I got into a conversation the other day and, part-way through, I realized I had no idea what I was talking about. The discussion was over where the sound comes out of a flute. Among those present, opinions varied over whether it was from the embouchure, the lowest tone hole, the (open) end of the flute, or all of the above. Could someone more technically oriented clarify this for me?
Thanks and best wishes.
Steve
[… in which the responder trots out their own little bit of alleged knowledge …]
I have seen one bit of research, on fipple flutes as I recall, that found about half the sound came from the window and half from the lower part of the flute. If there are open toneholes, they would share the load: much of it would come out of the top open tonehole, but those lower down and the end of the flute would also participate. I imagine the same would be true of transverse flutes.
My understanding (which, admittedly, could be flawed):
The sound in a flute is caused by the air vibrating at the embouchure hole. As you blow across the edge of the embouchure, a pressure wave is set up inside the flute body which causes air to alternately go into and out of the flute. This alternating action causes a vibration, making the tone. The speed of this vibration is caused by the resonance of the wave, which is determined by the tube length and determines the pitch. Opening tone holes shortens the tube length, changing the resonance of the pressure wave, changing the pitch.
Since the flute body mostly serves mostly as an air pressure regulator, and the sounds are caused by vibration at the embouchure, I would imagine that the majority of sound comes from the far edge of the embouchure hole (where the air is getting split). Of course, being a vibration, sound probably propagates and there may be some measurable amount from other areas of the flute, but I imagine it’s negligible by comparison.
Some links for further reading:
https://www.yamaha.com/en/musical_instrument_guide/flute/mechanism/mechanism002.html
https://newt.phys.unsw.edu.au/jw/fluteacoustics.html
Second this.
It comes out pretty much everywhere I think 
Having said that, the embouchure hole is primary. I say this based upon making recordings of the flute in my studio and the microphone tells the story. Also, watch any flute player on stage in front of a mic and notice where the mic is relative to the flute. No one is miking the bottom of the flute, for example.
In studio settings, the best results I had came from pointing the mic in the general direction of the embouchure hole, but far enough back to also include the finger holes. In a live setting, you want the mic in close so you don’t pic up other instruments and room noise, so you close-mic it at the place that counts the most: the embouchure. I know of one recording engineer at a pro studio who favored a ribbon mic up at the ceiling above the flute player! This ensured that he captured all of the nuances of the flute, including how it sounded in the room itself. Very interesting but not practical in a live setting.
Pointing the tail end of a flute or whistle at a sound level meter while playing the bell note demonstrates that there’s quite a bit of sound coming out that end.
On a few occasions I’ve placed a dynamic mic pointing up into the foot of the flute while I play. You definitely will get sound waves coming out of there, but if I remember correctly it was biased toward lower frequencies (gave a “bass-y” sound).
Flutes are weird!
I agree that, when playing into a mic, a lot of sound comes from the embouchure hole. However there is somw conflict of interest here: the embouchure hole stays stationary while playing-- and thus is easier to mic-- while whatever is ‘the open tone hole’ or ‘open end’ shifts around as you play-- and thus is difficult to mic.
I dont know the physics thaat well, but heres some food for thought: flutes are mostly not resonating bodies (like a guitar or fiddle or drum). Rather it is almost entirely the air space itself that acts as a resonator. Very very very little sound is emanated from the exterior wall of the flute (fact from Nederveen).
Secondly, at the edge of the embouchure the air stream is oscillating back and forth, over and under the edge. As far as I understand, this is based mostly on a balance between the inside and outside of the flute. Thus I would guess that about half of the sound emanates from the embouchure, while approximately the other half is either emanating from the open end, or radiating from the pads of closed holes, or being lost to friction and heat with the wall and turbulences and moisture etc.
Then again, the players mouth and throat and sinuses are part of the system, so maybe most of it is resonating from their eyeballs! Who knows!
Blow into a bottle and will have the answer.
everything you’ll ever need to know about it right here.
Essentially the same on a simple-system wood flute
https://newt.phys.unsw.edu.au/jw/fluteacoustics.html
That’s a great article, that I’ve looked at every now and then over the last few years. I’ve just looked at it again. I can’t see the answer to the poster’s question. Maybe I just can’t see it for looking …
Whenever you go and see a great flute player in concert, the mike is directed straight at the embouchure hole. Typically, there are no mikes directed anywhere else. I think that says something.
I looked at the “gearslutz” forum, which is run by and for audio engineers, and the consensus seemed to be that most of the action was at the embouchure hole, but that you needed to get some of the sound escaping from the finger holes or it wasn’t going to sound right. The general recommendation was micing a feet feet back from the flute. Same with the sax–the general view was don’t mic the bell only, because the soundholes are important. That’s obviously not going to work live. Sax players Ive worked with have used a clip on mic at the bell. Flute players just stand near a vocal mic.
The OP continues to be confused by this topic—but feels better as it seems that confusion is broader than just in his mind. The closest that the article from RudallRose seems to come is in the summary in the intro:
Once the air in the flute is vibrating, some of the energy is radiated as sound out of the end and any open holes.
This seems to suggest that the sound comes out, to one degree or another, from any open hole,l but I can’t tell if it is assuming that the bulk of the sound comes from the embouchure hole. Other comments seem to suggest that the nature of the sound varies depending on the source of the sound leak. Maybe the OP should just shut up and blow…
Best wishes.
OP Steve
You’re not the only one confused about that. I’ve wondered about how the tone holes contribute anything as well. If the sound is caused by a column of air being “excited” by the air split at the far end of the embouchure hole, then it makes sense to me that you would hear at least a lttle of that resonating air through an open tone hole. But then it gets complicated by however the open hole modulates what we hear, whether it acts as a Helmholtz resonance port, and so on.
I don’t know the acoustic theory, but just from practical experience, it doesn’t seem to me that much sound is coming from the tone holes. We can get a nice loud D note in both octaves with all the holes covered. If the tone holes were making a significant contribution to the volume, I would expect the D to be weak, and the flute would get progressively louder as more tone hole were opened up, until we get a C# as the loudest note. It doesn’t seem to work that way, which to me indicates we’re not hearing a significant contribution from the tone holes, except in the sense that venting along the air column contributes to strengthening other notes being played. Well that’s my two-bit theory anyway.
Maybe the OP should just shut up and blow..
Well, we would all be better flute players if we weren’t spending time yakking on the Internet instead, but it’s too early to wake up the house. So…
Try looking at it like water flowing downhill: the sound takes the easiest way out. With all the toneholes covered, the only ways out are the embouchure hole and the bottom end. As you uncover toneholes, more sound comes out of the open toneholes, and less out of the bottom end.
If the sound takes the easiest way out, then why doesn’t the flute get progressively louder as more tone holes are uncovered? The volume of each individual note is almost perfectly matched with each other on my flute.
The pressure wave inside the bore exits the flute at the nearest opening. If all holes are closed, the foot of the flute and the embouchure hole are the escape routes. If you open the lowest hole, the pressure wave traveling down the bore will take the nearest exit which is now the open hole. If you open the next hole, it will take that one. This is simplified, I’m sure, since my own understanding of the physics is limited.
So it doesn’t get progressively louder because it’s not like the pressure wave is escaping out all of the open holes equally. It is going to favor the nearest open hole.
This is what tuning holes do on flutes. I can make something like a Chinese xiao, which has a tone hole matrix below the finger holes, and it also has several inches of flute bore down below the tone holes. But the topmost tone holes effectively cut off the frequency at a certain pitch–literally as if you sawed the foot of the flute off at that point. So when you open each finger hole, its as if you simply cut the bore off at that point. Hope that makes sense.
So what happens when I work up the scale from the bottom D on a keyless flute? Bottom D is nice and loud, E is softer and lower in volume, F# is louder, etc. The difference seems to be related to the size of the first uncovered hole. What’s up with that?
And second octave d generally has the top finger hole open, or all holes can be closed and it’s pretty much the same note and volume.
Best wishes.
Steve