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In Reply to: RE: Barry's Isolation Platforms - marble isn't hard enough posted by tomservo on January 26, 2011 at 07:47:21
nt
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has been into acoustic isolation for decades. All nuclear subs have all their equipment on isolation mounts and indeed their entire platforms are floating within the pressure hull. The military was one of the first to use high resolution /DA converters, as I have found 20 bit converters dating way back into the 70's.
EMI and RFI isolation is also nothing new for the military. In fact the optical com links were designed for the military to avoid spying into lines. Many items in the military inventory designed for security and other aspects have great audio applications.
Stu
Talking about submarines. . . I was really fascinated reading up on ELF waves and the Russian ZEVS transmitter for underwater communications.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
You do know about that massive antenna grid the US Navy has in Minnesota, IIRC. In order to transmit the ELF signals you need a correspondingly large grid. Only problem is that it takes a really long time to receive the signal, too, since those wavelengths are so long., but you probably already know that.
Stu
Actually, it's in Wisconsin. But you were close, really close.
If I remember correctly there are only a few places on the Earth where they can set up ELF antennas, the Laurentian Plateau/Canadian Shield in North America because it's a good insulator as opposed to a good "Earth."
Hey, my old project!
Edits: 01/26/11
Not exactly, we used the wire rope isolators in the high temperature acoustic levitator we flew on shuttle flights STS-7 and STS-51a. It just means they have all the documentation and testing available for mil-spec use.
One point was that when you look at products designed to do this job, they look / work nothing like hifi trinkets.
Frankly, the wire rope isolators look every bit as funky as many audiophile isolation products, if not more. Not terribly surprising as there are only so many ways to isolate something when space is limited.
What kind of isolation are you using? Not to worry, it's a rhetorical question.
Cheers
Actually the wire rope isolators were the only ones that would work and then only snubbed with rubber blocks, the vibration in the shuttle is very intense at low frequencies, picture a vibration test were the entire experiment moves a foot peak to peak at 10Hz.
As I recall one of the earlier sounding rocket tests was a 9 G vibration load up high and 4G down low. Keep in mind, the part that was isolated weighted about 250lb.
Actually, my trusty old Thorens td125 is the only thing I have that is microphonic / converts vibration into audible sound. It sits on a 1 1/2 inch thick granite slab which is supported by four of an isolator like these;
http://www.vibrationmounts.com/RFQ/VM02005.htm
That was available in a mounting like these
http://www.vibrationmounts.com/RFQ/VM02009.htm
which makes a foot under it.
To maximize the isolation, one needs to also remove the cover while playing, it acts as a diaphragm so far as picking up airborne sound.
The amount of turntable pick up is EASY to demonstrate too, you can hear “how much” is picked up if you place the stylus on a record between songs with the record NOT turning.
Now turn up the level to near normal and see how much is or isn’t picked up by the TT. Now, take one of your speakers into another room, set up a separate speaker so that you can play music in the same room as the TT.
Then go in the other room and listen to the sound from the other speaker that the TT picked up.
For transcribing records into the computer, I do that without the speakers on and in a quiet time without walking around etc.
Most springs used in isolation devices such as the Mil Spec springs you linked to are too stiff to be effective in dealing with the extremely low frequencies that audio electronics are prone to. Realistically, the resonant frequency of the isolating system should be 2 Hz or less which means the spring constant of the springs must be quite low, i.e., the springs must not be very stiff at all. And I know only one person who can build an isolating system with only one spring.
Edits: 01/26/11
Actually for audio purposes one is making a low pass filter from a mass, spring AND resistance (damping).
In the case of the Turntable isolator, there are four of the small isolators at each corner. These are elastomeric springs which are also very lossy as well as springy in all 3 planes..
Here is a company who products I have used, examine this page
http://www.earsc.com/grommets.asp?mode=list
I have a golf ball and a couple sheets made from the blue material, it is amazingly lossy, if you hold the golf ball at eye height and drop it on a concrete side walk, it bounces about 1-2 inches.
Like I said, these systems have to be designed around the weight / mass being isolated but in general increasing the mass is like increasing the C in an electrical filter. Plain steel springs with a mass will have much too high a Q without added damping.
Funny there, one can make a resonant damper.
I had a Celestron telescope that didn’t have a stiff mount / tripod, it took a good long time to stop wiggling. As a band aid, I added a little damper with an adjustable nut which held the end of a 1/4 inch shaft about a foot long. I turned a weight that could slide over the rod with a thumb screw to lock it at any point (adjusting the moment or resonant frequency). By eye, I adjusted the weight so that it had approximately the same resonant frequency as the telescope tube it was attached to. Right off the bat, the view through the telescope was much more stable, with a little tweaking, the annoying wiggle was gone (at least until you moved the telescope significantly).
A rubber glove is not a perfect fix for a leaky pen.
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