ETSI ETS 300 381 ed.1 (1994-12)

SLOVENSKI STANDARD
SIST ETS 300 381 E1:2003
01-december-2003

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Telephony for hearing impaired people; Inductive coupling of telephone earphones to

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the

Whilst every care has been taken in the preparation and publication of this document, errors in content,

typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to

"ETSI Editing and Committee Support Dept." at the address shown on the title page.

Foreword .......................................................................................................................................................5

1 Scope ..................................................................................................................................................7

2 Normative references..........................................................................................................................7

3 Definitions and abbreviations ..............................................................................................................7

3.1 Definitions ............................................................................................................................7

3.2 Abbreviations .......................................................................................................................7

4 Introduction..........................................................................................................................................8

4.1 General ................................................................................................................................8

4.2 Background..........................................................................................................................8

5 Requirements......................................................................................................................................8

5.1 Magnetic field strength level ................................................................................................9

5.2 Linearity of the magnetic field strength ................................................................................9

5.3 Frequency characteristics....................................................................................................9

6 The probe coil....................................................................................................................................11

6.1 Dimensions ........................................................................................................................11

6.2 Calibration of probe coil .....................................................................................................11

6.3 Distortion............................................................................................................................11

6.4 Connecting leads for probe coil .........................................................................................12

7 Test procedures ................................................................................................................................12

7.1 Calibration of receive sound pressure level.......................................................................12

7.2 Measurement of the magnetic field strength level .............................................................13

7.3 Measurement of the linearity of the magnetic field strength ..............................................13

7.4 Measurement of frequency characteristics........................................................................13

Annex A (normative): Packaging, labelling and user instructions .........................................................15

A.1 Packaging and labelling ....................................................................................................................15

A.2 User instructions................................................................................................................................15

Annex B (informative): Step-by-step measurement procedures.............................................................15

Annex C (informative): Symbol indicating facilities for the hearing impaired...........................................16

Annex D (informative): Typical characteristics of probe coil ...................................................................17

Annex E (normative): Calibration of the probe coil................................................................................17

Annex F (informative): Components of the inductive field ......................................................................18

Annex G (informative): Bibliography........................................................................................................19

History..........................................................................................................................................................20

This European Telecommunication Standard (ETS) has been produced by the Terminal Equipment (TE)

Technical Committee, in co-operation with the Human Factors (HF) Technical Committee, of the

Several administrations and operating agencies have expressed the opinion that it would be acceptable if

this ETS were to be made mandatory for public telephones and wherever telephones are installed on the

basis of safety, e.g. in lifts. It would not be expected to be mandatory in all countries for all telephone

terminals. However, if such a facility is made available, it should conform to the requirements of this ETS,

and labels and literature advertising the facility should not be related to a particular product unless it does

Annexes A and E of this ETS are normative while annexes B to D and annexes F and G are informative.

Date of withdrawal of any conflicting National Standard (dow): 30th September 1995

This ETS applies to all telephones having supra-aural earphones (see ITU-T Recommendation P.57 [4])

that can be connected to the Public Switched Telephone Network (PSTN) or Integrated Services Digital

Network (ISDN), which are intended for direct application to the ear (e.g. traditional handsets, operators'

headsets) and which provide, at the earphone, a magnetic field for coupling to hearing aids. It specifies

the level linearity and frequency dependence of the magnetic field strength produced by the handset and

This ETS incorporates by dated or undated reference, provisions from other publications. These

normative references are cited at the appropriate places in the text and the publications are listed

hereafter. For dated references, subsequent amendments to or revisions of any of these publications

apply to this ETS only when incorporated in it by amendment or revision. For undated references the latest

level of magnetic field strength: The maximum value of the magnetic field strength is given in

subclause 5.1 and is measured in accordance with subclause 7.1. The units are Amperes per metre

permissible range: The range into which the measured level of the magnetic field strength needs to fall

plane of measurement: A plane parallel to the earcap plane at a distance of 10 mm.

preferred range: The range of magnetic field strength likely to be required for satisfactory performance by

hearing aids designed primarily for coupling to magnetic loops often installed in auditoria.

sound pressure level: Acoustic sound pressure level is expressed in decibels relative to 1 Pascal (or

It is recognised that there is a sizeable proportion of telephone users that have difficulty in conversing over

a telephone connection due to hearing loss. To alleviate these difficulties special means have been

provided in many national systems to enable hearing impaired users to couple their hearing aids

inductively to the telephone receiver, and a number of national/international specifications define

characteristics for this form of coupling. This ETS addresses the requirements for successful inductive

Furthermore, it is also recognised that many hearing impaired users are able to have satisfactory

telephone conversations while coupling their hearing aids acoustically to the telephone receiver, or even

using the telephone handset without a hearing aid. This latter situation is possible due to the fact that,

under good conditions, a telephone connection can be louder than a face-to-face conversation over a 1

The inclusion of inductive coupling does not reduce or replace existing technical standards that apply to a

handset. Inductive coupling can be combined with other additional functionality, such as amplification or

Provision of additional amplification in the mouth-to-ear path can greatly increase the proportion of

telephone conversations involving hearing impaired users that are rated as "good". A separate ETSI

Standard is planned to cover this form of coupling and it is also addressed by ITU-T Recommendation

P.37 [1]. Certain national standards also exist to enable direct electrical connection of hearing aids to

telephone apparatus. It is hoped that this form of coupling will be covered in a future ETSI standard.

Magnetic induction systems incorporated in telephone handsets generate an alternating magnetic field

with special characteristics which make the field detectable by hearing aids equipped with induction

Reception of an audio-frequency signal via an induction pick-up coil can often allow an acceptable signal-

to-noise ratio to be achieved in cases where the acoustical reception would otherwise be degraded by

The magnetic field strength, which enables induction pick up coils in hearing aids to function effectively,

shall be high enough to produce an acceptable signal to noise ratio but not so high as to cause

The value of the magnetic field strength given in this ETS has been chosen so that these requirements

Measurement methods used in this ETS are in accordance with those given in ITU-T Recommendations

NOTE: Care should be taken when designing hearing aids to include sufficient immunity to

radio frequency interference to avoid disturbances arising from the detection of radio

The following requirements in respect of magnetic field strength as a function of frequency shall be met at

all settings of the volume control, if provided. These requirements concern the sensitivity at 1 000 Hz, the

frequency response and the linearity at 1 000 Hz as a function of the earphone sound pressure level

measured with an artificial ear conforming to ITU-T Recommendation P.57 [4], Type 1.

NOTE: In respect of low acoustic impedance earphones, when the artificial ear according to

ITU-T Recommendation P.57 [4], Type 3.2 is validated then the level definition will

The level of the magnetic field strength at 1 000 Hz when measured in accordance with subclause 7.2

for an electrical drive to the telephone that gives a sound pressure level of - 14 dBPa at the artificial ear.

NOTE: Hearing aids with magnetic pick-up coils primarily intended for coupling to magnetic

loops in auditoria in accordance with IEC Publication 118-4, are likely to require a field