Focusing the clarinet sound

If we hear overtones; 11.9.7.5 it is easy to understand that the root note is 3.

Players can have wider or narrower sounds and still be playing with a focused sound. The difference of narrow vs. wide is an esthetic and stylistic distinction the difference between focused or not focused (using my definition anyway) is less so.
 
(It is important when reading articles like this to figure out what the terms really mean to the author and even more important that you do a lot of listening and take every opportunity hear a great players play up close and in concert to establish this concept)
 
Players usually start with too diffuse a sound
To fix this most often we find the player simply bites on the Reed or tries to use excessive lip pressure. Since the lips are quite weak, as muscles, this will not work and the jaw will have to be used.
Using the jaw reduces the lower lip to a lump of dead tissue that can no longer control the tone or the intonation since it is now trapped between the reed and the teeth.

A much better way of controlling the tone is to use your tongue position.

To focus the sound with your tongue. You must have your tongue very high in your mouth. You'll feel the sides of your tongue, hitting your molars on your upper jaw.

Say he (or tee) to demonstrate this tongue position to your self.
7u9
Because your tongue is now high in your mouth, it makes the air go faster, it's the same as blowing harder, which of course usually focuses the sound as well. It is also important that the center of your tongue remains firm.

Play a scale slowly full range on the clarinet. Observe your tongue position, closely, in particular notice, whether the center of your tongue relaxes or not. It shouldn't. Note that the back of your tongue should be higher for low pitches E and lower for high pitches O

Don't get the idea however, that your lips are not involved in the tone production or the shaping of the clarinet sound. The sides of your mouth should be drawn in towards the center is if you are saying dew and your top lip should be pushing down. You should feel a stretching of the chin, because while your lower lip is pushing up towards the Reed your chin is pointed.

It is very important that you think of bringing the corners in first then pointing the chin. If you just point your chin in isolation from any other gesture it pulls your corners back.

(Unfortunately "point your chin" is the first thing one tends to say to students when it should be the second or third, what I usually say (depending on what is going on with the student is this;

Clarinet in, acute angle, pushed up against top teeth
Sides in Top lip down and chin pointed
The shape inside your mouth can be variously described as tu ( eh throat plus eu tongue) or whistle a throat Bb c# for higher Pitches( courtesy Robert Marcellus ))

The amount of lower lip you have over your bottom teeth can vary from note to note and phrase-to-phrase or piece to piece (but it is best if you think about the external part of your embouchure as being static) . If you put out more lip over your bottom teeth your sound will tend to be darker, and yet, in certain ways more resonant because you'll be able to take more mouthpiece and reed into your mouth.

If you have less lip over your bottom teeth, your sound will be generally brighter and more resonant. Your top lip pushing down tends to firm up the bottom lip, and also opens the muscles of the throat, this tends to broaden out your sound and make it sound little bit deeper and darker. Bringing the corners of your mouth towards the center makes a bigger pad under the Reed. So the Reed is better able to vibrate and you are better able to control the amplitude of the vibration.
 
Again, it is very important that you think of bringing the corners in first then pointing the chin. If you just point your chin in isolation from any other gesture it pulls your corners back. After a while most good players will do whatever is required to bring the instrument mouthpiece and reed into resonance situation that produces the desired disposition of harmonics.