Resonance question

[cadesignr]

While researching "resonance", I came across this little gem, which raised a question in my mind. I thought "resonance"
was a standing wave. This is an excerpt from an article on Tesla, and his experiments with "resonance".

"Tesla performed his first experiments with resonance
technology at his New York laboratory by firing up a small oscillator, which caused a minor amount of vibration. Suddenly, an alarmed squad of police officers stormed into the lab, demanding that Tesla stop at once. Manhattan was shaking for miles around. Tesla had not taken into account how resonance waves grow stronger the further they travel from their source. He had unintentionally created what became known as Tesla's earthquake machine. "

Would anyone care to enlighten me a little. I do not understand how "resonance", which I thought was a sound wave, could grow "stronger" the farther it travels. My assumption was contrary to this notion. If this is indeed true, this leads to some pretty profound conclusions. Especially in recording studios. Since sound decays 6db for every doubling of distance from the source, then exactly what are "resonance waves"?

Thankyou.

Oh, btw, here is a link to the whole article for those of you who are interested.

http://www.parascope.com/en/0996/tesla5.htm

fitZ

[lovecow]

Dear fitZ,

"Tesla performed his first experiments with resonance
technology at his New York laboratory by firing up a small oscillator, which caused a minor amount of vibration. Suddenly, an alarmed squad of police officers stormed into the lab, demanding that Tesla stop at once. Manhattan was shaking for miles around. Tesla had not taken into account how resonance waves grow stronger the further they travel from their source. He had unintentionally created what became known as Tesla's earthquake machine. "

For a complete account of this story, you might check out Margaret Cheney's biography of Tesla: Tesla - Man Out Of Time. After discovering this effect, Tesla supposedly went out to a partially finished building on Wall Street to try his gadget out on some unsuspecting construction workers...

Would anyone care to enlighten me a little. I do not understand how "resonance", which I thought was a sound wave, could grow "stronger" the farther it travels. My assumption was contrary to this notion. If this is indeed true, this leads to some pretty profound conclusions. Especially in recording studios. Since sound decays 6db for every doubling of distance from the source, then exactly what are "resonance waves"?

Vibration in a mechanical system is really what this is about as opposed to sound traveling through air. Sound traveling through air does generally obey the "6 dB/doubling" rule if you are in the freefield of a source. However, a mechanical system adds its own "flavor" to sound vibrating through it.

I cannot hope to explain mechancial vibration in such a small space as this. But that doesn't mean I won't try!

Any mechanical system has at least one resonant frequency. If a system is exposed to vibratory energy, there is a relationship between the "forcing" frequency of the vibration and the resonant frequency. This relationship produces one of two results:

(a) Nothing. I.e., the forcing frequency's relationship to the resonant frequency of the system is such that the system is in the "isolation region" and nothing happens. (This concept is crucial to "floating floor" designs. More on that in the "Studio Construction" section.)

(b) Resonance. I.e., the forcing frequency is related to the resonant frequency such that the system is in the "amplification region." As resonant waves travel through the system, there is a varying amount of amplification of the waves. The amount of amplification will depend on the relationship between the two frequencies (forcing and resonant). In theory, perfect resonance creates infinite amplification. But this rarely occurs in the real world since most systems have some amount of inherent damping. But when it does, you get some beautiful results. And sometimes, catastrophic failures: Ever see the video footage of the Tacoma Narrows Bridge - aka, "Galloping Gert"? Now that's resonance.

Best regards,

Jeff D. Szymanski
Chief Acoustical Engineer
Auralex Acoustics, Inc.

[barefoot]

Fitz,

Resonances are standing waves. These can be acoustic waves, electrical waves, light waves, or whatever. Electrical oscillator circuits, lasers, and electrons orbiting an atomic nucleus are all examples of resonant systems. Each of these, in its own way, can be modeled as a set of standing waves.

Unless there is some sort of lossless transmission line to conduct the energy, all waves decrease in amplitude as they radiate away from the source. Nearby resonant systems can capture and store some of this energy, thereby increasing their local amplitude. But the further they get from the source, the smaller their amplitude will be - because there is simply less energy available. Nothing mysterious was going on in the Tesla experiment. The whole city wasn't really shaking..... just certain structures that liked to vibrate at the frequency he was generating.

Thomas

[Extreme Mixing]

Sort of like when an acoustic guitar goes crazy in response it certain low notes. I used to compare mixing in my living room, with it's raised wooden floor, to mixing inside an acoustic guitar.

Steve

[cadesignr]

Hello gentleman. Thank you very much for the explanation. Sorry it took so long to get back to you. Site was down here at the bbs, bad weather and lifes callings here at home. Anyway, I always enjoy a real explanation to things of this nature.

fitZ