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Perikles
2011-Oct-31, 02:08 PM
If you hold a stone chisel against rock and wack it with a hammer, the result (if you manage to hit the chisel, not your hand) is a note of a particular frequency. I can't work out what determines that frequency.

I'm in the process of trying to hack away pieces of rock from a very large piece of rock. This is volcanic rock, so it's as hard as - er - rock. The thing is, if I hammer away at one point and the note is of very high frequency, then I know I'm on a loser if the pitch remains constant. If, however, the pitch changes (always falling) then I know I'm making progress and eventually a chunk of rock will come loose and fall off. At the point of coming loose, the pitch has dropped to lower than a musical note. What is happening here?

HenrikOlsen
2011-Oct-31, 05:12 PM
Basically as micro-cracks develop, the overall stiffness of the rock drops while the mass is constant.

profloater
2011-Nov-01, 12:27 PM
there is a ring associated with the hardness of the chisel in fact this ring can be used to determine if there is sufficient hardening by a skilled ear. The ring has both odd numbered harmonics plus even numbered "bar" harmonics as does for example a piano string . the different harmonics can be altered in relative strength by the position of your hand (like a bridge) and the reflection of sound waves from the rock. the harmonious blacksmith story has a ring of truth to it! A soft mild steel chisel as well as being useless would have a dull sound in rock but a high tinkle in the air.

grapes
2011-Nov-01, 04:05 PM
At the point of coming loose, the pitch has dropped to lower than a musical note.I'm curious about what you mean by this. Do you mean, it's a low-pitch smeared thud, or do you mean that there is a certain frequency below which a pitch is no longer musical?
A soft mild steel chisel as well as being useless would have a dull sound in rock but a high tinkle in the air.Ah. The giving-way creates the spread of frequencies that we hear as a dull sound, I think. Whether the rock is giving way, or the chisel.

profloater
2011-Nov-01, 04:53 PM
i think i did not put it well. the chisel has two primary modes; transverse like a string and axial as a bar. energy from the latter is removed at the ends as well as by internal losses. if the stone cracks it will allow the chisel to ring wheras if it absorbs the blow without cracking the sound will quickly fade as a thud. the hand holding also takes away energy. If the chissel is supported with ligaments at its natural nodes 1/4 L for a cylinder, it will ring like a bell in transverse mode. I think the hard skin from either nitride or carbon induction hardening changes the damping rather than the elasticity.

Perikles
2011-Nov-01, 07:50 PM
I'm curious about what you mean by this. Do you mean, it's a low-pitch smeared thud, or do you mean that there is a certain frequency below which a pitch is no longer musical?.A low-pitch smeared thud is a very good description.

HenrikOlsen
2011-Nov-02, 03:30 AM
A low-pitch smeared thud is a very good description.
That does sound (sorry) like it's a sound from the rock rather than vibrations in the chisel which is what profloater is talking about, there's no reason for the latter to change pitch as the rock gets weaker, because the chisel isn't changing, which is why I think it's the wrong place to look for an answer.

profloater
2011-Nov-02, 10:08 AM
My reasoning is that the chisel has many frequencies and the rate of damping caused by the end energy absorbing characteristic of the impacted rock causes sound changes. of course the rock also makes sound especially the crack of energy release as a split occurs but the unimpaired chisel will have a longer ring as a relatively low damped mass spring object. The energy of the hammer imparts a lot of internal ringing energy to the chisel and of course has to travel at the speed of sound in the chisel toward the rock. If the rock takes a lot of that energy away it changes the chisel energy. In that case it would figure that the rock will emit sound rather than the chisel

Perikles
2011-Nov-02, 12:52 PM
So are you saying that to begin with, when the pitch is high and constant, the ringing is from the chisel. Then, if I make progress in affecting the rock, more energy is transferred to the rock, and this causes the decrease in pitch. Is that right?

profloater
2011-Nov-02, 01:11 PM
So are you saying that to begin with, when the pitch is high and constant, the ringing is from the chisel. Then, if I make progress in affecting the rock, more energy is transferred to the rock, and this causes the decrease in pitch. Is that right?Yes I think it is the favouring of the lower pitch in the overall mix of frequencies and harmonics. the free air ringing at high pitch is damped by transferring energy to the rock. The rock sound will be some kind of crack, not transferred back to the chisel. What I am now wondering afresh since my experience was with piano string design, is the way transverse and axial vibrations interact in a chisel. At a first level the axial vibrations are much faster and thus higher pitch than the transverse vibrations which are in bending. There will be some bending due to to euler instability even if the hit is dead true. Transverse will couple to air better than the axial vibrations but then if the chisel is in the mode where the nodes are 1/4 L the rock will kill those too. The steel will have much less internal damping than the rock I think.

jfribrg
2011-Nov-02, 05:22 PM
This may be like the string of a violin. Maybe the pitch is a function of the distance from the rock to the end of the chisel (where the hammer hits it). Normally the pitch decreases the farther in you go ( unlike a violin where the pitch increases with the shorter string). Another conjecture is that the wavelength of the sound is determined by the area of the chisel that is in contact with the rock. The farther in, the more area, and so the longer wavelength.

profloater
2011-Nov-02, 06:19 PM
This may be like the string of a violin. Maybe the pitch is a function of the distance from the rock to the end of the chisel (where the hammer hits it). Normally the pitch decreases the farther in you go ( unlike a violin where the pitch increases with the shorter string). Another conjecture is that the wavelength of the sound is determined by the area of the chisel that is in contact with the rock. The farther in, the more area, and so the longer wavelength.but a string is constrained both ends and the axial frequencies count for less although more with steel massive strings. the energy goes in mostly axially unlike a violin bow tangential excitation when the rock stays solid and stops the end vibrating it changes the available vibration modes