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|Year : 2009 | Volume
| Issue : 44 | Page : 132--140
Output sound pressure levels of personal music systems and their effect on hearing
Ajith Kumar, Kuruvilla Mathew, Swathy Ann Alexander, Chitra Kiran
Dr. MV Shetty College of Speech and Hearing, Vidyanagar, Mangalore - 575 013, India
Dr. MV Shetty College of Speech and Hearing, Vidyanagar, Mangalore - 575 013
This study looked at output levels produced by new generation personal music systems (PMS), at the level of eardrum by placing the probe microphone in the ear canal. Further, the effect of these PMS on hearing was evaluated by comparing the distortion product otoacoustic emissions and high frequency pure tone thresholds (from 3 kHz to 12 kHz) of individuals who use PMS to that of age matched controls who did not use PMS. The relationship between output sound pressure levels and hearing measures was also evaluated. In Phase I output SPLs produced by the PMS were measured in three different conditions - a) at volume control setting that was preferred by the subjects in quiet b) at volume control setting that was preferred by the subject in presence of 65 dB SPL bus noise c) at maximum volume control settings of the instrument. In Phase II pure tone hearing thresholds and DPOAEs were measured. About 30% of individuals in a group of 70 young adults listened to music above the safety limits (80 dBA for 8 hours) prescribed by Ministry of Environment and Forests, India. Addition of bus noise did not increase the preferred volume control settings of the subjects significantly. There were no significant differences between the experimental and control groups for mean pure tone threshold and for mean DPOAE amplitude comparisons. However, a positive correlation between hearing thresholds and music levels and a negative correlation between DPOAE measures and music levels were found.
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Kumar A, Mathew K, Alexander SA, Kiran C. Output sound pressure levels of personal music systems and their effect on hearing.Noise Health 2009;11:132-140
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Kumar A, Mathew K, Alexander SA, Kiran C. Output sound pressure levels of personal music systems and their effect on hearing. Noise Health [serial online] 2009 [cited 2020 Jul 5 ];11:132-140
Available from: http://www.noiseandhealth.org/text.asp?2009/11/44/132/53357
There are growing concerns over noise exposure via personal music systems (PMS) use by young adults. It is said that with the massive growth in popularity of portable MP3 players, exposure to high noise levels has increased, and millions of young people are potentially putting themselves at risk for permanent hearing loss every time they listen to their favorite music. As no evidence-based definition exists for hazardous sound levels of music, as a substitute, standards for exposure to occupational noise have been proposed for use.  The ISO 1999 standard for occupational noise  defines a time-weighted average (TWA) level of 85 dBA for an 8 hour period per day as the maximum permissible dose of sound energy. The limit 85 dBA is not fully harmless, as a few percent of people may still incur a permanent hearing loss if exposed to it. The ISO standard recognizes that there is a tradeoff between the exposure time and the sound level, which is quantified by a '3 dB exchange rule': Every 3 dB increase in the exposure level must be compensated by halving the exposure time to keep the risk constant. This means that an 8 hour exposure to 85 dBA bears the same risk for hearing loss as 4 hours of exposure to 88 dBA, or a 2 hour exposure to 91 dBA, and so on. In India, the Ministry of Environment and Forests  has proposed a TWA level of 80 dBA for an 8 hour period per day as the maximum permissible exposure.
Early studies by Wood and Lipscomb  and Katz et al.,  looked at the maximum output levels produced by the personal cassette players through headphones. These investigators reported levels as high as 124 dBA and 110-128 dBA respectively for compact disc (CD) and cassette players. Based on these reports both the papers concluded that CD and cassette players produce sound levels that are hazardous to hearing. However, these reported output levels were measured at maximum volume control settings which do not represent the everyday use settings. Moreover these measurements were made on a 9BS 9A coupler. Other researchers , asked users to set the volume control level to their preferred setting and measured the output levels. Results showed that output levels were substantially less than what is reported by Wood and Lipscomb  and Katz et al. ,  but even so a significant percentage of people set the listening levels higher than the permissible level of 80 dBA. Fligor and Cox  recorded the output levels of the different commercially available CD players in combination with a variety of earphone styles on a KEMAR. They concluded that output levels varied across different manufacturers of CD players and style of the earphone. But generally smaller insert earphones produced sound pressure levels higher than the permissible limits compared to bigger headphones. Output levels from PMS have been reported to be as low as approximately 80 dBA  to as high as 121 dBA.  Airo et al.,  measured the output levels of personal cassette players in an acoustic coupler at maximum volume control settings, comfortable volume control settings in quiet and in the presence of background noise. They reported that personal cassette players were able to produce high sound levels but the typical listening levels chosen by the users were not alarming. The output levels on average exceeded 85 dBA when the background noise level was 72 dBA, potentially creating some hearing loss risk when cassette players are used in noisy conditions at work or among traffic. Hodgetts et al.,  measured the preferred listening levels for a MP3 player in normal hearing adults. Using probe microphone measures, Hodgetts et al.,  measured the dBA weighted sound pressure levels produced by a commercially available MP3 player for different types of earphones (ear bud, over-the-ear, and over-the-ear with noise reduction circuitry). Preferred listening levels ranged from 75-78 dBA for different types of earphones. Preferred listening levels were higher for the ear bud style of earphones compared to the over-the-ear style. Furthermore, preferred listening levels were increased by 6-10 dB when background noise was introduced to the listening environment to simulate the 'real world' situation. Torre  measured the output SPL of PMS in the ear canal of 32 participants at four loudness categories: Low, medium, loud and very loud. Their results showed that mean output SPLs values were 62, 72, 88 and 98 dB SPL for low, medium, loud and very loud categories respectively. Based on these measurements they concluded that output SPLs produced by PMS at medium or loud volume control settings may not be hazardous as most of the subjects reported they listen to music at these volume control settings for about 1 to 3 hours a day.
Meyer-Bisch  found significantly poorer pure tone thresholds in people using PMS longer than 7 hr/week than in the matched control subjects. Lepage and Murray  measured the transient evoked otoacoustic emission in 700 individuals and found reduced amplitudes of these in individuals with a positive history of noise exposure or PMS use. Recently, Montoya et al.,  compared the amplitude, incidence and spectral content of transient and distortion product otoacoustic emissions in normal hearing MP3 player users to those of control group who were non-users of MP3 players. Results showed that subjects who had used MP3 players for greatest number of years and for more hours each week exhibited a reduction in incidence and amplitudes of both types of otoacoustic emissions and an increase in distortion product otoacoustic emission thresholds.
Previous research has shown that maximum output levels produced by cassettes and CD players are above the permissible limits. However, more recent research has shown that output SPLs of newer generation PMS like iPods, MP3 players at medium or loud volume control settings are within the permissible limits and may not be hazardous. However, there is a paucity of data regarding the output levels and their effect on auditory system of new generation MP3 players and mobile phones MP3 systems. In developing countries like India, mobile phone ownership is growing rapidly. About six million new mobile subscriptions are added every month and three quarters of India's population will be covered by a mobile network by the end of 2008. Moreover, the studies mentioned above have been carried out in Western population and it is known that output SPL values as well as susceptibility to noise-induced hearing loss depends on the race and ethnicity. ,, Hence, it is important to measure and document the output sound pressure levels and their effect on hearing of these new generation PMS including mobile phones in different racial/ethnic groups. This study specifically looked at output levels produced by newer generation PMS in an Asian-Indian population, when measured at the level of the eardrum by placing the probe microphone in the ear canal. Output levels were measured in three conditions - a) at the volume control setting that was preferred by the subject in quiet b) at the volume control setting that was preferred by the subject in presence of bus noise and c) at the maximum volume control settings of the instrument. The effect of this new generation PMS on hearing was evaluated by comparing the distortion product otoacoustic emissions and high frequency pure tone thresholds (from 3 kHz to 12 kHz) of individuals who use PMS to that of age matched controls who did not use PMS. Furthermore, the relationship between output sound pressure levels and hearing measures was also evaluated.
Materials and Methods
Participants comprised two groups. The experimental group had 70 adults (35 males and 35 females; age range of 17-24 years; mean age 20.5 years) who reported listening to music regularly through their PMS. A detailed history regarding the PMS and its usage was collected from each participant using a questionnaire [Appendix I] All the subjects in the experimental group had been using PMS for a period of over two years. The control group had 30 adults who never/very rarely listened to music through PMS. Participants in the control group had their hearing thresholds less than 15 dB HL at octave frequencies from 250 Hz to 8000 Hz. Subjects in both the groups showed no evidence of occluding cerumen or middle ear pathology on otoscopy and tympanometry. Furthermore, subjects in both groups did not report of any history of occupational noise exposure or ototoxic drug usage. Subjects were recruited from different graduate schools in and around Mangalore, a city in south India. Written consent was obtained from all the subjects. The study was approved by the research review board of the Dr. MV Shetty College of Speech and Hearing.
The study was conducted in two phases. In Phase I output sound pressure levels (SPLs) produced by the PMS were measured in three different listening conditions - at volume control settings preferred by the subjects in quiet, volume control settings preferred by the subjects in the presence of background of 65 dB SPL bus noise and at the maximum volume control setting of the PMS - using a probe microphone.
Output SPLs in the presence of background bus noise was measured to simulate a real life listening situation. Bus noise was chosen as the background noise source as our preliminary survey showed that more than 90% of the participants listen to music while commuting to the college by bus. To simulate this "real world" listening condition we measured the SPL at a distance of 2 m from the bus engine (to represent the maximum noise that the commuter may be exposed), inside the bus, using a sound level meter (Quest 1800) and a microphone (Quest 4180). The noise was measured while the bus was running in fourth gear (approximately 1600 RPM) and travelling approximately at 40 kilometers/hour. These conditions represent the city ride to which listeners are generally exposed. Four sound level measurements were done under these conditions and the average noise level produced by the bus engine was 65 dB SPL. This sound source was then digitally recorded and used as the background noise source in the experiment. Noise was played back through sound field speakers connected to a personal computer. The volume of the speakers was adjusted to produce a SPL of 65 dB at the position of the subject's head. The amplitude of the noise at the position of the subject's head was measured using sound level meter (Quest 1800) and a microphone (Quest 4180). In Phase II pure tone hearing thresholds and DPOAEs were measured on these same subjects and were compared with age matched normal subjects who did not use a PMS.
Phase I: Measurement of output SPL of PMS
Only the experimental group participated in this phase of the study. Output SPLs produced by the PMS were measured in the subject's ear canal using a probe microphone. A commercially available real ear probe microphone measurement system (Siemens Unity Ver 2.7) was used for this purpose. Insertion depth of the probe was 28 mm from the tip of the tube to tragal notch. This insertion depth is the standard insertion depth used while doing real ear probe microphone measurements in adults.  All the measurements were made with the subject's own PMS and earphones. After placing the probe tube in the ear canal, the earphone was placed. Subjects were asked to play one of their frequently played songs. Output SPLs were measured in three different conditions
by asking the subjects to adjust the volume control to their preferred listening setting in quietby asking the subjects to adjust the volume control to their preferred listening setting in the presence of 65 dB SPL bus noiseby setting the volume control to maximum level.
Position of the probe microphone was not changed between any of the measurements. All the measurements were done in a semi-acoustically treated room. Ambient noise in the test environment ranged between 40-45 dB SPL during the measurements.
Output SPLs were measured at individual frequencies from 125 Hz to 8000 Hz in octave and mid octave intervals. These ear canal SPLs were converted to equivalent diffuse field SPLs to which an ear was exposed, by subtracting the transfer function of the open ear.  The transfer function of the open ear was obtained by calculating the difference between the reference location at the opening of the ear canal and the probe microphone SPL near the eardrum for a sweep frequency tone presented at 60 dB SPL. The output SPLs at individual frequencies were converted to dBA values by adding the A-weighting adjustment values. The overall SPL in dBA was then determined by logarithmically adding dBA values at each frequency. From this data 8 hour equivalent continuous A-weighted noise exposure (L eq8h ) was calculated following the same procedures as Williams.  This was mathematically calculated from the equation
L eq8h = L T + 10 log 10 [T/8]
where L eq8h is the 8 hour equivalent continuous noise exposure, T = exposure time in hours, L T = Level of noise exposure over the time period T.
Phase II: Pure tone audiometry and DPOAE
Pure tone audiometry
Pure tone audiometry was done using a calibrated audiometer (GSI 61 with TDH 50 head phones fitted with supra aural ear cushions). All the subjects were screened at 15 dB HL in octave frequencies between 250 Hz to 1 kHz. Pure tone thresholds were measured at octave and mid-octave frequencies from 2000 Hz to 12000 Hz using the modified version of Hughson and Westlake procedure. This approach was utilized since previous research has shown that frequencies above 2 kHz are more sensitive to noise exposure and will get affected first. , Both experimental and control groups participated in this experiment.
Distortion product otoacoustic emission
Both the experimental and control group participated in this experiment. A computer based DPOAE analyzer (GSI AUDERA) was used to record DPOAEs. DPOAEs were recorded for seven pairs of frequencies in which f2 was at 1031, 1594, 2098, 3152, 4184, 4816, 6340 and 7277 Hz. The f2:f1 ratio was 1.20. , In this article, the DPOAE data is represented with reference to f2. The L1/L2 was 65/55 dB SPL. Data acquisition lasted for 30 s for every frequency irrespective of signal to noise ratio. A frame was rejected if it exceeded the 30dB SPL rejection criterion or if L1 and L2 differed by more than 2 dB from the target values. These test acceptance criteria and test rejection criteria were selected because they are consistent with the values that are employed in clinical settings with this instrumentation. ,, Subjects sat in a comfortable chair and the OAE probe was adequately sealed in the external ear canal and otoacoustic emissions were recorded with the above mentioned parameters. An intrinsic real ear intensity calibration was used to determine the quality of the DPOAE probe seal before starting the DPOAE measurement. DPOAE amplitude and SNRs at each frequency were measured.
Both pure tone audiometry and the DPOAE testing were carried out in an audiometric testing room where the ambient noise was within the permissible levels and the testing was done by a qualified audiologist with a bachelor's degree in audiology and speech language pathology. The order of Phase I and Phase II was counterbalanced among subjects to avoid any order effect and there was a gap of about 15 minutes between the two phases during which subjects completed the questionnaire. All the statistical analysis was performed using SPSS (version 13) software.
On average, young adult subjects listened through PMS for a period of 1.5 hours a day (range: 10 min-4 hours). Most of them reported that they listened to music while commuting to college/hometown. Of the 70 subjects, 62 subjects used mobile phones to listen to music, 4 used iPods to listen to music and 4 used locally made MP3 players to listen to music. As the iPod or MP3 players were fewer, these subjects were not included in further statistical analysis. Therefore, the results pertain to mobile phone PMS only. None of the subjects used cassette players or CD players to listen to music. All the subjects used insert kind of headphones to listen to music. None of them reported any symptoms of temporary threshold shifts like blocking sensation, tinnitus after listening to music. Though the information regarding the different manufacturers and models was obtained, it was not used in data analysis as the main aim of the study was to see if young adults listen to music at potentially hazardous levels and not to compare the output produced by PMS of different manufacturers.
Phase I: Measurement of output SPL of PMS
PMS used by young adults mainly consisted of mobile phones, and very few used iPods and locally made MP3 players. [Figure 1] shows the mean L eq8h at preferred volume control settings for males and females for mobile phone users. An independent samples ' t -test' revealed no significance difference in mean L eq8h between males and females in all three listening conditions tested ( t = 1.43, 0.89 and 0.65 respectively for quiet, in presence of bus noise and at maximum volume, p > 0.05). Since there were no statistically significant differences between mean output SPLs preferred by males and females data from both the genders were combined for all further analysis.
[Figure 2],[Figure 3],[Figure 4] illustrate the mean output SPLs at preferred listening settings in quiet, in the presence of 65 dB SPL bus noise and at maximum volume control settings for mobile phones, iPods and locally made MP3 players respectively. The mean L eq8h were 73 dBA for mobile phones (range: 40 dBA to 93 dBA), 76 dB for iPods (range: 56 dBA to 86 dBA), and 79 dBA for locally made MP3 players (range: 70 dBA to 84 dBA), at subject preferred volume control settings in quiet. Listening in the presence of bus noise did not increase the output SPLs significantly. A paired ' t -test' did not show a significant difference between means L eq8h produced by mobile phones in quiet and in the presence of bus noise ( t = 1.71, p > 0.05). Statistical analysis was not carried out for the other types of PMS as the number of instrument users was limited. At maximum volume control settings the output levels increased compared to the subject preferred volume control setting. Paired ' t -test' revealed that L eq8h produced at maximum volume control settings were significantly more compared to other two listening conditions ( t = 12.8, 13.2, P eq8h at the preferred volume control settings obtained in the presence of 65 dB SPL bus noise for individual subjects. As can be seen from the [Figure 5], a majority of the subjects (70%) listened to music at less than L eq8h of 80 dBA. Nineteen mobile phone users, 1 MP3 player listener and 1 iPod listener listened to music at higher than L eq8h of 80dBA.
Phase II: Pure tone audiometry and DPOAE
Pure tone audiometry
Based on the results of Phase I, subjects in the experimental group were divided into two groups: Individuals who used PMS at L eq8h of less than 80 dBA and individuals who used PMS at L eq8h of more than 80 dBA. Data from all three types of PMS users are pooled as the main purpose was to see if listening to music through PMS causes damage to auditory system or not. ANOVA was done to find out the significance of difference between mean pure tone thresholds of individuals who used PMS at L eq8h of less than 80 dBA , individuals who used PMS at L eq8h of more than 80 dBA , and individuals who did not use PMS. ANOVA failed to show any significant main effect of subject groups on pure tone hearing thresholds in both right [F = 2.7 (2,98) p >.05] and left ear [F = 1.8 (2,98) p >.05]. Descriptive analysis showed that none of the subjects who used PMS had pure tone thresholds more than 25 dB HL in 3 kHz to 8 kHz region in both ears. [Figure 6] shows the average pure tone thresholds of individuals who used PMS at L eq8h of less than 80 dBA , individuals who used PMS at L eq8h of more than 80 dBA , and individuals who did not use PMS.
Distortion product otoacoustic emission
All the subjects in the experimental group showed normal DPOAEs amplitudes and signal to noise ratios (re: Clinic normative values). ANOVA was done to find out the significance of difference between mean DPOAE amplitudes and SNR values between individuals who used PMS at L eq8h of less than 80 dBA , individuals who used PMS at L eq8h of more than 80 dBA , and individuals who did not use PMS. ANOVA did not show a main effect of subject groups on means of DPOAE amplitudes and SNR values at all the tested frequencies in both ears. [Figure 7] and [Figure 8] show DPOAE amplitudes and SNRs in different subject groups across different frequencies. Data is represented with reference to f2. Error bars show 1 SD variation.
Relationship between L eq8h and hearing measures
To evaluate the relationship between output SPLs and hearing thresholds Pearson's product moment correlation was carried out between subjects' pure tone thresholds as dependent variable and L eq8h measured in presence of bus noise as the independent variable. Data from all the PMS users were pooled for this analysis. The results are shown in [Table 1]. There was a significant positive correlation between hearing thresholds at 6000 Hz and exposed music levels, in both ears. Results of correlation analysis between exposure levels and DPOAE measures are shown in [Table 2] and [Table 3]. There was a significant negative correlation between DPOAE amplitudes and dBA L eq8h at 6340 Hz in both ears and at 2098 Hz in right ear. It can be inferred from [Table 3] that there was a significant negative correlation between DPOAE SNR and dBA L eq8h at 4816 Hz, 6340 Hz and 7277 Hz in right ear and at 4816 Hz in left ear. These negative correlations indicate that DPOAE amplitudes and SNRs were lesser in individuals who tend to listen to music at higher output levels.
The majority of the young adults studied here used their mobile phones to listen to music. Of 70 subjects 62 subjects used mobile phones to listen to music (88.6%), 4 used iPods to listen to music (5.7%) and 4 used local made MP3 players to listen to music (5.7%). On average, young adults listened to music through PMS for a period of 1.5 hours a day. Most of them reported that they listened to music when commuting to college/hometown. The mean dBA L eq8h at preferred volume control settings was 73 dBA for mobile phones, 76 dBA for iPods, and 79 dBA for local made MP3 players. These preferred listening levels are similar to what participants selected as "sounds best to you" in the Hodgetts et al.,  study or "medium/comfortable" in the Torre  study. Hodgetts et al.,  reported that participants increased the level of the music approximately 6 to 10 dB when either street noise or multitalker babble was added to the listening environment. But in the present experiments, with the addition of 65 dB SPL bus noise, participants did not increase the preferred output levels. This discrepancy between the results of the two studies may be due to intrinsic differences in the background noise source used in each study. Bus noise was utilized as a background noise as this simulates the most realistic condition replicating the environment where the majority of our participants used their PMS in daily life. However, it is surprising that when 65 dB SPL bus noise was added, subjects did not significantly raise the output levels of the PMS. Most likely, this is due to the fact that all the subjects used insert earphones and these earphones may have attenuated the bus noise reaching the ear. As a retrospective experiment we measured the attenuation created by the earphones on ten subjects who had participated in the original experiment. [Figure 9] shows the mean attenuation provided by the earphones. As can be seen from the figure, insert ear phones provided maximum attenuation of about 23 dB around 3 kHz. Due to the attenuation of mid-frequency bus noise, young adults may not have increased the preferred volume control settings in the presence of bus noise.
The majority of the individuals listened to music at an over SPLs less than 80 dBA even in the presence of 65 dB SPL bus noise. No evidence based definition exists for hazardous sound levels of music. As a substitute, standards for exposure to occupational noise have been proposed for use.  In India, the Ministry of Environment and Forests  has proposed a time weighted average level of 80 dBA for an 8 hour period per day as the maximum permissible limit. '5 dB exchange rule' has been proposed by the Ministry of Environment and Forests as a tradeoff between exposure time and sound level. Referencing this criterion about 30% individuals in a group of 70 young adults listened to music above the safety limits prescribed by Ministry of Environment and Forests. This is an alarmingly large proportion as one in three individuals who listen to music through PMS may be putting themselves at risk for permanent noise induced hearing loss if exposed for extended periods of time (years).
However, the results of pure tone audiometry and otoacoustic emissions showed that there are no statistically significant differences between pure tone hearing thresholds and amplitudes of DPOAEs of individuals who use PMSs at L eq8h of less than 80 dBA, , individuals who used PMS at L eq8h of more than 80 dBA and those who don't. Readers should be cautious about interpreting these results of the present study as
the measurement was done when subjects were listening to one of their favorite songs and the listening level cannot be generalized to different kinds of music that individuals may be listening to during any particular day.all participants in the present study were young adults and were using PMS for a period of two years or more. These young adults are less likely to show the signs of noise induced hearing loss due to their limited length of listening time and because of their age.we measured the output levels in the ear canals of young adults. These results cannot be generalized to children as output levels produced in their smaller ear canals may be significantly more than what is reported in this study.
However, the results of correlation analysis between exposure levels and audiological measures are interesting. A significant positive correlation between hearing thresholds and exposed music levels at 6000 Hz in both the ears suggests that individuals who listened to music at high levels tend to have higher pure tone hearing thresholds at 6000 Hz. A negative correlation between DPOAE measures at high frequencies and exposure levels suggest that individuals who listened to music at higher levels had reduced DPOAE amplitudes and SNRs. It should be noted that all individuals who used PMS had "clinically normal" hearing thresholds, DPOAE amplitudes and SNRs and there was no group difference between these measures among individuals who uses PMSs at L eq8h of less than 80 dBA , individuals who used PMS at L eq8h of more than 80 dBA and those who don't. Miller et al.,  reported that amplitudes of DPOAEs are more sensitive to noise induced hearing loss than pure tone hearing thresholds. Our results show that listening to music through PMS at preferred volume control settings for a period of two years may not result in "clinically significant" elevation of hearing thresholds or reduction of DPOAE amplitudes and SNRs. The correlations between output levels and auditory measures suggest that listening to music through PMS at higher intensities may cause subtle pre-clinical damage to the auditory system and over the years such behavior may be hazardous to hearing. However, these long-term effects need to be studied through long-term controlled experiments.
Authors would like to thank Dr. T.A. Subba Rao, Principal, Dr. M.V Shetty college of speech and hearing and management for all the support.
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