dBA vs dBc vs dBz

Hi, there. Which scale is the most accurate to use in occupational and non-occupational environment?

 

<<

• Z Weighting (dBZ):
  ALWAYS use the Z weighting to evaluate whether a hazard to hearing exists.
  
  Uses:
  
  1. Most sound and noise above 85 dB SPL which does contain infrasound or frequencies below 100 Hz
  2. Measuring machinery noises that can be hazardous to human hearing
  3. Measuring music that can be hazardous to human hearing
  4. Level of loud classical music
  5. Measuring louder sounds at the source
  6. With a tone generator, finding the frequency response of the sound system.
  7. With a tone generator, finding the frequency response of the room >>

<<

• A Weighting (dBA):
  NEVER use the A weighting to evaluate whether a hazard to hearing exists.
  
  Uses:
  
  1. All sound and noise below 55 dB SPL
  2. Measuring noises that are annoying, but not dangerous (e.g. sounds that keep people awake)
  3. Measuring the annoyance factor of most machinery noises.
  4. Measuring the annoyance factor of traffic
  5. Measuring the annoyance factor of commerce
  6. Ambient noises inside an auditorium or concert hall
  7. Level of quiet classical music
  8. Measuring louder sounds at the source, where the listener will be far enough away to hear it below 55 dB SPL
  >>

<<

• C Weighting (dBC):
  NEVER use the C weighting to evaluate whether bass notes are a hazard to hearing.
  
  Uses:
  
  1. Most sound and noise above 85 dB SPL not containing infrasound or frequencies below 100 Hz
  2. Measuring most machinery noises that can be hazardous to human hearing
  3. Measuring loud music that can be hazardous to human hearing
  4. Level of loud classical music
  >>

source: https://midimagic.sgc-hosting.com/spldose.htm

 

Theoretically true, but practically - we are at the mercy of phone apps and the various levels at which Android or iOS microphones are able to capture and process sound. Few (and all of them VERY expensive) SPL meters actually include Z-weighting. Apps like Decibel X and NIOSH SPL do include Z-weighting, but you have to know how accurate your phone's ability to process sound is. From my personal experience (I have a calibrated Class 2 sound level meter), iOS devices are closer than Android devices to the actual dBA reading that a calibrated SPL meter shows. When switching to dBZ, by the way, the reading usually jumps up by something like 6-8 dB.

Just to give an example: according to my calibrated sound level meter (an Extech SL510), the loudness of my white noise machine (about 2 feet away) is around 41 dBA/49 dBC. Decibel X on an iPad shows a reading of 37 dBA. When I switch Decibel X to dBZ mode, it indicates a noise level of around 43 dBZ. Given that the Extech sound level meter is more accurate through calibration, I would conclude that the dBZ reading for my white noise machine would be probably around 46-50 dB.

Oddly, the NIOSH SLM app on my iPad gives high values that tend to get closer to the reading of the calibrated sound level meter as the sounds get louder. Its reading may be as much as 10-12 dbA over that of other apps and then gradually the differences get smaller. According to it, my white noise machine generates 51 dBA/58 dBC/59 dBZ.

In conclusion, I believe that iOS and Android apps are not entirely accurate, though if one has a properly calibrated sound level meter, usually most varieties of sounds below 70 dBA or 75 dBC should be fine. If I am unsure, I would use the dBZ reading on Decibel X and give it a few extra dB, knowing that it tends to read a little below the actual value that the calibrated sound level meter shows. We are constantly exposed to micro-spikes that exceed these levels, but our ears are usually more resilient than we think.

 

The problem with dBC, dBZ or other non-dBA measures for hearing protection is that ANSI, NIOSH and other industrial standards are all based on dBA. The principle characteristic of dBA weighting that's of concern here is that it rolls off low frequencies very substantially, much more so than any of the other weightings discussed here.

When I was studying acoustics in the mid-1970s a question I had was, does the A weighting accurately reflect the sounds that can damage hearing? After all, noise-induced hearing loss is characteristically focused most in the 4 kHz octave band, to the extent that many audiometric tests of those who have known noise-induced loss show by far the greatest loss in that band. The 4 kHz octave band also corresponds with the most sensitive frequencies of normal human hearing, as evidenced by the Fletcher-Munsen and later curves.

Hearing sensitivity in ranges outside of the 4 kHz bands is also damaged by noise, though apparently not as much. But nobody suggests using a weighting that reflects a spectrum of what we know are the harms caused by excessive noise exposure. In other words, it's not AT ALL clear that A weighting reflects the relative hazards from the frequency components of high SPLs. I suspect that the reason dBA is used may come down to that filter prioritizing sounds that can most readily be excluded by noise reduction measures and protected against through use of hearing protection. Writing a standard based on unfiltered SPL, or dBZ, or even dBC would mean huge costs to industry in addressing the lower frequencies where a great deal of industrial noise resides.

Nearly half a century on we seem no closer to an answer, let alone a consensus.

On a related topic, I've wondered about cabin noise in automobiles, even those which are relatively luxurious, where low frequency highway noise can be substantial. Again, our hearing isn't especially sensitive to low frequency noise as it is to the upper-mid frequencies where we typically see noise-induced loss, but that doesn't mean that high SPLs in the lower registers aren't also damaging to some degree even before those sounds become objectionable to most people.

I wore hearing protection when I played in bands in the 1970s and 1980s, which was unusual in those days. Years later I had a home near a wetland, and sitting out on the deck of an evening I could clearly detect the bats echo-locating as they went after our friendly mosquitoes. I say this only to point out that I'm sure my hearing came through my rock band days in good shape. Well, those days are long past. Now I have substantial roll-off within the limited range that audiometric testing looks at, as well as tinnitus that's come on strongly in the last year. I've wondered whether my loss was related to my many years of commuting, because apart from that I've always taken care to protect my hearing. Highway traffic laws in every jurisdiction that I know of prohibit wearing any kind of hearing protection, which I suspect we may one day learn has been a mistake.

 

At what age did your hearing start to significantly roll off? And have you seen it continue to progress by itself?

I think that maybe noise damage simply doesn’t show for the first 80% of damage, then becomes increasingly apparent when we start to lose the final 20% in a given frequency.