MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

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Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)3. Brightness of the signal4. Length of time the signal is present5. State of adaptation of the eye.The level of luminance recommended for characters on avisual display terminal is 170 cd m 2 (50 fc) (Ref. 13).9-4.2 HEARINGHearing is an important sense in terms of informationgathering. Sound waves reaching the ear vary in frequency,amplitude, and complexity. Sound is usually describedin terms of its two major characteristics, i.e.,frequency and intensity. Frequency is measured in cyclesper second expressed as hertz (Hz). Generally, the humanear responds to frequencies between 20 and 20,000 Hz,though few adults hear near the 20,000-Hz limit. Frequenciesabove the 20,000-Hz limit cannot normally beheard by humans, but they do produce some biologicaleffects.The intensity of sound, or loudness, is usually measuredin decibels (d B). Weighting networks consist ofthree alternate frequency response characteristics designatedA-, B-, and C-weighting. Whenever one of thesenetworks is used, the reading obtained must be identifiedproperly. For example, if an A-weighted sound pressurelevel of 90 is obtained, it would be reptorted as a 90-dB(A).The A-weighted network is particularly valuable if aquick estimate of the interference of noise upon speech isrequired (par. B.1.1.2.1.2. Ret. 3). Areas requiring occasionaltelephone use or occasional direct communicationat distances up to 1.5 m (5 ft) should not exceed a 75-dB(A) level (par. 10-5.1. Ret. 13).The intensity and source location of audio alarms andsignals should be selected for compatibility with theacoustic environmenl of the intended receiver (Ref. 14).When discrimination of warning signals is critical to personnelsafety or system performance, audio signals shoulddeselected to signify different conditions requiring difierentoperator responses i.e. one signal for maintenance,another for emergencies, etc. For the purpose of thishandbook, auditory signals will be considered to be ofthree basic types i.e., tones, complex sounds, andspeech. A comparison of the strengths and weaknesses ofthese types with respect to their use for various functionsis presented in Table 9-5 (Ref. 3).FUNCTIONQUANTITATIVEINDICATIONQUALITATIVEINDICATIONSTATUSINDICATIONTRACKINGGENERALTABLE 9-5. FUNCTIONAL EVALUATION OF AUDIO SIGNALS (Ref. 3)TONES(Periodic)POORMaximum of 5 to 6 tonesabsolutely recognizable.POOR-TO-FAIRDifficult to judge approximatevalue and direction of deviationfrom null setting unlesspresented in close temporalsequence.GOODStart and stop timing. Continuousinformation whererate of change of input is.FAIRNull position easily monitored;problem of signalresponsecompatibility.Good for automatic communicationof limited information.Meaning must be learned.Easily generated.TYPE OF SIGNALCOMPLEX SOUNDS(Nonperiodic)POORInterpolation between signalsinaccurate.POORDifficult to judge approximatedeviation fromdesired value.GOODEspecially suitable forirregularly occurring signals.e.g., alarm signals.POORRequired qualitative indicationsdifficult to provide.Some sounds available withcommon meaning, e.g., firebell. Easily generated.SPEECHGOODMinimum time and error inobtaining exact value in termscompatible with response.GOODInformation concerning displacement,direction, and ratepresented in form compatiblewith required response.POORInefficient: more easilymasked; problem of repeatability.GOODMeaning intrinsic in signal.Most effective for rapid (butnot automatic) communicationof complex, multidimensionalinformation. Meaningintrinsic in signal and contextwhen standardized. Minimumof new learning required.9-10
Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)Occasionally in maintainability design, auditory signalsare preferable to visual ones, and vice versa. Table9-6 summarizes the situations in which one form is preferredover the other. However, when the maintenancetechnician must monitor several audio signals (channels)which must be recognizable above the sound of theequipment being tested or maintained, the signals shouldbe of different frequencies—e.g., high, medium, andlow—to avoid confusion.Noise is defined as any undesirable sound. Excessivenoise in a maintenance or operations area reduces theefficiency of the personnel and thus may impair or reduceoverall system effectiveness. Excessive noise also affectspersonnel psychologically; they fatigue more rapidly,their ability to concentrate decreases, and their annoyanceincreases. In addition to affecting the performanceof maintenance technicians, excessive noise can renderoral communications ineffectual or impossible. Fig. 9-4(Ref. 2) shows the difficulty in communicating at variousnoise levels and speaker-to-listener distances.Exposure to high noise levels can also produce physiologicaleffects—exposure to noise levels exceeding 85 dBmay result in a temporary or permanent hearing loss; theextent of damage depends on the length of the exposure.Table 9-7 lists the intensity of common sounds and theireffect on exposed personnel. TB MED 501 (Ref. 15)defines the noise hazards and the hearing conservationprogram of the US Army. Appendix B of Ref. 3 presents adetailed discussion of hearing loss resulting from bothsteady state and impulse noise together with the factorsinfluencing hearing loss and recovery; the resultant effectson performance are also discussed. Personnel in acousticenvironments where the risk criteria are exceeded shouldbe protected to avoid damage to their hearing. The mosteffective method of protection is to control the noise at itssource. If noise control is not practical, personnel must beprotected with ear protectors such as earmuffs or earplugs.9-4.3 TOUCHAs equipment becomes more complex, it is necessaryfor maintenance workers to employ all their senses to thefullest. Man’s ability to interpret visual and auditory stimuliis closely associated with the sense of touch. Thesensory cues received by the skin and muscles can be used,to some degree, to convey messages to the brain, whichrelieves the eyes and ears of part of the load they wouldotherwise bear. For example, the control knob shapesillustrated in Fig. 9-5 (Ref. 3) can be recognized easily bytouch alone. These knobs have been specifically designedand experimentally validated for tactile recognition (Ref.3). Many of these knob shapes could be used when themaintenance technician must rely completely on his senseof touch, for example, when the user is busily lookingelsewhere when that control must be moved.It is an Army tradition that personnel must be able tofield strip weapons based on touch alone. Therefore, it isincumbent upon weapon designers to make the shape andsize of each part unique to facilitate field stripping.Figure. 9-5. Easily Recognizable KnobShapes (Ref. 3)TABLE 9-6. WHEN TO USE AUDITORY OR VISUAL FORM OF PRESENTATIONUse auditory presentation if1.2.3.4.5.6.7.8.The message is simple.The message is short.The message will not be needed later.The message deals with events in time.The message calls for immediate action.The visual system of the person is overburdened.The receiving location is too bright or dark; adaptationintegrity is necessary.The person’s job requires him to be moving comtinually.Use visual presentation if1. The message is complex.2. The message is long.3. The message will be needed later.4. The message deals with location in space.5. The message does not call for immediate action.6. The auditory system of the person is overburdened.7. The receiving location is too noisy.8. The person's job allows him to remain in oneposition.9-11
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