There is a very common decibel-based sound level scale that I see casually tossed around all over the place with 0dB often described as "the threshold of human hearing" and typical non-specific entries like this:
- 30dB: Whisper
- 60dB: Normal conversation
- 110dB: Sporting event
- 130dB: Ambulance
Examples (there are many) include:
- Hundreds of colorful charts
- CDC page on hearing loss
- US DOD's "Hearing Center of Excellence" (TIL we have that) page about hearing injury
- Some spammy looking blog
As these charts get passed around and reproduced, there seems to be a bit of a drift in the specific numeric values, as well as inconsistencies in signal description (e.g. "dB" and "dB(A)" being used interchangeably).
The Question: What are the specifications of this scale (or do none exist)?
A Note: By "specifications", I mean the typical set of expected information:
- Does it have a technical name / standardized definition?
- What is the actual reference level and measurement unit? (In this scale, it usually seems identified as "the threshold of human hearing", but that obviously has a lot of variance and also I'm not sure what that is.)
- Assuming it's referring to pressure, is there some specific distance to the sound sources identified in this scale?
- Is it meant to apply to unmodified measurements, or is it meant to use some frequency weighting?
- When using it, is there a standardized, or at least widely-recognized, prefix/suffix to the "dB" abbreviation that indicates use of this scale?
- Where did the typically given measurements even come from in the first place?
You can stop reading here if you are not interested in my guesses about answers so far. You can continue reading if you want to see my reasoning and find it helpful to build off it.
Regarding the weighting:
I often see it related to hearing in humans, and also "the threshold of human hearing" in absolute units is presumably frequency-dependent. So from that context, it seems like it would describe weighted signals -- most probably A-weighted -- but it's hard to tell because usage of "dB" vs. "dBA" in such charts seems inconsistent, and most of them seem to just use "dB". Still, given that the target audience for these charts are generally human, it would be misleading to try to relate the decibel levels to familiar events ("whisper", "refrigerator", "jack hammer") while referencing a non-weighted signal, so A-weighting does seem likely. Is that the case?
Regarding the reference level and units:
Firstly, I'm assuming that this scale has sound pressure in mind (albeit at some unknown distance). It doesn't seem like it's describing power -- that wouldn't make sense in the contexts I always see this scale in, where distance to source matters. It also doesn't seem like it's describing intensity, because ... well, just a gut feeling (plus something about basing it off the "typical human's cross-sectional ear canal area" or whatever feels odd). That said, I've never seen one of these charts refer to "dB(SPL)" or similar.
So I've noticed, in real observations, a rough correspondance to the numbers in this scale when using various SPL meters in the past. For example, I have a Reed R8070SD that registers in the 75-95 dB(A) range when I am standing ~10ft away from a busy city street during rush hour, this is roughly similar to the 80-90 "dB" typically given for "loud traffic" in these scales.
Now, the SPL meter reference level is sort of (afaict) defined in IEC 61672 as "conventionally" 20µPa, but that "convention" is solidified by IEC 60942 (sound level calibrators; when calibrating the meters), which clearly defines the same 0dB = 20µPa reference level (note: most calibrators I've seen use a 1kHz tone [not sure if that's standardized] which has identity gain in all standard frequency weightings, so the weighting is irrelevant there).
Anyways, because I've noticed that SPL meters (calibrated with IEC 60942 compliant calibrators) seem to be in the general ballpark of the numbers in the mystery scale, and also because it seems likely that the scale is describing pressure, that makes me believe that the reference level in these scales is also 20µPa. Is this correct?