Document 2J69Ov2DXKYNzjp43GVrYonyr

92 ROSS A. MC FARLAND B. OTHER SENSORY DISPLAYS Although much of the information that a worker obtains is visual in nature, displays may involve other types of sensory stimulation. For example, the be havior of a worker may depend on, and be controlled by, auditory cues, either spoken words (as in a communication network) or mere sound stimuli (such as the noise made by a motor at different speeds). The use of different knob shapes and sizes on levers and switches may constitute a "tactual display." Skilled machinists often refer to the "feel" of their controls. The use of these sounds, shapes, and built-in "feels" on controls should thus be considered as auxiliary ways of insuring that the worker receives all the information he needs. Some of the highlights of what is known about the average worker's capabilities in these sensory fields are discussed below. - 1. Auditory Displays Auditory displays may be used effectively as warning or information signals to the worker, thus relieving his visual senses of additional responsibilities. To be most effective, of course, such an auditory information system must be employed in the sound range where the human ear is most sensitive. This applies with respect to the frequency, intensity, and duration of the sound. The ability of the human ear to detect a change in the frequency of a sound increa8es'with`the-pit'ch-fTom-a-very-low-valuerand-thenJ.essensJox-extrem.e]y_high_ tones. The greatest pitch sensitivity is found for tones (of moderate loudness) whose frequency is between 400 and 800 cycles per second. This is the range that should be utilized for auditory information. Intensity discrimination is also poor at low frequencies when the sounds are faint. Under such conditions, the greatest sensitivity to a loudness change is at about 2000 cycles per second. For moderately loud tones (that is, about 40 decibels), however, a normal ear is equally sensitive to loudness changes at all frequencies.24'71 To avoid continuous tones, which might become distracting or annoying, auditory cues should be as short as possible. However, they must be sufficiently long to afford the opportunity for adequate pitch and loudness discrimination. `Th^best^eyidepce.^ndicsites^hat-both-frequenGy-and^tenatymayithfijidiscnHffli. nated from durations as short as 20 to 40 thousandths/ofia.'seqond. ; 2. Tactual Displays ..... ^thougS^^iaeBiaT sense ofTorm arid'size is greatly inferiorto that-of-the-1 visual sense, it is nevertheless possible to code equipment knobs in this respect in order to reduce errors in control operation.55 The use of knobs of various shapes reinforces visual information and assists in avoiding accidental movements when "P. M. Fitts, Engineering psychology and equipment design, in S. S. Stevens, ed.. Handbook oj Experimental Psychology. Wiley, New York, 1951, pp. 1287-1340. HUMAN ENGINEERING AND rINDUSTRIAL SAFETY 93 i A *'Vr ontrols must be selected and used without, the aid of vision. Shape coding is particularly useful in situations where a specific control is located in different positions in different situations. The "habit interference" that is experienced in aese circumstances can be alleviated by this method of coding. A considerable knowledge now exists concerning the shapes that are maximally discernible.56 These are illustrated in Figure 9. During blindfold tests conducted by the Air I||rce, it was found that these shapes were the most easily distinguishable, caused heiieast hesitation and the fewest mistaken identifications. lh Mm; m <> Sb % JilE'Ure ^even 8hapes found most suitable for the coding of control knobs. During ex- ^^^^ta-l-tests-'errors-of'-identification-of-the-knobs-in this-group were either absent or ex- jePffecrare. (After Jenkins" and Chapanis et al.u) $ *; S. Kinesthetic Displays ||esistance to movement may also give the operator of a control a cue as to Ijgnt of its activation and thus may be considered as a form of display. Jjjf. only limited knowledge concerning kinesthesis, (muscle sense) so only ^tiy.ejrecommendations can be given. It is known that people tend to over- small pressures and underestimate large pressures. We also know that J;lpg pressures become more accurate and consistent after a certain small gpready exerted. In order to provide as many cues as possible, the range Ip&t the-controhshould extend from this small^ force^up itiUarr amount not cause fatigue when pressure must be maintained for a prolonged C. CONTROLS gloria] workers are sometimes required to perform repetitive tasks; to iigh rates of speed; to reach for, locate, and operate controls without the lion; to make sudden or corrective responses; or to operate over long l||:time without undue decrement of performance. Under these and ol'her MjJenkins, "The Tactual Discrimination of Shapes for Coding Aircraft-Type Con_____l|M. Fitts, ed., Psychological Research in Equipment Design, Rfipt. No. 19. USAAF |}1l)'!][V|hQlogy Program. U. S. Government Printing Office, Washington, 1&7, Ch. 14.