Part 1 in of this series of blog posts gave an overview of why musicians may have different degrees of hearing loss in their right and left ears. Typically, hearing loss is thought to be equal in both ears, or symmetrical, unless there is a reason for a difference. Clinically the reasons may be a growth or schwanoma on the VIIIth auditory nerve, or perhaps something less significant such as a virus or a previous acoustic trauma near one ear, such as a firecracker or blast occurring off to one side.
Music is an entirely different animal from noise. Well, perhaps I should say that music is a different species from noise but in the same genus. Music and noise are both vibrations in the air that people can hear and can easily measure using instrumentation, but that is where the similarities end.
In many industrial workplaces, noise emanates from machinery, where metal is in contact with metal. The noise tends to be lower frequency in nature with most of its energy on the left side of the piano keyboard.
In music, the sound emanates from contact between horse hair strings or plastic picks, and wire (stringed instruments such as violins and guitars), thin wood reeds and plastic or wood (woodwinds), sudden bursts of energy (percussion), and resonances of hollow, or semi-occluded tubes (flutes and brass). Not only is the production of music much more variable than the noise of industrial machinery, but the resultant sound tends to have significant energy on the right hand side of the piano keyboard. Music has more significant treble energy content than noise.
So what does this have to do with asymmetrical hearing loss? The answer lies in the realm of the properties of different wavelengths of sound. Audiologists typically speak in terms of frequency (α 1/wavelength) and not in terms of wavelength. The higher frequency sounds have shorter wavelengths than the lower frequency sounds; they are inversely related to each other.
There is a rule of thumb in physics that is relevant here: all sounds whose one half wavelength is less than the obstruction width are attenuated by that obstruction. In English, this means that the shorter wavelength, higher frequencies sounds are more obstructed than the lower frequency sounds that have longer wavelengths.
In the case of a violinist (playing the violin off to the left side), the human head is 20 cm wide (or 8” in American). All sounds will be obstructed by the head if their wavelength is less than 10 cm (or 4”). Low-frequency sounds that are commonly seen in industry do not see the head as an obstruction. Low-frequency sounds that emanate from machinery near the left ear are almost the same sound level by the time they reach the right ear. The 20-cm wide head is invisible to these low-frequency sounds.
In contrast, much of the harmonic (and some of the fundamental) energy of a violin, being in excess of 1000 Hz, does see the human head as an obstruction, and therefore music that is generated near the left ear can be significantly reduced in energy by the time it reaches the right ear. Music exposure can be asymmetrical.
We occasionally come across industrial environments that have significant mid- and high-frequency noise energy. When we do, the same reasoning that applies to music also applies in these industrial situations, but this is relatively rare. Stamping plants and riveting come to mind. In the case of riveting, a right-handed person would hold the riveter in their right hand with their left ear nearer the noise source (the rivet hitting the metal structure that is being secured). Riveting, like stamping plants, are characterized by sudden noise pulses and calling a sound “sudden” is a less-fancy way of saying “high frequency”. ABR clicks are sudden, and indeed have most of their energy in the higher frequency region.
Music, being characterized by a rich higher frequency harmonic structure, does see the human head and body as a baffle that attenuates the sound as it traverses from the left ear to the right ear. Audiometric asymmetries are therefore common among musicians, but relatively rare in industrial workers.
Musical instruments that can yield audiometric asymmetries consist mainly of violin, viola, and percussion drum kits. Drum kits typically have their high hat cymbals on the left side and the exposure, because of wavelength phenomena discussed here, is greater on the left hand side. Audiometric asymmetries are routinely observed with the left ear being worse than the right.
BUT these asymmetries are on the order of 10-15 dB and not 30 dB or more. If there are larger asymmetries between ears, other causes should be investigated with the appropriate retro-cochlear audiologic and otologic tests.
