Damping Factor, What is it & how does it effects drivers & drive systems?
Damping factor relate to the output impedance of the amplifier, The lower the output impedance the higher the damping factor. The main way of getting a high damping factor is to have an amp with very high negative feedback. In the olden days high feedback amps were frowned apon as most had really bad sound. This was blamed on the feedback & for the most part this was in fact true but it wasn't the only factor. The biggest issue was having high negative feedback combined with poor phase margin. You see high negative feedback combined with poor phasemargin caused intermodulation distortion as well as in really bad cases oscilation because as you go higher in frequency & you get beyond a cirtain frequency the negative feedback actually will turn into positive feedback which can cause oscilation. Modern high feedback amps tend to have a better phase margin & less likely to devolop these issues so many sound way better than early designs.
High feed back desigms did not really come into being until transistor amps were deveoped. It was discovered that high feedback amps could take tighter control of speaker cone motion. What most people don't tell you is that the extra control costs in terms of power available as it actually takes some power to shunt the driver back electromotive force which it does through the power supply. This wis whi many amps that just barely deliver thier rated power into a 8 ohm resister may actually start to clip at significantly lower power into a speaker because some of the available power is being used to shunt the back electromotive force of the driver. It's like driving your car with the emergency brake on.It takes more power to drive it to the same volume. This is why the claim that tubes play louder for a given power output than transistor gear. Usually by about 3db & sometimes even more.
During the early days of transistor amps they came up with a speaker design that would make use of the increased damping factor. That type of speaker is known as the acoustic suspension speaker which the first commercialy available to my knowledge was from Acoustic Research. These speakers have a sealed speaker cabinate & the bass extension was determined by the drive system resonance with a 12db/oct rolloff after you reach this resonance on the way down in frequency. Many of these speaker were designed with a drive total Q of greater than .707, .707 being optimally flat. Higher Q's gave a bump in the response at the resonance of the drive system. These speakers if hooked to a low feedback vacume tube amp would sound to varying degrees a little sloppy in the bass due to the natural bump in the response that already existed in these speaker at resonance. What would happen is the impedance peak of the drive system would interact with the high output impedance causing the voltage to rise along with the impedance of the driver thus essentuating the peak in the drive system. Transistor amp having higher feedback & lower output impedance did not do this making them perfect for this design of speaker. You could have a reasonably small cabinate combine with a high compliance driver & still get decent bass extension.
This of coarse is not true of vented spakers as they have thier peak response generally at the port resonance which actually is at the bottom of the impedance curve , not the top like it is with the sealed speaker. For this reason the vented speaker responds quite well to amps with a somewhat higher output impedance as in many cases the response will be smoother & slightly more extended I might add with the somewhat higher output impedance. Bear in minde these speaker designs were designed in the hayday of higher output impedance vacume tube amps. Horn speakers were likewise designed during this era & many of these designs sound horrid on low output impedance transistor gear.
Crossover design must also take into account the output impedance of the amp that is driving them as well as the speaker impedance which is why you rarely see balaced crossover designs anymore as speaker & amp impedances are no longer matched as they were on the best tube gear of old. Here you could easily build a balanced cossover (using same parts for woofer & tweeter crossovers) & not end up with a hole in the response, Any more the woofer & tweeter parts are different as there would be a hole in the response if not with low output impedance amps.
While I've been talking speakers here similar applies to earphones as well, some designs work well with high output impedance & some don't & of coarse some don't really seem to care either way. It is foolish to say there is only one proper way to drive a speaker of any type. You have to drive them the way they are designed to be driven. Etymotic earphones tend to work beat with a higher output impedance.