Insertion gain is the difference between the individual's unaided
response and his/her aided response. It refers to measurements actually
made in the ear with a probe microphone. Insertion loss is the hearing
loss that results from the insertion of a hearing aid or earmold into
the ear. The physical presence of the aid causes a loss that must be
overcome by amplification. At least these are how these terms are now
used in the hearing aid industry.