HEARING CONSERVATION PROGRAMME

Contents:

A.   NEED OF HEARING CONSERVATION PROGRAMME AND STEPS IN HEARING CONSERVATION PROGRAMME.

B.   EAR PROTECTIVE DEVISE

Ø TYPES – EAR PLUGS, EAR MAFFS, HELMETS, HELMETS, SPECIAL PROTECTORS, MERITS AND DEMERITS OF EACH

Ø PROPETIES OF EPDs

Ø EVALUATION OF ATTENUATION CHARACTERISTICS OF EPDs

Ø TOUGHENING

A.   NEED OF HEARING CONSERVATION PROGRAMME AND STEPS IN HEARING CONSERVATION PROGRAMME.

 INTRODUCTION:

                            In industry, to prevent the employees from occupational hearing loss, Occupational Safety and Health Administration-Hearing Conservation Amendment (OSHA-HCA, 1983) made a Hearing Conservation Programme (HCP) under Occupational Noise Exposure Regulation Act  1910.95

It is important to differentiate between an HCP that meets the requirements of the OSHA- Noise Exposure Regulations, and an effective HCP.  A programme that contains the minimal programmatic elements required by the noise standard may be far from effective. But on the other hand, an effective HCP should incorporate various activities and documentations that are not included in the OSHA noise standard to make it as effective.

       WHAT IS HEARING CONSERVATION PROGRAMME(HCP)?

·        An occupational Hearing Conservation Programme is a systematic plan implemented to protect the hearing of employees from damage due to hazardous sound exposures in the work place.

- Cyril M. Harris

·        An effective Hearing Conservation Programme is one that accomplishes the goals established for it .The primary goal of an industrial  HCP must be the prevention (or at least limitation) of permanent hearing loss associated with exposure to industrial noise(Royster and Berger).Other goals may be formulated such as -compliance with OSHA regulations, reduction of employee stress and absenteeism, reduction of work place accidents due to plant noise levels and reduction of the company's liability to worker compensation claims for occupational hearing loss.

                                                                        - David M. Lipscomb

NEED OF HEARING CONSERVATION PROGRAMME:

          An effective Hearing Conservation Programme is needed-

1.     To measure work area noise levels

2.     To identify the over exposed employees

3.     To reduce hazardous noise exposures to the extent possible through engineering and administrative controls

4.     For the provision of personal hearing protectors if other controls are inadequate

5.     To reduce liability for potential workers compensation claims for occupational hearing loss

6.     To provide a better working environment which leads to reduced absenteeism and reduced employee turnover.

HEARING CONSERVATION PROGRAMME (HCP)

 

STEPS /PHASES IN HEARING CONSERVATION PROGRAMME:

1. SOUND SURVEY PHASE:

          The primary purpose of a sound survey in a work environment is to determine the sound exposures to which employees are subjected in order to make appropriate decisions about how to protect workers from developing occupational hearing loss. These decisions include:

                               i.            Determining whether employee's sound exposures are hazardous enough to require the establishment of a hearing conservation programme

                             ii.            Identifying those employees who must be included in the hearing conservation programme (to comply with either a governmental regulatory criterion or the company's own internal criterion)

                          iii.            Classifying the sound exposures for employees into categories that trigger increasingly protective actions for increasing levels of hazard(such as the selection of hearing protection devices, design of appropriate educational materials and the setting of priorities for noise control projects).

          Sound level measurements are needed to -

                               i.            Select the equipment (noise source) to be controlled.

                             ii.            Select the order in which the noise control measures will be carried out.

A sound survey also:

                               i.            Identifies work areas where background noise is sufficiently high to interfere with communication or warning detection and therefore constitutes a safety hazard

                             ii.            Documents sound levels in the work environment and sound exposures of employees for purposes of regulatory compliance and for evidence in future workers' compensation claims.

                          iii.            Identifies work areas where annoying (but nonhazardous) sound levels may interfere with job performance.

SOUND SURVEY DATA COLLECTION:

          The sound survey should generate a noise exposure classification for each job category of employees. Depending on the governmental regulations which apply, the relevant measure of noise exposure may differ. In the USA, the OSHA of the federal government specifies the 8-hour time- weighted average(TWA)A-weighted sound pressure level based on the 5-db rule. Some countries use the 8-hour equivalent continuous sound level based on the 3-db rule. In either case, it is adequate to classify the noise exposures of employees in ranges of 5-db increments for the for the purposes of selecting appropriate hearing protection devices and deciding whether the use of such devices will be voluntary or mandatory.

          Sound survey should also measure A-weighted and C-weighted sound pressure levels in all areas where employees work, both for noise control planning purposes and for selecting hearing protection devices. Documentation of the sound levels is also useful for purposes of providing evidence in workers' compensation claims and to verify data obtained with dosimeters.

DISSEMINATION OF SOUND SURVEY DATA:

Sound survey results must be shared with all company personnel who require this information. The findings of the sound survey should be explained to the employees so that they will understand -

                                                       i.            The degree of the noise hazard in their work areas or for their  job categories

                                                     ii.            Where hearing protection devices are required

                                                  iii.            Which employees are to be included in a hearing conservation programme

Sound surveys should be repeated 1)annually or 2)whenever changes in production equipment or practices may increase (or decreases) the sound exposures of employees enough to justify adding employees to the hearing conservation programme (or removing employees from it ) or  changing the requirements for  using hearing protection devices.

2.     ENGINEERING AND ADMINISTRATIVE NOISE CONTROL PHASE:

                   Engineering noise controls are modifications of current or newly purchased equipment and/or facilities to reduce the sound exposures of employees. Administrative noise controls are plans to reduce the sound exposures of employees through i) work task reassignments ii) the design or purchase of quieter equipment or facilities. The primary goal of the engineering and administrative noise control phase of the hearing conservation programme is to eliminate hazardous employee sound exposures, if practical and economically feasible. A secondary goal of the engineering noise controls is the significant reduction of the employees' sound exposures, which increases the probability of adequately protecting the employees from noise- induced hearing loss through the use of hearing protection devices.

                   Reduction of noise levels can be achieved by modification of existing machinery and the use of damping materials or machine enclosures to reduce sound radiation.

                   One effective administrative noise control option which is emphasized in some governmental regulations is for management to implement a plan of slowly replacing older equipment with quieter machinery. Other administrative control options include modifications of employees' work schedules and job tasks to spread the hazardous exposures among more workers, resulting in a lower risk of hearing damage for many workers rather than a high risk for few workers.

                   To develop appropriate plans for engineering and administrative noise controls, the dominant sound sources which contribute most strongly to employee sound exposures must be identified. Also determine the number of employees whose exposures are affected by various pieces of equipment .After the dominant sources are identified, priorities for controlling them can be set based on  1)feasibility   2)the anticipated reduction in sound exposures of employees, and 3)the schedule for replacing equipment with quieter units. Regular maintenance of installed noise controls is required to ensure that the noise reductions achieved are not lost through neglect.

3.     EDUCATION PHASE:

                   The education phase of a hearing conservation programme provides all noise-exposed employees and their associated supervisors and managers with meaningful education about the risk of noise-induced hearing damage and their responsibilities in the hearing conservation programme, as well as motivational information to influence them to participate actively in the programme. Where sound exposures have been reduced to nonhazardous levels, employee education is required to teach operators and maintenance personnel how to use and maintain the noise control applications. Where a noise hazard exists, the educational programme should also include information about how to fit and wear hearing protection devices properly, as well as details about the purpose and procedures for the audiometric evaluation phase.

                   Educational phase emphasize the basic facts about noise-induced hearing loss: how it develops, how it adds to inevitable age-related hearing loss and how the risk of hearing damage can be reduced by proper utilization of hearing protection devices and noise controls.

                   The education phase in a hearing conservation programme is also required as the noise-exposed individuals needs to understand how much a hearing loss would degrade his /her own quality of life by causing 1) communication problems 2)social isolations from friends and family 3)the loss of enjoyment of music and other recreational activities and 4)the potential loss of job advancements and opportunities.

4.     HEARING PROTECTION PHASE:

                   Hearing protection devices are the prime mechanism of reducing the employee's sound exposure to safe levels. Such devices are ear-plugs, ear-muffs, semi-inserts etc. The employer should select hearing protection devices which provide adequate noise reduction. For sound exposures that require more than 10 db of protection, hearing protection devices should be selected that have been shown to provide the required attenuation under actual work conditions.

                   Because the protection achieved depends on proper and consistent utilization of hearing protection devices:

          i.            The individual wearer must be trained carefully how to use the devices and taught why it is so important to use them consistently.

        ii.            The regular replacement of worn-out or damaged devices is essential.

     iii.            The personnel who fit, issue and replace devices must be educated to perform these important tasks well.

     iv.            The supervisors who enforce correct use of these devices must conscientiously monitor utilization.

5.     AUDIOMETRIC MONITORING PHASE:

                   The audiometric monitoring phase includes the periodic measurement of the hearing thresholds for noise-exposed employees and evaluation of the results:

       i.            To detect hearing changes this may indicate the need for greater protection against noise.

     ii.            To identify employees whose pre-existing hearing impairments may warrant special considerations in job placement or the selection of appropriate hearing protection devices

  iii.            To detect hearing characteristics or hearing changes suggesting medical conditions unrelated to sound exposure

  iv.            To provide information concerning the hearing trends and level of protection for the entire group of noise-exposed employees, as determined by audiometric database analysis

A reference (baseline) audiogram should be obtained for each employee before the employee is assigned to a noisy work area. Annual audiograms to monitor changes in employees' hearing should be given during the course of the workshift, not before the workshift begins, so that the results will show any temporary threshold shift resulting from inadequate use of hearing protective devices.

All employees should receive information about their hearing status after each annual audiometric evaluation, both verbally (immediately after the audiogram) and later in written form. When any significant hearing change is identified, the employee's hearing protection device should be refitted and the worker retrained in its proper use.

The ability to detect hearing changes in individuals depends on proper calibration of audiometric test equipment and the use of consistent employee instructions and testing techniques by audiometric technicians.

DESIRED CHARACTERISTICS OF EFFECTIVE HEARING CONSERVATION PROGRAMME:

        i.            Strict enforcement of the use of hearing protection devices should be a condition of employment  for employees who have hazardous sound exposures

      ii.            Hearing protection devices that are potentially effective in preventing on-the-job noise-induced hearing loss must be available to the employees.

   iii.            A key individual who is committed to the programme must be present to motivate others to place a priority on hearing conservation.

   iv.            There must be active communication among the personnel involved in all phases of hearing conservation programme.

PERSONNEL INVOLVED IN THE PROGRAMME:

                   Personnel involved in the programme include various levels of the company's hierarchy to at least some degree, even if they do not spend significant time in noise hazard areas

EXTERNAL INFLUENCES:

                   In the above chart there are several types of influences that may affect the hearing conservation programme's implementation or its effectiveness.

CONCLUSION:

Although an effective HCP cannot be guaranteed by the presence of each of the element prescribed by OSHA, any programme that does not include all of the elements will doubtlessly be ineffective. The actual degree of effectiveness experienced by a company in its HCP depends:

1.     On the commitment of company management to the adequate implementation of each programme element;

2.     On the enthusiasm and interest shown by the company's hearing conservation technician in each employee's hearing health;

3.     On the competence and dedication of each member of the HCP team, both professional and non-professional; and

4.     On the motivation of each worker to protect his/her own hearing.

Hearing conservation programme must be vigorously and continuously supported by management and must include the technical assistance of professional supervisors to guarantee that each aspect of the hearing conservation programme is functioning with maximum effectiveness.

A.   EAR PROTECTIVE DEVISE

Ø TYPES – EAR PLUGS, EAR MAFFS, HELMETS, HELMETS, SPECIAL PROTECTORS, MERITS AND DEMERITS OF EACH

1)    EAR PLUGS:-

                                    These are inserts that fit directly into the ear canal. They come in many configurations and made of rubber, plastic or wax impregnated cotton or other materials. A Correct fit depend on a proper seal along the entire circumference of the ear canal walls. These EPDs are fairly small easily portable. They usually can be easily cleaned in soap and water.

Ear plugs are, however, not usually tolerated in the ear for more than two hours or so at a time. On an average, they provide an attenuation of between 15-35dB depending on the type of ear plug. Venkatesh (1980) reported the attenuation of earplug using REAT method with earphones as 27.75 at 250Hz and 46dB at 8 kHz.

          Chandrasekhar et.al (1930) did a study to obtain the NRR (noise reduction ratting) values of EPDS available, indigenously. The NRR value was calculated using the method recommendation by Berger in 1983. They reported the NRR values of earplugs that were manufactured in India to range between 1998 and 21.83. Imported were found to have a NRR values of 22.5-23.

Advantages of ear plugs

•             More comfortable to wear in hot environment

•             Can be worn easily with other equipments and eye glass

•             Are small and carried easily

•             Convenient to use under frequent changing head position

•             Cost effective

Disadvantages of ear plugs

•             They are less visible ;  difficult to monitor their use

•             Require more skills and attention in preparing them for use and inserting them into the ear canal

•             Some times painful when removed rapidly

•             Prior ear examination is essential to rule out infection

•             Some individual may be allergic to ear plug material

Some ear plugs are susceptible for hardening and shrinkage

DIFFERENT TYPES OF EARPLUGS AVILABLE ARE:-

a)    Cabricated/pre-moulded ear plugs:-

             They are manufactured from flexible materials such as cured silicones and other elastomeric formulation of the most common device is the V-5IR earplugs. It is available in 5 sizes. premolded plugs are available with having number of flanges (between 1-5) . Generally greater the number of flanges, better is the seal and grater in the attenuation . Values of attenuation for premolded earplugs as reported by Agnew (1987) are between 20-30dB with grater attenuation in the high frequencies then at low frequencies.

Berger (1994) reported values of attenuation for premolded earplugs as around 25dB at 1 kHz & approximately 40dBat higher frequency.

b)     Formable /moldable earplugs :-   

            They are also known as disposable and malleable earplugs. They may be manufactured from cotton, wax, fiber glass silicon putty and slow recovery foam.

           While disposable and moldable earplugs may be more comfortable to wear than prefabricated ones they require greater standards of cleanliness from the wearer.

c)     Custom- moulded earplugs:-

              These are made either from two part curable silicon putties or vinyl. These fill the over portion of the ear canal as well as the concha and pinna.

              Berger (1994) repotted that the average attenuation provided by these earplugs was between 15and 35dB. In general, earplugs need to be handled carefully and regularly, or else they could lead to infections and irritation to the skin of the ear canal. They require skill to insert and remove them without damaging the plug and hurting the user

d)    Semi inserts:-

These are also referred to as choncha seated hearing protectors or canal caps. They consist of pads or flexible tips or rubber caps attached to a tight weight headband that presses the caps against the entrance to the external ear canal. They are far easier to wear and remove than earplugs and are also easily portable.

2) EAR MUFFS:

                                              Most types of ear muffs are of a similar design and are made of rigid cups specially designed to cover the external ear completely .They are held against the sides of the head by a spring loaded adjustable band and sealed to the head with circumaural cushions.

                                             For maximum attenuation of sound, the protector cups should be made from a rigid, dense non-porous material. Each cup is partially filled with an absorbent material to reduce the high frequency resonances that may otherwise occur within the shell.

 

Advantages of ear muffs

•             Single size ear muff fit the large population

•             Usually more readily accepted by the employee

•             Comparatively more comfortable

•             Easy to monitor

•             Not as easily misplaced or lost as ear plugs

•             Provide more attenuation and the amount of attenuation is less variable between the users

•             Can be used for protection even in ears with infection

 

Disadvantages of ear muffs

•             Are larger ,bulky

•             Uncomfortable for prolonged usage and in hot environments

•             Cannot be carried easily or stored

•             They are not as compatible with other personally worn items I.e, eye glass as ear plugs

•             Muff can become loose during the course of work & during head movement and thus reducing the amount of protection provided

•             More expensive

•             Difficult to localize the sound source and to follow conversation

•             The metal portion of the muff can cause certain occupational hazard i.e, SHOCK.

 

3) HELMETS:-

                                         This is also called earmuff attached to a hard  when the use of protective headgear is require , hardhats with attached earmuff provides a convenient alternative to the use of earmuff attached with a head- band . however these are more difficult to properly orient and fit since the attachment arms can never provide as adaptable an adjustment s do the head- band attached muff . nor can they fit as wide a range of head sizes. For these devices the attachment arms conservation program than earplugs.

Advantages :

w When used with in conjunction with one form of EPD they cover bony portion of head and reduce bone conduction sound to some degree .

w Can be used in industries where protection of the workers head from impact or flying object is required in addition to hearing protection

Disadvantages :

w Are very bulky

w Are expensive and hence rarely used

Ø PROPERTIES OF EPDs.

1)    ATTENUATION

There are significant variation in the average sound attenuation (and standard deviations) of the same hearing protection devices measured with the same procedure at different laboratories.

Therefore, rank or laboratory variability are

1.     Difference in interpretation and implementation of the standard measurement methodology,

2.     Uncertainty of obtaining the proper fit to avoid acoustic leaks,

3.     Difference in subject selection,

4.     Differences in subject training, and

5.     Difference in data reduction techniques.

The accuracy and repeatability of hearing protection device attenuation measurement in the same laboratory also vary (but to a lesser extend) because of changes in,

1)    The laboratory s experimenter,

2)    The subject population,

3)    Selection criteria,

4)    Fitting and sizing techniques,

5)    The instrumentation, and

6)    The characteristics of samples of the hearing protection device being tested.

One consequence of this variability is that differences in the NRR of less than 3dB have no practical importance, and even 4 to 5 dB changes are of questionable importance unless closely controlled data are being compared.

2)    COMFORT

                Comfort is a critical feature of a hearing protection device, equivalent in importance to the attenuation that the device can provide.

A device that is not comfortable will not be worn consistently and correctly, or perhaps will not be worn at all.

Comfort can be improved by properly matching the device to the wearer and by providing instruction in its proper use.

Common sources of discomfort include improperly sized or inserted earplugs, worn—out device with hardened or ragged cushions or flanges, overly tight earmuff headbands, and the use of earmuffs in very hot environments.

                                            Hearing protection device are items of personal equipment and must be dispensed accordingly.

Users should be involved in the selection process and allowed to choose from a variety of suitable options, including a minimum of three different devices.

Generally, this will consist of an earmuff and two models of insert earplugs, but a semi- insert device or additional brands of each protector may be warranted.

It takes time for wearers to become accustomed to hearing protection device – both how they feel and how they sound. It may require a week or two for some novice users to fully adapt to the feeling of wearing them and to appreciate the benefits that their use provides.

3)    Comparison of hearing protective devise:-

Consideration	Custom plugs	Disposable plugs	muffs	Canal caps
Insertion	Easy to insert after initial learning period	Takes care and instruction	Easy to slip on and off	Easy to slip on and off
Hygiene	May get dirt w/handing generally easy to clean	May get dirty w/handing	Easy to clean	May get dirty w/handing but can be held by band to keep caps cleaner
Comfort	After initial adjustment period-excellent	After initial adjustment period good	Hot, sweaty headband presser may bother some	After initial adjustment period good
Safety	May be difficult to retrieve from equipment small size	May be difficult to retrieve from equipment small size	May be integrated with other equipment (hard hat; visor)	Band under chin may interfere with other equipment
Daily maintenance	Cleaning and drying	No required	Wipe clean	Wipe clean
Anticipation length of use	1-2 years	1-3 years	6month-2year	months
Cost per unit visibility susceptibility to ineffective use (inadequate protection)	$ 100 and up low low	$ 0.60 and up low high	$15 and up high moderate	$1.50 and up moderate high

Implementation for the effective use of EPD’ s

w Material:They should be made of imperforate material

w Comfort:It should be comfortable for the wearer

w Motivation :User should be motivated and should be trained for proper utilization of the EPD ‘s

w Compatible: they should be compatible in terms of shape / dimensions to each individuals need

w Periodic monitoring:Status of EPD should be monitored periodically

w It should have a seal compliance that will maximize the protector vibration

w Muff type protector should not be worn over long hair poorly fitted eye glasses or other obstacles

w  Air leaks:An ear plug must make an air tight seal against the sides of the canal and an ear muff must make an air tight seal  against the sides of the head to provide maximum attenuation

w  Combined effect :Combined use of ear muffs and ear plug was proved to be effective

w  Infrasonic frequencies:–ear plugs

w  Ultrasonic frequencies :- ear plugs / ear muffs

w  Comfort:To provide maximum comfort users should be involved in the selec5tion process

w  Hygiene and maintenance: EPD’ s should be cleaned regularly acc to manufacturers instructions

w  Cleaning improves the acceptability , durability of the device and reduces the possibility of skin and ear irritation and infection

w  Safety:Care should be taken while inserting or removing an EPD.

w  Abuse :Abusing of the ear protective devices should be avoided

Ø EVALUATION OF ATTENUATION CHARACTERISTICS OF EPDS.

1)    Octave band calculations method:-

The attenuation of an EPD is described by bthe no. of decibel by which it reduces the level of sound signal. The octave band calculations method have been proposed by NIOSH (National institute of occupational safety and health,1975),where a single number attenuation value is obtained for each of the octave frequencies from 125 to 8 KHz. This is a laboratory technique for obtaining the noise reduction by calculating the possible attenuation .The step for calculating are as follows:

STEP 1:

          The sound pressure level at each octave frequency is measured.

STEP 2:

          The correction for A- weightening network for each octave frequency is computed

STEP 3:

          These correction factor are subtracted from the measured sound pressure level to obtain. A-weighted sound level at each frequency.

STEP 4:

          A typical attenuation provided by the ear protective device is tabulated for each frequency.

STEP 5:

          The standard deviation 2 is computed for each octave frequency.

STEP 6:

          To calculate the estimated protected A-weighted sound levels, are obtained by adding the value in step 4 and 5 and this amount is subtracted from A-weighted sound level obtained in step 3. The protective levels are then logarithmically summed to determine the A- weighted sound level under the protection. This analysis requires an octave band analysis of the noise spectrum and perform appropriate calculation for each individual noise spectrum to calculation the attenuation provided by the EPD. The standard deviation correction used in this method is performed so that the result can be generalized to 98% of the laboratory subjects.

                                Eg.- 250-> 87 dB of noise

                                       87-8.7= A weighted SL at 250

2)    SHORTED METHOD:-

A.   Noise reduction rating (NRR):-

The NRR   is a single number descriptor to estimate the A- weighted sound level under a hearing protection device..The NRR is suggested by both ,NIOSH (1975) and the Ear Protective Agency(1979).It is defined as an attenuation index that represent the overall average A- heightened noise reduction in decibels that a hearing protector will provide in an environment with a known C-weighted sound level. It is calculated in a manner similar to octave-band method, however a pink noise spectrum is used instead of the actual noise spectrum, where the pink noise has an SPL of 100dB in each octane band from 125 to 8KHz this also includes the 25.d inclusion adjustment & an additional 3dB safety factor to amount for errors that may arise due to the usage of pink noise. This step of NRR computation is as follows:-

1.     The assumed SPL at each octane frequency is tabulated.

2.     The C- weighting correction factor is applied.

3.     From (1) & (2) weighting sound level are obtained.

4.     To these attained sound levels, the A- weighting correction is comported.

5.     From (1) & (4) the A-weighted sound levels are obtained.

6.     The attenuation of the ear plugs is calculated.

7.     To accurate for population dispersion, 2, s, l, s, are taken.

8.     Estimated protected A- weighed sound level are obtained by subtracting the ear plug attenuation +2 s. d. from the estimated A-weighted level obtained in step5.

9.     The NRR is obtained by subtracting 3dB spectral uncertainty factor from the valve obtained in step

                   E.g.-      Step 1-> 125Hz,

                                          Step 2-> 0.2(correction factor)

                           Step 3-> 100 – 0.2 = 99.8

                           Step 4 & 5 -> 100 – 16.1 = 83.9

                           Step 6-> 27.4 ->125Hz

The NRR is used to estimate the wearer noise exposure by subtraction it form C-weighted sound level by this equation i.e.

                    Estimated exposure= work place noise level – NRR

B.   SUBJECTIVE EVALUATION

Subjective method of evaluation the ear plug performance is done through EAG. It utilizes a subjective judgment to determine the evolution provided by a certain ear plug. The REAT is a fold standard in measuring subjective evaluation and is also endorsed by the ANSI. It closely capture the EPD performance by the user is least effective by the artifacts. REAT measures shift in threshold between the occluded and unconcluded conditions for a group of subject. REAT can either be conducted in a sound field or can also be carried out under circumaural headphones for measurement of circumaural headphones suitable correction factors home to be applied, between the occluded thresholds to match the sound field later .However for the circumaural headphone measurements are only possible on circumaural EPDs. Advantage of REAT is that it accounts for the entire sound path, i.e. including the BC pathways. 

          To carry out this procedure first the aided thresholds are noted down with sound presented and later occluded threshold are taken and the values are again noted down. The difference between the occluded and unconcluded thresholds gives the REAT. This method is ideal for those HPD’s which provide linear attenuation. For level dependant EPDs REAT tends to underestimate the attenuations provided. For measuring the REAT of a particular earplug thresholds are obtained from 10 normal hearing listeners in a diffused sound field. The test signals are pulse noise, narrow band noise with centre frequencies 125,250,500,1k,2k,3156 Hz,4k,6300 Hz and 8k(according to ANSI3.19,1974). The difference between open ear and occluded ear threshold is calculated for each frequency. Each condition is repeated thrice and the average of the 3 calculations is noted for all the 10 db for each frequency. This total is divided by 30to get the grand mean.

According to ISO 4869(1992) 16 subjects is used instead of 10.A more recent ANSI standard recommends employing fit qualified subjects who are trained  the audiometric test but don’t have a knowledge of hearing protectors

·        Disadvantage of REAT:

          1) This cannot be used for all the devices which are not insert EPDs, when testing in circumaural condition.

          2) Can be carried out with linear EPDs only

          3) Subjected to contaminations from the physiological noise which occurs as an artifact usually seen for low frequency below 1 KHz.

C.   OBJECTIVE METHOD OF EVALUATION:-

          The objective analog of REAT is MIRE (Microphone in real ear). In this condition the sensor is a probe microphone that is positioned inside the ear canal. This is a physical measurement that estimates the SPL developed inside the ear canal with and without the ear protective device. The difference in value which show the SPL loss or insertion loss. Therefore, the MIRE parading utilizes real insertion loss to calculate the amount of attenuation. 

          Advantage of MIRE is that it can calculate the attenuation provided by level dependant EPDs by measuring the SPL that actually develops at different sound levels. Hence it should be used for non-linear EPDs. However, the disadvantage of conventional MIRE methods is that requires the open ear condition to be unprotected. Which can be problematic for high level sounds? To account for this, the ANSI MIRE standardized (s 12.42:1995) recommends a procedure when two probe mics are kept, one inside the EPD and one outside the EPD. The difference in the two recording will yield the insertion loss. MIRE correlates well with REAT except for 2 KHz, where REAT thresholds are low.

·        DISADVANTAGES:

          This can be only implemented with insert ear phones. Since insertion loss is studied, it does not account for the BC hearing that affects the person during presentation of noise. It requires experienced personnel to carry out this method.

Ø AUDITORY TOUGHENING:-

Auditory toughening was first noted by Miller, Watson, and Covell in 1963.  They defined auditory toughening or “the training effect” as the auditory system’s ability to modify its susceptibility to damage from noise, depending on previous exposures.  Specifically, when the auditory system was “toughened” by non-damaging exposure to noise for a number of days, ensuing hearing loss as a result of a damaging level of spectrally similar noise is LESS than that which would occur if there was no previous toughening.

This phenomenon has been observed for permanent threshold shift (PTS) in a wide range of mammals (not humans!) as well as for temporary threshold shift (TTS) for teenagers (Miyakita et al, 1992).

In 1991, Campo, Subramaniam and Henderson found that after a toughening or training of the ear for 10 days to a 500 Hz band of noise (even after a 5 day period of recovery), a smaller PTS was found in chinchillas that were not toughened (by as much as 15 dB).  It should be noted that in this experiment Campo et al. toughened the chinchillas to a non-damaging level of 500 Hz, and then exposed the chinchillas to a damaging level of 500 Hz noise.  That is the same spectrum was used for the toughening as was used to create a hearing loss.

Subramaniam, Hendersonand Spongr (1991) then looked at what would happen if chinchillas were toughened with a 500 Hz stimulus, but later were exposed to a high frequency (4000 Hz) stimulus.  In this case, the reverse was found- the toughening actually INCREASED the PTS over that of the non-toughened, or control group.

In summary, it seems that if the non-damaging toughening stimulus is the same frequency as the more traumatic one presented later then there will be some protection from this effect.  However, if the toughening stimulus is low frequency and the traumatic one is high frequency, then the low frequency “toughening” effect may not be toughening at all and may exacerbate the hearing loss.

The physiology of the effect is not well understood.  Possible explanations may be related to the effects of the efferent pathways and the effects of the presence of proteins in the outer hair cells.  More research on this topic is certainly required.

Auditory toughening is an interesting effect but at this point my clinical recommendation is to wear the hearing protection as much as possible and not to rely on the potential benefits of auditory toughening.

References:

·        Handbook of acoustical measurement and noise control

                                                                        - Cyril M. Harris

·        Hearing Conservation In Industry, Schools, and The Military

                                                                        - David M. Lipscomb