Frequency Response?

62bakes - One way they increase extension of the speaker is to put second woofer for the lowest bass only (higher frequencies filtered out by xover). For two way speakers it is called 2 1/2 way speaker (pretty much like adding subwoofer). I suspect that small drivers, being point source, will have more interaction with the room than large woofers where bigger percentage of the sound goes direct.

Distortion might be caused by by nonlinearity of the motor especially at max extension of the membrane. Most of drivers are designed as overhanged type where magnetic gap is narrow but coil is long extending outside of the gap. Some drivers (like in Acoustic Zen Adagio) have underhung motor meaning that gap is very wide and the whole (narrow) coil is inside of the gap. This scheme is more linear but requires drivers with very large magnets - more expensive. Some tweeters are made that way as well (to lower distortion at max power) including Morel Supreme.

Kijanki - the speed of sound in dry air is 343.2 M / Sec = 1126 ft/Sec

Pbnaudio, We don't have exactly dry air here (Chicago) but I will settle for 1135ft/s which is speed of sound in dry air at 75degF - and this is my last offer.

Same example with new numbers: 1135/18/2=31.5Hz

62bakes: You can use just the high pass section of an active crossover, like a Marchand. Doing something similar with a NHT X2, which has a relatively simple choice of 3 fixed 12 dB/octave filters. Except for the Pass XVR-1, most active xovers use opamps which are not always totally transparent. 24 dB/octave filters are usually cascaded (series) but have their advantages. Then, there's digital xovers which require another ADC/DAC conversion but can offer incredible versatility.

A 5 string bass with a low B string creates, at least at the fundamental tone, a 31hz signal...using test tones I've found that my system (including a sub) easily reproduces that frequency, and a nice test is the bass note at the beginning of Donald Fagan's "Morph the Cat"...a solid B. I do use a phono preamp with a rumble filter (20hz), and before that (when I had a preamp with a phono stage) I found an older pair of Nakamichi line filters that worked really well to tame woofer rumble floppage. I think in live music there's a lot of stuff going on under 40hz, like kick drum overtones (undertones?) subway tunnels under the floor, or heavy footed waiters bringing my bar tab check.