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ARTICLE  
Year : 2011  |  Volume : 13  |  Issue : 50  |  Page : 9-15
Preferred sound levels of portable music players and listening habits among adults: A field study

1 Department of Audiology, Institution for Neuroscience and Physiology, Sahlgrens' Academy, Göteborg University, Box 452, SE- 405 30, Göteborg, Sweden
2 Audiological Research Center, Örebro University Hospital, SE-701 85, Örebro, Sweden

Click here for correspondence address and email
Date of Web Publication15-Dec-2010
 
  Abstract 

The main purpose of this descriptive field study was to explore music listening habits and preferred listening levels with portable music players (PMPs). We were also interested in seeing whether any exposure differences could be observed between the sexes. Data were collected during 12 hours at Stockholm Central Station, where people passing by were invited to measure their preferred PMP listening level by using a KEMAR manikin. People were also asked to answer a questionnaire about their listening habits. In all, 60 persons (41 men and 19 women) took part in the questionnaire study and 61 preferred PMP levels to be measured. Forty-one of these sound level measurements were valid to be reported after consideration was taken to acceptable measuring conditions. The women (31 years) and the men (33 years) started to use PMPs on a regular basis in their early 20s. Ear canal headphones/ear buds were the preferred headphone types. Fifty-seven percent of the whole study population used their PMP on a daily basis. The measured LAeq60 sec levels corrected for free field ranged between 73 and 102 dB, with a mean value of 83 dB. Sound levels for different types of headphones are also presented. The results of this study indicate that there are two groups of listeners: people who listen less frequently and at lower, safer sound levels, and people with excessive listening habits that may indeed damage their hearing sensory organ in time.

Keywords: Distortion and sound fatigue, exposure time, hearing loss, hyperacusis, listening levels, portable music players, tinnitus

How to cite this article:
Kahari KR, Åslund T, Olsson J. Preferred sound levels of portable music players and listening habits among adults: A field study. Noise Health 2011;13:9-15

How to cite this URL:
Kahari KR, Åslund T, Olsson J. Preferred sound levels of portable music players and listening habits among adults: A field study. Noise Health [serial online] 2011 [cited 2019 Jul 16];13:9-15. Available from: http://www.noiseandhealth.org/text.asp?2011/13/50/9/73994

  Introduction Top


The use of portable music players (PMPs) and the risk for acquired noise-induced hearing disorders is currently widely discussed. Unfortunately, our knowledge about listening habits is sparse. During the early 1980s, when the first PMPs arrived on the market ("Walkmans"), questions concerning the risk for hearing damage were quickly raised. Research done during that time showed contradictable results concerning listening habits and hearing loss. [1],[2],[3],[4] The PMP technique underwent a rapid and dramatic development, and today research shows that data are still as contradictory as they were earlier and that many studies cannot confirm a clear connection between PMP listening habits and noise-induced hearing disorders. [5],[6],[7],[8],[9]

In 1987, both in England and in Italy, Rice Rossi and Olina, and Rice, Breslin and Roper reported equivalent A weighted sound pressure levels for over 60 users of PMPs (cassette players). [3],[4] The users were asked to set their preferred listening volume, in a noisy or a quiet surrounding. Measurements were then made on the KEMAR manikin. These measurements showed a mean free field (FF) listening level (LAeq ) of 83.4 dB. About 5% preferred listening above 90 dB LAeq . In 1996, Airo, Pekkarinen and Olikinuora measured listening levels among 45 subjects and found an averaged listening level of 82 dB; here, the calculated weekly exposure (LEP,w ) was 75 dB. [10] In 2005, Williams measured preferred sound pressure levels in noisy surroundings in 55 individuals. [5] The averaged daily A weighted exposure level (LAeq,8 h ) was 79.8 dB, which is less than those reported above and considered to be a safe sound level. Nevertheless, the researchers found a slight, although not statistically significant, connection between self-reported hearing loss and higher listening levels when compared to persons who listened to lower levels. [5]

Hodgetts, Rieger and Szarko found differences in listening levels in a comparison of different types of headphones and listening surroundings; the noisier the surroundings, the higher the preferred listening level (PLL). Fligor and Cox measured high output levels from PMP output levels using different types of devices and earphones. They found 7-9 dB higher volumes when using ear canal phones. [6],[11] Mostafapour, Lahargoue and Gates (1998) made an audiological examination of 50 subjects. [12] They concluded that there was no correlation between hearing thresholds and any single or cumulative exposure to noise. When Axelsson and Jerson investigated leisure time activities and hearing thresholds among 538 persons in 1981, they did not find any correlation between hearing loss and a high frequency of noisy activities. [1]

On the other hand, other studies raise serious concerns over findings of a correlation between high listening levels and hearing symptoms. Jokitulppo, Toivonen and Björk (2006) asked 1054 Finnish conscripts about exposure to noisy leisure time noise and hearing symptoms. [13] They found a correlation between an increased exposure and a higher prevalence of hearing symptoms.

Vogel, Verschuure, van der Ploeg, Brug and Raat studied listening behavior in 2009 among adolescent Mp3 users and found that about one third were frequent listeners. [14] Those frequent listeners were about four times more likely to listen to higher volumes than those who were non-frequent users. The researchers conclude that listening frequency is associated with risky behaviors among adolescents and adults and the use of PMP players.

Based on The European Directive, Noise at Work Regulations 2003/10/EC and the report "Potential health risks of exposure to noise from personal music players and mobile phones including a music playing function" released in September 2008 by the European Commission and the "Scientific Committee on Emerging and Newly Identified Health Risks" (SCENIHR), current directives for PMPs (EN 60065:2002 and EN 60950-1:2006) are being revised. [16],[17],[18] Today's maximum sound limit of 100 dB (A) for those devices sold within the EU will most likely and in the near future be more rigorous and specific toward two types of products: those that are child appealing and those that are not. The future sound limit for adult products has now been proposed to have two steps, one base limitation and a higher one that will be possible to access only after a conscious manual action by, e.g., typing in a password. The future sound limits for child appealing PMPs are proposed to be harmonized with the sound pressure limits of toy standard EN71-1, thus a maximum output of 80 dB (A) when measured in FF.

Today we have nearly no updated research on PMP listening habits in Sweden, but studies of the prevalence of tinnitus have shown that approximately 10-15% of adolescents, 15-18 years old, report having tinnitus after exposure to music. [15] One study in the US that collected data between 1988 and 1994 showed that 14.6% of 5249 children of age 6-19 years had a characteristic noise-induced threshold shift commonly associated with a history of high noise exposure. [16]

We know that Mp3 listening is a common, everyday activity among young people today. [16] This raises questions about listening habits and how hearing health will develop in this large group of music lovers.

Research questions

The research questions in this study were: what listening level is preferred when people listen to music using PMPs, what are the current listening habits, have hearing symptoms (e.g. tinnitus and sound sensitivity) been found and can differences be observed between sexes?


  Methods Top


The study is a descriptive and explorative field study.

Data collection

Data were collected over 1 day between 9 AM and 9 PM (12 hours) in the main hall at the Stockholm Central Station. The measurement set-up was one part of a public and informative hearing week arranged by the Karolinska Institute and the Swedish Association of Hard of Hearing People [Figure 1]. People passing by and using PMPs were invited to measure their preferred volume and to use a PMP and listen to music. Sixty persons answered a questionnaire (n = 41 men and n = 19 women; average age 33 years, SD: 13.2) and 61 sound level measurements were made. Five of these sound level measurements were excluded when it was found that the chosen exposure was audio books and not music and another 15 measurements had to be excluded because of poor measurement conditions. This left us with 41 measurements for analysis.
Figure 1: Percentage of the 60 subjects choosing to listen to music via their PMP during different activities in school and leisure time

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The questionnaire

A questionnaire used in another ongoing Mp3 study in Örebro, Sweden, was employed to collect data on listening habits and hearing health. The questionnaire was earlier tested on 10 adolescents for its usability. The questions formed three cluster areas:

Cluster 1: History of usage included three items: "what type of PMPs and headphones are most often used and at what age did usage start?"

Cluster 2: Contemporary usage habits included 10 items: "do you use PMPs, what type of PMP and headphones do you use, how long have you used your latest and most used PMP, how often have you used it, how long have you used it and in what types of environments do you listen to it, how much of the maximum volume do you use, what type of music do you listen to the most and do you go to sleep with your PMP on play?"

Cluster 3: Hearing Health included seven items:

Good hearing: "I have no hearing problems at all."

Bad hearing: "I have bad hearing."

Occlusion: "I often or always have occlusion problems."

Tinnitus: "I often or always have tinnitus with a duration of > 5 minutes."

Sound sensitivity: "I am often or always sensitive to sounds that others normally do not react negatively to (such as rattling of china, rustling of paper, traffic noise).0"

Sound fatigue: "I often or always feel sound fatigue in my ears, especially after sound exposure.0" We regard this to be a mental state and not a hearing symptom but to be intimately connected to excessive hearing and mental stimulation.

Distortion: "Clean sounds that others do not complain over often or always sound distorted, unclean, fuzzy or broken to me."

The last question in this third cluster was: "How long have you had hearing symptoms?"

Measurement equipment

The artificial KEMAR manikin type 45BA (Knowles Electronics, Illinois, USA) was used to measure sound pressure levels. The Bruel and Kjaer Pulse Lab Shop software, with 7700 Fast Fourier Transform (FFT) and Constant Percentage Bandwidth (CPB) analysis was used together with a 3560B Frontend. The KEMAR was equipped with a Bruel and Kjaer 4157 coupler (IEC60711) and a Bruel and Kjaer 4134 microphone. The microphone was connected to a Bruel and Kjaer 2669 preamplifier and to the 3560B Frontend.

The PULSE software was used to sample and analyze all data collected. All measurements were made at the KEMAR's left side. A miniature microphone was mounted on the KEMAR's right side so that the music being played could be monitored simultaneously.

Sound pressure measurements

The subjects chose their favorite music and PLL. The music started and, after 30 seconds, a 60-second sampling time started measuring LAeq 60 sec and max and peak levels in 1/3 octave bands from 20 Hz-10 kHz. Data were recalculated (in real time) to equivalent FF and RMS according to ISO-11904-2. Both FF corrected and uncorrected data are presented here. Background noise levels were measured repeatedly during 1 day to ensure acceptable measurement conditions. For this study, we chose a minimum signal-to-noise ratio of 5 dB for the equivalent sound pressure level as an acceptable measurement condition.

Analysis

The PULSE software was used to sample and analyze all measured data. For the CPB analyzer, we used both averaging and PULSE's maximum hold function. This can be used in many ways: for each individual 1/3 octave band or, as in our case, it can take a snapshot of the frequency spectra when the A weighted Leq value reaches its maximum. This CPB was set to exponential averaging with a time period of 1/4 second. We also used a C weighted overall peak detector.

Measured and questionnaire data were then computed and analyzed using Microsoft Office Excel 2003 and SPSS 17.0. The SPSS independent sample t-test was used to test significant differences between sexes. A P value of 0.05 was chosen as the level of significance.


  Results Top


Questionnaire

Both men and women started to use personal music players on a regular basis in their early 20s.

Fifty-seven percent of the 60 subjects listen daily to their PMP, and no significant difference was found between sexes (P = 0.674). Neither could a significant difference be observed when comparing listening time per session (P = 0.222). These daily listeners are here called "frequent listeners" and 43% of those who listened one to a few times per week are called "non-frequent listeners". No significant difference in listening habits was found in a comparison between sexes. More women (2%) than men (20%) went to bed and slept with their personal music player on and played during the night , but this finding did not show a significant difference (P = 0.489). This exposure is extra and not included in their reported exposure time during time awake.

Of the 60 subjects, 50% had unaffected hearing, followed by "bad hearing" (2%), "often or always tinnitus" (17%), "often or always sensitive to sounds" (12%), "often or always sound fatigue" (5%) and "often or always have feelings of occlusion" and problems with distortion (3%). The preferred listening times per session, choice of volume and reported hearing symptoms are shown separately in [Table 1] for men and women.
Table 1: Listening habits while using personal music players and hearing problems reported in 34 men and 26 women

Click here to view


Temporary problems such as hard to hear in noisy environments, tinnitus after concert, sound sensitivity and sound fatigue were found in 14% of the subjects and only among men. Most subjects listened to pop, rock and hard rock music, and the PMP was typically used during transportation in some form [Figure 1].

Sound level measurements

The results of the 41 sound level measurements are shown in [Figure 2],[Figure 3],[Figure 4].
Figure 2: The averaged FF compensated LA60 sec levels in 1/3 octave bands for all 41 sound level measurements. The bold line shows the subject with the highest preferred listening level. The dashed line shows the calculated average for all 41 subjects. The thin line shows the subject with the lowest preferred listening level and the dotted line shows the average background noise level

Click here to view
Figure 3: The averaged FF compensated LA,max60 sec levels in 1/3 octave bands for 41 sound level measurements. The bold line shows the subject with the highest preferred listening level. The dashed line shows the calculated average for all 41 subjects. The thin line shows the subject with the lowest preferred listening level and the dotted line shows the average background noise level. The typical music style was rock and pop

Click here to view
Figure 4: FF compensated LA60 sec levels for each of the 41 separate measurements in 1/3 octave bands. The bold line shows the subject with the highest preferred listening level. The dashed line shows the calculated average for all 41 subjects. The thin line shows the subject with the lowest preferred listening level and the dotted line shows the average background noise level.

Click here to view


The measured LAeq 60 sec levels corrected for FF ranged between 73 and 102 dB with an average value of 83 dB. Men seemed to use their PMPs more often and for longer sessions than the women, but this was not a significant finding. We found no differences when we compared age and choice of different types of headphones [Figure 5]. An average listening level at ≥ 85 dB was chosen by 29 persons (71%) and they listened primarily to pop, rock and hard rock music. The average LAeq level for 11 measurements made before lunch was 83.1 dB, for 15 made after lunch until 5 PM was 84.5 dB and for the remaining 15 measured between 5 PM and 8 PM was 81.5 dB.
Figure 5: Equivalent levels for each type of headphones used by 41 subjects. Black bars: overall (n = 41); dark gray bars: clip-on/regular earphones (n = 18); white bars: canal phones/ear buds (n = 17), and light gray bars: other types (n = 6)

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Subjects who chose the highest exposure level

Of the whole group of 41 measurements, 19 (46%) persons chose a listening level ≥ 90 dB and 17% (seven persons) chose a listening level of ≥ 95 dB. Of these 19 persons, 43% were women.

Fifty percent (20 persons) listened between 1 and 2 hours per session and 53% of the persons chose to use the maximum volume (100%) when listening, while 26% chose 75% of the maximum volume. Thirty-nine percent (seven persons) went to bed and slept with their PMP on play. Twenty-eight percent (five persons) reported having "poor hearing", 17% (three persons) had often/always tinnitus and 11% (two persons) reported often/always having sound fatigue after sound exposure.


  Discussion Top


Measurements made, as done in this field study, give one important piece of information concerning the actual situation of exposure to PMPs. Since the measurements were made in Stockholm Central Station's main hall which is a noisy public place, there was any little opportunity to reach the criteria stating that each 1/3 octave band should be at least 10 dB louder than the background noise. These difficult measuring conditions forced us to accept a signal-to-noise ratio of 5 dB for the equivalent sound pressure level.

Most people rushed past our measure set-up stand, most of them being on their way either to or from their jobs. This could of course mean that those who were in the greatest hurry or who were uninterested listened to totally different sound levels than those who participated in the study. On the other hand, the measurement set-up made it possible for people to participate who would perhaps never have come to a clinic.

A risk for bias is the measurement technique, as Hammershøj and Møller thoroughly pointed out in their study of sound emission from sound sources close to the ear. [19] Averaged sound pressure data measured with the use of a manikin recalculated for the FF conform poorly to the individuals' personal sound pressure level measured at the ear drum due to the large individual differences. Furthermore, the positioning of the ear buds in the manikin KEMAR's ear canal is difficult and the ear buds are difficult to keep in place. [18] However, this is the one method that is reasonably practical to use at the moment.

This study does not attempt to generalize the preferred listening level among adults with an average age of 33 years but to display the measuring results derived during one day. Thus, some of the test persons could have retrieved a temporary threshold shift (TTS) during the day by the time that they took part in our study. First, this could indeed be a problem if we found remarkably higher levels in the measurements done during the afternoon and evening, but this was not found. Secondly, the main point is that regardless of whether the persons had a TTS or not, their actual and chosen exposure level was measured and is reported here.

Both men and women were in their early 30s and started to use PMPs on a regular basis in their early 20s. Results showed that the most used headphone types were ear canal headphones and canal phones/ ear buds. Fifty-seven percent of the study population used their PMP on a daily basis. The sound level results agree well with those found by Rice, Breslin and Roper, and Rice Rossi and Olina in 1987 and with those of Airo, Pekkarinen and Olikinuora in 1996. [3],[4],[10] One question that arises is whether the listening level is comparable for adolescents or even younger age groups. The average age of the subjects in this study was a mature age. This fact could influence the results. In this study, men tended to listen more often and for somewhat longer sessions than women did, but the findings did not differ significantly. Nor could we find any difference in the preferred sound level between the sexes or that, for example, hard rock was listened to at higher volume levels than ballads or pop.

The persons included in this limited study listened to pop, rock and hard rock music. There are variations in dynamic levels in many musical pieces to make them more interesting to listeners and to increase the emotional impact. The distribution of dynamic levels varied from song to song, of course, but most modern songs are quite similar in structure, length and dynamic range. Today, the music industry is trying to get listeners' attention by making music as loud as possible by bringing up soft passages and "pushing down" loud passages by compressing the dynamic range, even to a single decibel in extreme cases. Thus, while some genres preferably have as little dynamic range as possible, pop music often has a dynamic range of 6-10 dB. [20]

In this field study it was important not to discourage people by measuring whole songs, thus detaining them for several minutes. The measurements are thus 60-second long segments measured after 30 seconds from the start of the song. In this way, the measured segment should contain parts of both the louder and softer passages in the songs. Even if some of the highest or lowest passages are lost in our 60-second measurement, it still gives a good estimate of the levels.The averaged sound levels measured showed no critical sound exposure but since this study is limited, we want to interpret the results with caution. However, it must be kept in mind that this daytime sound exposure is prolonged by approximately one fifth in this sample as they reported exposure during sleep.

Another additive risk factor for hearing damage is other leisure time exposure such as noise at clubs and pubs and music at concerts. In Sweden, the National Board of Health and Welfare has issued limits for public music of 100 dB LAeq during performances and 115 dB LAfmax measured at the loudest possible position and in public music events where children under the age of 13 are not allowed. [21] These levels should be relatively safe, but only in the case of an exposure of 2 hours per week or 15 minutes per day. When this exposure is added to listening to a PMP or other noise sources, there is a rapidly increased risk of hearing damage.

Surprisingly, we did not find any high proportion of hearing symptoms in the group who exposed themselves to excessive sound levels. Some subjects report experiencing "sound fatigue". This mental symptom is something we observe (observe?) frequently among musicians and adolescents after excessive music exposure. This phenomenon is also reported among preschool and other teachers. Many of them seek silence and withdraw from social life during their leisure time after a workday loaded with sound and noise. We believe this is worth investigating further.

We could therefore not, as Williams in 2005, establish any correlation between listening level and hearing loss, but this study is limited. We know that noise-induced hearing loss and other hearing symptoms caused by overstimulation of the inner ear usually accumulate over time, even if the sound levels are not critically high.

We acknowledge the lack of studies on PMP listening habits and urge the need for further studies. To be able to work with hearing preventive actions, we need to learn more about PMP listening habits, what the preferred listening levels are and how great the risk is of developing hearing disorders over time.


  Conclusions Top


Both men and women in their early 30s started to use PMPs on regular basis in their early 20s. Results showed that the most used headphone types were ear canal headphones and canal phones/ear buds. Fifty-four percent of the study population used their PMP on a daily basis. The actual measured LAeq 60 sec levels corrected for FF ranged between 73 and 102 dB, with a mean value of 83 dB. Men in this study seemed to use their PMPs more often and for longer times than the women, and those who listened more frequently had more distributed sound levels compared to those who listened two to four times/week. The averaged sound levels measured showed no critical sound exposure related to the averaged listening time but, since this study is limited, we interpret the results with caution. We know that noise-induced hearing disorders due to overstimulation of the inner ear usually accumulate over time, even if the sound levels are not critically high.

There is a lack of studies on PMP listening habits and we emphasize the need for more studies. To be able to work with hearing preventive actions, even at early ages, we need to learn more about PMP listening habits and study what the preferred listening levels are in order to be able to assess how great the risk is of developing hearing disorders.


  Acknowledgments Top


We are grateful to Prof. Claes Möller at the Audiological Research Center in Örebro and Prof. Mats Ulfendahl and Associate Prof. Björn Hagerman at the Karolinska Institute for technical and overall support. Thanks also go to the Swedish Association of Hard of Hearing People for financial support.

 
  References Top

1.Axelsson A, Jerson T, Lindgren F. Noisy leisure time activities in teenage boys. Am Ind Hyg Assoc J 1981;42:229-33.  Back to cited text no. 1
    
2.Hellstrφm PA. The effects on hearing from portable cassette players: A follow-up study. J Sound Vib 1991;151:461-9.  Back to cited text no. 2
    
3.Rice CG, Rossi G, Olina M. Damage risk from personal cassette players. Br J Audiol 1987;21:273-8.  Back to cited text no. 3
    
4.Rice CG, Breslin M, Roper RG. Sound levels from personal cassette players. Br J Audiol 1987;21:273-8.  Back to cited text no. 4
    
5.Williams W. Noise levels from personal stereo use. Int J Audiol 2005;44:231-6.  Back to cited text no. 5
    
6.Hodgetts WE, Rieger JM, Szarko RA. The effects of listening environment and earphone style on preferred listening levels of normal hearing adults using an MP3 player. Ear Hear 2007;28:290-7.  Back to cited text no. 6
    
7.Peng JH, Tao ZZ, Huang ZW. Risk of damage to hearing from personal listening devices in young adults. J Otolaryngol 2007;36:181-5.  Back to cited text no. 7
    
8.Torre P 3rd. Young adults′ use and output level settings of personal music systems. Ear Hear 2009;29:791-9.  Back to cited text no. 8
    
9.Shah S, Gopal B, Reis J, Novak M. Hear today, gone tomorrow: An assessment of portable entertainment player use and hearing acuity in a community sample. J Am Board Fam Med 2009;22:17-23.   Back to cited text no. 9
    
10.Airo E, Pekkarinen J, Olkinuora P. Listening to music with earphones: an assessment of noise exposure. Acta Acustica united with Acoustica 1996;82:885-94.  Back to cited text no. 10
    
11.Fligor BJ, Cox LC. Output levels of commercially available portable compact disc players and the potential risk to hearing. Ear Hear 2004;25:513-27.  Back to cited text no. 11
    
12.Mostafapour S, Lahargoue K, Gates G. Noise-induced hearing loss in young adults: The role of personal listening devices and other sources of leisure noise. Laryngoscope 1998;108:1832-9.  Back to cited text no. 12
    
13.Jokitulppo J, Toivonen M, Björk E. Estimated leisure-time noise exposure, hearing thresholds and hearing symptoms of Finnish conscripts. Mil Med 2006;171:112-6.  Back to cited text no. 13
    
14.Vogel I, Verschuure H, van der Ploeg C, Brug J, Raat H. Adolescents and Mp3 players: Too many risks, too few precautions. Pediatrics 2009;123:953-8.  Back to cited text no. 14
    
15.Directive 2003/10/EC of the European Parliament and of the council of 6 February 2003 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (noise).  Back to cited text no. 15
    
16.Personal music players and hearing: EU Commission report, SCENIHR. Available from: http://www.ec.europa.eu/health/opinions/hearing-loss-personal-music-player-mp3-l1_en.pdf [last accessed on 2008].  Back to cited text no. 16
    
17.EN 60065:2002. Audio, video and similar electronic apparatus. Safety requirements. ISBN 0 580 39800 5.  Back to cited text no. 17
    
18.EN 60950-1:2006. Information technology equipment - Safety - Part 1: General requirements. ISBN: 9780580673399.  Back to cited text no. 18
    
19.Hammershöj D. Determination of noise immission from sound sources close to the ears. Acta Acustica united with Acoustica 2008;94:114-29.  Back to cited text no. 19
    
20.Katz Bob. Mastering Audio: The Art and the Science. Canada, Elsevier Science, Focal Press ISBN: 0-240-80545-3, 2007:99-139.  Back to cited text no. 20
    
21.National Board of Health and Welfare, Sweden SOSFS, 2005:7 (M) Public advice, 2005.  Back to cited text no. 21
    

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Correspondence Address:
Kim R Kahari
Department of Audiology, Institution for Neuroscience and Physiology at the Sahlgrens’ Academy at Göteborg University, Box 452, SE- 405 30, Göteborg
Sweden
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Source of Support: Swedish Association of Hard of Hearing People, Conflict of Interest: None


DOI: 10.4103/1463-1741.73994

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1]

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