Discussion - Quantitative Vibration Evaluation Of Modified Rock Drill Handles

J. Dasher Regarding the March 1991 ME technical paper by De Souza and Moore: For more than a decade since my February 1981 article on how to use modern metric, which SME-AIME had decided to do, I have monthly pointed out metric errors to the editors. In part, I do this because there has been no action to allow editors to fix figures and tables or to allow them to require authors to do so. The latest resulting atrocity provokes this discussion of vibrating drill handle units being stated in decibels. Reply by T. Moore We have read the discussion of our paper by Mr. Dasher. Our reaction is one of surprise and incredulity. It would seem that Mr. Dasher takes exception to the use of the decibel scale to present vibration acceleration data, and the use of hertz as the unit for frequency. The basis for his objection to the decibel appears to be that it has no dimensions (which somehow invalidates its use), that it is "non-metric" and, finally, that it is parochial (of limited or narrow scope). His objection to the use of the term hertz is not stated, but we will assume that it stands condemned as "non-metric" and parochial. Obviously we disagree with Mr. Dasher's views and will now outline our reasons. Although the decibel scale originates from transmission line theory and telephone engineering, it is also at present widely used, not only in the fields of electronic engineering and acoustics, but also in the area of vibration. The original definition of the decibel (dB) was based on power ratios: dB = 10 log 10(W/W0) where Wo is a reference power. However, as the power measured across a given impedance is related to the square of the force acting upon this impedance, Z, a more commonly used definition is: [2 dB = 10 logF /Z) = 20 log F/F 10\ F0 2 /Z(0)] where F and F0 are the r.m.s. values of the forces. Now, if the measurements are related to one and the same impedance, the decibel notation in the form of 20log10(X/Xo) may be used as a convenient relative magnitude scale for a variety of quantities. Thus, X may, for instance, be an r.m.s. displacement, velocity or acceleration. It is only required that XD always be a reference quantity of the same type as X. That is, when X represents an acceleration, then X0 represents a reference acceleration. This is the formulation used in our paper. This was not an arbitrary choice on our behalf but reflects standard practice as specified in the International Standard ISO 5349-1986(E) Mechanical Vibration - Guidelines for the Measurement and the Assessment of Human Despite the metric prefix, the decibel is a parochial expression of (l) the logarithmic ratio of the loudness of a sound to what is normally audible or (2) the logarithmic ratio of two power signals in radio or electronics. A decibel is not a unit, much less an SI, unit and has nothing whatsoever to do with the acceleration of drill handles. Stating that m/s2 (acceleration) is decibels is without reason. Whoever reviewed this material should not have allowed publication of figures of dB and H.[ ] Exposure to Hand-Transmitted Vibration. This was clearly stated in the "measurement protocol" section of our paper. This quantity is then referred to as the acceleration level and is expressed in dB. We may have inadvertently caused some confusion when we simply used the term acceleration to refer to acceleration level on our diagrams. At the time, we felt the use of dB or m/s2 would make the context clear to the reader. For any confusion this decision may have engendered, we apologize. Since the decibel expresses the ratio of two like quantities, it certainly has no dimensions. It is, however, common practice to treat "decibel" as a unit as, for example, in the sentence, "The acceleration level measured at the operator's hand was 160 dB." The expression of measured quantities in dimensionless form is not inherently unacceptable. In fact, in many areas of engineering it is standard practice (consider the use of Reynolds Number, Nusselt Number, etc.). The fact that the decibel is a dimensionless quantity makes the question of whether it is a SI unit nonsensical. However, it is valid to insist that the dimensional quantities used to obtain the decibel values be expressed in SI units. A careful reading of our paper will make it clear that the measured acceleration was, in fact, expressed in units of m/s2 as was the reference acceleration (l x 10-6 m/S2). These are the accepted derived SI units for acceleration. See, for example, the standard ASTM E380-89a Standard Practice for Use of the International System of Units (SI) (The Modernized Metric System). Concerning Mr. Dasher's implication that hertz (Hz) is an unacceptable unit of measure for frequency, we would again refer him to the standard ASTM E380-89a. Here, he will find (section 2.4.2) that hertz is an accepted "special name" for the derived SI units-1. This is in keeping with numerous other international standards including ISO 5349-1986(E) to which we referred in our paper. In conclusion, we agree with Mr. Dasher on the desirability of expressing measurements in modern SI units. But we would remind him that the standards that define the use of these units, and the accepted means of presenting measured data, are in a continual state of refinement. It is, therefore, incumbent upon him to keep abreast of these changes if he wishes to constructively critique the work of others.[ ]