I always am amazed by how much the study of music and musicians teaches me about basic audiology. One of the first things we learn in our first course on noise and its control in school is that the audiogram for noise exposure should be symmetrical. Asymmetries in the audiogram are most likely related to non-noise etiologies such as the protective aspects of a long-standing conductive component or even some retro-cochlear pathologies.
The reasons given in my Audiology 101 class is that occupational noise is predominantly low frequency and that workers typically live their lives in highly reverberant environments. Low frequency sounds have long wavelengths and therefore do not “see” the head as an obstruction (our heads are only about 8-9″ wide). Even if these low frequency sounds emanate from the right side, the sound pressure level hitting the right ear would be almost the same as the sound pressure level at the left ear- there is no head shadow for lower frequency sounds. In addition, even if there was significant mid and higher frequency sound in the noise, such as in a stamping plant or one with a lot of riveting noise, the environment is so reverberant that even if that noise emanates from the right ear, the echoes at the left ear would be almost as intense.
Violin, viola, and drummers typically have a slightly greater hearing loss in their left ear than their right. This is shown below. These instruments contain significant mid and high frequency sound energy and the sound pressure at the right ear is shadowed by the presence of the head. Most, but not all drummers, have their high hat cymbals on their left side. This is the main culprit with drummers and its related to a combination of hitting the cymbal with the right arm so that there is more leverage in hitting it harder, and the mechanical/acoustic characteristics of the cymbal itself. Among other things, drummers should be encouraged to practice with their high hat cymbals closed or even a rubber absorbing practice pad placed between the top and bottom cymbal parts of the high hat. Also musicians don’t play in highly reverberant locations with the exception of a few unusual venues such as subway platforms…
Audiometric asymmetries do not mean 40 dB differences- just up to about 20 dB. Larger differences should be investigated with the appropriate test for a retro-cochlear lesion such as ABR or an MRI.