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|Year : 2011
: 13 | Issue : 51 | Page
|Symphony orchestra musicians' use of hearing protection and attenuation of custom-made hearing protectors as measured with two different real-ear attenuation at threshold methods
KH Huttunen1, VP Sivonen2, VT Pöykkö1
1 Institute of Clinical Medicine, Department of Otorhinolaryngology, University of Oulu, Oulu, Finland
2 Department of Signal Processing and Acoustics, Aalto University, Helsinki, Finland
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|Date of Web Publication||1-Mar-2011|
Despite a high level of sound exposure and a fairly large selection of earplugs available, musicians have often been reported to use personal hearing protectors only seldom. For better hearing conservation, it is important to identify and eliminate the causes for the low motivation to use hearing protection. We explored the usage rate of custom-molded musician's earplugs (ER-15) among 15 symphony orchestra musicians with a questionnaire, and measured the attenuation properties of their earplugs with a Real-Ear Attenuation at Threshold (REAT) procedure in a sound field. Earplug use was found to be low, and the musicians reported that earplugs hampered listening to their own and their colleagues' playing; earplugs affected either timbre or dynamics, or both. Additionally, several reasons related to discomfort of use were itemized, but the musicians who consistently used their earplugs did so in spite of problems with use. The REAT values obtained in sound field were relatively close to the manufacturer's nominal specifications, being 13.7 dB, on average. In the frequency range studied (0.125-8 kHz), individual variation in REAT was, however, up to 15 dB across the measured frequencies. Fluctuation in attenuation might be related to low use of hearing protectors, and REAT measured at fixed center frequencies may be too robust a method to uncover it. We therefore tested 10 additional subjects to find out whether a sweeping signal used in Bιkιsy audiometry would bring more detailed information on earplug attenuation. Mean attenuation was found to be somewhat closer to the nominal attenuation of the ER-9 and ER-15 earplugs up to about 1 kHz, whereas REAT measurements in sound field revealed more even attenuation at frequencies between 1 and 6 kHz. No significant association was found between earplug attenuation properties and earplug use. It was concluded that support and determination to get accustomed to hearing protector use are important factors in hearing conservation.
Keywords: Békésy audiometry, earplug, hearing conservation, measurement
|How to cite this article:|
Huttunen K H, Sivonen V P, Pöykkö V T. Symphony orchestra musicians' use of hearing protection and attenuation of custom-made hearing protectors as measured with two different real-ear attenuation at threshold methods. Noise Health 2011;13:176-88
|How to cite this URL:|
Huttunen K H, Sivonen V P, Pöykkö V T. Symphony orchestra musicians' use of hearing protection and attenuation of custom-made hearing protectors as measured with two different real-ear attenuation at threshold methods. Noise Health [serial online] 2011 [cited 2018 Dec 15];13:176-88. Available from: http://www.noiseandhealth.org/text.asp?2011/13/51/176/77210
| Introduction|| |
Music as a hobby or a profession often exposes the listener to sound levels that are hazardous to hearing. While the vulnerability of the ear is somewhat individual in long-term high-level exposure, music-induced tinnitus, hyperacusis (a reduced tolerance of suprathreshold sounds), diplacusis (perception of a single sound as double), temporary threshold shift (recoverable worsening of hearing thresholds) and permanent hearing impairments, especially at higher frequencies, have often been reported after years of continual high-level exposure. ,,,, As a countermeasure, many types of hearing protectors are available on the market, including (1) disposable, foam earplugs, (2) generic, premolded earplugs, and (3) custom-molded, so-called musician's earplugs (such as the ER-15 and ER-25 by Etymotic Research Inc., Elk Grove Village, IL, USA).
The pinna, concha, and ear canal of a normal ear act together as a resonator, which amplifies sounds emanating from the environment to the eardrum. The resonance peak frequency of an open ear canal of an adult human is between about 2.5 and 3.3 kHz, depending mainly on the length of the ear canal. However, when the ear canal is occluded, for example, by an ear mold, by an in-ear hearing aid, or by an earplug, its resonance will be altered.  In essence, the quarter-wave resonance of an open ear canal turns into half-wave resonance in the fully occluded case. 
Foam earplugs dampen the natural resonance of the ear canal and typically provide increasing attenuation as a function of frequency. Consequently, they are often reported to make music sound muffled. In the musician's earplugs, the diaphragm of the earplug's attenuator module functions as an acoustic compliance, while the volume of air in the sound bore of the ear mold serves as an acoustic mass.  The aim of this design is to provide acoustic amplification at higher frequencies, and therefore, to achieve uniform attenuation across frequencies.
| Use of Hearing Protectors|| |
Despite high sound levels and the availability of hearing protection, only 6% of the Finnish classical music orchestra musicians studied (N = 196) reported always using (mostly custom-molded) hearing protectors during orchestra rehearsals and/or performances.  The corresponding figure among Danish orchestra musicians (N = 145) was 15%.  Of German symphony and opera orchestra musicians (N = 429), 8% reported using custom-molded hearing protectors often or very often,  and only 1-2% reported using other types of hearing protectors. Higher prevalence figures related to the use of (mostly disposable) hearing protectors have been reported by musicians of Dutch symphony orchestras.  Namely, 52% of the musicians studied (N = 245) in the Netherlands reported using hearing protection during orchestra rehearsals, 29% during concerts, and 36% during noisy leisure-time activities.
Reluctance to use hearing protectors is a widely reported problem in many kinds of work environments ranging from construction work , to pop and rock bands  and classical music orchestras. ,, Among musicians, loss of monitoring ability, alteration of timbre (sound characteristics that distinguish a certain sound from other sounds of the same pitch and loudness), uncomfortable fit, a feeling of pressure from the earplugs  and deteriorated localization ability  have been mentioned as the most important reasons for non-use of hearing protection. Specifically, according to Laitinen (2005),  Cunningham et al. (2006)  and Wright Reid (2001),  the most often reported reasons for not using earplugs are that they hinder monitoring of the musician's own performance. More specifically, the reasons include poor overall sound quality and difficulty in monitoring dynamics (e.g., when very loud parts of music are followed by very quiet parts), difficulty in hearing others perform,  and discomfort or difficulty in inserting the plugs into the ear canal. Many musicians also deny the need to use hearing protection or perceive earplugs as a stigma or as having a negative effect on appearance.
Contradictory reports have been published on the association between hearing symptoms (tinnitus, hyperacusis, hearing impairment) and commitment to hearing protection use. In a study by Laitinen (2005),  hearing protectors were used more often by classical orchestra musicians with hearing symptoms than by those without symptoms. On the other hand, Schmutziger, Patscheke and Probst (2006)  and Cunningham with his colleagues (2006)  reported a more pronounced hearing impairment to be associated with the absence of hearing protection among pop/rock musicians. However, Lusk and colleagues (1998)  did not find any correlation between the use of hearing protector and self-reported hearing impairment among 398 construction workers. The discrepancy between the results of different studies may be related to skewed sampling of the subjects, to musicians with hearing impairment possibly denying their problems and hence avoiding participation (as discussed by Jansen et al., 2009  ), or to those with hearing problems eagerly volunteering as subjects (as discussed by Cunningham et al., 2006  ).
Could the difficulties with the use and non-use of hearing protectors be related to the attenuation characteristics of custom-made earplugs? According to reports on clinical experience, , custom-made earplugs designed to produce uniform or near uniform attenuation of, for example, 15 or 25 dB across all frequencies often do not prove to work linearly; attenuation may vary as much as from 0 to 35 dB.  This may result from possible failures in taking of the wax ear impressions and/or making the ear molds. According to Chasin and Chong (1991),  ER series earplugs are also prone to variability in attenuation due to the acoustic amplifier of the plugs.
| Measurement Methods for Examining Attenuation of Hearing Protectors|| |
Several different measurement methods exist for exploring the sound attenuation of earplugs and earmuffs. Acoustic test fixtures (ATFs; in this case, artificial heads) have been used in a laboratory environment to measure hearing protectors. The main differences between the test results acquired with an ATF and data collected from human subjects are bone conduction, the occlusion effect (blocking of the ear canal, amplifying low frequencies and making some sounds hollow and echoing) and physiological masking in the ear canal, which are often present when subjective measurements are made with users of hearing protectors.
There are two commonly used methods of testing the attenuation abilities of hearing protectors on human subjects.  The first one published is the subjective psychoacoustic "Real-Ear-Attenuation at Threshold" procedure (REAT; ANSI S12.6-1997).  REAT is a subjective method of determining the attenuation of a hearing protector by subtracting the open-ear (unprotected) hearing threshold from the occluded ear threshold (with the hearing protector in place). The other method is the objective "Microphone-in-the-Real-Ear" (MIRE). 
Both techniques typically utilize a loudspeaker to present a sound field stimulus. In a sound field, however, control over the exact stimulation at the subject's ear is somewhat compromised. This is due to difficulties in keeping the position of the subject's ear exactly constant between measurements with and without an earplug. Furthermore, problems with position-dependent stimulation are increased when utilizing these measurement techniques in a room with reflected sounds. This fact has often been discussed in relation to procedures for sound field audiometry (see, e.g., Walker et al., 1984).  The REAT technique, based on subjective responses, is always a binaural measurement, while the results from the MIRE method can be analyzed one ear at a time, thus possibly yielding more information on the acoustic functioning of a single earplug. Furthermore, although the REAT technique better resembles the use of earplugs in field conditions, it is susceptible to subjective biases in determining hearing thresholds. 
Both measurement techniques also have their even more specific pros and cons. For example, MIRE measurements do not account for bone-conducted transmission of sound through the skull, which can lead to overestimation of thresholds when hearing protectors providing good attenuation are tested. While the REAT measurement takes bone conduction into account, the technique is susceptible to physiological noise (breathing, heartbeat, blood flow, and stomach rumble) in the ear canal when low signal levels (close to the threshold of hearing) are used. This means that various noise sources can mask the weak signals the person being tested is listening to. Toivonen and colleagues (2002)  reported that with the REAT measurement, attenuation provided by a hearing protector is greater than when a miniature microphone is used. This is most likely due to air leaks and difficulties in positioning the microphone together with the earplug in the ear canal to achieve proper insertion in MIRE measurements. , According to a general conclusion, REAT measurements are reported to be more accurate than MIRE measurements.
Musician's earplugs are measured with standard real-ear measurement protocols. , Comparisons between real-ear in situ measurements (utilizing probe tubes of different hearing aid fitting instruments) and MIRE measurements have also been made, but according to our own testing experiences with a continuous sweeping tone (Huttunen et al., unpublished data) and, for example, a thesis by Custer (2007),  there is a significant risk of earplug leakage in the lower frequencies when probe tubes are used. This often results in an underestimation of the real attenuation of the earplugs.
In Finland, neither medical audiologists working in the tertiary setting of the public healthcare system nor those in the private sector conduct fitting of musician's earplugs. In Finnish legislation related to medical care, the fitting of personal protectors has been included as a measure of preventive care, and hence, it does not belong to the domain of tertiary care. Instead, musicians and other people interested in custom-made hearing protectors get their ear impressions taken at private companies, and they usually receive their earplugs by post from the ear mold laboratory. The attenuation properties of the earplugs are therefore usually not measured. In this situation, no one is in the role of taking care of quality control and quality assurance of the whole custom-earplug fitting process.
As musicians so commonly complain about discomfort in using their rather expensive, custom-molded earplugs, we wanted to examine in Study 1 whether the amount of use of hearing protection, as assessed with a questionnaire, is associated with perceived discomfort of earplugs and possibly with their nonlinear frequency characteristics of attenuation. Further, in Study 2, we wanted to explore if Békésy audiometry with a continuous sweep technique would bring more information about the frequency dependency of earplug attenuation (when compared with REAT measurements, which conventionally utilize warble tones or bands of noise with lesser frequency resolution). Additionally, we were interested in the reliability of Békésy audiometry results in measuring earplug attenuation.
| Study 1|| |
We wanted to explore factors that hamper hearing protection. According to Chesky et al. (2009),  there is a lack of prospective field studies of earplug use among musicians. If the main reasons for the low rate of hearing protection are found, it would be easier to plan effective hearing conservation programs and target them in a proper way.
Approval of the study plan was received from the ethical board of the Northern Ostrobothnia Hospital District, and the subjects gave their written informed consent to the study.
Altogether, 15 musicians all from Oulu Symphony Orchestra who owned a pair of musician's earplugs volunteered to take part in REAT measurements. The subjects filled out a questionnaire exploring, for example, the degree of hearing protector use both at work and during their leisure time, experiences with hearing protection and with cleaning the earplugs, as well as reasons for possible non-use.
These subjects, 10 males and 5 females, were, on average, 43 years old (range 31-54 years) and had played a variety of instruments from small and large strings (majority) to brass and woodwinds in the Oulu Symphony Orchestra for an average of 19 years (range 3-31 years). They were provided with custom-made earplugs (ER-15 by Etymotic Research Inc., Elk Grove Village, IL, USA) mostly by the employer, while some had bought their hearing protectors themselves while working in other orchestras. At the time of the tests, 21 of the orchestra members owned a pair of ER-15 earplugs; hence, the 15 subjects tested represented 71% of those who had earplugs in the orchestra. The subjects had owned their earplugs generally for 1-2 years (median 15 months).
An otoscopic examination, with wax removal when necessary, was done on all the subjects before the measurements. A clinical audiometer (Madsen Midimate 602) equipped with earphones (TDH-39 with MX-41/AR cushions) was calibrated according to the ISO 389-1 standard (ISO, 1991)  and used for all the hearing tests, which were performed by a trained audiometrician in a standard sound-treated audiometric cabin. Air and bone conduction pure tone thresholds were obtained at frequencies from 0.125 to 8 kHz using a descending/ascending technique of 5 dB steps (bracketing; ISO, 1989). 
The REAT technique was then used to examine the attenuation of the ER-15 custom-molded earplugs. The sound field hearing thresholds of the subjects from 0.125 to 8 kHz were first measured binaurally without hearing protectors. The measurements were conducted in a sound-treated room using a warble tone (frequency modulation by a 3.7-Hz triangle wave, ±5% from the center frequency). Hearing thresholds were then obtained after (usually) an experimenter-supervised fitting of the earplugs. In the experimenter-supervised fitting, the ER-15 earplugs were put on by the subjects themselves and insertion of the plugs was then checked by an audiometrician. If the subjects had unconquerable difficulties in inserting the earplugs, they were assisted by the audiometrician. The REAT was calculated by subtracting the hearing threshold measured with earplugs from the one measured without a hearing protector for each test frequency. The difference between the thresholds indicated the attenuation achieved with the plugs at each frequency. Test-retest reliability of the REAT measurements was not performed.
Results and discussion
In pure tone audiometry of the 15 musicians studied, the mean air conduction pure tone average over the frequencies 0.5, 1, 2 and 4 kHz (PTA 0.5-4kHz ) was 5.25 dB HL (hearing level; range −2.50 to 13.75 dB) in the right ear and 7.00 dB HL in the left ear (range −3.75 to 20.00 dB). Seven subjects (47%) had at least one air and/or bone conduction threshold that exceeded 20 dB HL; the worst thresholds, from 55 to 65 dB HL, were found at the higher frequencies [Figure 1]. PTA 0.5-4kHz in the sound field [Figure 2] was, on average, 8.50 dB (range 2.50-16.25 dB).
|Figure 1: Average pure tone thresholds of the 15 musicians with their minimum and maximum values, obtained with earphones (right ear: dotted line, left ear: dashed line)|
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|Figure 2: Average sound field hearing thresholds of the 15 musicians with ±1 standard deviation (dashed lines) at each frequency|
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Use of and experiences with musician's earplugs
Despite elevated hearing thresholds at several frequencies in some subjects, the rate of earplug use was low; only one to three of the 15 musicians reported using their ER-15 earplugs always or nearly always (>95% of the time) either during orchestra rehearsals or concerts, when rehearsing alone or when teaching [Figure 3]. One musician used the earplugs often (>80% of the time) during orchestra rehearsals and another used them during rehearsing alone. The main reasons mentioned for non-use were an occlusion effect, difficulties in monitoring one's own playing and oversensitivity to sounds immediately after earplug removal. The occlusion effect hampered the use of earplugs, especially of musicians playing wind instruments. Almost half (46%) of the musicians had problems in using their earplugs. Most of the problems itemized were related to monitoring one's own playing. Itching, a feeling of pressure and difficulties in inserting the earplugs into the ear canal were also mentioned. According to Fisher's Exact Test, the existence of at least one hearing threshold worse than 20 dB HL was not associated with the use of ER earplugs while rehearsing, performing or teaching music. Hence, if a musician had one or more thresholds worse than 20 dB HL, he or she did not use ER earplugs more often than those with all hearing thresholds equal to or better than 20 dB HL. To run the Fisher's Exact Test, variables related to earplug use were dichotomized (no or occasional use/use over 80% of playing or teaching time). Fisher's Exact Test also showed that the low rate of use of custom-molded hearing protectors was not statistically significantly dependent on the musicians' view on the general usefulness of ER-15 earplugs in hearing protection, or perceived problems in their use (reported as yes or no). This indicates that the subjects who consistently used their earplugs did so in spite of perceived problems in their use.
|Figure 3: Use of ER-15 earplugs by the 15 musicians in different situations related to music exposure|
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A majority (60%) of the subjects considered their earplugs slightly or very uncomfortable during use, whereas 40% (6 out of 15) considered them rather comfortable. According to Fisher's Exact Test, use of ER earplugs in different situations was not statistically significantly dependent on the musicians' views on how comfortable they were in use. The subjects were asked with a multiple-choice question whether the use of ER-15 earplugs possibly negatively affected hearing of music and speech. In the questionnaire, hearing of timbre and dynamics was asked in particular. Feelings of distorted or missed timbre/nuances and/or dynamics of music produced by colleagues were reported by 80% of the subjects, and 100% reported that perception of timbre/nuances and/or dynamics of their own playing was affected [Table 1].
|Table 1: Difficulties in hearing music and speech as reported by 15 musicians when using their ER-15 earplugs |
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McNemar's Test did not reveal any statistically significant difference in ER-15 earplug use during orchestra rehearsals and concerts as a function of problems in perceiving ones' own playing or in perceiving others' playing. This result probably arose from the low variability in the use of earplugs (i.e., so few musicians used their earplugs regularly) and the fairly large prevalence of problems in hearing the performance of other musicians. Similarly, the frequency of earplug use was not associated with the reported complaints of hearing the speech of the conductor.
Regardless of so many difficulties in using earplugs at work, 40% of the subjects used their custom-made earplugs during their leisure time, when mowing the lawn, when doing repair work and while using public transport, for example. Instead of ER-15 earplugs, three musicians (20%) used other types of hearing protectors, mostly disposable plugs, during leisure-time noise exposure. According to McNemar's Test, the use of ER earplugs at work in the orchestra or while playing alone and during leisure time was not dependent on one another.
REAT measurements of the ER-15 earplugs
When we examined the attenuation properties of the custom-molded hearing protectors of the subjects in the sound field, we found that the real-ear attenuation of the ER-15 earplugs was, on average, 13.7 dB. The group mean was fairly linear (mean from 11.3 to 15.7 dB, SD from 2.4 to 5.6 dB) across the measured frequencies. The average individual range in REAT across the measured frequencies was 7.8 dB, varying from 5 to 15 dB (SD 4.3 dB) [Figure 4].
|Figure 4: Individual, binaural REAT results of ER-15 earplugs (in dB). Sound field measurements with and without hearing protectors were performed in 5 dB steps. Consequently, the REAT values were calculated in 5 dB steps|
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Compared with the attenuation data for ER-15 earplugs that the manufacturer has reported for each frequency (however, not at 3 and 6 kHz  ), the individual REAT measurement results were, on average, 3.8 dB (min 0.8 dB at 4 kHz, max 6.3 dB at 0.5 kHz) less than those specified by the manufacturer. This difference is probably due to the different measurement technique; the figures given by the manufacturer are obtained with an ATF, while ours were based on psychoacoustic measurements. The results of the REAT measurements can be found in [Table 2].
|Table 2: Manufacturer's specifications of the attenuation ability of the ER-15 earplugs (in dB), and the group-level real-ear attenuation at threshold measurement results by measured frequencies of the 15 orchestra musicians studied |
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We were also interested in knowing whether the uniformity of attenuation at each frequency was associated with the frequency of use of the ER earplugs at work. According to Spearman's correlation coefficients, neither the average REAT nor the standard deviation of the REAT values of single subjects was statistically significantly associated with the frequency of ER earplug use while playing in the orchestra, alone, or, for example, when teaching music. However, when examining the REAT values at adjacent frequencies, we noticed that seven subjects (47%) had a difference of 10 dB or more in REAT values at adjacent frequencies at least once. We found a tendency of these subjects to use their earplugs either occasionally or never [Figure 5]. Additionally, we found a similar tendency in the range of attenuation: the larger the range of the REAT values of an individual subject across the measured frequency range (0.125-8 kHz), the less frequently the ER-15 earplugs were used [Figure 6]. These were interesting findings, although Kruskall Wallis test did not reveal a significant difference in the use of earplugs between those musicians who had and those who did not have such a large fluctuation in attenuation at adjacent frequencies or in the range of attenuation across the measured frequency range.
|Figure 5: Use of ER-15 earplugs by the 15 musicians as a function of REAT fluctuation of 10 dB or more at adjacent frequencies in individual subjects (y-axis: 1 = always or nearly always, i.e., over 95% of playing time, 2 = often i.e., over 80% of playing time, 3 = occasionally, 4 = never)|
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|Figure 6: Use of ER-15 earplugs by the 15 musicians as a function of the range of their measured REAT values (x-axis: range in REAT values of individual subjects, y-axis: 1 = always or nearly always, i.e., over 95% of playing time, 2 = often, i.e., over 80% of playing time, 3 = occasionally, 4 = never)|
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In summary, several reasons for the low rate of earplug usage were uncovered. It has to be noted, however, that as many as 80% of the musicians complained that the ER-15 earplugs distorted the timbre and/or dynamics of music in the orchestra. As already slight hearing problems and the properties of hearing protectors affect musicians' performance and comfort when playing, we wanted to examine in Study 2, specifically, whether hearing and earplug attenuation measurements with a continuous signal using Békésy audiometry would provide more detailed information on the attenuation properties of earplugs than conventional sound field REAT measurements do.
| Study 2|| |
The REAT measurements of Study 1 were performed at nine fixed frequencies between 0.125 and 8 kHz. The aim of Study 2 was to assess the properties of earplug attenuation using a finer frequency resolution with the help of Békésy audiometry. We wanted to investigate whether there are peaks or dips in the attenuation, which a standard REAT measurement would not reveal. If such phenomena were to exist, they might negatively affect listeners' use of their earplugs.
In Békésy audiometry, the stimulus frequency is automatically increased at a fixed rate while the person tested continuously manipulates the stimulus level to keep it barely audible. The subjects are asked to press a button when they hear the test stimulus and to release it when they no longer hear the stimulus. In output, this results in a zigzagging curve approaching the sound pressure level that corresponds to the subject's hearing thresholds. Each threshold is calculated as the mean of the midpoints of the runs recorded by the audiometer.
Due to frequency-dependent standing waves and reflected sound in typical clinical test facilities,  performing Békésy audiometry in a sound field is generally not feasible, and the use of calibrated headphones is therefore advisable. Although there is variability in positioning the headphones on the subject's head, it is relatively safe to assume that use of headphones retains better control over at-the-ear stimulation than does testing in the sound field. Additionally, the use of headphones allows the experimenter to disregard any position dependencies of the subjects in the test room between repeated threshold measurements (e.g., before and after earplug insertion).
For REAT measurements with fixed frequencies, Carter and Upfold (1993)  and Franks et al. (2003)  have suggested that the average REAT results using the sound field method can be predicted fairly well from the average results obtained with TDH-49 earphones combined with circumaural headphone cups. Furthermore, Berger (1999)  considered large circumaural cups with built-in speakers as the most practical equipment for REAT measurements. However, to achieve comparable results of the frequency characteristics of earplug attenuation between headphones and a loudspeaker in a sound field, the headphones used need to be as immune as possible to the impedance difference in the ear that is noticeable between an open ear canal and an ear canal occluded with an earplug. Thus, to achieve the required immunity, the acoustic output impedance of the headphones needs to be relatively low. This characteristic is often found in open-back headphone designs (for information on headphone acoustics, see, e.g., Borwick 2001  ).
The suitability of headphones for REAT measurements may be tested by measuring their frequency response at the blocked ear canal entrance and at the eardrum. This procedure is well known in the field of binaural synthesis, where headphones are used in the reproduction of spatial sound captured at the blocked meatus. Ideally, the headphones used should be of the free-field equivalent coupling (FEC) type,  that is, differences in headphone response between different measurement points in the ear canal should be comparable with changes noticed when the same signal is reproduced with a loudspeaker. When the results are comparable, utilization of FEC headphones in assessing earplug attenuation yields a result similar to a standard REAT measurement performed in a sound field.
In the ER series, ER-15 earplugs are designed to produce a linear 15 dB attenuation across all frequencies, whereas ER-9 earplugs have reduced diaphragm stiffness for attenuation of about 9 dB at low frequencies and 14-15 dB at high frequencies.  In our Study 2, in addition to using a finer frequency resolution with Békésy audiometry, we explored the possibility of using open-back, circumaural, high-fidelity headphones to assess the frequency characteristics of both ER-9 and ER-15 earplugs.
A group of subjects (N = 10, six males and four females) consisting of musicians, students of music and persons engaged in listening to music and/or playing an instrument as a hobby volunteered for tests in which the attenuation of the ER series earplugs was examined with a REAT measurement employing Békésy audiometry. The mean age of these subjects was 35 years (SD 12, range 22-55 years). They had used their individually fitted ER-15 plugs, on average, for 3 years, and their current custom-made ear molds were, on average, 2.5 years old. Nine of the subjects were free from loud sound exposure for at least 12 hours preceding the audiometric measurements. For one subject, only 1 hour had elapsed since the last sound exposure in the form of orchestra rehearsals. A sound-insulated, double-walled audiometric cabin was used as a test environment. The background noise level in the cabin was measured with a sound level meter (Brüel and Kjaer 2260 Observer, Brüel and Kjær Sound and Vibration Measurement A/S, Nærum, Denmark), and it was found to comply with the maximum permissible ambient sound pressure levels in 1/3 octave bands for sound field audiometry (ISO 8253-2  ). The subjects were seated in the cabin with their back toward the monitoring window from which the experimenters could see them.
After an otoscopic examination, and wax removal when necessary, the air conduction thresholds of the subjects were first determined without earplugs by using a clinical audiometer (Madsen Orbiter 922, GN Otometrics A/S, Taastrup, Denmark) attached to earphones (TDH-39 mounted in MX-41/AR cushions). The audiometer was calibrated according to the ISO 389-1 standard  prior to the examinations. Hearing thresholds were determined according to the ISO 8253-1 standard  at frequencies from 0.125 to 8 kHz using a descending/ascending technique of 5 dB steps. We used this conventional audiometric measurement as a means of quality control for the Békésy audiometry. It was found that these hearing thresholds were in fair agreement with those obtained with Békésy tracing. Altogether, four subjects had at least one threshold worse than 20 dB in one or both ears. The mean PTA 0.5-4kHz of the 10 subjects was 4.4 dB HL (range −2.5 to 22.5 dB) in the right ear and 4.1 dB HL in the left ear (range −2.5 to 22.5 dB).
For Békésy audiometry, the clinical audiometer was calibrated (in 1/6 octave bands) with circumaural, open-back headphones (Sennheiser HD-650, Sennheiser electronic GmbH and Co. KG, Wedemark, Germany) according to the ISO 389-8 standard.  On the basis of a series of acoustic measurements (presented in detail in Appendix), we determined that the open-back headphones were suitable for measurement of earplug attenuation. When assessing the earplugs with the REAT technique over headphones, half of the subjects were first examined monaurally with Békésy audiometry without hearing protectors, and then with hearing protectors. Half of the subjects were examined in reversed order. Testing the attenuation of the ER-9 and ER-15 earplugs was performed in a counterbalanced order across the subjects. As the last test of the session, test-retest measurements were also performed, for half of the subjects in the right and for the other half in the left ear. Throughout the testing, the test ear was changed so that the order of the testing was either right-left-left-right, etc., or left-right-right-left, etc. The aim of this counterbalancing was to level out possibly emerging decrements in vigilance during the course of the test session, which lasted 2 hours, on average, for each subject.
For the Békésy audiometry, a previously unused pair of both ER-9 and ER-15 attenuation modules was attached to the subjects' custom-made ear molds that they had brought along to the test session. All but one subject inserted their earplugs without the experimenter's help. In all cases, the insertion result was checked when putting the headphones on the subject for audiometry. As recommended in the case of musicians, an extended frequency area was included, and hearing thresholds from 0.125 to 12.5 kHz were tested monaurally in both ears with the Békésy continuous sweep frequency method (attenuation rate 2.5 dB/sec, 1 octave/min, 1 Hz frequency resolution). The duration of one sweep was 7 minutes and the subjects were given a short break after every two sweeps. A 1-minute practice round starting from 1 kHz preceded the actual testing.
The Békésy data obtained were digitized when transferred from the audiometer to a computer (with an RS-232 interface) for further analyses. The transferred raw data were saved as a text file, and possible breaks were corrected (imputed). Analyses of the measurement data were performed with Matlab V12 software. The original zigzagging Békésy curves were smoothed with a moving average function calculated between four contiguous points. The subject's thresholds were then determined using a critical-bandwidth resolution  of the cochlea. As in Study 1, the REAT was calculated by subtracting each hearing threshold measured with earplugs from the ones measured without hearing protectors.
Questionnaire data on the use of earplugs were also obtained. These data included questions on the duration of use of ER series hearing protectors, instrument(s) played, frequency of use of custom-molded earplugs, as well as reasons for possible non-use of the earplugs.
Results and discussion
The subjects reported using their custom-molded ER-15 hearing protectors most frequently during orchestra rehearsals and concerts [Figure 7]. During orchestra rehearsals, three musicians reported using their earplugs always or nearly always (>95% of the time), and one used them often (>80% of the time). Additionally, one used the earplugs always or nearly always and three used them often during concerts. According to Fisher's Exact Test, earplug use was not significantly different among these 10 subjects compared with the 15 musicians in Study 1. Seven of the subjects itemized reasons for non-use; four of them mentioned alterations in the music perceived or unpleasantness of use of the earplugs. Other responses were related to low sound exposure levels not seen as problematic and forgetting to take the earplugs along. Six subjects (60%) reported using their ER-15 earplugs during leisure time, most often at loud rock concerts (five responses), or while traveling or sleeping.
|Figure 7: Use of ER-15 earplugs by the 10 subjects in different situations related to music exposure|
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The mean attenuation properties of the ER-9 and ER-15 earplugs across the 10 subjects, obtained with Békésy audiometry, are plotted in [Figure 8]. The nominal attenuation properties of the earplugs, as reported by the manufacturer, are also plotted in the figure. The results of these 10 subjects matched the earplug attenuation properties announced by the manufacturer up to about 1 kHz fairly well. Additionally, the dip in the attenuation properties of both earplug models at around 8 kHz was well reproduced by the present results. There was, however, a difference of about 4 dB between our results and the nominal attenuation. These mismatches occurred mainly between 1 and 3 kHz, and for the ER-9 hearing protector, also at the lowest frequencies and at around 6 kHz. Slightly worse attenuation results between 1 and 3 kHz were noticed not only in the means, but also in the median values, indicating that the results were not strongly influenced by, for example, a single subject, but represented a more common phenomenon among the subjects. All in all, the mean attenuation of both earplug models was relatively flat.
|Figure 8: Mean attenuation of the ER-9 and ER-15 earplugs (measured with 10 subjects using Sennheiser HD-650 headphones in Békésy audiometry) compared with the manufacturer's specifications|
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All measurement types of earplug attenuation have their vulnerabilities, and caution is urged against over-interpretation of the present results. There may be several reasons for the deviations between the obtained mean and nominal attenuation characteristics. Firstly, the nominal attenuation has been tested by the manufacturer with the ATF (KEMAR) method.  In the standard REAT technique, which was used in Study 1, real subjects were used instead. Secondly, our headphones in Study 2 did not fully give the free-field equivalent frequency response, which further resulted in some inaccuracies in the results. The frequency dependence of these inaccuracies can be estimated from the lowermost panel of [Figure A1] in Appendix.
|Figure A1: The uppermost panel contains the results of five repeated measurements of the frequency response of the headphones (Sennheiser HD-650) measured in an anechoic chamber on an artificial head with adjustable ear canal length. The middle panel presents the average pressure division between the blocked ear canal entrance and the eardrum position for the headphones and for a reference loudspeaker. The bottom panel presents the ratio of the pressure divisions of the middle panel. See the text for details|
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In addition, all psychoacoustic hearing measurements are susceptible to subjective biases (see, e.g., Voix and Laville, 2009  ). Because the reproducibility of attenuation measurements may be compromised for many reasons, we wanted to check the reliability of our results. [Figure 9] depicts the test-retest difference of the ER-15 earplugs in Study 2. The thin lines stand for individual curves, while the thick line represents the median across all 10 subjects. For the most part, the individual curves were below 5 dB, and the mean of the medians across the frequencies measured was 2.5 dB. As the intrasession test-retest repeatability of the hearing thresholds of the same subjects, measured either in the free field or with headphones, has generally been found to be within 5-10 dB, ,, the repeatability of our Békésy audiometry results can be considered very good.
|Figure 9: Test-retest difference in attenuation response of the ER-15 earplugs measured in Békésy audiometry. The thin lines stand for individual data and the thick line represents their median|
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| General Discussion and Conclusion|| |
Questionnaire data regarding the use of ER series earplugs and their attenuation characteristics obtained with two different measurement methods are reported in the present study. The attenuation of the ER-15 earplugs obtained with the two methods (standardized REAT measurement in Study 1 and Békésy audiometry with headphones in Study 2) is plotted for comparison in [Figure 10].
|Figure 10: Mean and standard deviation of the attenuation of the ER-15 earplugs obtained in a sound field (REAT, N = 15 subjects) and via headphones (Békésy audiometry, N = 10 subjects), as well as the manufacturer's specification obtained with an artificial head. The error bars and the area between the two gray lines denote the inter-subject standard deviations of the attenuation measured in the sound field and with headphones, respectively|
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The results of the Békésy audiometry in Study 2 were closer to the nominal attenuations specified by the manufacturer at low frequencies and at 8 kHz, whereas the REAT measurements of Study 1 proved to be more even at higher frequencies up to 6 kHz. Overall, however, the deviations from the manufacturer's specification were not large. In sound field assessment, obtained attenuation is affected by both ears, and the risk of, for example, a poor seal (causing compromised attenuation) is therefore higher than in assessment concentrating on one ear at a time. However, inter-subject deviation in Study 2 was generally somewhat greater than that in Study 1. This finding may be related to higher requirements that the automated measurement technique (Békésy audiometry) poses on the vigilance of the subject. In the REAT measurements, slightly greater dispersion in the attenuation results of the earplugs at higher frequencies compared with lower frequencies was an expected phenomenon. Examination of higher frequencies in the sound field is more vulnerable to movements of the subject being tested due to the decreasing wavelength and the increased directivity of the loudspeaker.
It has to be noted, however, that it is not possible to draw definitive conclusions from [Figure 10] because of differences in data collection (the measurement technique used, measurement environment, subjects, a single, unused pair vs. used pairs of earplug attenuation modules, good fit of the earplug in the ear canal), described above. Additionally, the REAT measurements in Study 1 were performed in 5 dB steps, which may make the results somewhat crude compared with the fine-grained measurement results obtainable with the continuous sweep (both in amplitude and in frequency) in the Békésy audiometry.
For both the ER-9 and ER-15 earplugs, the standard deviation covers the nominal attenuation given by the manufacturer, which suggests that reasons other than average frequency-dependent attenuation must be considered. For a given subject, however, the attenuation of the earplugs varied up to 15 dB when measured in the sound field across the frequencies from 0.125 to 8 kHz (Study 1). Since the subjects in our Study 1 especially complained about distorted or missed dynamics and the timbre of music, we cannot therefore exclude the possibility of variation in the attenuation response being associated with the use/non-use of earplugs on an individual basis.
Even if the earplugs change the perceived dynamics and timbre of music, the key point in hearing protection is a strong motivation to use hearing protectors during sound exposure and a determination to get accustomed to a changed auditory input. Neitzel et al. (2008) have detailed factors related to hearing protector use.  They grouped different factors of belief, knowledge and use into an eight-component model. Reluctance to use hearing protection is a widely reported phenomenon among musicians, and no single cause related to either the attenuation properties of earplugs or factors of belief, knowledge or social situation seems to explain the low use of hearing protection. , The use of earplugs needs to be implemented in hearing conservation programs for musicians, and at least 2-3 months is often needed for the musicians to get accustomed to their use. Similarly, acclimatization to hearing aids has been reported to take several months. , Additionally, hearing aid use is often recommended to be started in quiet surroundings with a gradual shift in the use of amplification in noisier, more difficult listening conditions. The use of hearing protection often succeeds better if it is started while rehearsing alone, and only after that earplug use is started in orchestra rehearsals, and finally, in concerts. Musicians also need continuous support in the learning process of getting accustomed to wearing earplugs.
Training can help users of hearing protectors insert the earplugs deep enough in the ear canal to minimize or prevent the occlusion effect. Studies by Toivonen et al. (2002)  and Joseph et al. (2007),  for example, have shown that training in the use of formable and premolded earplugs improves attenuation achieved with the hearing protectors. Therefore, although the insertion depth issue is more related to the use of disposable foam plugs than custom-molded hearing protectors, training in proper earplug insertion also needs to be implemented in hearing conservation programs.
However, no matter how effective the attenuation of the hearing protectors is, they are effective only when used properly and almost always when significant noise exposure is present. For example, in the study of Neitzel and Seixas (2005),  construction workers used their hearing protectors only for 25% of the time that they were exposed to noise that exceeded 85 dBA. In this case, the effective hearing protection was less than 3 dB. Therefore, any kinds of obstacles to hearing protector use, whether they are uncomfortableness, attitudes (working culture underestimating the need for hearing protection and the results of noise exposure) or use of hearing protectors preventing proper performance of work assignments, should be identified, thoroughly examined, and when possible, eliminated. Better interventions and more research on the effectiveness of noise prevention intervention programs are also needed. 
In summary, our main findings are the following:
- Among the musicians studied, use of earplugs was not very active and not related to any single background factor explored.
- Based on our verification measurements, open-back headphones may be used for measurement of earplug attenuation.
- Frequency response of ER-9 and ER-15 earplugs was fairly flat although the attenuation amplitudes in both REAT measurement types were about 4 dB lower compared to the nominal attenuation.
We therefore conclude that attenuation response of special earplugs developed for musicians is rather even and does probably not explain their low use among musicians. Because musicians complain of problems in hearing timbre and dynamics of music when using earplugs, special effort must be taken to help them to get accustomed to earplug use and in this way support hearing protection.
Artificial-head measurements of the headphones utilized in Study 2
To investigate the suitability of the Sennheiser HD-650 headphones for REAT measurements of earplug attenuation, a series of acoustic measurements were performed. The frequency response of the headphones was measured on an artificial head (DADEC; Dummy with ADjustable Ear Canals, see Hiipakka et al., 2010, for details  ) with an ear canal adjustable in length having the microphone at two positions: 1) at the blocked entrance to the ear canal and 2) at the eardrum. These responses were then compared to the corresponding responses obtained with a loudspeaker to verify how close to FEC the headphones were. The acoustic measurements were carried out using FuzzMeasure software that utilizes sine sweeps to obtain a time-domain impulse response. The artificial head was placed inside an anechoic chamber, and the measurement data obtained were analyzed with MatLab V12 software.
The frequency response of the headphones (excluding calibration with the audiometer) is plotted in the uppermost panel of [Figure A1]. Data from the measurement points representing the eardrum level (dashed line) and the blocked ear canal entrance (solid line) are shown. As seen in the panel, the headphone response covers frequencies up to about 20 kHz, but the response differs considerably between the two measurement points. This difference is mainly an outcome of a lack of ear canal resonance in the blocked meatus measurement. Each measurement was repeated five times to determine the reproducibility of the measurements on the artificial head.
For the middle panel of [Figure A1], the blocked entrance response was divided by the eardrum response using complex transfer functions in the frequency domain (deconvolution). The average of the five repeated measurements is displayed in the panel. The solid line represents the (pressure) division for the Sennheiser headphones used in the present study, and the dashed line represents the division obtained with the same measurement setup for a reference loudspeaker (Genelec 1029A, Genelec Oy, Iisalmi, Finland). The middle panel illustrates the difference in the measured pressure between the two points in the ear canal. At low frequencies up to about 1.5 kHz, the difference between the headphones and the loudspeaker is small, suggesting that the FEC characteristics were met for the headphones. However, above 2 kHz, differences up to about 4 dB can be seen. This is further illustrated in the bottom panel of [Figure A1], where the mean ratio of the two pressure divisions is plotted with their standard deviations. The bottom panel serves as an estimate of the inaccuracy of using the particular headphone type for REAT measurements, when compared with sound field reproduction via a loudspeaker. For details regarding the FEC, see Møller et al. (1995). 
For frequencies below 10 kHz, deviations in the pressure division ratio from 0 dB (indicating a complete FEC) are, however, relatively small, and most likely, on the same order or less than position-inaccuracies imposed by sound field testing in typical clinical environments. Based on these measurements, we considered the Sennheiser HD-650 headphones to be suitable for measurement of earplug attenuation.
| Acknowledgments|| |
Professor Martti Sorri, Johanna Palonen, MA and Heidi Vastamäki, MD are kindly acknowledged for their help in various stages of the project. Additionally, Jopi Penttilä of Danalink/GNOtometrics, Finland, and Manne Hannula, PhD, are acknowledged for their help in providing information on transferring measurement data from an audiometer to a PC environment. The subjects are thanked for their participation in the experiments.
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K H Huttunen
Institute of Clinical Medicine, Department of Otorhinolaryngology, P.O. Box 5000, FI-90014 Oulun yliopisto
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure A1]
[Table 1], [Table 2]
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