I found this study Hearing loss in motorcyclists: occupational and medicolegal aspects
Andrew W McCombe, MD FRCS(ORL) 2003 to be most thorough, it also has links to six other studies which you could use as further reading:
However, over the past 10-15 years, as motorcycle development has led
to quieter machines with radically improved performance, there has
been increasing concern that riders are exposed to excessive noise
through turbulent airflow around the helmet—so-called wind-noise. The
seminal report on this topic (although not the first) was that of Van
Moorhem et al. in 1981. Apart from the work of our group, there have
since been only six published reports on the subject. (See article for
All groups have used essentially similar techniques: a miniature
microphone is placed at the rider's ear under the helmet and sound
levels are measured in various riding conditions. All these studies
show excessive wind noise around the helmet—about 90 dB(A) at 60 km/h
and increasing linearly when plotted against the log of speed, to
reach 110 dB(A) at 160 km/h.
Modern helmets, we found, offered very poor low-frequency sound
attenuation and we also demonstrated a phenomenon of resonance at 250
Hz. The source proved to be a turbulent boundary layer, vibrating
against the outside of the helmet shell, with its maximum sound energy
focused between 250 and 500 Hz.
When we used more suitable controls, we did confirm hearing loss in
motorcyclists. We also identified a temporary threshold shift
after only 1 hour of high-speed riding and a corresponding subjective
complaint of tinnitus. After long periods at high speed, riders
commonly report other non-specific complaints such as fatigue,
headache and even disequilibrium. Similar symptoms have been described
in industry and elsewhere.
60 km/h is common on urban streets, but if you are regularly traveling along highways or touring you can be sitting at 100 km/h for reasonable stretches which means above 90 dB for an extend period of time. According to several sources this is sufficient to cause hearing damage.
Example sources for above claim: