The name of a particular phenomena... 2

[Manini]

Dear people
Could someone tell me the name of the phenomena for witch when you move your finger with a specific speed under a surface (for example the border of a glass) you can hear a noise?
Thank you guys
By
Manini

 

[izax1]

I think I would call it "resonance"

 

[MikeyP]

The reason you hear a sound is because the glass resonates. The reason it resonates is becasue you are appling a force (with your finger) close to one of its natural frequencies.

I think what you are looking for is the term which describes how the force is generated. I don't know whether this is a generally accepted term, but I have heard the phrase "Slip-stick excitation" used in association with bowed stringed instruments. The principle of a bowed string and the finger on the glass is the same. The wave-form is often simplified to a sawtooth for modelling purposes.

M

--
Dr Michael F Platten

 

[Manini]

Thank you friends
Sure, there is a resonance, but why? My force is not a periodic force
It could be “sleep stick excitation", like Michael told me.
I’ve hear also “self-exitation friction” name. This means that friction increase when you apply a particular force, but I don’t know other.
Could someone explain me more?
Thank you
By
Manini

 

[objengrs]

Yes, it is self-excited vibration. Wine glasses do it, railroad wheels do it (wheel squeal), violin strings, etc.

It will happen whenever the dynamic coefficient of friction is less than the static coef.

Say you have a box of cereal lying flat on a (stationary) grocery store checkout belt. Further, assume your kid has stick one end of a rubber band to the box with a piece of gum, and is holding the other end behind you in the cart. When the clerk turns the belt on, the rubber band will stretch more & more as the box moves forward, until the band is too tight (just exceeding the static coef. of friction, say 0.3), whereby it will jerk the box backward several inches (against the dynamic coef. of friction, say 0.28). The box will then stop, static friction will take over again, and the cycle will begin again.

Often the topic is covered in the controls section of a dynamics/vibes text. If you want a all-time great one, find Den Hartog's "Mechanical Vibrations" book at Barnes & Noble, first published in the 30's (later updated twice). It's still available from Dover Books, and rrather cheap ($14?) because it is so old.

 

[brad]

MikeyP is right in his term "slip-stick". That is the common term I have seen used in literature to describe such a behavior. It seems like I've also heard it referred to as "slip-stick instability".

This is the forcing mechanism which generates brake squeal and other friction-induced noises. The frequency comes from the fact that your translational motion does in fact drive an oscillating force due to the continual slipping and sticking. In your example, this phenomenon happens at too small of a scale for one to see and readily appreciate.

(In all seriousness...) To demonstrate it on a large-scale so that you can visualize it, the best thing to do is scratch your fingernails across a chalkboard. You will see and feel your fingernails coming into and out of contact with the chalkboard. (Just don't complain to me when the hairs on the back of your neck stand up). If the chalkboard is sufficiently dusty, you will even recognize a pattern that demonstrates the slip-stick. This is the same mechanism as you observe in your glass example.

Brad

 

[brad]

One last note--
of course for the glass to "hum", the resultant oscillating frequency must match closely to a resonant frequency of the structure (as MikeyP has said above). This is why the humming happens at certain speeds--different translational speeds will drive a different oscillation, resulting in different forcing frequencies.
Brad

 

[Manini]

Thank you Brad
Your answer is very clear but… I had some problems when I scratched my fingernails across a chalkboard!!!
Thank you also to Dr Michael, izax1, objengrs
By friends!
Manini

 

[strokersix]

Slip-stick describes the concept but I feel some refinement is warranted. Friction coefficients are a function of velocity. Slip-stick or dynamic versus static friction is a simplified way of describing mechanical systems. In reality friction coefficients are continuous functions rather than step functions.

To describe this effect imagine a sliding machine element (skin on glass) driven by an elastic member (fat under your skin). Assume friction coefficient is inversly related to velocity. Elastic driving member deflection will decrease sliding velocity which in turn will increase friction force and deflection. Conversely, increased sliding velocity decreases friction force and deflection further increasing velocity. This system is unstable and results in oscillations in elastic member (fat cells) deflection and in sliding (skin on glass) element velocity. These oscillations then excite a resonant frequency in the glass and the party trick is complete! My point is that the sliding velocity does not need to reach zero or "stick" for the effect to occur.

 

[MikeyP]

Strokersix also raises the issue of there being a dynamic interaction between the "exciter" (ie the finger or the violin bow) and the "structure" (ie the wine glass or the violin string). Both exciter and structure are dynamic systems in their own right. If you tried drag a rigidly held bow across a violin string it makes a dreadful noise. This means that sound of a small child practicing the violin is the most irritating sound known to man (it even beats Brad's fingers on the blackboard)!

M


--
Dr Michael F Platten

 

[antti]

Just to expand a little on a slightly different (but related) topic. In the literature concerning brake squeal noise, a variety of models exist explaining the mechanism for instabilty in system with sliding friction. The negative coefficient of friction vs sliding velocity can indeed introduce some neagative damping into a system and lead to instability, but even a conststant coefficient of friction can be shown to produce the same. Ultimately the friction force drives energy input into the system, so variation in this driving function can lead toward intability. Even with the simplest Ffriction = mew*Fnormal point of veiw, if the normal forces at a friction interface vary, the friction force can vary even in mew is contant. This can occur during elastic deformation in coupled vibration - ie, the normal forces vary, hence varying the friction forces. So you don't necessarily need mew to vary for friction induced instabilty to occur.

I don't mean to over complicate the situation, but the continuing reasearch, particularly in brake NVH, seems to be only slightly improving our undersatnding of this complex phenomnom. This "simple" problem is much deeeper and more difficult than it first appears