Inductive hearing system

from Wikipedia, the free encyclopedia

An inductive hearing system , also induction loop system , induction loop , less often ring loop system , is a technical device with which audio signals such as music or speeches in event rooms can be made accessible to the hard of hearing . The audio signals are converted into analog electrical currents and these are transmitted as an electromagnetic alternating field via an induction loop laid out in the room . With hearing aids that have a special built-in receiving coil , these sound signals can be received and reproduced with little interference.

Logos for the reference to inductive hearing systems
Reference to inductive hearing systems in a Danish church

Structure and functionality

The system consists of a signal source (e.g. microphone or the usually existing microphone system), an electronic amplifier circuit (technically a current amplifier as opposed to a voltage amplifier for the loudspeakers), an induction loop with usually only one turn and the hearing aid or an induction receiver as a receiving device .

The induction loop is laid around the room to be supplied, e.g. along the inner walls, and connected to the amplifier device. If the room is larger (e.g. more than 6–10 m wide) or if there is steel reinforcement, then special types of laying make sense (e.g. eight, double eight, clover leaf or loop array). During operation, this cable loop emits an alternating magnetic field that generates an electrical voltage in the receiving coil of the hearing aid through electromagnetic induction , the course of which is similar to that of the audio signal. In the hearing aid, this is amplified by the audio amplifier, adapted to the individual hearing loss and transmitted to the wearer's eardrum via the earphones .

In order to be able to use such induction loop systems, the hearing aid must have a so-called telephone coil ( “T-coil” for short ), which absorbs the alternating magnetic field of the induction loop. The microphone of the hearing aid is usually deactivated when the telephone coil is used. Most hearing aids (approx. 85%) have such a T-coil, but most of the time it was not activated by the hearing care professional. He has to do this at a later date free of charge, the effort is a few minutes.

application

Inductive hearing systems are mainly used in public buildings and event rooms such as B. churches, cinemas, theaters and lecture halls application. According to the Federal Building Guideline, if a loudspeaker system is installed, public buildings must also - if feasible - be provided with a sound system, preferably with an induction system. Locations equipped with an inductive hearing system are often indicated in the entrance area by a sign in the form of a blue or yellow square with a stylized ear and the letter "T".

For alleged cost reasons or presumably due to a lack of a large number of users, often only delimited areas are provided with a ring loop. These are only certain seating groups in cinemas or in a church, which are then marked accordingly. However, in the sense of inclusion, this is not to be regarded as barrier-free. Induction systems are actually stationary systems. But mobile FM systems or stationary infrared systems can also use the advantages of inductive transmission into the hearing aid with the help of a mini induction loop (neck loop).

advantages

  • As a rule, the use of an inductive hearing system improves the signal-to-noise ratio for the respective user, or more precisely, the interference to useful noise ratio, as the pure audio signal is selectively transmitted via the induction loop and background noises are faded out or significantly reduced.
  • Many lecture rooms (especially churches) have room acoustics that have a negative impact on the intelligibility of speech through reverberation or echo , which has an increased effect with impaired hearing. (Reverberation time> 0.5 sec) Since the signal fed into the inductive hearing system is picked up with a microphone directly at the sound source, such disturbing effects can be effectively attenuated.
  • Disturbing background noises that are generated near the hearing aid wearer can also be suppressed by the inductive transmission. However, this only applies if the internal microphone of the hearing aid can be switched off when the telecoil is used, which is possible with most devices.
  • It is the only hearing system technology that is worldwide and completely manufacturer-independent compatible.
  • It is the only hearing system technology that can be considered barrier-free, as no additional equipment is necessary for hearing aid wearers and they do not have to come out, i.e. H. somewhere as a "supplicant" have to borrow a special device necessary at the transfer location, such as the z. B. with infrared or radio technology is the case or have to bring your own additional device, such. B. with streamer technology a smartphone or tablet with a special manufacturer-specific app.
  • It is the only hearing aid technology that can reach at least 85% of all hearing aid users.
  • It is the only hearing system technology that does not incur any additional costs for hearing aid wearers, i.e. it is socially acceptable, because all cash register hearing aids have the T-coil, it only has to be activated free of charge by the hearing aid acoustician, i.e. H. be activated electronically.
  • It is the only hearing system technology that is not limited in the number of users. (The alternative FM or IR technology is limited to the number of receivers kept in stock by the operator; the streamer technology common in Germany is limited to the 50 participants per transmitter.)
  • As a rule, it can be installed without structural measures, since the loop (usually a standard cable with a cross-section between 0.75 and 2.5 mm²) is laid on the floor and not in the floor. The effort is usually less than installing a loudspeaker system.
  • The costs are kept within reasonable limits, usually between 2000 and 3000 EUR per room to be supplied.
  • If the hearing aid wearer uses the T-coil built into the hearing aid, their hearing impairment is compensated to the maximum level, while the use of a conventional FM or IR under-the-chin receiver does not take into account their individual hearing impairment because the hearing aids have to be removed to be able to use the standard Headphones to hear. The latter would be completely inadequate for the severely hard of hearing.
  • There are mobile (transportable) systems that can be set up operationally within an hour. However, hearing aid wearers sometimes have to live with compromises and compromises in terms of quality, as there is neither time nor qualification for a calibration according to DIN EN 60118-4: 2014.

disadvantage

  • Strong electromagnetic sources of interference (such as older fluorescent lamps , tube monitors , dimmers, notebooks at a distance of up to about 1/2 meter and, allegedly, cell phones) can have a disadvantageous effect in the application environment of an induction system. In practice, it has been shown that mobile radio devices do not influence the induction field itself, but rather - if at all - the upstream amplifier system, in particular radio microphones or technically outdated microphone amplifiers or damaged signal lines. Interferences are consequently transmitted equally to the loudspeaker system and the induction system, so they are not a specific problem of induction technology. Interferences in the actual induction amplifier have never been reported when installed correctly, since all common induction amplifiers have been shielded according to the state of the art for years. Technical background: The induction system works in the low frequency range (100–5000 Hertz), while the mobile radio works in the gigahertz range. Audible interference can only arise from interference in the amplifier circuits and demodulation there into the audible frequency range, which would also affect the amplifiers of the microphone and loudspeaker system. If the system is correctly installed, there will be no interference from mobile communications. In practice, there are very seldom faults due to incorrect electrical installation, e.g. B. when the power supply (phase) does not take the same route as the return (neutral), or in the immediate vicinity of distribution cabinets (up to about 3 m). Disturbances are also to be expected in the immediate vicinity of electrified federal railway lines or tram lines. However, it must also be said that interference due to interspersed mains hum (50 Hz) is transmitted by very few hearing aids and is perceived as less disturbing in terms of hearing physiology.
  • The electromagnetic audio signal is neither encrypted nor coded , therefore it is in principle possible to listen to the broadcast information outside the room with simple means without authorization . However, it is possible to limit the horizontal overspill extremely through a suitable loop layout. If strong security against eavesdropping is actually necessary, the only alternative is encrypted digital FM or PCM radio technology or encrypted streaming (WLAN) technology, which are currently not available on the free market. Infrared technology would only be bug-proof if the room was sealed light-tight. Incidentally, eavesdropping security is limited by nature to an extremely limited audience. In an extremely limited audience, however, it is possible to communicate adequately with the hard of hearing through appropriate behavior. In purely practical terms, the problem of security against eavesdropping does not arise, but rather arises than:
  • Problem of the separation between rooms: If inductive transmissions are to take place in closely spaced rooms at the same time, this can lead to mutual crosstalk. A standard loop can also be received by a hearing aid about 30% outside of its width / length, albeit considerably less than inside the loop. If two rooms are next to each other, one of the loops overlaps the neighboring one. A suitable loop layout (cancellation loop) can largely prevent crosstalk between horizontally adjacent rooms. However, this is considerably more difficult, if not practically impossible, for rooms lying one above the other. Such problems are usually only dealt with with the help of experts at the renowned induction amplifier manufacturers who use proprietary software for this purpose.
  • If the building contains considerable steel reinforcements, a considerable amount of additional work is necessary to achieve the necessary field strength according to DIN EN 60118-4: 2014 (loop array with at least two amplifiers operated out of phase). In such a case, the loop must be laid "under the carpet" or "under the parquet". In most cases, this also requires planning by experts from the renowned amplifier manufacturers.
  • The establishment of a stationary induction system must therefore be preceded by expert advice, it must be planned and installed by demonstrably competent electroacousticians who are state of the art and in possession of the measuring equipment in order to install it in accordance with DIN EN 60118-4: 2014 can. It therefore makes no sense to buy it ad-hoc in stores on the assumption that it can be used immediately. For the church sector, most regional churches offer free and non-binding preliminary advice from the respective hard-hearing pastoral care, which enables you to understand the offers of specialist companies.

See also

Web links