http://blog-imgs-52-origin.fc2.com/m/o/n/monoadc/Insertion_depth_AURVANA_In-Ear2.png
http://blog-imgs-52-origin.fc2.com/m/o/n/monoadc/Insertion_depth_MA850GB.png
http://www.audio-technica.co.jp/atj/html/hp/whatis/img/zu_29.gif
https://www.youtube.com/watch?v=Dtb-eAP7IxQ