Who Plays Saxophone? - Questions on Resonance Modes

[Anton Alice]

Hello people,

I have started 2 days ago with alto saxophone, since I have to take a year off violin because of several shoulder injuries due to overpractise.

I observed a strange thing with the sax, which I would like to discuss with you:

The sax is delivered in three parts: mouthpiece, saxophone neck, and the the heavy body.
Sometimes it is useful to practice sax separately on the mouthpiece, without the body.
I have done that with the mouthpiece + neck, so I put the mouthpiece onto the neck, and practiced toot-technique. What I observed was the following:

If you blow into it, then you will hear some resonance note. Now if you put your finger from the other side into the neck, then the frequency goes down. Of course the finger does not completely close the neck, so that air can still pass by.
The naive expectation would be, that the frequency goes up, because the length of the resonator has become shorter? (apparently?)

Why does the frequency go down?

EDIT:

I use the index finger, because it fits into the neck, such that air can still travel past.
Maybe the reason is, that the finger has a certain thickness, and the resonant modes are not simply excitations along the length of the neck, but also along the thickness (diameter). But why should the freq decrease, if the thickness decreases? (The thickness only decreases in that section, where the finger has been intruded. The finger can not be intruded all the way up to the mouthpiece)

 

[pbuk]

When you introduce your finger into the end of the tube you do three things:

1. Reduce the length of the tube: this would tend to decrease the fundamental wavelength and raise the pitch proportionately
2. Close the end of the tube: this would tend to halve the fundamental wavelength and increase the pitch by an octave
3. Change the tonal characteristics i.e. change the amount by which different modes of oscillation are damped

As 1 and 2 increase the frequency but you observe the opposite you can conclude that the pitch you are hearing with the open tube is not the fundamental pitch but an overtone; when you insert your finger the mode of oscillation completely changes (with the fundamental changing from 1/4 wave to 1/2 wave) and a lower overtone becomes dominant.

 

[Anton Alice]

Why should the higher freqs in the overtones be damped more, the more the finger is intruded?
I mean it could also be the case, that the low-freq overtones are damped away, so that the avarage heard impression goes up.
Another way of asking: Why should the spectrum of overtones depend on the penetration depth exactly in such a way that deepening the finger leads to lowering the freq?

when you insert your finger the mode of oscillation completely changes (with the fundamental changing from 1/4 wave to 1/2 wave)

If I intrude the finger slightly, then the freq goes down slightly, while the character of the tone does not change too much. So it is not that the "mode of oscillation changes completely". The characteristics do change gradually, very gradually as the finger goes deeper into the neck. What you are describing would lead to a sudden change of characteristics.

you can conclude that the pitch you are hearing with the open tube is not the fundamental pitch but an overtone;

Why Am I not able to excite the fundamental?

 

[Anton Alice]

Do my questions make sense?

 

[pbuk]

Why should the higher freqs in the overtones be damped more, the more the finger is intruded?
I mean it could also be the case, that the low-freq overtones are damped away, so that the avarage heard impression goes up.

It could be, but experience shows that in general higher frequency sounds are more easily damped.

Another way of asking: Why should the spectrum of overtones depend on the penetration depth exactly in such a way that deepening the finger leads to lowering the freq?

If I intrude the finger slightly, then the freq goes down slightly, while the character of the tone does not change too much.

That is strange - I must admit I haven't got a sax here to experiment with, I was just trying to explain your observations.

So it is not that the "mode of oscillation changes completely". The characteristics do change gradually, very gradually as the finger goes deeper into the neck. What you are describing would lead to a sudden change of characteristics.

Yes indeed, something else must be happening which I can't explain.

Why Am I not able to excite the fundamental?

Again this was an idea to explain the observation - note that it is often hard to play low notes on reeded instruments, particularly quietly or very loudly, without breaking up an octave. A good site for the physics of musical instruments is http://newt.phys.unsw.edu.au/music/

 

[Anton Alice]

Again this was an idea to explain the observation - note that it is often hard to play low notes on reeded instruments, particularly quietly

Ouh yes, this is exactly what I am struggling with for a day. The damn lower C always jumps to the first harmonic. Especially when I am trying to do it quietly, because of neighbors.

 

[Svein]

Ouh yes, this is exactly what I am struggling with for a day. The damn lower C always jumps to the first harmonic. Especially when I am trying to do it quietly, because of neighbors.

Not unusual. You are in the situation that:

• If the reed you are using is too soft, squeaking is a common problem
• If the reed is too hard, the low tones are hard to get to speak.

Try pushing a lot of air through the instrument without making a sound. Then, when you are ready, play low F and hold it. When that tone is stable, change suddenly to low C.

For more information and saxophone-related stuff, consult http://www.saxontheweb.net/.