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Power Level and Volume

If a given reference value is chosen, the power level, or power being transmitted past any point in a system,1 may be expressed as so many decibels above or below the selected reference. For several years the reference point or zero level chosen was approximately 0.006 watt, or 6 milliwatts. On this basis, if a circuit were delivering 60 milliwatts, it was operating at a "plus 10 db power level." Or, if operating at a "minus 10 db level", the power at that point would be 0.6 milliwatt. A zero level of 0.001 watt is common.

It is also common practice to select some strategic point in the circuit as a reference point and express the power level in decibels in other parts with respect to this arbitrary point. Thus, care must be exercised in interpreting data because of the different zero levels used. Because of this, there is a growing tendency to use the designation dbm when the zero level is 1.0 milliwatt (or 0.001 watt).

Power levels are used in circuit tests where steady-state sine-wave currents are employed. Such factors as the ballistic characteristics of the measuring instrument do not, therefore, affect the measured values. For measuring program levels on circuits transmitting speech and music, where the currents are of transient nature, the characteristics of the instrument do affect the measured value.

The volume at any point in a telephone circuit is a measure of the power of a voice-frequency wave at that point,1 Volume is expressed in decibels with respect to some arbitrary reference standard, A special volume-level indicator or VI has been developed23 for measurements in program circuits. This reference specifies (a) the characteristics and method of use of the volume-indicator instrument, and (6) that the steady-state reference power is 1.0 milliwatt in a circuit of 600-ohms characteristic impedance. The term volume units (abbreviated vu) is used on the scale of the volume indicator, one volume unit denoting that the program volume level is one decibel above zero reference volume.

Last Update: 2011-05-30