Rob Baum then provided a thorough review of all things headphone and earphone, reminding us that many factors need to be considered in a given design.

He began by reviewing the human auditory system, then a little headphone history. The headphone came about with the advent of radio communications. Popularity took a big jump with the personal cassette player in the 70s, again with the iPod, and now with smartphones.

It was noted that with headphones, you hear none of the acoustic room effects you get from loudspeakers. IHL (in-head-localization) is yet another headphone design consideration. He pointed out that different headphone designs alter the outer ear in different ways, changing the HRTF (head related transfer function) and localization. It is hard to make a headphone that localizes nicely.The performance and value from headphones can be nice, but there's also a greater potential for deafening SPLs and hearing loss.

Moving on to headphone and hearing testing, several standard methods of testing were discussed. One can measure human ear response with a small measurement mic almost against the eardrum, at the entrance of the ear canal, or one can block the eardrum with the mic. In any case, human response is not flat, so a good headphone is not designed flat itself, but to give net flat response to the human. Tests of headphones and speakers were described and results shown.

Many design goals were discussed, such as non-accurate sound, greater intelligibility, free field vs diffuse field simulation, air seals, leakage considerations and bass, and mechanical cable noise.

Traditional headphone styles include circumaural (cups around the whole ear); supraaural (sits on top of the outer ear); and off the ear (sits off the ear). Their many characteristics and pluses and minuses were explained. Then earbud and in-canal devices were discussed.

Finally, business aspects of 'phones was discussed, including research, offshore manufacturing, and marketing. Cell phone requirements are driving a lot of development.

After a break, door prize winners were chosen, then a final Q&A session covered many topics, from experiences with different in-ear monitor materials to hearing loss and loudness limit laws.