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ARTICLES Table of Contents   
Year : 2004  |  Volume : 6  |  Issue : 24  |  Page : 75-84
Does the presentation of audiometric test data have a positive effect on the perceptions of workplace noise and noise exposure avoidance?

1 National Acoustic Laboratories, Chatswood, Australia
2 New England Area Health Service, Tamworth, NSW, Australia

Click here for correspondence address and email

Research and 'common knowledge' has for many years accepted that education and feedback supplied to individuals during and immediately after workplace health assessments provides valuable information to workers about their health. Further, if more relevant and detailed information could be supplied then awareness and preventative action may increase proportionately. This research carried out with a rural Australian population has shown that preventative action did not increase in proportion to a corresponding increase in the amount and variety of information provided in connection with hearing health status. Two research groups underwent hearing tests, both with pure tone audiometry (PTA) while the second group also underwent otoacoustic emission (OAE) testing. Test results were presented to the subjects at the conclusion of their test session. An analysis of questionnaire responses at six week and twelve months follow up showed that more information did not result in increased preventative action. Barriers seem to exist such that individuals feel that they are not able to effectively act to reduce overall noise exposure. While self-efficacy initially increased, it declined to close to its initial value over the longer period. Other measures such as perceived susceptibility to hearing loss and the benefits of exposure reduction significantly increased and remained at the same increased level after twelve months. So, while overall awareness of noise and the risks of exposure were increased after both types of hearing test there was no increased hearing health benefit due to additional testing and hearing information.

Keywords: hearing loss, barriers, self-efficacy, feedback

How to cite this article:
Williams W, Purdy S C, Murray N, Dillon H, LePage E, Challinor K, Storey L. Does the presentation of audiometric test data have a positive effect on the perceptions of workplace noise and noise exposure avoidance?. Noise Health 2004;6:75-84

How to cite this URL:
Williams W, Purdy S C, Murray N, Dillon H, LePage E, Challinor K, Storey L. Does the presentation of audiometric test data have a positive effect on the perceptions of workplace noise and noise exposure avoidance?. Noise Health [serial online] 2004 [cited 2023 Mar 26];6:75-84. Available from: https://www.noiseandhealth.org/text.asp?2004/6/24/75/31653

  Introduction Top

Hearing loss due to noise exposure is an ever increasing problem throughout the industrial world (WHO: 1997). "Hearing loss is one of the most pervasive occupational health problems in America today" (US Dept of Health and Human Services: 1996). In the US it is estimated that there are between 5 million and 30 million workers exposed to hazardous levels of noise on a daily basis (Berger, Royster, Royster, Driscoll and Layne: 2000, p 3). This has been translated into a figure of 10 million who have a hearing loss at least partially attributable to noise (Berger, Royster, Royster, Driscoll and Layne: 2000, p 3). In the UK it is estimated that "at least 1.3 million employees are exposed to noise levels above 85 dB(A)" during the regular course of their work (RNID: 1999, p 4).

The incidence of hearing loss in the Australian community due to noise exposure has been recognized for some time (AAS: 1976; NOHSC: 1989), while more recently the particularly high incidence in rural communities has been examined (Williams, Forby-Atkinson, Purdy and Gartshore: 2002). Estimates of hearing loss across the general Australian community lie between 5.7% (ABS: 1993) and 27% (Wilson, Walsh, Sanchez and Read: 1998). Noise levels in the workplace need to be reduced, and individuals and groups who work in noisy areas need to develop specific knowledge and techniques in order to protect themselves from excessive noise exposure (NOHSC: 2000).

A "Noise at Work" questionnaire was developed by Purdy and Williams (2002) as a means to better understand the knowledge and perceptions which individuals have concerning noise and its impact on hearing, and whether individuals feel that they can affect their noise exposure in the workplace. This questionnaire looks at perceptions in relation to the benefits of reducing workplace noise; the barriers to reducing noise; the ability to reduce noise exposure; attitudes to noise in the workplace; and the individual's perception that their hearing could be damaged through exposure to noise ("susceptibility").

Purdy and Williams (2002) found that simple audiometric testing was beneficial as it resulted in enhanced perceptions of the benefits of reducing workplace noise exposure.

Zohar, Cohen and Azar (1980) showed that ongoing feedback regarding hearing loss had a positive effect on workers' hearing protector use while Lusk et al (1999) recognized that "limited changes in behavior can be expected from a "one-shot" intervention" (p 493). Unfortunately one-shot interventions are the reality for many organizations where access to workers for training purposes is limited.

This study had three main hypotheses that:­

1) there would be a high incidence of hearing problems amongst rural workers and that these problems would be revealed by audiometric testing and a self-report questionnaire;

2) having a hearing test performed by a nurse audiometrist and having the results explained would have a positive effect on perceptions about noise and hearing protector use, and these effects would be sustained over time; and

3) subjects who have both conventional pure tone audiometry and a more sophisticated presentation of the results from otoacoustic emissions (OAE) testing of inner ear performance (LePage, Zhou, Murray and Seymour: 2000) would be more likely to believe that noise exposure would further damage their hearing and thus, more likely to increase their use of hearing protectors and avoid noise. Also, as a specific corollary, that, all other things being equal, the presentation of individual OAE results to subjects would prompt subjects with low emission strengths to take greater preventative action providing their hearing is still 'normal'.

  Method Top

In this study the approach was similar to that used by Zohar, Cohen and Azar (1980), differing only in that the feedback from the hearing test results was presented in the same session as the hearing test was carried out. There were no other formal or informal presentations to the participants.

The study design involved two groups of subjects, one group that underwent a conventional pure tone audiometric (PTA) hearing test, and a second group that had the PTA test plus OAE testing. All subjects completed a self-report questionnaire, immediately before and twice after hearing testing, so that the short and long-term effects of the two hearing tests could be evaluated.

Because individuals were presenting at a health clinic for PTA testing as part of their workplace Occupational Health & Safety program the reasonable assumption was drawn that they were employed in a noisy workplace. For the purposes of this project the degree of noise exposure was not particularly important only that the fact that the workplace was noisy to some degree. Hence there was no estimation of noise exposure from work or leisure attempted.

Subjects were 113 men and 23 women aged 20 to 65 years (mean 39.6 years, SD 11.4 years) presenting to a nurse audiometrists for a work ­related hearing assessment because they work in areas that are routinely noise exposed. Subjects were recruited from the New England area in northern New South Wales, Australia. The industries in this area include diverse agricultural activities and many small manufacturing and processing businesses. On average, subjects had been in their current employment for some time (mean 13.8, SD 10.7, range 0.5 to 45 years). Only one subject spoke a language other than English at home.

Subjects were asked if they would be interested in participating in the study when they came in for routine audiometric testing. When individuals indicated that they were prepared to participate, the study was explained to them and they then completed a written consent form. All of those approached indicated that they were willing to participate (no payment or other reward was involved).

Before testing was conducted, subjects completed the 20-item "Noise at Work" questionnaire developed by Purdy and Williams. This questionnaire has 5 sub-scales with items assessing:­

1) Benefits - the perceived benefits of reducing noise and noise exposure, for example through the use of hearing protectors;

2) Barriers - the perceived barriers to reducing noise exposure;

3) Self-efficacy - the perceived ability to reduce noise exposure and/or protect hearing (Bandura: 1986,p 390);

4) Attitude - attitudes to workplace noise and noise exposure; and

5) Susceptibility - an individual's perceived susceptibility to hearing loss, interpreted as whether they think noise exposure can/will damage their hearing.

The wording of some of the items was altered compared to the original so that 10 of the 20 items were negatively worded. These were subsequently reversed for scoring. Subjects also completed a 16-item questionnaire with demographic items as well as questions about type and duration of work experience, and self ­reported hearing loss, noise exposure and use of hearing protectors. At six weeks and 12 months after the initial visit and testing the questionnaire was administered again by the nurse audiometrist who contacted each subject by telephone and obtained responses.

At the initial visit audiometry was carried out on all subjects using a fully calibrated Beltone Model 110 audiometer with a TDH-50 headset to determine pure tone hearing thresholds. PTA testing was performed in accordance with the background noise requirements and conditions as required by the combined Australian/New Zealand Standard AS/NZS 1269.4:1998 Occupational noise management Part 4: Auditory assessment. Two composite pure tone averages were computed and used in the statistical analyses: a) 0.5, 1 and 2 kHz and b) 3, 4 and 6 kHz (high frequency average).

Half of the subjects were randomly assigned to a second group that had transient click-evoked otoacoustic emissions (OAE) testing in addition to pure tone audiometry. OAE testing was carried out by the same nurse audiometrist who performed PTA testing. The nurse audiometrists were trained by one of the authors (NM) in using an Otodynamics™ ILO88 Click-Evoked Ototacoustic Emissions Analyser and the NAL­OAE software analysis package (see LePage, Zhou, Murray and Seymour (2000), for a full explanation of the process).

The results of the otoacoustic emission tests were compared to population data and displayed on a computer screen in a way that highlighted damage that had occurred to ears and implications for further damage (LePage, Zhou, Murray and Seymour: 2000). This presentation and accompanying explanation was hypothesized to make a significant impression on the subjects and their subsequent attitudes and future actions in looking after their hearing. In particular it was hypothesized that subjects who experienced 'normal' hearing and exhibited lower than expected OAE results may be prepared to take more precautions.

  Results Top

Group 1 that had only pure tone audiometry testing included 67 subjects, 97% of whom were men. Group 2 that had both pure tone and OAE testing included 69 subjects, 70% of whom were men. Because of the imbalance of men and women in the two groups an initial analysis was undertaken to determine any group differences as these could influence the effects of audiometric testing on the outcome measures. Unfortunately the gender imbalance did result in the two groups differing significantly for a number of factors including: type of work carried out; level of work (e.g. managerial, supervision, machine operator); length of time working; high-frequency average pure tone thresholds in each ear; and self-rated percentage of time with high noise exposure (p <_ 0.013 for all items). Due to time constraints it was not possible to test additional subjects to correct this imbalance, hence the females were excluded from any subsequent group analyses. Details of the two groups of males and the 23 females are shown separately in [Table - 1].

  Audiometry Top

Average pure tone thresholds of the males and females are shown in [Figure - 1]. Audiometric results of the males are combined since there was no significant difference in hearing threshold levels between the two groups once females were excluded (females excluded Group 1 mean and SD, 23 dB HL and 19.8 dB; Group 2, 23 and 17.7; with Group 1 & 2, 24 and 13.5 respectively). On average, men had a mild high frequency hearing loss, with the notched pattern characteristic of noise exposure. Although both males and females had minimal hearing loss on average, there was a very wide range of thresholds in both groups, from normal to profound hearing impairment in men and from normal to severe hearing impairment in women.

  Otoacoustic emissions Top

Of the 69 subjects tested with both otoacoustic emissions and pure tone audiometry 14 (20%) had coherent emission strengths (CES) of 0 dB SPL or less in one or both ears. An OAE CES of 0 dB (SPL) has been shown to correspond on average to a hearing threshold of around 25 dB computed as an average of thresholds measured at 0.5, 1, 2 and 4 kHz (Le Page & Murray, 1993). The results of the OAE tests were presented to the subjects according to the NAL-OAE software analysis package (LePage, Zhou, Murray and Seymour: 2000). These results are presented as a comparison of the particular individual to a similar group with respect to age and gender. This is basically: green for satisfactory or above average results; amber for just below average, indicating a warning for the future; while red indicates that results are well below the average and represent significant risk.

CES results versus the PTA average for the frequencies 3, 4 and 6 kHz are plotted in [Figure - 2]. This shows the wide range of measured CES values and that a number of subjects with relatively high hearing acuity had low CES values.

  Group comparison Top

Without the females, groups 1 and 2 were equivalent at the first visit, for all but one of the measures, self-rated noise exposure. Self-rated noise exposure was higher (t = 2.2, df = 110, p = 0.033) for Group 1 compared to Group 2. Noise exposure was therefore included as a continuous predictor (covariate) for a three-way analysis of variance using 'Group' as a between-group factor and sample time and sub-scale as repeated-measures factors. This analysis showed no significant differences between the two groups for questionnaire subscale scores (F = 0.6, df = (1, 86), p = 0.436). There was, however, a significant effect of test occasion on questionnaire scores (F = 8.1, df = (2, 172), p < 0.001). This is illustrated in [ Figure 3], which shows that subscale scores improved at the 6­week assessment except for the Attitudes subscale that showed no change across the three assessment occasions.

There were no group effects or interactions with group and hence the groups were combined prior to doing post-hoc analyses to examine the repeat effect. Scheffe posthoc analyses for the individual sub scales showed significant improvements between the Baseline and 6-week ratings for the Benefits (p = 0.002), Barriers (p < 0.001), Self-efficacy (p < 0.001) and Perceived Susceptibility (p = 0.048) sub scales. For Benefits, Barriers and Perceived Susceptibility this improvement over the baseline was maintained at 12 months (p >_ 0.862). For Self ­efficacy, however, ratings dropped significantly between 6 weeks and 12 months (p = 0.005) and consequently the final 12-month result did not differ from the initial result (p = 0.379).

To summarize the subscale results, Benefits, Barriers and Perceived Susceptibility ratings showed an improvement after hearing testing which was sustained, Self-efficacy showed an initial improvement but this was not sustained over time, and there was no change in Attitudes.

Other outcome measures that might have changed as a result of the hearing testing include self-rated hearing loss, if subjects became more aware of hearing difficulties after the test. A Friedman non-parametric ANOVA of the responses to the questions about hearing problems, conversational difficulties in noise, family reports of hearing problems or tinnitus showed no significant changes across test occasions for either group (p >_ 0.071).

One anticipated outcome was that subjects would be predisposed to use hearing protectors more often after having hearing testing and becoming aware of hearing problems and risks to hearing. Although the figures (see [Table - 2]) show there was a weak trend towards increased use of hearing protectors, a repeated measures ANOVA of self-reported hearing protector use showed no statistically significant change over the three test occasions (F = 1.5, df = (2, 212), p = 0.215).

A two-way ANOVA of self-reported exposure times, with Group as a between-groups factor and time as a repeated-measures factor, showed the difference between groups mentioned above, with Group 1 exposed to high noise levels at work 45% of the time on average, and Group 2 being exposed 33% of the time on average at the first test. There was a significant effect of test occasion (F = 4.4, df = (2, 212), p = 0.014), with both groups reporting lower exposure at the 6 week interview (33% and 29% for Groups 1 and 2 respectively). This trend had sharply declined at the 12-month interview, at which time reported exposure times had returned to near baseline levels (46% and 31% for Groups 1 and 2 respectively). Thus there were no statistically significant effects of the hearing testing on either self-reported exposure times or hearing protector use when the long-term data were examined, and there were no interactions with group. As was seen for the questionnaire data, there were no differences in outcome for Group 1 receiving PTA only versus Group 2 who had PTA and OAE testing.

An interesting trend that was observed was the increased use of hearing protectors by a particular sub-group. It was desired to test the proposition that individuals who had a 'normal' audiogram but who exhibited a low OAE result would be more likely than those who knew only that they had a normal audiogram to take preventative action in order to protect their hearing. Normal audiogram in this case was defined as a PTA average for 3, 4 and 6 kHz of 25 dB HL or better, while low OAE results were defined as a mean CES of less than 7.5 dB (SPL). There were 47 subjects from Group 1 and 32 subjects from Group 2 met these criteria.

As illustrated in [Figure - 4] the individuals from Group 1 tended to increase their use of hearing protectors while individuals from Group 2 had a slight decrease in their use of hearing protectors. A two-way ANOVA (Group by sample time) indicated that the differing pattern for the two groups was close, but not significant (significance level for the interaction between test time and group F(2, 124) = 8.1, p = 0.067). At the same time the individuals' overall estimate of their noise exposure did not vary significantly.

  Discussion Top

The questionnaire results in general showed that there were more positive attitudes towards noise and hearing loss prevention after hearing testing. These positive attitudes were sustained for at least twelve months. However, there was no significant impact of hearing testing on hearing protector use, the one immediate and obvious action that individuals could take to reduce their personal noise exposure.

The results support previous findings (Purdy and Williams: 2002) that having a hearing test is a significant positive factor in raising an individual's awareness of noise as a risk to hearing health. Presenting additional information about the status of the cochlea, as evidenced by the OAE results, did not significantly increase awareness or affect preventative action. Thus the third hypothesis of the study that the presentation of more relevant and 'sophisticated' information would enhance awareness was not supported by this project.

The combined results from both groups showed that after hearing testing there was a statistically significant increase and sustained awareness concerning exposure to noise and noise problems. Improvement was demonstrated for Benefits, Barriers and Perceived Susceptibility. Thus the feedback provided during the testing raised awareness with respect to these sub­scales. This educational effect on peoples' perceptions did not translate into preventative action.

Prochaska, DiClemente and Norcross (1992) concluded that, in order to effect successful change, not only must the means of causing the change be known and available but also there must be "insight" (desire and willingness) on the part of the individual as to why that change is necessary. According to Krause and Sloat (1993) and Feyer and Williamson (1998) peer group support and management support and responsibility are also needed. Thus for a practical and successful health promotion outcome four key elements must be simultaneously present. These elements are: 1) the availability of appropriate information; 2) the individual's desire and willingness to initiate change; 3) the required change must be actively supported by peer groups; and 4) workplace management must support change and take responsibility for the change. Significant improvements in noise perceptions for three of the questionnaire sub-scales suggest that elements (1) and (2) had occurred.

Self-efficacy, which initially increased, returned close to the baseline at the end of twelve months. Thus individuals initially felt that there was something that they could do to prevent noise exposure but for some reason they then felt that they could not easily accomplish change. This lack of action was evidenced by the lack of change of noise exposure levels and hearing protector use before and after hearing testing. Ideally preventative action would include methods other than increased use of hearing protectors. Since exposure levels remained constant it is reasonable to assume that no alternative noise management methods were employed. As hearing loss prevention behavior did not change, presumably elements (3) and (4) were not in place.

The inability or reluctance to initiate change is referred to as self-inefficacy (Bandura: 1986) and can occur when individuals feel that it is futile to attempt to initiate change. Bandura suggested two distinct possible solutions to self­-inefficacy: (a) the "development of competencies and strong percepts of self-efficacy"; or (b) the "changing of the social environment so that people can gain the benefits of the competencies they already possess" (p 446). Tackling the four key elements as discussed previously would also address Bandura's solutions.

Inefficacy may be used, for example, to explain the trend displayed in [Figure - 4]. Feelings of futility and self-inefficacy could arise when individuals who experience no problems with their hearing are informed that "they could do better" or "need to do better". This in turn could be taken as efforts to look after their hearing not being really successful. As they may not understand the processes involved, the problem of looking after their hearing is then seen as 'too hard' so they do not make any further effort and in effect give up. Atherton (2002) has described this process in relation to the disaffection of students and the formal learning process as 'learned helplessness' where individuals "have lost (or never gained) any sense of the connection between their efforts … and any meaningful achievement".

The lack of response to a greater amount of 'educational' information could also relate to the observation made by Detweiler et al (1999) that a "gain framed" [positive] message "is far more motivating in encouraging a preventative behaviour such as sunscreen use" (p 189) than a "loss-framed" [negative] message. Traditionally the results of hearing tests have been presented in a negative (loss framed) manner where after the discovery of a loss of hearing the subject is told that they 'should look after their hearing so that they won't lose any more'. Messages may need to be framed more positively in order to achieve positive behaviorral change.

Although perceptions about barriers did improve (i.e. they were perceived as less of a problem) after the intervention, they were still rated as moderately high. [Figure - 3] shows that subjects were very aware of their susceptibility to hearing loss since average ratings exceeded 4 on the 5­point scale. They were less positive about their perception of barriers, indicating that they did perceive barriers to the reduction of noise exposure. This project did not endeavour to discover the exact form of these barriers. For people exposed to significant workplace noise, self-inefficacy in terms of noise reduction and hearing loss is not likely to be overcome unless the barriers specific to individual workers and workplaces can be addressed.

  Acknowledgement Top

The authors would like to acknowledge the assistance of S Hynes, P Foster, L Smith, M Sampson and the New England Area Health Service for their assistance with hearing testing and the collection of the data.[23]

  References Top

1.Atherton J S (2002) Learning and Teaching: Learned helplessness [On-line]: UK: Available: http://www.dmu.ac.uk/~jamesa/learning/learned_helpless ness.htm Accessed: 11 July 2003.  Back to cited text no. 1    
2.Australian Academy of Science (1976) Report of a Committee on the Problem of Noise. Report Number 20, Australian Academy of Science, Canberra, March 1976.  Back to cited text no. 2    
3.Australian Bureau of Statistics (1993) Disability, Ageing & Carers - Hearing Impairment.  Back to cited text no. 3    
4. Bandura, A (1986) Social Foundations of Thought and Action - A social cognitive Theory, Prentice Hall, New Jersey.  Back to cited text no. 4    
5.Berger, EH, Royster, LH, Royster, JD, Driscoll, DP and Layne, M (2000) The Noise Manual, fifth edition, American Industrial Hygiene Association, Fairfax, Va.  Back to cited text no. 5    
6.Detweiler, JB, Bedell, BT, Salvoney, P, Pronin, E and Rothman, AJ (1999) Message Framing and Sunscreen Use: Gain-Framed Messages Motivate Beach-Goers in Health Psychology, Vol 18, No 2, pp 189 - 196.  Back to cited text no. 6    
7.Feyer, AM and Williamson, A (Eds) (1998) Organisations, management, culture and safety in Occupational Injury: Risk, Prevention and Intervention, Taylor & Francis  Back to cited text no. 7    
8.Krause, TR and Sloat, KCM (1993) Attitude alone is not enough, Occupational Health & Safety, January 1993.  Back to cited text no. 8    
9.LePage, EL, and Murray, NM (1993) Click-evoked otoacoustic emissions: comparing emission strengths with pure tone audiometric thresholds, Aust.J.Audiol. 15 1:9­22.  Back to cited text no. 9    
10.LePage, EL, Zhou, D, Murray, NM and Seymour, J (2000) NAL OAE Software Analysis, paper presented at the Australian Society of Audiology Conference, Adelaide, 2000.  Back to cited text no. 10    
11.Lusk, SL, Honh, OS, Ronis, DL, Eakin, BL, Kerr, MJ and Early, MR (1999) Effectiveness of an Intervention to Increase Construction Workers' Use of Hearing Protection, Human Factors Vol 41, No 3, September 1999, pp 487 - 494.  Back to cited text no. 11    
12.National Occupational Health and Safety Commission (1989) National Strategy for the Prevention of Occupational Noise-Induced Hearing Loss, Australian Government Publishing Service, Canberra  Back to cited text no. 12    
13.National Occupational Health and Safety Commission (2000) Occupational Noise, National Standard for Occupational Noise[NOHSC: 1007(2000)] and National Code of Practice for Noise Management and Protection of  Back to cited text no. 13    
14.Hearing at Work [NOHSC: 2009(2000)] 2nd Edition, Canberra, Australia, July 2000.  Back to cited text no. 14    
15.Prochaska, JO, DeClemente, CC and Norcross, JC (1992) In Search of How People Change, American Psychologist September 1992, pp 1102 - 1114.  Back to cited text no. 15    
16.Purdy, S and Williams,W (2002) Development of the Noise at Work Questionnaire to assess perceptions of noise in the workplace, J Occup Health safety - Aust NZ, 2002, 18(1): 77 - 83.  Back to cited text no. 16    
17.Royal National Institute for Deaf People (1999) Indecent Exposure - A joint report on noise at work by RNID and TUC", TUC/RNID noise at work campaign, London.  Back to cited text no. 17    
18.Standards Australia (1998), Australian/New Zealand Standard AS/NZS 1269.4: 1998 Occupational noise management Part 4: Auditory assessment, Homebush.  Back to cited text no. 18    
19.US Department of Health and Human Services (1996) Preventing Occupational hearing Loss, NIOSH, Cincinnati, Ohio.  Back to cited text no. 19    
20.Williams, W, Forby-Atkinson,L, Purdy, S and Gartshore, G (2002) Hearing loss and the farming community, J Occup Health safety - Aust NZ, 2002, , 18(2): 181 - 186.  Back to cited text no. 20    
21.Wilson, D, Walsh, PG, Sanchez, L and Read, P (1998) Hearing impairment in an Australian population, Department of Human Services Centre for Population Studies in Epidemiology, Adelaide.  Back to cited text no. 21    
22.World Health Organization (1997) Prevention of Noise­Induced Hearing Loss, Report of an Informal Consultation held at the World Health Organization, Geneva, October 1997.  Back to cited text no. 22    
23.Zohar, D, Cohen, A and Azar, N (1980) Promoting Increased Use of Ear Protectors in Noise Through Information Feedback, Human Factors 22 (1), February 1980, pp 69 - 79.  Back to cited text no. 23    

Correspondence Address:
W Williams
National Acoustic Laboratories, 126 Greville Street, Chatswood, NSW 2067
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Source of Support: None, Conflict of Interest: None

PMID: 15703143

Rights and PermissionsRights and Permissions


  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]

  [Table - 1], [Table - 2]

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