u/ClockAutomatic3367

Most noise guidelines are set based on the assumption that death of hair cell is the only kind of damage that can occur, which is basically what causes permanent threshold shifts. However temporary threshold shifts (which likely many of us experienced before the onset of hyperacusis) are thought to be caused by damage to synapses connecting to inner hair cells. The classical diagnostic criteria is that pure-tone audiometry readings are normal, while "digits in noise" test shows lowered performance.

Many of us also note that sounds which are supposedly classified as "safe" in brief durations (e.g. shower, blender, vaccum cleaner) for most people can nonetheless cause setbacks. This might possibly be a reason why.

Quoting from https://pmc.ncbi.nlm.nih.gov/articles/PMC11876062/

>NITTS [noise induced temporary threshold shift] is a real and measurable phenomenon. Additionally, any auditory symptoms after noise exposure, including tinnitus, likely indicate that permanent auditory damage has occurred [50]. The sound pressure level needed for the human ear to recover from NITTS, the effective quiet level, is approximately 55 dBA [51]. This is probably the safe noise exposure level to prevent NIHL from a single exposure, with 55–60 dB time-weighted average being the actual safe noise exposure level for a day.

So unlike standard NIOSH guidelines which say that 80dB LEq per day is "safe", here we have a claim that ~60dB is the safe daily (presumably per 24hr) LEq dose to avoid any ear discomfort.

Another source is mice models of cochlear synaptopathy. Jensen et al. show that 94 dB exposure for 2 hrs causes temporary threshold shift but no permanent damage to synapses, while 97 dB for 2 hours causes TTS that leaves lasting damage. So there really is quite a narrow gap, and humans auditory neurons are likely more sensitive than all other mammals so the thresholds for humans might be even lower.

The take away is that even under liberal guidlines, the thresholds that NIOSH provides should likely be shifted down by 5dB to account for the possibility of damage from cochlear synaptopathy. Whereas niosh claims that 91 dB is safe for 2hrs, this is might only be safe for up to 40 minutes. (The standard 3dB exchange rate is assumed due to doubling of overall signal power).

If you want to be really cautious and conservative however, the true 100% safe thresholds assume that only an LEq of 60dB for 24hrs is safe. Then using the 3dB exchange rate to scale appropriately, that gets us:

Sound Level (dBA)	Current Guidelines	Adjusted standard	Real-World Equivalent
60	No limit	24 hours	Normal conversation
63	No limit	12 hours	Background music
66	No limit	6 hours	Normal laughter
69	No limit	3 hours	Vacuum cleaner
72	No limit	1 hour, 30 min	Busy street traffic
75	No limit	45 min	Inside a moving car (highway)
78	No limit	22 min, 30 sec	Washing machine
81	No limit	11 min, 15 sec	Garbage disposal
84	No limit	5 min, 37 sec	Blender
87	~3 hours, 10 min	2 min, 48 sec	Heavy city traffic
90	~2 hours, 30 min	1 min, 24 sec	Power mower
93	~1 hour, 15 min	42 seconds	Tractor
96	~37 min	21 seconds	Power saw
99	~18 min	10 seconds	Hand drill
102	~9 min	5 seconds	Motorcycle
105	~4.5 min	2.5 seconds	Leaf blower

You'll notice that many daily activities (e.g. simply driving for an hour) can blow past the limits. Now I don't want to be alarmist and fuel anxiety by claiming that every setback means there's additional damage (since there are other valid proposed mechanisms for hyperacusis that are purely on the brain/sensitivity side of things rather than on the synapse damage side). But the fact that limits for recovery are this low does provide evidence that this is around the "safe" level we best operate in, and that people who cannot tolerate supposedly "minor" noise like street traffic are in a sense right to protect themselves from such noise that could them outside the safe window.

Lastly most people here are probably familiar with the pure-tone audiometry tests you can do yourself with an iPhone and airpods. If you're on Android, there's an equally good app for digits-in-noise test which has had results validated by published research. At least for me with relatively "minor" hyperacusis, it does seem that my speech intelligibility—while still in the green region—is closer to the cutoff than I'd like it to be. (The app also has pure tone audiometry, but its inbuilt calibration for headphones seems off by ±10dB, so I would not trust the absolute numbers for that test).