Sound & Units: dB-A /B/C/D/Z
In addition to the ubiquitous dB(A) sound level frequency weighting standard set by Fletcher and Munson back in the 1930s and previously written about here [link], there are many other sound-level frequency-weighting standards. Those of us that have used a basic sound-level meter—purpose made and calibrated or app on phone—are likely familiar with the dB(C) scale too, or at least wondered what the A/C or A/B/C-weighting switch did. There are dozens and dozens of additional sound weighting standards and each specification has an intended use. Today the letter standards are regarded as overly simplified and and are rarely used exclusively in technical sound work. Even so, these standards continue to be used by cities governments, venues, bands and meter-in-hand sound sniffers as objective standards, particularly dB(A) and sometimes dB(C), both mandated as part of the IEC 61672 class-1 sound level meter.
A-weighting is commonly used for environmental noise like roadways and aircraft, mechanical sound levels like dishwashers and air compressors; and in assessing potential hearing damage from sound exposure. Originally the equal-loudness dB(A) weighting was developed for low-level sounds.
The dB(B) sound level frequency weighting specification gets a short shrift as it is rarely used. In fact, I can't think of a time it was referenced by anyone outside historical reasons.
C-weigthing averages a large swath of orchestral and other big musics and corresponds to that averaged power spectrum.
D-weighting is rarely used, developed primarily as correlative weighting for aircraft noise.
The end of the letter-weights, this weighing has zero emphasis or de-emphasis throughout the bandwidth of 10 to 20,000 cycles per second (Hz).
For easy reference here are the curves for these common sound level frequency weightings: