Two on the same axis, turning at the same speed in opposite directions, would cancel.
It is fluids that support only compressional acoustic waves.
Within solid materials, acoustic waves can propagate in three modes. 1 Compressional waves, 2 Transverse (or shear) waves with a speed of propagation at around half of the compressional wave's speed, 3 Surface (Lamb) waves with a speed of propagation around a quarter of compressional wave's speed.
With any propagation mode, the speed is the same regardless of the excitation frequency (acoustic or ultrasonic frequency). Metals and alloys are only weekly acoustically dispercive materials.
When the thickness of the solid is close to the wavelength of the Lamb wave, the dominant mode of disturbance transmition within the solid is surface waves.
Compressional wave velocities
Material Sound Velocities |
George
Honda Goldwing has a less elegant version with the alternator as the opposing gyro. But my ancient airhead Beemer has a very noticable flywheel. Just part of its "charm", to be generous.
Thanks,
Chris
Thanks very much for posting this.
Does this apply to thin-walled tubes in the same way?
Thanks,
Chris
For clarity, the propagation velocity is very different in each of those modes, of course. In cantilevers, only transverse flex propagation is slow enough to be interesting, and even that only at audioband hf.
Transverse waves in cantilevers, structures like beams, rods etc, is dispersive - that is to say velocity increases significantly with driven frequency. For a given rod like structure such as a cartridge cantilever, velocity scales with the 4th root of the ratio of flex rigidity to mass per length, and with the square root of frequency.
So both transverse velocity and dispersion depends on material properties, as well as the cross-section profile, and it's definitely not fair to say that metals aren't dispersive as far as transverse propagation is concerned. Real life evidence of this is transverse wave propagation in train rails, where we hear the high frequencies first for an approaching train.
Compression and surface waves aren't important in cantilevers, or more to the point the frequencies at which they become significant is way beyond the audioband. Not so transverse (flex) waves.
I wonder if there's anything at all in the last half of the book? Might just be random letters, maybe no one has ever found out!
Speed reader? Once at MIT I sat around with a bunch of the guys after dinner talking nonsense for and hour or so but one of us speed read an entire Doc Smith sci-fi novel while we talked and could recall it almost word for word.
Eidetic memorists are fun, although sometimes the ability compromises other aspects of thought, like good conceptual reasoning. This was evidently not the case with von Neumann, who had memorized Dickens' A Tale of Two Cities. Friends challenged him to recite it as they followed along in a copy of the book. I think they decided he was telling the truth somewhere around page 30, and thought it was enough.
Interesting thought ----- I know a man ('best man' at my first wedding) who got straight A's in High school and same in getting two MS degrees from University of California..... ME and CS/EE. Went on to study for PHD. He would read the chapters/book the night before a test and ace it the next day. but whe n it came to practical issues... like reassembling his car engine, he couldn't figure which cylinder head went to which side of the block. Seriously. he was so abstract in his thinking..... I got him an interview in CS dept at LLNational Lab. he was working on PHd in Computer Science. They didn't hire him.
I'll let you-all figure the moral to the story.
[btw I escaped Nepal... cholera starting to spread.... sitting up here on the 30th floor looking out over Bangkok
I have an prototype PA to go measure and listen to on Monday] THx-RNMarsh
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It is what a guy who was the manager of an office teaching "speed reading" told-me.
I suppose it was because i learned to read with a photographic method called "Méthode globale". (I don't look at the letters, i recognize entire words in a photographic manner). Result ? If i can see the spelling mistakes in books or other's hand writing, i don't see my owns, because my writing is familiar to me. At the end, i can say it was more an handicap than a help in my life.
Most people read by scanning words not letters, the brain fills in the missing bit... never tried speed reading is there a words per minute definition of what is speed reading.
http://www.slideshare.net/yatender/power-of-brain
http://www.slideshare.net/yatender/power-of-brain
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The guy I was talking of made standard tests with me. It was timed pages of text to read, followed by questions about the content.
Yes - not even whole words, often endings are missed off. It's exposed when one tries to read aloud from a written text one hasn't seen before, and get any sense of natural flow. Unless one is used to reading aloud, that is.
I'll look into it, I tend to read pretty fast mainly due to constant reading, from an early age. Many cant understand me these days because I refuse to have modern mobile phone (I use mine for phone conversations and the occasional text) and I will read a book rather than watch TV, I don't use social media (apart from DIYAudio!) and I'd rather go out and look at the beauty of the world than mess about online.... Computers are for work and developing photos in my world...
Interesting, this video. And talkative about the absolute accuracy of measurements in HIFI Illusions. (I have a corrupted mind ;-)
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