EP1273204B1 - Acoustic transducer for broad-band loudspeakers or headphones - Google Patents

Abstract

The invention relates to an acoustic transducer for broad-band loudspeakers or magnet-free, electrodynamic head-phones for generating sound, especially for the use in the homogenous and/or inhomogeneous magnetic field of a magnetic resonance tomograph. According to the advantages of the invention, sound having defined characteristics can be generated in such a way that said sound is provided with good quality and high effectiveness within the strong magnetic field of a magnetic resonance tomograph. In addition to music and voice, this comprises the generation of sound for actively controlling noise by generating sound by means of one or several membranes (1) that form air pockets. Said membranes (1) consist of elastic, non-magnetic or slightly magnetic material and are connected to strip conductors (2) in a two-dimensional and solid manner. A Lorentz force which is caused by the magnetic field of the magnetic resonance tomograph is exerted on said strip conductors as the driving power when current flows.

Description

The invention relates to an acoustic transducer for Broadband speakers or magnetless, electrodynamic Headphones for sound generation Use in a homogeneous and / or inhomogeneous magnetic field of a magnetic resonance tomograph.

The generation of sound, especially with strictly defined ones Properties and in high quality such as Music, speech and anti-sound is in areas with strong magnetic fields are problematic since conventional electrodynamic speakers or headphones strong acoustic transducers in such environments Are exposed to forces and additionally those on the strong magnetic field based application can interfere. Modern methods of magnetic resonance imaging for imaging Representation of z. B. Brain performance and cardiac dysfunction are due to their high noise emissions in the low-frequency range only through passive measures can be met inadequately, in application and clinical distribution limited (magazine "British Journal of Radiology ", volume 1994, issue 67, Pages 413 to 415; Journal of "Radiology", vintage 1994, Issue 191, pages 91 to 93 in connection with the "Recommendation of the Radiation Protection Commission, adopted in the 131st meeting on June 22, 1995 ", page 17). Also below the legal limits Acoustic emissions reduce patient comfort and communication options and thus patient safety Active noise abatement ("anti-noise") represents a promising process for reduction of the noise emissions from MRI systems (magazine "Radiology", 1989, issue 173, pages 549 to 550 and magazine "Proceedings of the Society of Magnetic Resonance "Volume 1995, Issue 2, page 1223) effective noise cancellation is for frequencies up to about 1 kHz but only possible if the anti-noise speaker a very small spatial distance from Noise source (the gradient tube inside the MRI magnets) and the anti-noise speaker has the acoustic Can reflect the field of interference.

So far, it is not compatible with magnetic resonance imaging Sound generator has been described using this Requirements met to a sufficient extent.

Speakers previously designed for noise control directly in the tomograph, which use the inhomogeneous portion of the magnetic field for electrodynamic coupling, are only suitable up to pitches of approx. 1 kHz due to their design and can also not be installed in the homogeneous area of the magnetic field (DE 197 27 657 C1 ).
Arrangements for the extinction of sound waves, based on the principle of generating a signal that is phase-shifted by 180 °, have often been described outside of magnetic resonance tomography (DE 195 28 888 A 1), but cannot be used in the field of magnetic resonance tomography due to the circumstances there ,

Previous developments in noise suppression systems restrict in magnetic resonance imaging relying on noise reduction to improve patient monitoring outside the magnetic resonance tomograph, in one by the magnetic resonance tomograph acoustically isolated control room. This will also no sound generated, but one by use suitable adaptive filter reduced by the interference signal Signal with a conventional speaker inside of the magnetic field-free control room (EP 0 655 730 A1).

One for use within the homogeneous part of the Magnetic field of a magnetic resonance tomograph more suitable electrodynamic loudspeaker without its own magnet using ferromagnetic field inhomogenizers as well as hose line systems for forwarding the generated sound has already been described (US 005 450 499 A). However, this provides due to the used Hose system neither for generating Sound pressure required by anti-sound over the required Frequency band in a defined phase position, still allows the use of essential ferromagnetic Components that are absolutely essential to be demanded Handling in rooms penetrated by magnetic fields. Furthermore, when using ferromagnetic Components within the originally homogeneous part of the Magnetic field due to the local generation of undefined Magnetic field inhomogeneities interference with the Imaging.

The displacement principle on which the invention is based has already been described elsewhere in particular by reducing the effective mass Formation of air pockets (DE 2003 950, US 4039044, US 4160883). The effective mass is determined by the use a membrane which is suitably driven and which air in the trained air pockets is displaced, reduced. The desired big relationship from width to depth of the air pockets the relatively small spatial expansion of the magnetic field limited.

Newer solutions concern the detailed design of the Membranes and the magnet, however, all use own magnets in the form of pole pieces or the like (US 591 28 63) without the problem of spatial expansion and to generate the magnetic field.

An essential aim of the invention is noise pollution through the MRI system during the examination to reduce. Sound insulation with passive systems (e.g. earplugs) is psychoacoustically ineffective, because hearing is achieved to a comparable extent Damping reacts more sensitively. Active systems, existing from headphone system and soundproof capsules same damping and no psychoacoustic Effect weakening - on the contrary, loud music lowers it Sensitivity ("Noise Covering", effect enhancement) and the ability to communicate with music is called felt pleasant and increases comfort and lowers Dropout rate and the patient loses listening to music the sense of time is not.

Combined systems of soundproof capsules and headphone systems are known and described in the MRI. It are different systems with specific pre and Disadvantages, a modified is described for example Cone speaker without its own magnet as Headphone system. This system is robust and inexpensive, however, it is unable to produce low notes (below 300 Hz) to be transmitted. Also systems with piezo speakers are described, the above applies to these combinations said, but here is the lower cutoff frequency higher (due to design at 500-800 Hz). combinations with electrostatic headphones are also described and commercially available. These systems point a frequency response that is sufficiently deep is enough - their acoustic properties are very good. However, you have several disadvantages. In particular are they are expensive, the soundproofing of the earmuffs is greatly reduced since the large membrane does not contain any insulation materials allows more on the ear-facing side and they are very problematic in terms of security. The combination of headphone system (capacity) and cable (Inductance) can cause damage if damaged Represent the resonant circuit, the radio frequency energy of the MRI transmitter records. The system would work with the usual Transmitting powers may quickly get very hot and there is a risk of burns.

The invention has for its object an acoustic Converter for broadband speakers or headphones to create which in the magnetic field of a magnetic resonance tomograph can be used safely and reliably is high without interfering with imaging Quality requirements in a wide frequency range is enough, active noise abatement is made possible and simple and inexpensive to manufacture and in the form of implementation as headphones with an earmuff can be combined.

This object is achieved by the features in the claim 1.

Useful embodiments of the invention are contained in the subclaims.

The invention in the form of implementation as a loudspeaker the underlying problem is that the high noise levels effective MRI systems through active noise abatement can be lowered if a powerful Antisound generator in the gradient tube of the MRI system can be installed. In particular, here the fact that the Handling magnetic materials in today's world Applied magnetic field flux densities (1 T to 3 T in clinical operation) an extremely high risk potential represents.

The advantages achieved by the invention are that sound with defined properties in high Quality and high efficiency within the strong Magnetic field generated by a magnetic resonance tomograph can be. In addition to music and language, this also includes the generation of sound for active noise abatement, like they are not with hose line systems and others electrodynamic converters only in the inhomogeneous range of the magnetic field and in lower quality can be.

The advantages of folding membranes are used particularly well, if it succeeds, a big enough, enough strong and homogeneous magnetic field like one Magnetic resonance imaging to build up around the membrane.

Loudspeakers can be mounted anywhere in the magnetic resonance tomograph and thus optimally adapted to the respective purpose. This aspect is essential for use as an anti-noise speaker. The loudspeaker has a wide frequency band and in particular tones can be generated above a frequency of 1 kHz. At the same time, the principle applied of a drive that acts uniformly and uniformly on all parts of the loudspeaker diaphragm ensures that the bending vibrations and distortions that occur with conventional, local cone drives do not occur.
The use in a magnetic resonance tomograph enables a practically arbitrarily extended membrane due to the magnetic field which is spatially more extensive than the practical dimensions of the loudspeaker. The extremely low effective mass possible as a result leads to high efficiency and a uniform transmission behavior over wide frequency ranges. At the same time, the use of a large membrane surface offers the possibility of effectively producing deep tones.

In magnetic resonance imaging, one can be unrivaled large ratio of pleat depth to pleat height realized become. The effective mass of the speaker cone it becomes very small. For example, at a Pleat depth of 200mm and a pleat height of 10mm effective mass of the membrane compared to the actual Mass reduced by a factor of 420. This in turn means that the acoustic rigidity of the membrane is very high and almost independent of their mechanical properties becomes. This enables distortion-free Sound radiation even at high sound pressures. The Efficiency of sound generation is also increased since the losses due to positive and negative acceleration the effective membrane mass are low.

According to the invention, a headphones can be used up to now unmatched ratio of pleat depth to pleat height will be realized. The effective mass of the transducer membrane is also very low here and the acoustic rigidity of the membrane very high and almost regardless of their mechanical properties.

A large fold area enables the application of many electrically conductive elements arranged in parallel, preferably flat wires. The conductive elements are also electrically connected in parallel and thus result in a very low ohmic resistance of the arrangement. The electrical losses are minimized and there is no heat development in the individual elements. This considerably increases the operational safety and the lifespan of the sound generator. Furthermore, the arrangement has the advantage over the metal strips described (DE 2003 950) that only very low eddy currents can be generated in the conductive elements by the strong magnetic alternating fields generated by an MRI scanner with a frequency of up to 1500 Hz. This in turn prevents the conductive elements from heating up and in particular has no disruptive influences on the magnetic gradient fields of the magnetic resonance tomograph.
The magnetic field strengths (up to 3T) of an MRI scanner, which are atypical for acoustic transducers, impart a large driving force (Lorentz force) to the membrane firmly connected to them, even at low, auditory currents through the electrically conductive elements. This enables effective sound radiation even at low currents. The magnetic fields generated by these currents are correspondingly low and do not impair the homogeneity of the main field, which means that they have no disruptive influence on the imaging.

The complete absence of ferromagnetic materials enables safe handling under all circumstances of the anti-noise speaker. Otherwise you can ferromagnetic parts projectile in the direction of Center of the permanent magnetic field can be accelerated. There are no safeguards to control of ferromagnetic forces near MRI magnets required.

Some embodiments are in the drawings are shown and are described in more detail below.

Fig. 1a

eine gefaltete Membran mit parallel zu den Faltachsen verlaufenden in Reihe geschalteten Leiterbahnen,

Fig. 1b

die Veränderung der Membran gemäß Fig. 1a bei Durchfluß eines Stromes durch die Leiterbahnen,

Fig. 2a

eine gefaltete Membran mit orthogonal zu den Faltachsen verlaufenden parallelgeschalteten Leiterbahnen,

Fig. 2b

die Veränderung der Membran gemäß Fig. 2a bei Durchfluß eines Stromes durch die Leiterbahnen,

Fig. 3a

eine Membran mit einem Leiterbahnenblock,

Fig. 3b

die Veränderung der Membran gemäß Fig. 3a bei Durchfluß eines Stromes durch die Leiterbahnen,

Fig. 3c

eine Darstellung des Verhältnisses von Lufttaschenweite zu Lufttaschentiefe.

Show it

Fig. 1a

a folded membrane with parallel interconnects running in parallel to the folding axes,

Fig. 1b

1a the change in the membrane according to FIG. 1a when a current flows through the conductor tracks,

Fig. 2a

a folded membrane with parallel interconnects running orthogonally to the folding axes,

Fig. 2b

2a the change in the membrane according to FIG. 2a when a current flows through the conductor tracks,

Fig. 3a

a membrane with a conductor block,

Fig. 3b

3a the change in the membrane according to FIG. 3a when a current flows through the conductor tracks,

Fig. 3c

a representation of the ratio of air pocket width to air pocket depth.

Fig. 1a shows a possible embodiment in which the membrane 1, consisting of elastic, not or only weakly magnetic material, such as paper, nonwoven or plastic along an axis orthogonal or almost orthogonal to the magnetic field B of the magnetic resonance tomograph in one or more folds or waves is arranged, or a corresponding arrangement is formed by several movably connected individual membranes. Along the air pockets formed in this way by the surfaces 4, one or more conductor tracks 2 are in each case connected to the membrane 1 flatly and firmly in such a way that the direction of the conductor tracks runs parallel or almost parallel to the folding or bending axes. The conductor tracks 2 are interconnected by electrical connections 3 in such a way that the conductor tracks 2 on opposite surfaces 4 of an air pocket are flowed through by one and the same electrical current in opposite orientations.
FIG. 1b shows the arrangement shown in FIG. 1a when a current I flows through the conductor tracks 2. The external magnetic field B of the magnetic resonance tomograph transmits a deflecting force on the conductor tracks 2, the orientation of which is determined by the direction of the current flowing through. A conductor arrangement as described has the effect that the air pockets on one side of the folded or corrugated membrane surface 4 are narrowed by conductor tracks 2 moving towards one another, while the air pockets on the other side of the folded membrane are expanded. A hearing-frequency current leads to a simultaneous and common opening or closing of the air pockets on both sides of the folded or corrugated membrane 1. The pressure fluctuations caused by these movements within the swept air volume are emitted as sound waves on both sides perpendicular to the entire membrane surface. The efficiency of this arrangement is optimal if the magnetic field B of the magnetic resonance tomograph is oriented perpendicular to the folded or corrugated total membrane surface. Deviating from this geometry, be it due to the curvature or twisting of the entire arrangement or of parts thereof, reduces the efficiency without, however, questioning the functionality.

A deviation from this geometry makes sense for headphones, be it by curving or twisting the entire arrangement or parts thereof. The efficiency of the sound generation is reduced and thus the radiated sound energy is reduced to the usual and safe values for headphones. The headphones can then be operated with an electrical power, as is usually provided by headphone outputs of audio devices.
The membrane is inserted into the earmuffs in such a way that it is almost parallel to the main field of the MRI scanner when worn. The remaining space in the earmuffs is filled with acoustic insulation material so that passive attenuation is retained.

Fig. 2a shows an embodiment in which the Membrane 1, along one to magnetic field B of the magnetic resonance tomograph parallel or almost parallel Axis arranged in one or more folds or waves is, or a corresponding arrangement by several single membranes movably connected to each other becomes. One or more flexible conductor tracks 2 are connected flatly and firmly to the membrane 1 in such a way that the direction of the conductor tracks 2 is orthogonal or almost orthogonal to the folding or bending axes 5 runs. The conductor tracks 2 fitted in this way are electrically connected in parallel.

Fig. 2b shows the arrangement shown in Fig. 2a Flow of a current I through the conductor tracks 2. The external magnetic field B of the magnetic resonance tomograph imparts a deforming force on the conductor tracks 2, their orientation by the direction of the flowing Current is determined. A ladder arrangement as described causes the air pockets on a Side of the folded or corrugated membrane 1 narrowed be folded while those on the other side of the Membrane 1 air pockets are expanded. A hearing frequency current leads to a sense of the same direction and opening or closing the air pockets together on both sides of the folded or corrugated membrane 1. The pressure fluctuations caused by these movements within the swept air volume are sound waves on both sides perpendicular to the total membrane surface radiated. The efficiency of this The arrangement is optimal if the magnetic field B of the magnetic resonance tomograph perpendicular to the conductor tracks 2 is oriented. A deviation from this geometry is it by curving or twisting the entire assembly or parts thereof, reduces efficiency, without however the functionality in question to deliver.

Fig. 3a shows the effective part of an embodiment (without holder), in which the membrane 1 along one to magnetic field B of the magnetic resonance tomograph orthogonal or almost orthogonal axis is arranged in one or more folds or waves, or a corresponding arrangement movable by several interconnected single membranes formed becomes. One or more serving as feed lines 2b Conductors become flat and solid with the membrane 1 connected that the direction of the ladder orthogonal or runs almost orthogonally to the folding or bending axes. In addition, be parallel or almost parallel to the folding axis and thus orthogonal or almost orthogonal to the direction of the magnetic field B of the magnetic resonance tomograph one or more conductor tracks 2a flat and firmly connected to the membrane 1. The conductor tracks 2a are with the leads 2b on both sides of the formed by the membrane 1 air pocket electrically connected in parallel. The power must be supplied in such a way that one and the same electric current Conductor tracks 2a on opposite sides of the air pocket flows in the opposite orientation.

Several conductor tracks 2a are closed by means of supply lines 2b Conductor blocks 6 interconnected.

3b shows such an arrangement when a Current I represents one when current flows through the Conductor tracks 2 causes force mediated by magnetic field B, that the formed by the folded membrane 1 Air pocket is expanded or narrowed. A listening frequency Electricity causes within the swept air volume a pressure fluctuation in the same direction, which as Sound wave is emitted.

The arrangement shown in Fig. 3a and 3b enables by applying multiple parallel more effective Conductor tracks 2a a large area and eddy current free Driving the membrane 1. Taking into account the Large spatial compared to the dimensions of a transducer Extension of the magnetic field B of the magnetic resonance tomograph, so is the execution of a converter with additionally increased efficiency possible because the efficiency determining ratio of air pocket width a to air pocket depth b (see Fig. 3c) by enlargement the depth b can be reduced.

The invention is not limited to those described here Embodiments. Rather, it is possible by a suitable combination of the means mentioned and features to realize further design variants, without leaving the scope of the invention.

Claims (12)

1. Acoustic transducer for broadband loudspeakers or headphones for sound generation and for use in a homogenous and/or inhomogeneous magnetic field of a magnetic resonance tomograph using Lorentz force,
   characterized in that
   sound generation takes place by one or more membranes (1) that form air pockets, whereby the membranes (1) consist of elastic, nonmagnetic or weakly magnetic material and are connected two-dimensionally and securely to printed conductors 2 on which a Lorentz force conveyed by the magnetic field of the magnetic resonance tomograph acts as a driving power when current flows.
2. Acoustic transducer according to claim 1, wherein the air pockets are formed by the surfaces (4) of the membranes (1), and the surfaces (4) are bordered by the fold or bending axes (5).
3. Acoustic transducer according to claim 2, wherein the printed conductors (2) run parallel to the fold or bending axes (5) and are electrically connected in series.
4. Acoustic transducer according to claim 2 or 3, wherein several printed conductors (2a) are interconnected by means of feed lines (2b) into a printed conductor block (6), and the printed conductor blocks (6) run parallel to the fold or bending axes (5).
5. Acoustic transducer according to claim 2, wherein the printed conductors (2) run orthogonally or almost orthogonally to the fold or bending axes (5).
6. Acoustic transducer according to claim 5, wherein several printed conductors (2) are electrically connected in parallel.
7. Acoustic transducer according to claim 1, wherein to increase efficiency two or more membranes 1 are arranged such that deflection pointed in the opposite directions accomplishes displacement of a larger air volume.
8. Acoustic transducer according to claim 1, wherein current flow through the printed conductors 2 takes place in an orientation which is opposite on alternating sides by folding up one of more membranes 1 and by attachment of the printed conductors 2 along the surfaces 4.
9. Acoustic transducer according to claim 1, wherein the additional magnetic field which has built up in the vicinity of the loudspeaker is minimized by the compensatory arrangement of the printed conductors 2.
10. Acoustic transducer in an implementation as a loudspeaker according to claim 1, wherein it is made as lining of the inside walls of the tomograph with folds (volume loudspeaker).
11. Acoustic transducer in an implementation as a loudspeaker according to claim 1 for use as an antinoise loudspeaker.
12. Acoustic transducer in an implementation as a loudspeaker according to claim 1 for use within part of a two-way intercom system, the part being located in the magnetic resonance tomograph.

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