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

most things are spec’s +- dB so for example a speaker might be described as "flat" from 20 to 20kHz +- 3 dB. so it could be down 3 dB at 20 Hz which is half as loud at 20 Hz. No speaker is ruler flat thus when most speaker companies state their speakers are flat to 20 Hz they actually mean they are 3 dB down at 20 Hz or even -5 dB down at 20 dB or whatever.  Also if speakers are spec’s in an anechoic chamber their actual performance in a real room will usually vary considerably.

atmasphere
3 dB (down) 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.

actually decibels are already logarithmic; hearing is not on a logarithmic scale. the microphone used to measure the SPL is a listening device and measures what the human ear actually hears. 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.

mapman OP
"Actually here is what the ear hears and its far from flat at the extreme top and bottom range of our ears:

http://www.independentrecording.net/irn/resources/freqchart/ear_sensitivity.htm

For example according to this what measures correctly as flat at say 20 hz will be heard down more than 70db by the typical human ear. So significant equalization/boost is needed to "hear it as flat". Same true with the high end."

That’s absurd. You will pay big bucks to hear 20 Hz 70 dB down? Are you crazy? 70 dB down is almost as low as the noise. Besides, you're the one that says it's flat at 20 Hz. Hel-loo!

geoffkait: 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.

to which atmasphere replied,

"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."

Uh, the microphone hears what the ear hears. Forget the amplifier! You’re making it too complicated. You’re hearing 20 Hz at 3 dB down. Half as loud. Example - Suppose you were listening to a jackhammer and you measured the SPL where you were standing to be 110 dB. Then you stepped backwards - a few steps at a time - until the SPL measured 107 dB it would be then half as loud. 3 dB down.  Case solved.

Hartley 18" and/or 24" subs might be flat to 20 Hz. Everything else is a pig in a poke.

Of course there is always the *quality* of the bass to be considered. It's not simply a question of frequency response.