Telephony end to end levels and the hearing impaired user
G J Barnes, BNR Europe,
London Road, Harlow,
Essex, CM17 9NA
+44 1279 402720
Email G.J.P.Barnes@bnr.co.uk
Contents
End to End Acoustic Loss for an Average (Optimum) Telephone Connection
Typical Telephony Connection - Comparison with a one metre air path
Inductive Field Developed over a Typical Telephone Connection
Input Sensitivity for the Hearing Aid Inductive Coupling
Typical Telephone Connection Inductively Coupled to a Hearing Aid
ANNEX 1 - A Brief Review of the Relevant Standards
ANNEX 2 - Tolerances
ANNEX 3 - Essential Features for a Specification Covering Sensitivity for the Inductive Input of Hearing Aids
Abstract
For many years a number of standards relating to the measurement of the sensitivity of hearing aid inductive pickup coils have been in existence, e.g. IEC 118-1. Additionally other standards relate to the inductive field to be provided by room loops and, more recently, by telephone earphones (IEC 118-4, CCITT Rec. P.37, ETSI pr ETS 300 381, EIA/TIA RS 504) in order to provide, by means of the inductive coupling feature, satisfactory communication to a hearing aid user.
The paper, by means of a brief review of the standards mentioned, and an assumed standard for the inductive coupling input sensitivity to a hearing aid and, together with the long term international end to end telephone connection losses, looks at the grade of service that can be expected for a hard of hearing telephone user. It also relates the telephone connection levels to that of a one metre air path, and underlines the need for an international standard for the inductive input sensitivity for hearing aids. The paper suggests the essential elements of such a standard including values, and links this to the sound level occurring at a hearing aid microphone over a one metre air path. Paper given at International Human Factors Symposium, Melbourne, March 95
Introduction
For many years a number of standards relating to the measurement of the sensitivity of hearing aid inductive pickup coils have been in existence, e.g. IEC 118-1. Additionally other standards relate to the inductive field to be provided by room loops and, more recently, by telephone earphones (IEC 118-4, CCITT Rec. P.37, ETSI pr ETS 300 381, EIA/TIA RS 504) in order to provide, by means of the inductive coupling feature, satisfactory communication to a hearing aid user. (A brief review of these standards is given in Annex 1).
Unfortunately, in the case of telephony, instances have been reported, albeit often anecdotally, of telecommunications using the inductive coupling feature that have not been satisfactory. A number of reasons for poor communications can be suggested, but it is not possible to be objective and to suggest corrective action with any confidence as there is currently no agreed international standard for the input sensitivity of a hearing aid inductive pickup coil.
In order, therefore, to assess the likely effect of recently agreed standards for inductive coupling on hearing impaired telephone users this study has been carried out using an assumed standard. It has been based on the standard in use in the UK, and uses the same sensitivity values. These are linked to the sound level occurring at a hearing aid microphone over a one metre air path, thus avoiding the need to be specific about particular volume control setting on the hearing aid and leading to the possibility of calculating end to end levels over a telephone connection.
It has also been assumed that the loss of speech signals between the two ends of a telephone connection having characteristics similar to an IRS is approximately equal to the loss at 1 kHz. (A more accurate assessment of the relative effect of the telephone transmission path with hearing aid inductive coupling compared with a hearing aid acoustic input over a one metre air path, taking into account the different bandwidths, will need to be carried out at some stage.)
One of the objectives of developing a standard for the inductive coupling of telephones to hearing aids is to ensure that as far as possible conversations over a typical telephone connection result in sound levels at the ear of a hearing aid user that are similar to those experienced in face to face conversation using the hearing aid microphone. The study looks at this aspect also.
End to End Acoustic Loss for an Average (Optimum) Telephone Connection
The Figure 1 shows the end-to-end loss of a telephone connection meeting the ITU long term aim of 10 dB OLR (Overall Loudness Rating) given in Recommendation G.111. This is based on the use of telephones at the send and receive ends having the same sensitivity frequency characteristic as the IRS (Intermediate Reference System - ITU Rec. P.48). At 1000 Hz the IRS send sensitivity is -3.7 dBV/Pa and the receive sensitivity is 12.6 dBPa/V.
Figure 1
Speech path mouth to ear loss at 1 kHz for a connection having a typical OLR of 10 dB
If connected directly together, (giving by definition an OLR of 0 dB), at 1 kHz this
connection would have an overall gain of
12.6 -3.7 = 8.9dB
[Tests have suggested that for speech the overall acoustic gain is about 5 dB, ie 3.9 dB less. This however involves a bandwidth change which is probably not relevant for hearing aid listening. Two figures are given from here on on the understanding that the effective value lies somewhere in between, to be determined as mentioned in Section 1.] Thus given a typical overall loudness rating path of 10 dB, equivalent to connecting a 10 dB loss pad between the IRS send and receive parts, the end to end acoustic path would have a gain of
8.9 - 10 = - 1.1 dB @ 1000 Hz
8.9 - 10 -3.9 = - 5.0 dB for speech
One Metre Air Path Loss
The mouth to ear loss of a one metre air path under free field conditions is usually taken as 30.2 dB in a straight line horizontally from the MRP (Mouth Reference Point). [This can be deduced from the assumption for telephony purposes that the virtual source of signals from the mouth originates from a point 6 mm behind the lip plane and that the MRP is 25 mm in front of the lip plane. The inverse square law is invoked in the calculation].
Given the long term speech level at the mouth reference point this results in a figure of
89.3 - 30.2 = 59.1 dBSPL at the ear of a listener 1 metre away.
If the obstacle effect of the head is taken into account a further 1.8 dB (1 kHz) may be added. In reverberant conditions, assuming a reverberation radius of 1 metre another 3 dB may be added giving a final figure of 63.9 dBSPL. This compares well with the value of 65 dB often assumed in hearing aid design circles.
Typical Telephony Connection - Comparison with a one metre air path
For the typical connection shown in Figure 1 the end to end 1000 Hz acoustic loss of 1.1 dB is in fact louder than a one metre air path by 29.1 dB (free field) or 24.3 dB (reverberant conditions) depending on the conditions assumed. The figures for speech are estimated as 25.2 and 19.3 dB respectively.
This explains why many mildly hard of hearing people who do not use a hearing aid in fact find it easier to converse over a typical telephone connection than face to face. Also, acoustic coupling to hearing aids is quite successful in many instances, albeit there is some degrading of the frequency response of the overall connection due to the characteristics of typical telephone earphones that are designed to work into a 'closed cavity', such as an ear. The presence of a severe acoustic leak, as is present when coupling to a hearing aid microphone results in the loss of low frequencies. Recently however newer designs of earphone, for example as used in the Beocom telephone and having a low acoustic impedance, have resulted in frequency characteristics that are far less dependent on the load applied, thus preserving a good low frequency performance when coupled to a hearing aid and similar to that enjoyed when inductively coupled.
Inductive Field Developed over a Typical Telephone Connection
For a telephone at the receive end of the connection that provides an inductive field in accordance with the requirements of ETSI ETS 300 381, or ITU Rec. P.37 a sound pressure of 80 dB SPL (1000 Hz) at the ear would result in a field of -30 to -17 dB rel 1 A/m. Thus the typical connection shown in Figure 1 would give a sound level at the earphone 8.2 dB louder than the measurement condition (80 dBSPL) and an inductive field of
(-30 to -17) + 8.2 = -8.8 to -21.8 dB rel 1A/m @1000 Hz
(-30 to -17) + 8.2 -3.9 = -12.7 to -25.7 dB rel 1A/m for speech
Input Sensitivity for the Hearing Aid Inductive Coupling
Currently there is no internationally agreed standard for the inductive coupling input sensitivity of a hearing aid. In the UK however the UK Dept of Health do have a specification that requires all aids supplied with an inductive coupling feature to have a defined sensitivity. This requires the output sound pressure level at 1000 Hz, measured with 10 mA/m (-40 dB rel 1A/m) at the inductive input and with the aid gain set to give 40 dB air to air gain, to be 95 ± 5 dBSPL. Thus an inductive field of -40 dB rel 1A/m is equivalent to 55 dBSPL at the microphone input.
Using this input sensitivity one can see from Figure 2 that for telephone sets meeting the ETSI ETS 300 381 or ITU Rec P.37 the sound level (1000 Hz) at the hearing aid earphone would be equivalent to 65 to 78 dBSPL at the microphone.
Speech path 80 dBSPL at the ear results in For hearing aids meeting the UK spec. -30 to -17 dB rel 1A/m an inductive field of equivalent to 65 to 78 dBSPL at the hearing aid microphone For sets meeting ITU and ETSI standards F
Figure 2
Interpretation of the UK Dept. of Health hearing aid specification
Typical Telephone Connection Inductively Coupled to a Hearing Aid
A typical telephone connection having a an OLR of 10 dB results in a sound level (1000 Hz) at the receiving ear piece of 88.2 dB, i.e. 8.2 dB louder than the sound level used in the definition and measurement procedures of ETS 300 381 and ITU Rec. P.37, (see Figure 2). Using the information given in Section 6 a typical telephone connection with an OLR of 10 dB would give sound levels at the hearing aid earpiece equivalent to hearing aid microphone levels of;
(65 to 78) + 8.2 = 73.2 to 86.2 dBSPL @1000 Hz
(65 to 78) + 8.2 - 3.9= 69.3 to 82.3 dBSPL for speech
Put another way, at 1000 Hz this is 8.2 to 21.2 dB louder than the one metre air path assumed for hearing aid design, or 14.1 to 27.1 dB louder than the theoretical free field one metre air path, and for speech 4.3 to 17.3, or 10.2 to 23.2 dB respectively.
Conclusions
From this simple study it would appear that for hearing aids inductively coupled into telephone sets meeting the requirements of ETSI prETS 300 381 or ITU-T Rec. P.37 over a typical connection having an OLR of 10 dB, a hearing aid user can expect to experience conversational sound pressure levels at his/her ear that are equivalent to levels at least 4 dB (4 to 17) louder than those experienced in conversation over a 1 metre air path using the hearing aid microphone. For this study it was assumed that the aid has an inductive input sensitivity meeting the UK Dept of Health requirements.
It can therefore be expected that, given the conditions above, a hearing aid user changing from a face to face conversation to an inductively coupled, typical telephone conversation, would not have to increase the volume control setting on the aid, thus meeting one of the objectives for developing the prETS 300 381.
It is recommended that an international standard be developed covering the hearing aid inductive input sensitivity having characteristics similar to those of the UK Dept of Health Requirements. Annex 3 suggests the essential elements of such a standard.
ANNEX 1 - A Brief Review of the Relevant Standards
EIA RS-504 (1983) Addendum No1 (1994) extends to digital sets)
For a given input signal voltage (-10 dBV) at a local telephone exchange this standard sets the acceptable limits of both axial (>-22 dB rel. to 1 A/m) and radial (>-27 dB rel. to 1 A/m) magnetic fields emanating from a telephone earphone in order for the set to be able to claim "hearing aid compatibility". Measurements are made in a plane 10 mm away from the plane of the earcap and there are somewhat involved rules relating to the frequency response requirements. Recently there have been suggestions towards a change in this standard to make it similar to the ITU and ETSI standards by relating the magnetic field strength to the sound level at the earphone. This improves international consistency and avoids the need for digital telephones to be treated differently.
ETSI pr ETS 300 381 (1994)
Inductive field requirements are specified for an acoustic output of 80 dB into an artificial ear. The field is measured in a plane 10 mm away from the earcap plane and the maximum field strength, which can have any orientation, is 'Acceptable' if it lies between -30 and -17 dB rel. to 1 A/m at 1000 Hz. A 'Preferred' range of -17 to -25 dB rel 1 A/m is suggested as likely to be required for good coupling with hearing aids designed to couple to induction room loop systems. Frequency characteristics are also specified. An Annex points out that the radial field is the most important.
IEC 118-1 (1975)
Concerned entirely with measurement method for setting up and calibration of a known inductive field into which the hearing aid is placed for sensitivity and frequency characteristic measurements. The sensitivity is expressed as the output sound pressure into an artificial ear coupler for magnetic field strength of 1mA/m.
IEC 118-4 (1981)
Specifies the measurement methods and levels (100 mA/m) of the inductive field associated with room loop systems.
ITU-T Recommendation P.37 (1993)
Recommends measurement methods and field strength requirements for an acoustic output of 80 dB into an artificial ear. The field is measured in a plane 10 mm away from the earcap plane and the maximum field strength, which can have any orientation must lie in the range -30 to -17 dB rel to 1 A/m at 1000 Hz. The frequency characteristics are also recommended.
UK Dept of Health
Specification for hearing aid type BE 50 Series. Specifies the inductive coupling input sensitivity for hearing aids for issue in the UK National Health Service. The inductive coupling input sensitivity (10 mA/m) is specified at a known air to air gain setting (40 dB) and a known output sound level (95 dB). In this way it is possible to link the microphone and inductive inputs irrespective of the volume control setting and thus give equivalences with 1 metre air path conversational levels.
ANNEX 2 - Tolerances
In this brief and much simplified study the tolerances involved, which are considereable, have been ignored and concentration has been focussed on the average values. It is however worth taking space to consider these, albeit again somewhat simplified.
The ITU-T long term OLR recommended is in fact a traffic weighted mean, itself having a tolerance of ± 2 dB. For any one connection therefore there can be expected variations due to transmission loss, line length and transducer manufacturing tolerances that might add another ± 5 to 10 dB depending on whether the connection is entirely digital or entirely analogue.
The speech level used in telephony is another variable that will affect the final sound level reaching the ear of the listener and might amount to a standard deviation of 5 dB. Fortunately in this case, the talking level can be subject to feedback from the far end and if levels are too low the talker can be asked to speak up or to position the handset more correctly. Nevertheless there will be a variation which we might take as ± 5dB.
The variation in the conformance with the ITU-T P.37 or ETSI prETS 300 381 has been highlighted throughout this study and has a range of 13 dB.
Without an international standard for the inductive input sensitivity for hearing aids there is no guarantee of performance for this parameter. However if we assume that eventually there will be a standard similar to that of the UK the tolerance for conformance is stated as ± 5dB.
On top of all these mentioned there is the variability of the coupling caused by position of the handset relative to the hearing aid, decidedly unknown, and also the variability of the frequency characteristic end-to-end which will also affect the ability to converse easily.
One hesitates to attempt to assemble all of these tolerances into a single overall tolerance or even a worst case. It can nevertheless been seen that even though the end-to- end levels are likely to be good or excellent for the great majority of connections a few may expected where even normally hearing users will have difficulty, particularly if there are other factors such as noise or poor sidetone. Fortunately there are other standards that go some way towards alleviating the effects of the poorer connections, for example ITU-T Rec. P.37 Part II, ETSI Draft standard DE/TE 04092, covering the specification of amplified handsets, which, if used in conjunction with the inductive coupling facility should reduce to a minimum those connections that would otherwise be labelled Unusable by a hearing impaired user.
ANNEX 3 - Essential Features for a Specification Covering Sensitivity for the Inductive Input of Hearing Aids.
Sensitivity at 1000 Hz
-30 dB rel. to 1A/m at the inductive input shall give a sound level at the earpiece equivalent to 65 dBSPL (the level of speech at 1 metre) at the hearing aid microphone. Note: If another frequency is chosen as being more suitable it should be possible to relate to the 1000 Hz sensitivity to facilitate calculations of the kind presented here.
Noise
The noise level at the earpiece with no inductive input should be equivalent to less than -50 dB rel. 1A/m (<45 dBSPL at the microphone)
Frequency response Not specified at this stage.
Note: The sensitivity of the inductive input in the absence of shaping circuits is assumed to be proportional to frequency. ITU-T Rec. P.37 and prETS 300 381 permit characteristics for the inductive field relative toa sensibly flat electrical input signal that vary from falling at 6 dB/octave to rising at 3 dB/octave. These are to cope with the variations arising from the use of a telecoil in series with different earphones, eg. magnetic, moving coil or piezo ceramic devices. It may be that with no shaping of the frequency characteristic when coupled to a set with a piezo ceramic receiver the high frequencies will be over-emphasised.
References
EIA RS-504 (1983)
Magnetic field intensity criteria for telephone compatibility with hearing aids. Electronic Industries Association, Washington, DC, USA.
ETSI pr ETS 300 488 (1995)
Telephony for hearing impaired people; Characteristics of telephone sets that provide additional receiving amplification for the benefit of hearing impaired users, European Telecommunication Standards Institute, Sophia Antipolis, France.
ETSI pr ETS 300 381 (1994)
Telephony for hearing impaired people; Inductive coupling of telephone earphones to hearing aids. European Telecommunication Standards Institute, Sophia Antipolis, France.
IEC 118-1 (1975)
Methods of measurement of electro-acoustical characteristics of hearing aids. Part 4: Method of measurement of characteristics of hearing aids with induction pick-up coil input. International Electrotechnical Commission, Geneva, Switzerland.
IEC 118-4 (1981)
Methods of measurement of electro-acoustical characteristics of hearing aids. Part 4: Magnetic field strength in audio-frequency induction loops for hearing aid purposes. International Electrotechnical Commission, Geneva, Switzerland.
ITU-T Recommendation P.37 (1993)
Coupling hearing aids to telephone sets. Part I -Magnetic field strength around the earcap of telephone handsets which provide for coupling to hearing aids. International Telecommunication Union, Geneva, Switzerland.
ITU-T Recommendation P.37 (1993)
Coupling hearing aids to telephone sets. Part II -Characteristics of telephone sets that provide additional amplification for the benefit of hearing impaired users. International Telecommunication Union, Geneva, Switzerland.
ITU-T Recommendation G.111 (1993)
Loudness ratings (LRs) in an international connection. International Telecommunication Union, Geneva, Switzerland. UK Dept of Health Specification for hearing aid type BE 50 Series. Medical Devices Directorate, Department of Health, London, England.