Have you ever SEEN a sound wave, check it out

[XxDarkSidexX]

YouTube - Sound Waves

Darkside doesn't know how this works, darkside is actually scard for some reason(i think its the unknown). I found him here watching it. But darkside thinks this is really cool for some reason. He's drunk and making me(Bri) type this. He said to tell you guys, "That GN's rule, and supras can go poo on themselfs."

 

[Turbo Indian]

that is neat!! my dogs started to freek out when I turned up the volume.:biggrin:

 

[turbojimmy]

that is neat!! my dogs started to freek out when I turned up the volume.:biggrin:

My dog came running immediately and is now staring at me with the 'wtf are you doing?' look.

Jim

 

[wiked87gn]

YouTube - Sound Waves

Darkside doesn't know how this works, darkside is actually scard for some reason(i think its the unknown). I found him here watching it. But darkside thinks this is really cool for some reason. He's drunk and making me(Bri) type this. He said to tell you guys, "That GN's rule, and supras can go poo on themselfs."

Now you know where the crop circles come from and who makes them, and they keep denying UFO's don't exist.

 

[BJM]

You are not looking at a sound wave, you are looking at various modal resonance shapes of that specific plate. It is simply mounted on a large voice coil shaker (like a speaker coild but bigger). The specific mounting (how many mounting points and degree of fixity at each point) determines the modal frequencies where the plate will resonate. All vibration modes have a specific shape where portions of the metal stand still (these are called nodes) while other parts flail up and down. The sand sits still on the nodal regions which are different for each mode, hence the different patterns. Higher order modes are characterized by increasingly complex shapes, that is why the patterns get more intricate as the driving frequency gets higher. The other interesting thing to do is input multiple wave forms at once, they seemed to be doing a simple sine sweep.

 

[22lbChevelle]

More please, BJM. I am starting in my second year of engineering, with the first part of the physics with calculus. Where does a guy learn of what you spoke of? thanks

 

[BJM]

If you are already in engineering you should have an opportunity to take a course in vibrations. I suggest a course in acoustics since its basically fluid borne vibrations coupling into structures in interesting ways.

All structures vibrate in specific ways. One of the most fundamental improvements to cars that has been made over the years is the pushing up of the fundamental body modes. Its perceived of as tightness in the ride, no shaking and shuddering. Typically the 2 lowest modes are the most important, usually the lowest mode is the first bending mode followed by the first torsional mode. In the 70's, body on frame cars had a first natural frequency of around 10-12 Hz. That is, the car held at the suspension mounting points would shake up and down in the middle in response to a forcing vibration. Back then though the first twisting mode of the car end to end was often lower than the bending mode, especially in convertibles. Why do these frequencies matter? The suspension on old cars had to be quite soft in order to prevent the jouncing of the suspension from coupling into the body modes and causing the car to be a shuddering disaster. That's why raising spring stiffness causes ride quality to get poor on G bodies where modern cars are much tighter, even with stiff springs.

Over the years a lot work has been done and cars got rid of separate frames in order to stiffen the bodies up. The mushy bushings between the body and frame caused the frame to be heavy yet still floppy while holding up a body that just rides along for the most part. What matters most is the ratio and distribution of stiffness to mass in the body to determine the frequencies it will vibrate at. The natural frequency of something varies with the square root of the ratio of stiffness/mass. To double the frequency you must quadruple the stiffness as compared to the mass you used in the structure. Cars are especially a challenge since much of the mass does not contribute to the stiffness (seats, dash, people), its along for the ride making the structural bits work harder.

Nevertheless, modern modal frequencies are pushing mid 20's Hz nowadays and the torsional modes are now above the bending modes. I remember Buick bragging about hitting 40Hz in torsion a few years ago in some model.

Now with a stiffer body, suspensions can be tuned more optimally without shaking everything apart. Another huge change is unsprung mass. Not only does the body ride on the suspension, the suspension must keep itself under control. The is why independant rears are nice, the differential is now moving with the body and the suspension no longer has to control it. Our rear axles weigh 200+ pounds and the suspension has to work hard to keep it on the road.

 

[Nashty]

So you're saying crop circles aren't real?

J/K - excellent description, I'm a trained mfg engineer but spend most of my time doing crap production work. I miss the interesting physics classes.

I take it you work in auto chassis design?

 

[BJM]

I take it you work in auto chassis design?

Space craft design actually, several 1000's hours optimizing S/C structures for stiffness and mass. We optimize much further than car makers since engineering hours are a good trade off against launching extra mass.

One thing to add about car design is damping design. Shock absorbers have allowed design changes as well. Old style dampers were pretty crude, today's are non-linear in response letting you smack a harsh bump without feeling it (damping drops down at high stroke speeds) while controlling body motion (high damping for handling which occurs at slow stroke speeds).

 

[Nashty]

I toured McDonald Douglas when I was in college. One interesting fact they mentioned is that auto makers will spend ~$10 to lose a pound, aircraft makers will spend around $1000 to lose a pound and aerospace would spend around $10,000 to lose a pound. At the time, they were making F-16 & F18 aircraft there (St. Louis) and they milled I-beam structures down to around 0.030" thickness. They milled out over 500,000 # of titanium chips and around 2,000,000 # of aluminum chips. I suppose most are composite and held together with adhesives now. They had one clean room where they assembled some subs for the shuttle program. Very cool stuff.

I was gonna guess you worked for Tower Automotive. I worked with some TA guys integrating robot weld cells @ ABB in Ft. Collins, CO. They (ABB) talked about some pick and place robots for chip mfg's that could work at crazy speeds, like 1,000 or so bits placed A MINUTE.

I make big, lumbering combines. Not so exciting...

 

[BJM]

I toured McDonald Douglas when I was in college. One interesting fact they mentioned is that auto makers will spend ~$10 to lose a pound, aircraft makers will spend around $1000 to lose a pound and aerospace would spend around $10,000 to lose a pound.

My company once had to lose 50 grams due to a mass budgeting error on a Japanese satellite. We were given $1000/gram to carve out some mass. We eventually did only to discover that the savings were wiped out by a 50 g error in the original estimate.

I suppose most are composite and held together with adhesives now. They had one clean room where they assembled some subs for the shuttle program. Very cool stuff.

Not all S/C are composites. If you watch the Canadarm II on the Space Station doing its thing, I worked on the Force and Moment Sensors at each end of the arm (at the wrist locations). It is made of Aluminum 7075-T6. Yet to be launched is the hand for the arm, most of that is titanium 6Al-4V.

With respect to design for vibration, I was mentioning that the ratio of stiffness/mass (density) is the driver for what frequencies a structure will vibrate at. Divide the stiffness modulus of steel, titanium and aluminum by their respective densities you get roughly the same answer for each. With magnesium you get a slightly higher number. If you made identical trusses from each material, each would quiver at nearly the same frequencies. However the absolute stiffnesses of the trusses differ, if aluminum has a stiffness of 1.0 the titanium would be 1.6 while the steel would be 2.9. Despite being dense, steel competes against the others quite while since it brings stiffness along with it mass. When you add in non-structural mass like people, interiors, etc. the answer is much muddier. This is why you can choose any of the three and make a car body perform almost identically. Then when you consider strength (particularly fatigue strength) steel still dominates. Titanium is nice but expensive, and aluminum has poor fatigue life.