Perhaps you could clarify what article you intended to reference.
Your link actually points to an article on ototoxicity caused by salicylates. That article does not claim that caffeine has ototoxic effects. The graph appears to originates from a different article that does discuss caffeine but again does not claim that caffeine has ototoxic effects.
A brief review of the literature on caffeine, it's clinical monograph and pharmacology did not find claims that caffeine even in toxic overdoses is ototoxic.
Caffeine is implicated as causing or aggravating numerous otorhinolaryngological conditions, including tinnitus by clinicians. This is not claimed to be a causal relationship, but is ascribed to the vasoconstricting properties of caffeine. However two major reviews of the literature identified more than 417 papers a review of which concluded:
At present, there is little evidence in the literature to support the
notion that caffeine causes or aggravates otorhinolaryngological
conditions. In tinnitus, its withdrawal may actually worsen symptoms.
You want to know "what amount of coffee has an ototoxic effect after an acute noise exposure". As one might expect quantitative data in humans appears to be unavailable and it would be difficult to persuade a medical ethics review board to approve such a study. There is limited animal data available but it could not be applied to human cases with any meaningful credibility.
The characteristics of tinnitus in workers exposed to noise
The role of caffeine in otorhinolaryngology: guilty as charged?
Caffeine abstinence: an ineffective and potentially distressing tinnitus therapy