What are the smallest speakers that are clean and flat down to 20hz?

Headphones!

Grado made a line source array that they showed at CES one year, just for fun. It made decent bass but it had a lot of drivers in it. Seems to me it was over 6 feet tall and they won't be manufacturing it.

Otherwise you're simply going to have to have a big speaker.  My speakers go to 20Hz and have two 15" woofers. You probably can do it with one. The smaller the driver, the more excursion it needs and that's your limit. Bruce Thigpen got around that limitation with his subwoofer fan.

Click on the link below and then products (TRW-17):

http://www.eminent-tech.com/main.html

From the page:

A conventional speaker cones displacement must increase four times for each halving of frequency to maintain the same output. This is why conventional cone woofer companies are trying to develop “long throw” woofers. Although inefficient, cone woofers work fine above 40Hz. Below 40Hz however cone woofers quickly run out of travel and the output diminishes rapidly.

3 dB at 20 Hz which is half as loud at 20 Hz

To be audibly half as loud is usually considered 6-10db on account of the ear being on a logarithmic scale.

I really mean  the speaker does not roll off much or at at all at 20hz in a typical room.  How flat it is or not will largely depend on room acoustics from there right?  If it can only be done in a smaller room fine, just say so.  

The room will have some influence certainly, but if you really want 20Hz response **with any sort of dynamic range**, its not going to be a small speaker.  Note emphasis.

As far as 'the room will have some influence certainly'.  I completely disagree with Ralph on this because he is seriously underestimating the effect  the room has on bass response and has chosen to ignore the mountain of scientific data compiled on this subject

I think actually we're on the same page. But Mapman is talking about 20Hz response and so am I. That's a bit different than **perception** of bass, which is very much affected by the room (for example I have a room resonance at 26Hz in my room, which tends to reinforce bass impact, but its at 26Hz no 20Hz. I tend to be very literal it that helps...). IOW the room isn't going to change the actual frequency response of the speaker, although it will affect to a large extent the **perception** of bass coming from the speaker.

Duke LeJeurne of Audiokinesis makes an excellent subwoofer system he calls 'The Swarm'. It is a set of multiple subs that can be distributed so you don't have problems with bass in some parts of the room (like the side wall) and not in others (like the listening chair).

As a general rule of thumb I usually regard the room as half of the overall system sound.

If the SPL is 3 dB down at 20 Hz then you will hear 20 Hz as half as loud as any frequency that is measured as 0 dB. If 20 Hz was found to be - 6 dB it would be 1/4 as loud as a frequency at 0 dB.

Actually in the above case being down 3 db it would take double the amplifier power to make up the difference but the ear would not hear the -3db as half as loud. If the speaker were 6 db down then that tends to sound closer to 'half as loud' and would take 4x amplifier power to correct.