitude of the impedance for the screw-skull system was found to be generally between 10 and 30 dB higher than that for the conventional skin-covered mastoid bone. One conclusion is that the conventional point impedance of the skin-covered mastoid portion of the human skull is essentially due to the properties of the skin and subcutaneous soft tissue. Another conclusion is that a much lower stimulation velocity is needed, with skin penetration, to produce a given hearing sensation.

Linearity of sound transmission through the human skull in vivo

The linearity of sound propagation through the human skull was investigated. One male subject, equipped with bilateral skin-penetrating titanium fixtures for attachment of bone-anchored hearing aids, was studied thoroughly. Three different methods were used: comparison of the frequency response functions estimated at different signal levels (using stepped sine as well as random noise), comparison of the coherence function at different signal levels (using random noise), and the Hilbert transform of the estimated frequency response function. Frequencies from 0.1 to 10 kHz and signal levels up to 77 dB HL at discrete frequencies were used. No indication of any significant nonlinear behavior was found with the three methods used.

The bone-anchored hearing aid: reference quantities and functional gain.

&NA; One of the most important parameters of a hearing aid is its gain characteristics. Under ideal circumstances, the gain and the functional gain(FG) are the same for an air conduction device. This is not the case, however, with bone conduction devices, e.g., the bone‐anchored hearing aid(BAHA). In this article, the relation between the gain and the FG is derived for bone conduction devices including the BAHA. Reference quantities used when measuring bone conduction devices also are presented.

Force threshold for hearing by direct bone conduction

The bone-anchored hearing aid is connected, by means of a skin-penetrating bayonet coupling, to an implanted titanium fixture. Hence, direct bone conduction (dbc) excitation is used. Since no international standard of audiometric zero for dbc force threshold exists, it is of general interest to determine the dbc force threshold for normal hearing subjects. Two different methods have previously been applied to estimate the relation between bone conduction (bc) and dbc thresholds. One preliminary problem was to make a measurement of the output-force level of dbc transducers, which is equivalent to the situation in situ. A skull simulator, TU-1000, has been designed for measuring the output-force level of dbc transducers. The skull simulator does, in an adequate way, reflect the mechanical point impedance of the human skull. This opportunity to determine equivalent dbc force thresholds has motivated the present study in which a linear relation between the dbc force threshold and the bc force threshold was estimated. The estimate found in the present study conforms fairly well with the two previously found estimates. It is suggested that the estimate found in the present study be used as the reference equivalent threshold force level for dbc.

Percutaneous Vs. Transcutaneous Transducers for Hearing by Direct Bone Conduction

There is a substantial need for improvement of the hearing situation for patients with chronic middle ear or ear canal disorders. To improve hearing for these patients, two different bone conduction hearing systems have been developed. The Nobelpharma Auditory System HC 200—the bone-anchored hearing aid we present here—uses a percutaneous transducer; whereas the Audiant device, developed by Dr. Jack Hough, uses a transcutaneous transducer. In percutaneous transmission, the transducer is directly coupled to the bone by means of a permanent skin penetration, whereas in transcutaneous transmission one part of the transducer is implanted and the other part is kept outside the intact skin and soft tissue. Comprehensive audiologic assessments indicate great differences in performance between the two systems. These differences probably originate in differences in length of gap and in different suspension properties of the two transducer systems. This article will demonstrate that large gaps, such as in the transcutaneous transducer, can be devastating for power consumption, maximum output capability, and second harmonic distortion. Since the properties of the suspension in the transcutaneous transducer are not under adequate control and the complication risk of permanent skin penetration is low, we continue to concentrate our efforts on percutaneous transducer systems.

A Speech-to-Noise Ratio Test with the Bone-Anchored Hearing Aid: A Comparative Study

Hitherto, for persons with impaired hearing who cannot use an air conduction hearing aid, the only alternative has been a conventional spring-loaded bone conduction hearing aid. Now, with minor surgery, a titanium screw can be implanted in the bone behind the ear and a coupling, which penetrates the skin, can be attached, giving a new kind of hearing aid—the “bone-anchored hearing aid.” Improved quality of sound is one of the patients’ subjective assessments. Improvement was not confirmed by a standard speech-discrimination test. With new speech material consisting of sentences in noise, the speech-to-noise ratio (SN) has been determined for 24 patients. Patients who previously used a conventional bone conduction hearing aid improved their SN on the average by 3.3 dB. The most important difference between the two aids related to improved SN is probably the increased audibility between 600 and 6000 Hz.

A NEW MORE POWERFUL BONE-ANCHORED HEARING AID : FIRST RESULTS

In a pilot study seven subjects were asked to use a new more powerful bone-anchored hearing aid, the BAHA cordelle, for 5 days. All seven had a moderate or severe sensorineural component in their hearing loss and had been using a BAHA superbass on a daily basis for more than 2 years. Free-field thresholds and speech recognition were measured with the BAHA superbass and with the BAHA cordelle. A questionnaire was also filled in to obtain subjective information about speech recognition with the BAHA superbass compared to the BAHA cordelle. Aided thresholds in the high frequency range with the BAHA cordelle were better than with the BAHA superbass. In six of the seven subjects there was significant improvement in the speech recognition score with the BAHA cordelle. These six subjects expressed preference for the BAHA cordelle.