Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Having protective or sheilding feature
Reexamination Certificate
2000-04-28
2003-04-08
Kuntz, Curtis (Department: 2643)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Having protective or sheilding feature
C361S818000
Reexamination Certificate
active
06546107
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to both a method and an apparatus for controlling the propagation of magnetic fields by electrodynamic/magnetic transducers, in telecommunication devices wherein static magnetic fields produced by the transducers are substantially shielded and dynamic magnetic fields are radiated in a substantially unimpeded manner.
2. Description of the Prior Art
Electrodynamic/magnetic transducers which are a subgroup of electroacoustic transducers, are used wherever electrical or electronic signals are to be converted into speech and/or speech is to be converted into electrical or electronic signals. Typical fields of use are, therefore, the audio and hi-fi field, domestic engineering in areas where alarm and bell signals are output, and telecommunications engineering, for example.
In the latter field of use, the electrodynamic transducers are particularly used, for example, in handsets (cord-connected, cordless—e.g., mobile parts and mobile phones), headphones and headsets, usually in the form of earphones, but also sometimes in the form of a microphone. The use of electromagnetic transducers is less common.
A big disadvantage of these transducer types, particularly the electrodynamic transducers, is that they produce, as shown in
FIG. 1
(showing a basic sketch of an electrodynamic transducer a), static magnetic fields (stray fields) MF
s
as a result of a pot magnet TM, for example, wherein the magnetic fields penetrate non-magnetic materials (plastics) unimpeded. Magnetizable objects such as pins, paperclips, iron filings, particles (in industries which use iron or steel, metalworking shops, etc., are inevitably attracted toward the center of the transducer. If the particles are small enough to pass through the (voice inlet) voice outlet openings, they collect at the place of greatest field strength (clearance of the pot magnet TM) and permanently jam a diaphragm MB of the transducer. Depending on how sensitive the diaphragm MB, is and hence the transducer as such, toward such mini foreign bodies, the result is either an abrupt total failure or a gradual failure of the diaphragm MB.
Furthermore, the relative movement of the handset in the vicinity of electrical conductors, particularly inductors, results in unwanted induced currents (a key issue regarding cardiac pacemakers, medical devices etc.).
As a result of the devices being miniaturized (e.g., characterized by a short distance between the transducer's sound exit opening and the sound exit opening in the housing of the handset; cf. FIG.
1
), smaller and smaller cordless mobile parts or mobile radio phones are becoming available on the market. However, problems are intensifying because “rare earth” magnets (such as magnets made of Nd or Sm alloys) with relatively high remanences, and, hence, relatively strong stray fields, are frequently being used in flat and small transducers.
The result of the problems outlined is, on the one hand, in some countries (e.g., Australia, Great Britain, USA) licensing requirements limit the static magnetic field. On the other hand, there recently has been an increased number of cases, particularly with GSM mobile phones, arising from earphones having failed on account of jammed diaphragms. Although fine-meshed tissue (e.g., dust webs) in the acoustic openings prevent the diaphragm from becoming jammed, they clogged over time such that reproduction became continuously quieter as the magnetic force is permanently exerted on the magnetizable particles contained in the tissue.
To solve this problem, electrets and piezoelectric microphones, which represent equivalent alternatives, are called upon for implementing the microphones in the handsets.
The situation is different, however, for implementing the earphones.
Piezoelectric transducers used as earphones have no pronounced magnetic field. However, the transducer technology based on the piezoelectric effect has two clear disadvantages as compared with the transducer technology based on a magnetic field: 1) From the point of view of speech quality, electrodynamic transducers, particularly with small diameters, are clearly superior; 2) Some countries (e.g., Australia, Great Britain, USA, Italy) and, in addition, British Telecom and France Telecom, generally have the requirement of “hearing aid compatibility” (hac) for stimulating hearing aids. This stimulation is (virtually) exclusively inductive and is based on a dynamic magnetic field (alternating field). This means that the required (measurement of the alternating field in an hac measurement plane as shown in
FIG. 1
) magnetic alternating field needs to be produced using cumulative supplementary coils.
Thus, in the end, the electrodynamic ransducers, in particular, are again called upon after all and, at the same time, attempts are made to solve the aforementioned problem in a different way.
When electrodynamic transducers are used, it is naturally helpful, for reducing the static stray fields, to have a relatively large distance between the transducer's sound exit opening and the surface of the handset as has been done, for example, in the, handset of the cord-connected Siemens “Symphony D” devices available on the market. However, this procedure is absolutely contrary to the market requirements of smaller and smaller handsets, particularly mobile (cordless) handsets. To solve this problem, the “transducer's sound exit opening ⇄ surface of the handset” distance would have to be approximately 1 cm. This would allow the aforementioned negative influences of the static magnetic field to be eliminated.
However, by increasing the distance, a dynamic magnetic field MF
d
(as shown in FIG.
2
—a basic sketch of an electrodynamic transducer), which is needed on account of the hac requirement and is also large enough given proficient dimensioning of a plunger-type coil TS which produces this dynamic magnetic field MF
d
, in conventional electrodynamic transducers, is also attenuated to such an extent that it is no longer sufficient for the hac requirement. This means that additional air-core coils are again necessary here, too, to amplify the alternating field.
Taking
FIG. 1
as a basis, a cover AD (shield, e.g.,) in the form of a shielding plate) is therefore used, as shown in
FIG. 3
, in a familiar manner. This cover concentrates the field lines of the stray field MF
s
and allows them less projection into space. The cover must, of course, be provided with openings to be “transparent” to the sound pressure produced by the diaphragm MB. The magnetic alternating fields MF
d
(cf.
FIG. 2
) for the hac requirement are consequently attenuated again, however, and therefore need to be produced with supplementary coils fitted in front of the cover.
EP 0 422 424 A2 discloses an electrodynamic transducer with improved electrical and magnetic shielding, which has a moving coil, a diaphragm and a magnet system which are surrounded by a housing with a cover. Clamped between the inside of the cover and the internal pole plate of the magnet system, there is an insert which is made of a nonmagnetizable, electrically insulating material. The insert defines the axial position of the magnet system with respect to the cover, and the cover shields external magnetic alternating fields and has openings which let through a sound pressure.
DE 3 401 072 A1 discloses an electrodynamic transducer head for nondestructive testing of workpieces using ultrasound, wherein the transducer head has an electromagnet which has a magnetic yoke and includes an outer pole shoe and an inner pole shoe surrounded by the latter. These pole shoes have exciter and reception coils arranged on them. To allow the inner pole shoe to be placed directly on the workpiece, moving relative to the latter, without any risk, these exciter and reception coils are provided with a protective cap which is surrounded by a protective ring whose end face facing the workpiece projects over that of the protective cap, the inner pole shoe and/or the protective cap hav
Ensey Brian
Kuntz Curtis
LandOfFree
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