With the ubiquitous presence of Personal Stereo Systems (PSSs), namely iPods R and the like, many hearing conservationists have raised concerns about the temporary and permanent hearing damage that could result from long exposure to loud music playback, especially among adolescents and teenagers. The crux of the problem can be identiﬁed as an overexposure of the auditory system. In order to reduce the dose received by the PSS listener, the playback level and/or the duration should be reduced. Assuming that the duration of the music playback experience is really up to the user, the remaining parameter is the music playback level. The purpose of this study is to ﬁrst understand - from the available public and scientiﬁc literature - what factors are inﬂuencing the PSS playback level; and second, to investigate if the use of earphones featuring good attenuation of the ambient noise level would lead to a reduced playback level, hence a reduced dose and eventually less auditory damage. The third section of this paper will describe other beneﬁts regarding sound quality that are associated with isolating earphones. Finally, the paper will review other safety mechanisms that could be used in earphones and PSS to make them safer for hearing.1 Introduction sure duration may be quite long, resulting in PSSs users having poorer hearing than non users. Potentially evenMP3 players, Portable Multimedia Players and Smart more damaging is that Axelsson found that users withPhones with audio playback capabilities, commonly re- temporary threshold shift may compensate by turningferred to in this article as “Personal Stereo Systems” up the volume on their PSS!(PSSs) are very widely used in the developed world, es-pecially adolescents and teenagers. Indeed, worldwide 1.1.1 Listening levelunit shipments for PSSs reached 182 million in 2006, up42% from 2005 and are expected to grow to 275 million The sound pressure levels (SPL) at maximal power out-units by 2011 . The daily noise dose accumulated with put for PSSs can be quite high. Fligor and Cox  mea-the use of such music playback systems incrementally sured SPL between 95 and 120dBA. The World Healthadds to occupational noise exposure and the combina- Organization (WHO) stipulates that in order to avoidtion may result in excessive noise exposure. Even if the acute hearing impairment LAmax should always be be-safety (from an auditory perspective) of such practice low 110 dBA , making the use of PSSs at full powerhas been questioned  since the 1980’s (beginning with problematic [6, 7]. Hopefully, users are rarely using thethe introduction of the Sony WalkManTM ), not many PSSs at their maximum levels, but still, the Royal Na-answers nor practical tools have been developed to re- tional Institute for Deaf People  reported that 65%duce or manage the risk of Noise Induced Hearing Loss. of users were listening at levels above 85 dB, while Rice The purpose of this study is to ﬁrst understand from et al.  reported that 20% of the users tested had athe available public and scientiﬁc literature what factors preferred listening level (PLL) higher than 90 dB. It isinﬂuence the PSS playback level; and second, to investi- reasonable to assume that a too high listening level ingate if the use of earphones featuring good attenuation itself is not a danger for the hearing of a majority ofof the ambient noise level would lead to a reduced play- users.back level, hence a reduced dose and potentially lessauditory damage. 1.1.2 Listening time and accumulated noise The ﬁrst section presents analysis of the useage dosehabits of PSS users in terms of listening level and lis-tening time and the inﬂuence of background noise. The It is apparently not the listening level alone that is the danger, but rather the combination of a high listen-second section describes the actual remedies or potentialchanges in PSS use habits that may assist in reducing ing level and listening time, which constitutes the noise dose. While the WHO recommends  limiting dose torisk. The third section describes in detail one speciﬁc 70 dBA for a 24 h period or to 85 dBA for a 1 h period,application (the Sonomax solution, a custom expand-able isolating earphone). the study conducted by the Royal National Institute for Deaf People reported that more than 50% of the peo- ple surveyed reported listening to their PSSs for more1.1 Personal stereo systems (PSSs): a than 1 h per day. Accordingly if these users are exceed- risk for the hearing ing a level of 85 dBA, they are putting their hearing at risk. Catalano and Levin  have compared the dose re-A number of factors can explain why the use of PSSs is ceived by PSS users to the maximal dose advised by theso common and why they may pose a danger to hear- American Academy of Ophthalmology and Otolaryngol-ing health. Axelsson  explains the convenience of ogy (85 dBA for 16 h per day or 80 h per week over 10these portable devices with high sound levels capabil- years) and found that 31.4% of the users exceeded thatities, the feeling of “being inside the music” with high criteria.sound levels, the good impulsive sound reproduction,the fact that since they are relatively quiet to othersthan the user/listener, they do not disturb the environ- 1.2 Relation between the backgroundment (hence few complaints from neighbors). On the noise level and the listening levelhazardous side, he also noted that users tend to turnup sound levels in noisy environments , and that expo- One of the ﬁrst studies on background noise inﬂuence on PLL was made by Rice et al. in 1987 . This study
found that in silence the PLL for the studied population Based on this short literature review:was 80.7 dB LAeq . With road traﬃc background noise • Any PSS can emit damaging sound levels;of 70 dBA the PLL was 85.1 dB LAeq , giving a 15 dB • Most of PSS users have an acceptable PLL in quietsignal to noise ratio (SNR). settings; In 2005, Williams  interviewed PSS users in thestreet about their PLL habits. In average background • Background noise level has a signiﬁcant inﬂuencenoise of 73.2 dBA (standard deviation 2.3 dB ), the av- on PLL;erage PLL is 86.1 dB LAeq,2h (7.9 dB of standard devi- • PSS users favor the music quality (not too loud)ation), resulting in SNR of about 13 dB. rather than the SNR; In 2007, Hodgetts and all  examined the relation- • The average preferred SNR is about 13 dB;ship between background noise (quiet, street noise be- • Airo et al recommend that “continuous use of PSStween 70 and 80 dBA, multi-talker babble at 70 dBA), and earphones should be avoided in conditionsearphones types (in-the-ear, over-the-ear, over-the-ear where environmental noise level exceeds 70 dB”.with noise cancelling) and PLL. Their subjects wereuniversity students and testing was performed in a lab- It should be noted that the average noise level inoratory. Table 1 presents their results: average PLL the New-York subway is 85.7 dBA, with peaks as highin LAeq , standard deviation in brackets and estimated as 112 dBA . However, Ario et al.  recommendSNR. Whatever the earphone type used, higher back- not using PSS in a background noise higher than 70 dB. Indeed with such levels, is would be expected that the Table 1: Results (mean ear canal level in dBA PLL will be set higher than the WHO recommendation (standard deviation) SNR) extract from  of 85 dBA for one hour . Quiet Multi-talker Street babble noise 70 dBA 70 to 2 Solutions to PSS danger 80 dBA In-the-ear 77.82 86.66 88.83 Various options are available to limit the danger of PSSs (7.7) (4.3) (3.9) for user’s hearing health. ≈ 17 dB ≈ 14 dB Over-the-ear 75.17 82.93 84.46 2.1 Inform PSSs users (9.5) (5.5) (4.8) ≈ 13 dB ≈ 9 dB Raising public awareness on the potential of auditory Over-the-ear/NR 75.16 81.63 83.00 risks caused by abuse of PSSs is essential. Health (9.1) (5.6) (5.4) Canada is warning the public of the auditory damage possibly caused by the abuse of PSSs . The Royal ≈ 12 dB ≈ 8 dB National Institute for Deaf People suggests that PSSs manufacturers put warnings on their products , justground noise levels resulted in higher PLL and lower as required in the tobacco industry. Laroche et al. SNR. Moreover, the over-the-ear earphone with noise are currently evaluating a temporary threshold shift de-cancelling resulted in a reduction in PLL. The results tector  that could be run on PSSs intended to warnfrom the two ﬁrst studies from Rice and all.  and the user about auditory risk.Williams  match the results of the earphones with-out noise cancelling. In 1996, Airo et al.  examined background noise 2.2 Limit the listening levelinﬂuence on PLL in both a laboratory and ﬁeld study Some jurisdictions are promulgating rules regarding lim-with earphones without noise attenuation. In the lab- itation on PSS maximum levels. In France, in 1998,oratory, railway station, subway train cabin, traﬃc on maximum SPL has been limited to 100 dB  anda busy street, industrial noise and pink noise were con- more recently, in 2005, output voltage has been limitedsidered at LAeq of 60, 70, 80 and 90 dB. In the ﬁeld, to 150 mV . Some simple handheld devices havepublic places and streets were chosen. Table 2 shows been commercialized, like the Ear 3 (Hollins Communi-the average results of: background noise level, PLL in cations Research Institute, VA, USA), to help protectLAeq and SNR, with standard deviations in brackets. people from hearing loss by measuring sound levels and indicating when PSS emit potentially damaging levels Table 2: Average background noise levels, PLLs and of noise. Unfortunately, the absolute level on its own isSNR in ﬁeld, laboratory and combined measurements, rarely the cause of hearing damage, and listening time with standard deviation in square brackets, extract has also to be taken into account. from  Field Laboratory Combined Noise [dBA] 65 (4.8) 73 (11) 71 (10.3) 2.3 Limit the listening time and the ac- PLL [dB LAeq ] 82 (8.8) 85 (6) 84 (6.9) cumulated noise dose SNR [dB] 17 (8) 12 (7.2) 14 (7.6) Several “rules of thumb”, such as a playback level at 60% of maximum for 60 minutes only , have been de-Airo et al. determined the linear regression in all three veloped to attempt to limit both the listening level andcases (ﬁeld, laboratory and combined). The Airo et al. the listening time, and hence to limit the accumulateddata will be more thoroughly discussed later in paper. noise dose. Rules such as these are unfortunately very
gross approximations since they cannot account for the The soundbore that holds the speaker housing (visiblediﬀerence in the power output of the PSSs nor the sen- in Fig.1 on the soft silicone core) is ﬁrst used to insert asitivity of the earphones used by the user. specially-developed microphone for the measurement of the diﬀerence between the sound pressure levels outside2.4 Use noise isolating earphones the earpiece and in the occluded earcanal (see  for the assessment of the earpiece attenuation), to assessGiven that the listening level appears to be mostly re- the quality of the acoustic seal. This testing featurelated to the level of the background noise, the use of ensures the user that his custom expandable earphoneearphones that provide some degree of attenuation of is not only providing a good sound isolation but alsothe ambient noise may help the user reduce the listen- optimize the low-frequency response of the miniatureing level. receiver used.2.4.1 Active noise reduction of the background noiseSome earphones featuring active noise reduction (ANR)have been recently commercialized , like theQuietComfort R Acoustic Noise Cancelling R Head-phones from Bose Corporation (MA, USA). These ANRearphones generally use a feedback controller to gener-ate a sound wave that is of the same magnitude but op-posite phase as the initial disturbance (the background Figure 1: Exploded view of the custom expandablenoise) and should not be confused with less expensive isolating earphone (not yet adjusted to the earcanal):devices that are simply featuring a white-noise genera- transducer housing (left), soft silicone core (middle)tor to mask the surrouding noise. They can quite ef- and inﬂatable envelope (right).fectively cancel or reduce the background noise in thearea of the user’s ears. There limitations are two-fold. 3.1 The beneﬁts of custom isolating ear-On the performance side, the active controller is only phoneeﬀective on a limited frequency range and does not ef- Apart from the physical comfort beneﬁts discussed pre-fectively cancel or attenuate the high frequency content viously, the main auditory beneﬁt of the custom iso-of the background noise. The technology is also inef- lating earphone is reduced background noise, hence thefective for noises with too highly ﬂuctuating character- reduced PLL. To illustrate that decrease of the musicistics, such as human speech. From the perspective of playback level, a statistical analysis will be conductedcomfort, since the high pitched background has to be at- on athorough dataset made available by Airo et al. tenuated by passive means (like a circumaural headset), on earphones not providing any noise isolation.the ANR headset may be quite bulky. Fig. 2 shows the linear regression between the back- ground noise levels and music listening levels as well2.4.2 Passive attenuation of the background as the 95% conﬁdence bands for these data. Fig. 3 noise 105Another approach consist in merging the features of theearphone with the noise isolation provided by a pas- 100sive hearing protection device. Such earphone can be 95a headset or an intra-canal generic “one-size-ﬁts-most” M u si c L iste n in g L e v e l [ d B A ]earpiece, such as the Etym¯tic Research Inc. (IL, USA) o 90ER6 IsolatorTM earphones. This is a somehat similar 85approach that has been used in the current work withthe Sonomax isolating earphone and that will be further 80discussed in terms of pro’s and con’s in the following sec- 75tion. 703 A custom expandable isolating 65 earphone 60 30 40 50 60 70 80 90 100 B ac k grou n d N oi se L e v e l [ d B A ]Fig. 1 presents the custom expandable isolating ear-phone as designed by Sonomax Hearing Healthcare Inc. Figure 2: Linear regression ﬁtting and the 95%(QC, Canada), a manufacturer of custom instantly- conﬁdence bands for background noise levels and musicﬁtted earplugs. The earpiece is instantly adjusted to listening levels (free-ﬁeld equivalent sound pressurethe ear canal by the injection of a two-part medical sili- levels) with the data extracts from con ﬁller into a very thin inﬂatable envelope which sur- .rounds a soft silicone core of generic shape and size. This illustrates the auditory beneﬁts of using the Sonomaxunique feature ensures a custom and comfortable ﬁt of isolating earphone, featuring a N RSA50% = 26 dB at-the earpiece and secures it inside the wearer’s earcanal. tenuation (see  for details on NRS calculations): for
an average background noise of 67.9 dBA the average ing such isolating earpiece with a PSS in day-to-day ac-PLL is 84.2 dB LAeq , but since the background noise tivities.is now reduced by the amount of attenuation provided Given that simply driving a car while listening to theby the isolating earphone (down to 41.9 dBA), the PLL radio can create a distraction (see ), it is no surpriseis now accordingly reduced to 72.7 dB LAeq . Fig. 4 that the use of portable electronic devices (such as cell- phone or PSS) can more greatly increase distractions. 105 Pedestrians are also clearly distracted when using such 100 electronic devices , and their proved inability to con- centrate properly [24, 25, 26] can even be the reason for 95 lethal injuries  in up to 15% of pedestrian/vehicle M u si c L iste n in g L e v e l [ d B A ] 90 incidents. Use of isolating earphones with a PSS could clearly 85 A ver a g e mu s i c l i s t en i n g l evel lead to similar distraction, but one could also fear that some useful auditory cues (like the noise of an approach- 80 ing motorized vehicle ) might be masked and further 75 jeopardize the user’s safety. Although some very re- cent studies  suggested that pedestrians using a PSS 70 were actually behaving more safely than cellphone users 65 N R SA5 0 % and even more safely than regular pedestrians, such un- expected outcome is most probably due to an extra 60 30 40 50 60 70 80 90 100 cautiousness of the user and required further scrutiny. B ac k grou n d N oi se L e v e l [ d B A ] Meanwhile most of the manufacturers should warn, if Figure 3: Linear regression ﬁtting and the 95% they do not already do so, the users of noise isolatingconﬁdence bands for background noise levels and music earphone against using their use in activities where au- listening levels and the application of the N RSA50% ditory cues can be crucial (street jogging, car driving, etc.).shows the distribution of the PLL both for a PSS withnon-attenuating earpieces (in blue) and for a isolatingPSS featuring a N RSA50% = 26 dB noise attenuation 4 Conclusions(in red). The beneﬁts of the isolating earphone are evi- Risks associated with Personal Stereo Systems (PSSs)dent: using a non-isolating PSS, 45.1% of the users have use have been explored and some existing solutions havea PLL higher than 85 dB LAeq (the recommended max- also been presented. Given that the preferred listeningimum level for one hour listening without risk of hear- level (PLL) is, for a given individual, essentially drivening losses based on WHO recommendations), while this by the background noise level, the use of noise isolatingvalue drops to 3.7% of the users if the PSS used ear- earphone appears as a logical ﬁrst step. The auditorypieces with an N RS50% = 26 dB sound isolation, like beneﬁts of such isolating earphone use has been furtherthe Sonomax custom expandable isolating earphone. examined by evaluating the reduction of the PLL associ- 15 0.06 ated with the use of an isolating earphone. A concept of a custom expandable instantly-ﬁtted earphone featuring P = 0.451 an average 26 dB attenuation has been presented. Using 0.05 a simple statistical analysis on existing published data, the percentage of wearers using a regular non-isolating 10 0.04 earphone exposed to a potentially dangerous noise doseN u mb e r of su b j e c ts has been established to 45.1%. The use of the proposed Prob ab i l i ty 0.03 custom isolating earphone would lead theoretically to a substantial decrease (down to 3.1%) of the percentage 5 P = 0.037 0.02 of users putting their hearing at risk. While it remains unclear if the use of such isolating earphones is increas- 0.01 ing or not, the risk for the user of physical injury due to an auditory warning mis-perception, the precaution 0 0 principle should be applied. The use of noise isolating 40 50 60 70 80 90 100 110 M u sic L i ste n i n g L e v e l [ d B A ] earphones can bring healthy beneﬁts to PSS users, as long as they are not used in activities where hearing of Figure 4: Distribution of the music listening levels auditory cues is critical. with and without the NRS application3.2 Safety concerns associated with the References isolating earplugs  Stephanie Ethier. Worldwide demand remainsThe reduced PLL associated with the use of such a cus- strong for mp3 and portable media players. Techni-tom isolating earphone is associated with the fact that cal report, In Stat, Portable entertainment devices,good passive attenuation is oﬀered by the earpiece. This August 2007.can also raise some concerns regarding the safety of us-
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