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|Year : 1999
: 1 | Issue : 4 | Page
|Hearing in young adults : Report to ISO/TC43/WG1
Pauline Smith1, Adrian Davis1, Melanie Ferguson1, Mark Lutman2
1 MRC Institute of Hearing Research Clinical Section, Ropewalk House, 113 The Ropewalk, Nottingham NG1 6HA
2 Institute of Sound and Vibration Research, University of Southampton, Highfield, Southampton SO17 1BJ
Click here for correspondence address
|How to cite this article:|
Smith P, Davis A, Ferguson M, Lutman M. Hearing in young adults : Report to ISO/TC43/WG1. Noise Health 1999;1:1-10
| Introduction|| |
The UK National Study of Hearing carried out by the MRC Institute of Hearing Research and reported in 1994 by Lutman and Davis, showed that hearing in otologically normal young adults was not as acute as is defined by the relevant International Standard (ISO 389 1991). They reported that median pure tone air conduction thresholds were nearer 5dB than zero across the frequency range 0.5 to 4 kHz, and even more discrepant at the extremes of the frequency range. They discuss a number of possible explanations for this discrepancy which include the original sampling of the subjects used in the data collection. ISO 389 is rather vague in its definition of otologically normal: "a person in a normal state of health who is free from all signs or symptoms of ear disease and from obstructing wax in the ear canal, and who has no history of undue exposure to noise." Hearing testing was carried out over 30 years ago to establish the Standard and since then there have been other queries raised about the validity of the standard, particularly at 6kHz. (e.g. Robinson et al, 1981).
One main aim of this study was to measure hearing in a large group of randomly sampled young adults aged between 18 and 25 years, and to compare the findings with those reported by Lutman and Davis, and with ISO 389 (1991). The study began in July 1994 with a postal questionnaire to households selected at random in Nottingham, UK. At that time, we circulated our proposed protocols to members of ISO/TC43/WG1, and we received some useful feedback, which we incorporated. In 1996, ISO/TC43/WG1 published an article in Scandinavian Audiology outlining their reasons for wishing to revise ISO 389, and describing preferred test conditions for workers interested in contributing data to the revision. At this time, our study was in the data collection phase and so the "preferred test conditions" were not followed precisely. Rather, our definition of otologically normal described in this report, is more stringent than the one suggested in Scandinavian Audiology.
| Methods|| |
Initially we sent out a postal questionnaire to 5850 randomly selected households in Nottingham. We asked questions about hearing, tinnitus, use of GP and ENT services, as well as demographic variables and requested that the questions be completed for each member of the household aged 14 years and above. We then invited all the 18 to 25 year olds to attend a two hour session in our clinic, based at the MRC
Institute of Hearing Research, on the campus of Nottingham University. Subjects were paid an attendance allowance and travelling expenses and each subject was sent up to a total of four invitations to attend. Those who booked an appointment and then failed to attend were offered up to three appointments, to try to maximize the attendance rate.
Data were collected on 356 people over a period of two and a half years for logistic reasons. Each subject was seen by one of three experienced Audiologists.
Pure tone audiometry
The relevant International Standard (ISO 8253, 1989) was followed in terms of the following three areas: all subjects were tested according to the ascending protocol (shortened version), the ambient noise in the room in which threshold determination took place met the criteria for measuring down to -10dB by bone conduction, and the pure tone audiometer used (a Grason Stadler GSI-16 with TDH50P earphones and a Radioear B71 bone vibrator) was calibrated throughout the study to ISO 389 (1991) for air conduction and to ISO 389-3 (1994) for bone conduction. Stage A listening checks were carried out rigorously at the start of each session, stage B measurements were carried out approximately 4 weekly, and a full set of acoustical measurements were taken annually. Although the same audiometer was used throughout the entire study, two sets of transducers were used and alternated every 4 weekly calibration period.
The following frequencies were tested by air conduction: 0.125, 0.25, 0.5, 1, 1.5, 2, 3, 4, 6 and 8 kHz. In half the subjects, the left ear was tested first, and in the other half, the right ear was tested first. The subject was always given a break after air conduction measurements, before bone conduction measurements, as the protocol recommends that testing must not exceed 20 minutes uninterrupted. Masked bone conduction thresholds were measured at the following frequencies: 0.25, 0.5, 1, 2, 3 and 4 kHz. Narrow band noise was applied to the contralateral ear via an earphone at a fixed intensity level of 40dBHL. Whilst bone conduction thresholds were being measured at 3 and 4 kHz, an EAR plug was inserted into the ipsilateral ear to reduce the level of air-borne radiation, as reported by Lightfoot, (1979).
If contralateral masking was required because the hearing was asymmetrical, a plateau method was used.
Otoscopy and Otoadmittance measurements
Prior to tympanometry, otoscopy was performed on all subjects, and an assessment was made by the Audiologist at the time as to its normality.
All subjects underwent tympanometry on both ears using a Grason Stadler GSI 33 Otoadmittance meter. They were then tested for presence of an acoustic reflex, using a pure tone stimulus presented contralaterally at 1kHz at 100dBHL. If this reflex was present, no further testing followed, but if no reflex was seen in response to that stimuli, then the following stimuli were presented until a reflex was elicited, or the end of the sequence was reached:
1kHz 110dBHL contralaterally,
WBN 100dBSPL contralaterally,
WBN 110dBSPL contralaterally,
1kHz 110dBHL ipsilaterally,
WBN 95dBSPL ipsilaterally.
Subjects were all sent a questionnaire prior to their appointment which included questions on past and present ear disease, all ear and hearing related problems including tinnitus and vestibular symptoms, general health problems (e.g. meningitis) and medication (e.g. intravenous aminogylcoside antibiotics) known to affect hearing, and family history. It also included a section on noise exposure. The purpose of the questionnaire was to save time at the appointment, but also to allow the subjects time to recall the necessary details and to consult with their parents on such items as childhood ear disease and deafness. When the subjects were seen, this questionnaire formed the basis of the interview. Appendix 1 lists all the clinical factors which were covered in the interview.
Each subject was grouped as manual or non manual according to the Registrar General's classification (OPCS 1991), for the main occupation of the principal earner in the household .
A detailed history of noise exposure was taken which included occupational, gunfire and social situations. All situations were documented above which voices had to be raised for two people with normal hearing and 4' apart to hold a conversation. For each different noise to which a subject had been exposed, we estimated the sound pressure level, the number of times they had been exposed, the duration of exposure and whether any temporary effects followed: dullness of hearing, tinnitus or both, and in which ear. Details of any hearing protection were also recorded. Further details of the interview procedure can be found in Smith et al (1998).
Each subject was given a rating for each different activity, and these were then combined into 3 ratings: occupational, social and gunfire noise exposure. Noise exposure was classed as significant if the Noise Immission Level (NIL) was 97dB(A) or greater. This is equivalent to 81dB(A) or greater for a full-time (40 hour week) working lifetime of 50 years.
| Results|| |
A total of 356 subjects was seen over 22 years for logistic reasons. This represented 46% of the 18-25 year olds at the outset of the study. The attendees did not differ substantially from the total sample in terms of their reported hearing problems or tinnitus.
Results from 346 of the 356 subjects tested were analysed. Two of the subjects had audiograms which were deemed unreliable at the time of test, and 8 were just outside the age range at the time of test.
For 44 subjects, the threshold measurements at 250Hz for bone conduction were missing due to possible excessive harmonic distortion which only came to light at the subsequent 4-weekly calibration. In other cases there were individual thresholds missing for various reasons (e.g. masking dilemmas) and the numbers are given for each analysis presented in this report.
In 118 subjects for both air conduction and bone conduction, the hearing threshold levels were measured with headbands which had a measured force which was outside the range (too high) given in ISO 389. The effect of this was that we measured thresholds which were significantly (ttest, p<0.05) more acute than otherwise by 12dB at 125, 250 and 500 Hz for air conduction and 2-3dB at 2k, 3k and 4k Hz for bone conduction. Therefore, when considering the group data below, it will be useful to keep in mind that without this problem, the data would possibly have been a little further away from zero as defined by ISO 389.
In order to define a group of otologically normal (ON) individuals, we used a very strict set of criteria. Appendix 1 shows the criteria which had to be met for individuals to be coded as strictly otologically normal. It also shows the percentage of subjects who met each criterion. If the abnormality was present on one ear or both, or if there was any missing data, the whole subject was excluded from the strictly otologically normal group. The criteria in this table were defined before the study began, based on data from the National Study of Hearing and other literature.
Out of 346 individuals, only 93, or 27% were "otologically normal" as defined in this very strict way.
The pure tone thresholds are presented for two differently defined populations. Firstly, [Table - 1],[Table - 2] show the central tendency and distributions for the entire population, namely the typical population or TP. [Table - 3],[Table - 4] then show the same data but for the group we have defined as otologically normal or ON.
In the otologically normal group, the data do show a discrepancy from ISO 389. For air conduction, the discrepancy is smallest at 1kHz, and greatest (>5dB) at the extremes of the frequency range. For bone conduction, the greatest discrepancy (>5dB) is at 2kHz, and at other frequencies it is in the order of 2-3dB.
Age by gender distributions, and occupational group distributions are shown in [Table - 5],[Table - 6] for both populations.
| Discussion|| |
It can be seen from [Table - 3] that the data collected from the strictly defined otologically normal group in the present study do show a discrepancy from the relevant ISO standard for all frequencies for both air and bone conduction signals.
In order to compare the data in the present study with that of Lutman and Davis, it was necessary to use a different definition of "otologically normal": that described in their article. Detailed description of this is beyond the scope of this report but it is clear that the present data are also discrepant from those presented by Lutman and Davis (1994), in that their mean data were 2-3dB less acute at most frequencies, and 9dB less acute at 6kHz. The explanation for this large discrepancy at 6kHz is most likely to be the type of earphones used for the study, and the coupler with which they were calibrated. The National Study of Hearing was carried out using TDH39P earphones, which were calibrated to the relevant International Standard using a 9A coupler, whereas the present study was carried out using TDH50P earphones, calibrated to the relevant International Standard using an artificial ear. There are other data available to support this explanation, both laboratory data (Sherwood et al, 1995) and data on subjects tested with both sets of earphones (Lutman and Qasem, 1997). The smaller discrepancy at other frequencies is probably due to a number of other factors in combination: more careful audiometry, use of ISO 8253 rather than the BSA recommended procedure for pure tone audiometry, possible differences in earphones.
[Table - 5],[Table - 6] show the distributions of age, sex and occupational group for both the typical and the otologically normal populations. There is no significant difference between the numbers of males and females who are otologically normal, but there are significantly (chi-squared p<0.05) more with non-manual occupations in the otologically normal group. There are also significantly (chi-squared p<0.05) more 18 year olds in the otologically normal group than 19-25 year olds. Further analyses of the hearing threshold levels with age, sex and occupational group will be conducted and reported in a further publication.
| Conclusions and recommendations|| |
The data from this study strongly suggest that ISO 389 (1991) for air conduction and ISO 3893 (1994) for bone conduction are in need of revision. More data are needed, as has been requested by ISO/TC43/WG1 in Scandinavian Audiology. We would recommend that workers adopt our protocols for defining "otologically normal" rather than using those published in Scandinavian Audiology, which we consider are still too vague in places, for example, in definition of "undue exposure to noise".
We would also recommend in any revised International Standard that only one acoustic coupler is specified for calibration of earphones, that being the artificial ear, meeting IEC 318 (1970), typically B& K type 4153. In practice, this will mean that TDH39 and Beyer DT48 earphones will in future have to be calibrated to a different set of values, otherwise there are no implications. In the meantime, until such changes to ISO 389 occur, we recommend that audiologists restrict their use of earphones to exclude TDH39 and Beyer DT48, and include only the earphones (e.g. TDH49) that are specified in the standard as requiring calibration using the artificial ear meeting IEC 318 (1970).
Our major recommendation to WG1 is to change ISO389 so that the RETSPLs relating to the artificial ear, and RETFLs more accurately reflect hearing in otologically normal young adults. This change would be based on our data and on other data if available. Alternatively, keep ISO 389 unchanged in its table of RETSPLs relating to the artificial ear, and RETFLs, but re-word the standard as an audiometric zero that is arbitrary. That is, make the link between normal threshold and ISO 389 approximate and subject to variation. That way, ISO 389 does not pretend to represent normal hearing directly; it is just an agreed way to calibrate audiometers and to anchor the HL scale. (This would resolve problems with ISO 7029 as well). This latter suggestion would then need a new standard proposed that does aim to represent normal hearing threshold, using our data and other data if available. There can clearly be different versions for different populations and selection criteria, e.g. males and females.
| References|| |
|1.||British Society of Audiology (1982): Recommended procedures for pure-tone audiometry using a manually operated instrument. Brit J Audiol ; 15:213-216. |
|2.||IEC 318 (1970 ) An artificial ear of the wide band type for the calibration of earphones used in audiometry. International Electrotechnical committee (Equivalent to BS 4669: 1971) |
|3.||ISO/TC43/WG 1 (1996) Threshold of Hearing. Preferred Test Conditions for Determining Hearing Thresholds for Standardization. Scand Audiol ; 25:45-52. |
|4.||ISO 389 (1991) Specification for Standard reference zero for the calibration of pure tone air conduction audiometers. Geneva, Switzerland: International Organization for Standardization . |
|5.||ISO 389-3 (1994) Acoustics- Reference zero for the calibration of audiometric equipment- Part 3. Reference equivalent threshold levels for pure tones and bone vibrators. Geneva, Switzerland: International Organization for Standardization . |
|6.||ISO 8253-1 (1989) Acoustics- Audiometric test methodsPart 1: Basic pure tone air and bone conduction threshold audiometry. Geneva, Switzerland: International Organization for Standardization . |
|7.||ISO 7029 (1984) Acoustics-Threshold of hearing by air conduction as a function of age and sex for otologically normal persons. Geneva, Switzerland. International Organization for Standardization . |
|8.||Lightfoot G R (1979) Air borne radiation from bone conduction transducers. Brit J Audiol ; 13:53-56. |
|9.||Lutman M E and Davis A C. The Distribution of Hearing Threshold Levels in the General Population Aged 18-30 years. Audiology ; 33: 327-350. |
|10.||Lutman M E and Qasem H Y N. (1994) Deviation from audiometric zero at 6kHz with TDH-39 earphones. 3rd European Conference on Audiology, Prague. Book of Abstracts, 1997 |
|11.||Office of Population Censuses and Surveys. (1991) Standard Occupational Classification. Volume 3: London HMSO. |
|12.||Robinson D W Shipton M S and Hinchcliffe R. (1981) Audiometric zero for air conduction. A verification and Critique of International Standards. Audiology; 20: 409431. |
|13.||Sherwood T R McNeill H A and Torr GR. (August 1995) Differences in the performance of metal- and plastic-cased TDH-39 and TDH-49 audiometric earphones, and consequences for their calibration. National Physical Laboratory Report. |
|14.||Smith PA Davis AC Ferguson MA and Lutman ME. (1999) The prevalence and type of social noise exposure in young adults. Noise and Health; submitted. |
MRC Institute of Hearing Research Clinical Section, Ropewalk House, 113 The Ropewalk, Nottingham NG1 6HA
Source of Support: None, Conflict of Interest: None
[Figure - 1]
[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5], [Table - 6]