Dynamic (moving coil) transducers are often found on low to entry class IEM due to their ease of availability and lower cost (relative to BA transducer). They commonly range from 8mm to 16mm in diameter. Dynamic transducers are known for their ability to create a more powerful bass response since relatively more air is moved during sound reproduction. More air movement means stronger bass sensation as we tend to ‘feel’ low end bass more than actually hearing it. The downside of using dynamic transducer on IEMs is its larger size. It is also worth noting that many big brand name manufacturers use dynamic transducer of relatively low sound quality in their IEMs; therefore you should avoid most of them as much as possible if sound quality is your priority. Of course, there are also IEM manufacturers who specialize in producing high end dynamic transducer IEMs, most noticeably Future Sonics, one of the very first IEM manufacturers.
BA transducers have the benefit of being really small, therefore you can often find many IEM makers using 2 or 3 BA transducers in each side of their ‘phone. By dedicating at least one of the BA transducers as a woofer, multi-ways IEM tend to have better bass performances than most single-way BA transducer IEM (but not always). The use of multiple BA transducers partially solves the bass problem as BA transducers move a lot less air than dynamic transducers (and noticeably less low end bass). The downside of BA transducers is its higher price tag.
There are also hybrid IEMs (ie. UE Super.fi 5 EB) that utilize both dynamic and BA transducers together in order to have the best of both worlds. However, reviews of such type of IEM are often mixed.
SOURCE : Basic Guide To In Ear Canalphones - Head-Fi.org Community
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You can look up at wikipedia here for some information on headphone transducers.
