It's easy to get a sound out of a bass flute, but it is diabolically difficult to get a good tone. By "good" tone I mean a big rich, resonant warm tone that buzzes and projects. For myself and for many other flutists, bass flute is the most difficult of all the flutes to get a good sound. I believe the reason for this is because the bass flute requires great precision with the embouchure, more precision than seems to be required of piccolo, alto or soprano flute. In addition, it is a physical task requiring a big volume of air. One must work hard and breathe deep like exercising, without forcing it or losing precision with the lips. Meanwhile, because the bass flute has a crutch in the web between the left thumb and hand, moving the left thumb makes it want to shift around, which changes the lip plate position.
One of the problems with bass flute is that it is not very loud. Its lack of projection means one is usually playing it at or near its dynamic limits for any live performance. Many bass flute players simply amplify it. This works but amplification is not always an option. And even when it is, really learning to make a bass flute project will benefit one's playing of any flute. Bass flute can be a dead sounding instrument. Playing it near its dynamic limits adds upper harmonics that enhance its tone quality. Thus, a musician using amplification may not be getting the flute's full rich tone.
The bass flute must often be played at or near its dynamic limits. But like any flute, the cracking point, the natural dynamic, the airstream direction and response time are different for every note. For example imagine attempting to play a soprano flute just below the cracking point for every note - it would be quite difficult. First one would have to experiment to *find* each note's individual cracking point, then when playing one would have to be constantly adjusting the embouchure to hit each one precisely. That would be hard enough, yet on bass it's even harder. Bass flute requires greater embouchure adjustments for each octave, the individual personalities of the notes are more different - both in tone and response - and the notes are more finicky to small changes in airstream direction.
As one might expect, the bottom and top octaves are the most difficult. On the bass flute there also is a big difference between short tube notes (like C#) and long tube notes (like Eb).
I have played the both Jupiter basses (523 and 1123), the Emerson, a Flutemaker's Guild and an old vintage Rudall Carte. Most of what follows is specific to the Jupiter 1123 which is my main bass. Other bass flutes respond differently but what follows may still be useful to try with any bass.
Short tube notes tend to be flat, hollow sounding, and have fast response time. Long tube notes are the opposite: tend to be sharp, rich sounding, with slower response time. This is the opposite intonation tendency of most normal soprano flutes. I'm not sure why - it may be because my Jupiter is tuned A=443 and I'm playing it at A=440. That means I've pulled out the head & curve joints more than usual, which would flatten the short tube notes more than the long tube notes.
I'll start with the middle octave since it's the least difficult and provides a common ground for branching out.
The Middle Octave
One might say the middle octave is C2 to C3. But it's more useful to define the middle octave in terms of response and tone quality. This shifts it down somewhat - more like G1 to G2 (at least on my flute). Here is where the bass flute is easiest to blow and produce a big resonant sound. It's a good place to start practicing on bass flute to find what it takes to get a fat sound. But try to avoid Bb, B, C and C# for intial practice. These notes are the bastard children of the bass flute middle octave. The flute has less natural richness in these notes, so they require a bit more care & attention to sound their best. For the middle C# in particular, when possible I finger the lowest C# and overblow it. This produces a richer tone that matches better to the surrounding notes.
What works well to learn good bass flute tone is to begin with middle "D". This is a very stable, forgiving and resonant note that can take a lot of dynamic range before splitting. Move the headjoint around - slide up and down, roll in and out, move your lips around, etc. You will know when the sound locks in - you'll get a fat, resonant, projecting buzz to the tone. Don't be afraid to use a lot of air - you've got to get a very big flute to resonate. Air conservation comes later - first get a good tone. Now carry that embouchure up, note by note, to the G just above. You may notice that E and Eb are finicky notes - this is normal. If you can get a big rich sound on these notes, the rest will come out naturally. Now jump down to the A and G below. When you get that fat resonant sound with those notes, you've got the basic bass flute embouchure. You're ready to branch out, so come back and revisit the short tube notes. It takes a lot of practice to get the open fingering C# to sound good compared to the 1st harmonic of low C#. But the rich sound of that harmonic is your goal. It is not quite achievable - the harmonic of low C# will always be a little richer sounding. But striving toward this goal is excellent practice.
NOTE: middle E and F can be delicate and tend to split. I find this is worst on Alto flute, but it affects Bass more than a normal soprano flute. The trill keys can be used to stabilize them (left for E, right for F), but they also lean out some of the richness of the tone. Usually splitting occurs when the E or F is being arrived at via a leap from a lower note (though it can sometimes be an issue leaping down from a higher note). The trick to this is to realize that the flute has a certain inertia with the note it plays. E and F are more stable once they are sounding - it's just that getting them to sound in the first place can be delicate. Taking advantage of this knowledge, let the side of your finger flick against the trill key just briefly as you finger the E, but don't hold the trill key down. As your finger brushes past let the trill pop back up just as soon as the E begins to sound. This will force the flute into a perfect, smooth transition to the middle E or F, and once there it will be easy to maintain it by blowing alone. This way the trill key remains closed for the note so it doesn't get a chance to impair the tone. I call this "popping" the middle E and F.
The Bottom Octave
F1 is the note where the bass flute's response transitions. The G (and F#) above it are easy to blow with a fast response time. The E below is a totally different beast and has all the character of the bottom octave down to low C1. One must relax the embouchure but DO NOT OPEN IT UP too much. If you open up the embouchure the note will sound but it will go totally dead (this is OK if you need to "hit" a fast lower note but it will never resonate fully that way). To get that fat buzzy sound, you must not only direct the airstream more downward without rolling the head inward, but also think of flattening your lips into more of a flat oval shape - as it gets wider (horizontally) it gets shorter (vertically). This lets you use the entire surface area of the bass flute's rather large blowhole, which helps the sound get as loud as possible (and a bass flute needs all it can get in the bottom octave). AND it also tends to keep the airspeed more consistent.
Why does an elliptical hole provide more consistent airspeed? I don't know but I can guess based on Bernoulli principles - the speed of any given part of the airstream depends (to some extent) on how close it is to the edge (your lips). Now imagine two holes of equal area - one circular, one elliptical. With the circular hole, some air (in the center) is significantly further from the edge (lips) than other air. With an elliptical hole, there is less difference between the air closest and furthest from the nearest lip surface. Most of the air is close to a lip (upper or lower). By making that distance more consistent, you are eliminating (or at least reducing) one factor that leads to varying airspeed.
Another factor to keep in mind for a powerful bass flute bottom octave is what Robert Dick calls throat tuning. This is not specific to bass flute - it works on every flute I have played from bass to piccolo. Imagine that the cavity resonating is not just the flute, but the entire airstream including your mouth, throat and lungs (this doesn't necessarily have to be true (it may or may not be), but it is a useful model regardless). That's not to say that the entire cavity from your lips to your lungs isn't resonating, maybe it is. But it just doesn't matter whether or not it actually is because the idea just works either way.
Anyway, the idea is you want to tune the entire resonating cavity in your body to the note being played - as if you were going to sing that note. But keep your vocal cords totally relaxed. Don't actually sing it, but form the throat, mouth and lungs as if you were going to. It's similar to the mouth/throat position during yawning. Imagining this will help you drop the jaw, relax the throat and push a big volume of steady air through the flute.
In short, for the bottom octave you aim low, squash your embouchure flatter but still keep the lip plate loosely coupled to the lower lip. This helps keeps your airspeed consistent, controlled yet as fast as possible without going into the upper octave. Think of relaxing the lips and throat as you bring the corners of your mouth downward. The richest sound in the bottom octave is when the tone is just about to break to the higher octave. After the note initially "hits", there are two ways to grow it louder. You can tighten and focus your embouchure, and you can push more air deep from the diaphragm. It takes the right combination of both to get the fattest, richest buzziest tone. In a spectrum analysis of a fat low C, the fundamental is actually weaker than the first few harmonics. But the ear still perceives it as a fat low C; the harmonics add richness and buzz while having very little effect on the perceived pitch.
The Top Octave
The top octave of a bass flute tends toward the same hollow tone as short tube midrange notes, with a bit of honky or squeaky color added to the sound. The top octave is easy to blow, but incredibly difficult to get good tone quality. It can be downright poor sounding and requires lots of practice to blow it just right to get it to sound acceptable. To help fatten these notes it helps to blow them with the MINIMUM air velocity that will sustain them. If it just *begins* to split and a *slight* touch of the bottom note sounds, I think this is OK and actually beneficial as long as you keep it on the edge and don't let it actually split. You shouldn't actually hear both notes, but just the slightest hint of the lower note in the timbre of the note you are playing. This technique - blowing minimally with a hint of the lower octave in the note - also helps intonation, since the bass flute tends to go sharp in the top octave. Also it helps both with tone and intonation to direct the air as downward as possible. This lowers the pitch and fattens the sound.
In terms of response and tone, A2 is the transition note from the middle to top range. A2 still has mostly the response and tone of the midrange, but the Bb just above it is a different beast with the character of the top octave.
High E is very unstable on the basses I've played, far more unstable than on a normal soprano flute. The Jupiter 1123 has a split E which makes a huge difference. On a normal flute, split E doesn't do much for me but on bass flute it can make a world of difference. On the Jupiter 523, I always played high E with pinky up. This was the only way to stabilize the note and it also brought the pitch down a smidgen for better intonation.
Final Observations
Definitely experiment with the positioning of the curve joint and head joint. There is a lot of flexibility and wide range of different positions that work. Finger C# to Eb back and forth, and also finger B to C# to get your left thumb moving, and find a position that minimizes the motion of the flute when you do this because that motion affects your embouchure. Also, find a position in which the force of your lip/chin on the headjoint is minimized because you do not want to stress these delicate tenons. If the headjoint is rotated too low (horizontally between your face & the flute body), then spit flying from your lips will get on the key action near the LH C key. So rotate the headjoint generally above the body. The position that works best for me is having the headjoint rotated above the flute. Not directly vertical above the flute body, but mostly above and rotated a bit back toward my head.
How much you pull out from the headjoint versus from the curve joint, changes your hand position. For pitch it is the overall pullout that matters, so you have flexibility here to maximize your own comfort. Pulling out mostly from the curve joint puts the hands further away to the right. Pulling out mostly from the headjoint puts the hands closer in.
Rolling the blowhole inward toward the lips makes the high notes speak easier, and vice versa. It's educational to experiment with this and combine that with head/curve joint positioning to see how it affects tone in different registers. But remember the end goal is not to roll the flute or headjoint while playing, but to do all the adjustments with your lips (and perhaps jaw).