A full range speaker?

The bottom octave 20 to 40 Hz is critically important. You can not get the sense of a live performance without it. The breath and feeling of the music disappear. Drums and bass become two dimensional. Synthesized bass just dies. I have never heard a speaker system produce this correctly on it's own not that it is impossible but, it seems to be much easier with subwoofers. It takes more than subwoofers. You have to put them in the right places in a well treated room with additional room control and digital bass management. The subs have to have adequate power and be timed correctly. If not done well you have just mud and a detriment to the rest of the system. This is the most difficult part of the spectrum to get right and a zillion ways to get it wrong which is why there are so many opinions on the subject. A large part of the problem is room acoustics. Even with the best equipment the room can severely F up the bass. Rather, I should say that the room will F it up. +- 10 dB in an octave is not uncommon. The end result is what I call one note bass. There is no one solution to this problem. It helps if the room was designed as a listening room to begin with but most people do not have that option. So, you have to use multiple subs with a lot of power, room control and digital bass management with a two way digital crossover. Those of you who are digital phobic need to get over it. You are just shooting yourself in the foot. 

djones, a rising response from 100 Hz down to 20 Hz with 20 Hz up 6 dB does not produce fat bass. Fat bass is produced by a peak in the 80 to 160 Hz region. Pushing the response up at 20 Hz gives you that sense of a live performance at less than ear shattering levels. Push it too high and the bass becomes disjointed, you know you are listening subwoofers. You should never know you are listening to subwoofers. 

You have to forget about fundamentals. Things like the thumb striking low E produce a thump that is below the fundamental. Percussion will do this also. Music and venues also breath. There is a lot that happens below 40 Hz that provides the sense of a live performance. Without it you will never feel as if you are at a live performance. Many people give up on this issue thinking that HiFi's can't possibly do this. Bad assumption. They certainly can. It takes the right tonal balance, imaging and low bass to pull it off. 

William53, it is much harder to control a large cone, to keep them moving in a straight line and not flexing. I prefer multiple smaller drivers. The effect is the same with less distortion. My new subs will use a total of 8 12" drivers.

It is not an undertone cleeds. It is another low frequency sound that accompanies the note. Like a rim shot with a snare drum, the stick striking the rim is a different sound than the stick head hitting the skin. Why is it that you so like to miss interpret what I say. My typing and spelling stink but other people do not seem to have trouble understanding what I say. 

@cleeds , that is exactly what I wrote. That thump is not the note it is an associated percussive sound and I think that is pretty obvious. 

Oldhvymec has it right. There is a visceral aspect to live music that is missing in most HiFi systems. People will try to get it by turning up the volume way too high. This does not work and blows out your ears. This aspect is coming from bass below 40 Hz. Reproducing these frequencies in residential settings is difficult and it is easy to "damage" the the rest of the music trying. I believe this is why there are such polar opinions on the subject. 

@ivan_nosnibor , interesting project! You need drivers with a free air resonance below 20 Hz. Infinite baffle speakers are much more efficient but the enclosures are large and larger enclosures tend to have more resonance issues. I know one person who mounted the drivers in his floor so that the basement was the enclosure. I did not like it because there was a floor resonance that ruined bass detail. He needed a stiffer floor. I toyed with the idea of mounting drivers in the front wall which in my situation is an outside wall. Glad I did not. If you go outside when the system is running that wall buzzes and rattles like crazy. The Hardy Plank siding is creating most if not all of the racket. Fortunately, I used  staggered stud sound proof walls so you can't hear it inside. I used the same construction in the master bedroom so the kids could not hear mom and dad having fun:-)
I am not sure that 18" drivers are the way to go. I would use multiple smaller drivers. I would use two of these instead of one 18" driver.
https://www.parts-express.com/Dayton-Audio-UM15-22-15-Ultimax-DVC-Subwoofer-2-ohms-Per-Co-295-514  
It is easier to control a smaller cone. A cone's motion has to be pistonic. Larger cones have a tendency to move asymmetrically. 15" drivers are plenty big enough. I use 12" drivers to keep the individual enclosures small enough to fit in my situation. The new system will use 8 drivers. Dayton subwoofers are excellent and a great value. I have been very pleased with them. I got two Morel drivers and was not impressed. I sent them back.

You all have to understand that there are very few speakers that can project under 80 Hz with authority in a normal sized room. The specs are taken at 1 meter. Anyone here listen to their speakers at one meter? This does not even account for what the room does and most rooms are awful when it comes to bass. To me a full range speaker does 100 Hz to 20 kHz. Below 100 Hz is in the realm of a specialty system designed specifically for 20 Hz to 100 Hz with room correction (really speaker correction) and enough power to handle the correction. 
There is only one speaker made today that is truly one way, flat between 100 Hz and 20 kHz and with a perfect dispersion pattern minimizing room interaction and I'll give you one guess what that is. Hint; it also has the lowest distortion of any speaker and the fastest transient response. 

Mozartfan, you wish. I smell an excuse for not having a system that can do bass. There is a huge amount of information between 20 and 60 Hz. You can see it in real time with a measurement microphone. In about 15 seconds of observation you will eat your words and choke on them. It is painfully obvious you have no idea what you are talking about or you are just joking. If not you are spreading false information based on assumptions that are dead wrong. I hate that and I am happy risking getting kicked off this site to say so. 

Without adequate performance in the 20 Hz to 60 Hz range the FEELING that you are at a live performance disappears. It is just one factor required for the absolute sound but a very important one. I can demonstrate that to anyone in just a few minutes. 

Mozartfan, all you need is a transistor radio and you will be happy as a lark. But, what the heck are you doing here? Why don't you join a bridge club.

There is more to live music than the frequency of any given note especially when it comes to percussion. But, since you insist, the lowest note on a pipe organ is 8.17 Hz. The lowest note on a tuba is 16.35 Hz as is the lowest note on the Bosendorfer Imperial grand piano.

You obviously have not heard a proper system. In the minds of little people if you have not experienced it, it does not exist. 

@bache , good enough? Not if you are trying to reproduce the live experience. With 40 Hz and under missing you are just listening to a HiFi.
Many people have noticed something special when they add a subwoofer to their system and have remarked about it here. It seems to add more than just bass. People say things like it adds "air" or makes "everything else sound better." You can only hear at best down to 16 Hz. But you can feel right down to 1Hz if the pressure is high enough (that would be an explosion).  Anybody who has been in the presence of a large pipe organ will tell you it is like the hand of god shaking you, an incredible experience. For those of you who have not heard (felt) one there is a list of the largest pipe organs in the world. Find the largest one near you and attend a concert. It will blow all your fuses.

@bache , you are absolutely correct. Except for me there is no regular listening. I think you mean for background music like in the office. Then I am really not listening. But, when I am listening everything matters. I want to close my eyes and be transported to the venue real or fabricated. 
This will not happen without bass. Otherwise, you are correct. Speaker systems will not do it and really should not try. It does take large or multiple drivers. The smallest I would ever use is 10" but I greatly prefer 12". Four 12" drivers should do it in most home situations but if you have the room for larger enclosures 15" is fine. I have no trouble getting down to 10 Hz If I want to with four 12" drivers but I put a very steep digital subsonic filter at 18 Hz. Even with this filter the whole house sings. Glasses, dishes, pots and pans rattle. Mirrors shake and siding buzzes. Fortunately, you can not hear it all in front of the system. Masking is a beautiful thing.

There are several personal "don't buy under any circumstance" issues. 
 #1) Never buy a ported subwoofer. Power and digital EQ are a much better way to deal with frequency response. Port noise at very low frequencies is unavoidable.
 #2) Never by a driver with a whizzer cone. It is a sure fire way to ruin high frequency detail. The best tweeters are ribbons and ESLs. If you want a full range speaker an ESL is the only way to fly. 

  

Mozartfan, I think you need to listen to a pair of Quads.

daledeee1, absolutely incorrect. Specs as published mean very little. They are taken at one meter in an anechoic chamber. At 3 meters in a real room the story is entirely different. Most speakers start dying at 80-100 Hz with a peak or two below depending on room nodes which results in one note bass.

william53, I think that is what I have been trying to tell you. Unless you want to go for an ESL with subwoofers, the best you can do with dynamic drivers is a two way system with subwoofers. It is easier to control and engineer a passive two way crossover than it is to control and engineer the mechanical crossover you have in these so called full range loudspeakers. Anything with a whizzer cone is not full range, it is two way. The crossover is mechanical as the whizzer cone becomes decoupled from the woofer/midrange cone. The ESL is the only truly full range driver but it still IMHO requires subwoofers. An ESL will do 28Hz to 20 kHz but removing 100Hz down cleans up the sound so much that you are better off crossing to subs. I cross at 120 Hz. Above that it is all one way. Not that everything is perfectly rosy, it is not. There are two transformers, several resistors and caps involved between the amps and the ESL panel.  

Check out the upright bass in the article I posted. I never knew they made one like that! It is tuned to A0 which is 27.5 Hz. 

William53, I think the definition is rather arbitrary. What is important is how speakers perform in reality in a normal room at normal distances. You can not rely on specs particularly when it comes down to low frequencies. There are painfully few "full range" speakers that make it down to 20 Hz gracefully particularly if you try to run room control. Read the article I just posted in this section. Low bass is a problem that is best handled in isolation. IMHO as moon-audio defines it there is no such thing as a full range loudspeaker. Good performance certainly under 80 Hz requires entirely different parameters than the rest of the audio range particularly when it comes to positioning the drivers. You might want to use totally different types of amplification such as a tube amp above and a SS bruiser below. Not that it isn't impossible but I have never heard a "full range" speaker produce adequate bass. 

Then 18Hz it is and I do not mean 3 dB down.  That is always my target. 

William53b, in thinking about it I would to re-define my answer to reflect what I really believe. The full range that needs to be reproduced to create the "absolute sound" is 18 Hz to 20 kHz. However, no single speaker system should do this. Low bass requires a specific type of enclosure and location to perform well. Very low base also causes trouble with drivers that have to run up higher. Thus IMHO every system requires subwoofers. A main speaker should only be responsible for 40 Hz to 20 kHz at most on its own and 80 Hz to 20 kHz with subwoofers. So I would consider a loudspeaker that can do 40 Hz to 20 kHz to be full range. I have never heard a single driver do this successfully and my intuition says that you would be more likely to achieve satisfactory performance out of a well designed 2 way loudspeaker than any of these so called "full range" drivers