Multiphonics
"Multiphonics" are fascinating sounds which result from the column of air in the recorder being unsure of which of two or three possible notes it ought to produce. Generally a multiphonic fingering can produce any one of the notes by using slight variations or -- if you get it just right -- it can produce all of them at once.
The nature of acoustics is such that these sounds will generally only be useful for rather avant garde music, or at least music which isn't based on the tonal system, because they tend to be "out of tune" in relation to the tempered scale. Multiphonics are natural sounds, not artificial ones, and they should be used with care.
At this stage, I've not analysed all of the multiphonics I've found into their constituent notes, and they're offered here for your exploration and experimentation. You can use them as colourful effects in a solo, or you can even construct extended lines from them with enough care and patience.
Notation system:
T is the thumb hole closed; O means it's open
123 4567 are the finger-holes closed, from top to bottom (ie 1=left hand index finger, 7=right hand little finger)
- means the hole isn't covered
/ means you half-cover the hole; on 6 and 7 this means you cover one of the two holes only
X means you cover the hole in the bell using your thigh (this is best done sitting down...)
V means you do something with the vent on the headpiece (explained in the text)
So, T1/3--6/X means cover the thumb hole, and finger holes 1, 3 and 6; half-cover holes 2 and 7 and stop the bell with your leg. Extra indications of how to get the multiphonic are often needed, but never mind.
T123-567
This is a nice easy one. Blown very lightly, it gives the quarter-tone between B and Bb, but blow slightly harder and the multiphonic emerges.
If you gently vent the thumb-hole, you can hear the other two notes in the multiphonic, which are a G a high C#. Which of these two notes you get is quite unpredictable in this fingering.
/12--567
Trickier, this gives a slightly high D with rasping, very high notes. The thumb-hole should only be slightly vented, otherwise a more or less in-tune F# will appear. It's a very unstable sound, best used for colouring the low D, T12-----, in which case holes 567 may be left uncovered (they do seem to strengthen the high notes, though).
T1234-67X
This is quite an easy one, which combines a low note (a flat Bb) and two high notes. It works well with the next two.
T12345-7X
Try alternating -- or even trilling -- this with the note above, as the low note here is a flat A but the high notes actually stay the same. A trill is difficult because multiphonics are inherently unstable, but it can be done fairly easily.
T123456-X
This is the hardest of these three, and the least attractive (odd, isn't it? I actually find a lot of these sounds "attractive"). The low notes moves to a slightly sharp F#, and the high notes jump up about a semitone in comparison with T123457X. You can use it in combination with the two above, however.
T1234567X
Yes, by blowing very, very gently on this fingering, it is possible to get a combination of a low D with a very low note which I think might be a fifth below low F#. Works only at extremely low air pressure, with a very loose embouchre. It seems to be just possible to play the low note only, but at the moment I find the high note keeps intruding. T123456/X can also be used; it would be nice to think a trill between these two might be possible, but breath control there would be critical.
T12345/7
Another one to approach gently, it's possible to get a flat, low G# simultaneously with a rather flat A an octave above. Not a very pretty sound, though. If you under-blow it, you get the G#; overblow, and you hear the A. It's just possible to get T123456/X, but it's so elusive that I haven't included it here.
I want to hear some music using multiphonics!
Well, try solo music written after 1945 (roughly); Recorder repertoire is a bit thin, but there's plenty for clarinet, flute and saxophone employing these techniques. You should also definitely check out recordings by improvising saxophonist John Butcher.