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134 CHAPTER 8. NOISE ON THE WORK FLOORA loss of auditory functions can lead to:A social disability, when the person has difficulties to have a conversations.Incapacity for work: if he is unfit to continue working.Invalidity, if the suffered physiological loss is too large.It is important to make a distinction between these three concepts and, inparticular, between the last two : incapacity for work is a relative concept,while invalidity is assessed as absolute. Hereafter we will only focus on thecriteria for social disability and invalidity. The question arises which frequenciesshould be used to measure disability or invalidity? Since the speechfrequencies are located between 250 or 4000 Hz, it would be logical to definethe average hearing deficiency as the average of the loss at the octave bandfrequencies 500, 1000 and 2000 Hz. The most widely accepted scale w.r.t.hearing disability which is based on that average, is that of the AmericanAcademy of Ophtalmology and Otolaryngology:disability = 1.25D b +0.25D m −39 (8.4)with D b and d m the hearing loss in the best ear and the average hearingloss of the weakest ear respectively. So the disability is 0 if D b or D m < 26dB. Disability would therefore occur starting from a loss of 26 dB and wouldreach 100% (full deafness) for a bilateral loss of 93 dB. In the calculation ofthe average hearing loss is currently the loss at 500 Hz replaced by that at3000 Hz. It has been attempted to justify this change by underlining that itis not only the person’s ability to hear, but also to understand the words insound conditions that correspond to those of everyday life and not just in acalmenvironment, whichisimportant. Thefrequency of3kHzisincreasinglyimportant as the rate of speech increases and as the the distortion due tothe background environment increases. Based on these frequencies one candefine the disability rate as :Disability = 1.25D b +0.25D m −39 (8.5)so the disability is 0 if D b or if D m < 35 dB. The handicap threshold shouldtherefore be increased to 35 dB as the hearing loss is calculated based on thefrequencies 1, 2 and 3 kHz. The Fund for Occupational Diseases in Belgiumhas decided to choose the losses at 1, 2 and 3 kHz for the calculation of theaverage loss. The scale for physical incapacity for work is the following :50 to 55 dBA : 1 to 5 % incapacity for work
8.4. THE AUDIOMETRIC EXAMINATION 13555 to 65 dBA : 5 to 10 % incapacity for work65 to 75 dBA : 10 to 30 % incapacity for work75 to 85 dBA : 30 to 55 % incapacity for work75 to 100 dBA : 55 to 80 % incapacity for workNote that a person can never be 100% incapable for work by deafness. Aperson who in fact is socially affected by the sustained deafness, is thereforenot incapacitated (threshold of 50 dBA compared to 35 dBA for disability).8.4 The audiometric examinationThe audiogram is an objective measure of hearing loss or deafness. It is thegraphic representation of the hearing loss relative to the reference measuredwith an audiometer. In this context, one speaks of the Bekesy audiometry.This is a type of automatic tone audiometry. There is also speech audiometry.This is done by testing speech intelligibility of one-syllable words, theso-called PB-lists (PB stands for phonetically balanced). In the Bekesy audiometry,a device is used that automatically passes 7 test tones (500, 1000,2000, 3000, 4000, 6000 and 8000 Hz) and this for both ears separately. Thesubject presses a button when he hears the tone, he releases the button whenhe does not hear any sound. After the button has been pressed, the intensitydecreases gradually. After the subject releases the button, the intensity increases.This results in saw-tooth-shaped curve of the sound levels at whichthe respondent reacted. For standardization, the device must be calibratedon a so-called artificial ear. Audiograms of the automatic method can berecorded with a pulsating sound or a continuous sound. The pulsating soundis typically 260 ms on and 260 ms off. Tone audiometry learns us a lot aboutthenatureofthehearingloss. Inaudiometry, aboneconductionaudiogramisoften used as an additional diagnosis. A vibrating plate mounted directly onthe skull is used instead of a loudspeaker (in the case of the air-transmissionaudiogram). The vibration propagates through the head until it reaches thecochlea. Some examples of audiograms are shown in Figure 8.4.A number of diagnoses can be made from audiograms :1. If the bone conduction audiogram is normal and the air conduction audiogramshows a significant loss, then the problem is probably situatedin the ear canal or in the middle ear. With such an audiogram, thepatient would, for example, suffer of otitis (ear infection).
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VRIJE UNIVERSITEITBRUSSELAcousticsB
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iiWhen sound becomes noise, it will
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ContentsI Introduction to acoustics
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CONTENTSvii4.3 Measuring the acoust
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Part IIntroduction to acoustics1
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4 CHAPTER 1. FUNDAMENTAL CONCEPTS O
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36 CHAPTER 3. MEASURING SOUNDFigure
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Chapter 4Sound AbsorptionAll(constr
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4.1. ACOUSTIC TRANSMISSION BETWEEN
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4.1. ACOUSTIC TRANSMISSION BETWEEN
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Chapter 5Sound InsulationWhen consi
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