Updated here:
http://rinchoi.blogspot.com/2010/05/how-deep-do-you-insert-you-er4.html
Etymotic Research approved the article.
This means that most of the IEM manufacturers have been hiding the most vital part of the equation in using IEMs- proper coupling of the earphone and the ear itself! Usually, conventional IEMs, especially custom-moulded ones(you better double check the custom-manufacturer's capability), reach no deeper than the 1st bend due to technical difficulties & safety reasons. This kind of shallow insertion would not secure the residual ear canal volume of 0.5cc(or the distance of 13mm from the eardrum to the tip of a sleeve), and cause some degree of linear distortion on IEMs.
Edited by udauda - 12/21/10 at 4:13pm
