Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Electromagnetic
Reexamination Certificate
2002-09-09
2004-06-15
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Electromagnetic
C381S152000, C381S412000
Reexamination Certificate
active
06751334
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a bone transmitting hearing aid/bone transmitting vibrator for generating or monitoring vibrations in accordance with the variable reluctance principle comprising a coil for generating/monitoring a magnetic signal flux, a bobbin body of a magnetic conductive material, one or more yokes of magnetic conductive material, and one or more permanent magnets for generating a magnetic biassing flux.
BACKGROUND OF THE INVENTION
Bone transmitting hearing aids are used by patients who can not use conventional air transmitting hearing aids e.g., due to chronic middle ear disease or a congenital/acquired deformity. A traditional bone transmitting hearing aid consists of a bone transmitting vibrator enclosed in a polymer shell which is pressed with a constant pressure of 3-5 Newton against the skin over the bone behind the ear. Microphone, amplifier, and current source are placed in their own enclosure at a suitable site and at a secure distance from the vibrator to avoid feed-back coupling problems. The most essential drawbacks of this type of bone transmitting hearing aids is that it is uncomfortable to wear due to the constant pressure and that the soft skin over the bone deteriorate the transmission of vibrations to the bone.
Since the beginning of the 1980's there is a new type bone transmitting device—a bone anchored hearing aid (BAHA)—where the bone transmitting vibrator is connected directly to the bone via a skin penetrating and bone anchored implant of titanium, cf e.g., SE-A-81 07 161-5, SE-A-94 04 188-6 or Tjellström & Håkansson, The Bone Anchored Hearing Aid—Design principles, indications, and long-term clinical results, Otolayngol. Clin. N. Am. Vol. 28, No. 1, (1995). In this way a bone transmitting hearing aid is obtained which provides for higher amplification, splendid carrying comfort, and where all parts can be enclosed in the same housing. In a future solution the vibrator can be implanted completely and thereby skin and soft tissue can remain intact. Signal and necessary energy can in this case be transferred through intact skin by means of inductive connection. At more severe hearing damages where the energy demand is large the energy can be transferred by means of skin penetrating (percutaneous) electric connection device, cf e.g., SE 9704752-6. The advantages implanting the whole vibrator into the temporal bone compared with a vibrator being externally situated is, besides the pure medical ones, that an increased sensitivity is obtained, the size of the externally placed unit becomes smaller and stability margins becomes improved.
It is of course of utmost importance that BAHA vibrators in general and implantable ones in particular are (1) efficient, to keep current consumption down, (2) small, in order to be able to be placed in the temporal bone, and (3) reliable, as a repair/exchange of the vibrator requires a surgical incision. The need to improve conventional bone transmitting hearing aids in the above mentioned respects is perhaps the most important motif behind the present invention.
Another area of application where the bone transmitting vibrator is used is within clinical audiometry. At a conventional audiometry examination both air transmitting and bone transmitting threshold values are regularly determined The vibrator used at bone transmitting audiometry is of the same kind as used for bone transmitting hearing aids with the difference that the audiometry vibrator shall be capable of determining bone thresholds down to 250 Hz. It is commonly known the vibrators of today to be used in audiometry, e.g., B71 from Radio Ear, shows dissatisfactory high distorsion at low frequencies due to an intrinsic problem of this construction. Thus even here there is a great demand for improving the technology.
Vibrators based on piezo electricity, magnetostriction (magnetic elongation), and electromagnetism of the moving coil type are not used in bone transmitting hearing aids or audiometry vibrators mainly due to bad response at low frequencies. The devices used are electromagnetic vibrators of the variable reluctance type.
Prior Art
A cross-section of a conventional (State of the Art) vibrator of variable reluctance type of hitherto known type is shown in FIG.
1
. The vibrator of
FIG. 1
is substantially circularly symmetric. It consists of on one hand an annular permanent magnet, a coil coiled around an annular bobbin body, as well, and a counter mass connected in a suitable manner to a rigid unit (lower part), and on the other hand of a vibrator plate connected with a spring elemnt and a suitable adapter for connection to the load (top part). The bobin body and the vibrator plate are made of magnetic field well conductive material, suitably special treated soft iron. The vibrator plate functions as a yoke closing both the static (biassing) magnetic flux &PHgr;
0
generated by the permanent magnet and the signal flux &PHgr;_ generated by a signal current flowing through the coil. The total development of force in the air gap is determined under certain presumptions approximatively by
F
tot
∝(&PHgr;
0
+&PHgr;)
2
=&PHgr;
0
2
+2.&PHgr;.&PHgr;
0
+&PHgr;
2
Equ. 1
The term &PHgr;
0
2
represents the static force of the permanent magnet, the term 2.&PHgr;. &PHgr;
0
represnts the useful signal flux and the term &PHgr;
2
represents a non-desired distortion. The primary task of the counter mass is to add mass to obtain a suitable resonance frequency fr according to the relation
f
r
∝1
/m.c
Hz Equ. 2
Wherein m is the mass of the lower part of the vibrator (including the outer rigid part of the spring element) and c is the compliance (resilience) of the spring element. In the following the mass m is called the counter holding unit. The resonance frequency may, e.g., in accordance with Equ. 2 be lowered by increasing the weight of the counter holding unit (m) or increasing the compliance of the spring element (c).
Drawbacks at Conventional Variable Reluctance Vibrator
Of the above description it is evident that a conventional variable relucatance vibrator is construed in such a way that the magnetic signal flux coincides/follows the static flux (biassing flux) along is entire run, cf FIG.
1
. It leads to the fact that the properties of the electrodynamic change deteriorate as permanent magnets as a rule have a low relative permeability r (low permeability provides for a high magnetic flux resistance, which decreases the generation of signal force).
In order to prevent that the air gap which is created between the vibrator plate and the lower part of the vibrator does not collapse due to the static force (&mgr;&PHgr;
0
2
) a spring element is required that keeps the parts apart. This spring element consists normally of a plate spring package with or without dampening coating as described in SE-A-85 02426-3. In resting condition which corresponds to an air gap of 50 to 100 nm the spring is so bent out to such a degree that its returning force exactly balances the attracting force of the permanent magnet. The Attraction force of the permanent magnet thus all the time strives to reduce the air gap created by balancing the magnet force and the spring force. Ageing of the spring as well as outer mechanical strains may thus lead to that the air gap of the vibrator collapses. If this should occur the sound of the vibrator becomes strongly distorted and the vibrator has to be repaired.
Another problem is that this type of vibrator, at higher signal levels, creates a high harmonic distortion of the second order due to the term &PHgr;
—
2
, cf Equ. 1. In order to obtain a good linearity a high biassing flux (&PHgr;
0
) is required, which requires high stiffness of the returning spring which in turn leads to a higher resonance frequency. This increase of the resonance frequency can be counteracted by increasing the counter holding mass (cf Equ 2) but at the price of increased weight and size.
Balanced Armature
In order both to be able to maintain a high biassing flux and simultan
Gauthier & Connors LLP
Le Huyen
Osseofon AB
LandOfFree
Electromagnetic vibrator does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electromagnetic vibrator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electromagnetic vibrator will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3365034