A coil assembly is coupled to a diaphragm. The coil assembly has a number of coils that are fixed in a stacked, end to end manner along a length axis. The coils include a middle coil, a several upper coils and several lower coils. Each coil has a respective pair of coil terminals so that it can be independently driven by an audio signal. The coil assembly as a whole is overhung, while each of constituent coils is underhung so that there are at least two adjacent coils that are completely within the magnetic gap, and at least two other coils that are completely outside the magnetic gap, during a majority of the full excursion of the coil assembly. Other embodiments are also described and claimed.