Z Decibel…..

Audiologists in any part of the world know and usually understand the decibel, as it is an integral component of an education in Audiology.  Some learned this complex concept from basic Audiology textbooks, classroom instruction or, possibly, Chuck Berlin’s  classic “programmed learning” booklet from the 1970s.   The Bel, named for Alexander Graham Bell is too large a measure to be useful in the measurement of hearing, thus the decibel, 1/10th of a Bel, has become our standard of intensity measurement.  One decibel is considered the smallest difference (or just noticeable difference) in sound level that the human ear can discern and is extremely useful in the measurement of hearing. Created in the early days of telephony as a way to measure cable and equipment performance, the Bel is a relative measurement derived from two signal levels: a reference input level and an observed output level. A decibel is the logarithm of the ratio of the two levels. One Bel is when the output signal is 10x that of the input, and therefore one decibel is 1/10th of a Bel.

As knowledge and use of decibels grew to measure intensity, the unit became also a measure of noise and noise exposure. As these measures became popular in the 1970s due to increased awareness of the damage to hearing from noise, there was a need to adjust these intensity measures to the response of the human ear.  The human ear is more sensitive to sound in the frequency range 1 kHz to 4 kHz than to sound at very low or high frequencies. Thus, exposures to higher sound pressures are acceptable at frequencies below 1kHz and higher than 4 kHz, since sound pressures at these frequencies will not cause as much damage to the auditory system as those presented to the mid-range frequencies.   To compensate for this sensitivity, sound level meters are normally fitted with filters  [dB (A), dB (B) and dB (C)] adapting the measured sound response to the human sense of sound.   In A-weighted filtering, acceptable values for the measured decibel values of sound at low frequencies are reduced, compared with unweighted (unfiltered) measurements in decibels dB (C), in which no correction is made for frequency.  The differences among dB (A), dB (B) and dB (C) measures are presented in the graph from EngineeringToolbox.com and are typically familiar to audiologists around the world that routinely deal with noise and noise measurement.

BUT………What is a dB (Z)

 There is, however, another type of decibel that is not so familiar to audiologists. It is the decibel (Z) or dB (Z).   The Z weighting is a flat frequency response between 10 Hz and 20 kHz ±1.5 dB, excluding microphone response.  Measurements made using ‘Z’ weighting are usually shown with dB(Z).  All of these frequency weightings are defined in the standards to which a noise measurement instruments are designed. For example, the frequency weightings used on sound level meters will be defined in the international standard IEC 61672:2003 (BS EN 61672-1:2003 .  This standard specifies the performance and tolerances for the frequency weighting curves to be used and replaces the old linear scales or unweighted scales for sound level meters.

The Z scale for decibels, however,  has a totally different meaning from the simple measurement of unweighted noise levels.  While quite familiar to Meteorologists, this decibel (Z) is virtually unknown to audiologists.  The Z scale is used differently in predicting weather, but the fundamental concept is the same.  In meteorology, a stimulus to an object and the echo returned is the reflectivity of the object.  The display of this echo intensity (or reflectivity) is measured in dBZ (decibels of Z, where Z represents the energy reflected back). “Reflectivity” is the amount of transmitted power returned to the radar receiver. Base Reflectivity images are available at several different elevation angles (tilts) of the radar antenna and are used to detect precipitation, evaluate storm structure, locate atmospheric boundaries and determine hail potential. In weather prediction, the dB (Z) scale stands for decibels relative to Z.  It is figured using the formula:

This form of dB (Z) is a meteorological measure of  equivalent reflectivity of a radar signal reflected off a remote object.  The reference level for Z is 1 mm⁶ m⁻³, which is equal to 1 μm³, which is related to the number of drops per unit volume and the sixth power of drop diameter. Reflectivity of a cloud , for example, is dependent on the number and type of hydrometeors, which includes rain, snow, graupel, and hail, and the hydrometeors’ size. A large number of small hydrometeors will reflect the same as one large one. The signal returned to the radar will be equivalent in both situations, so a group of small hydrometeors is virtually indistinguishable from one large hydrometeor on the resulting radar image.  As they review a situation, meteorologists can determine the difference between a large hydrometeor and a group of small hydrometeors as well as the type of hydrometeor through their knowledge of local weather condition contexts by considering the amount reflected back to the radar measured in dB (Z).

While this reflectivity use of the decibel is unfamiliar to audiologists in general, it is nonetheless another use of this scale that could be called Z….Decibel!