PSN-L Email List Message

Subject: Re: School project
From: John or Jan Lahr JohnJan@........
Date: Wed, 29 Nov 2006 08:55:58 -0800

I've posted pictures of a simple seismic system
made from wood here:
The natural period is a function of the tilt of the spring.  As it becomes more horizontal (by attaching the top of the spring lower on the mast) the period becomes longer.  However, if one tries to make the period too long the pendulum will become unstable and it will be hard to keep it from drifting to the upper or lower stop.

At 02:10 PM 11/28/2006, you wrote:
Hi Folks,  Some discussion on a simple vertical seismometer got me to thinking about a possible school project for the kids.  At one time, I found a simple vertical sensor on the internet, it used pvc pipe and a small 9"? spring.  I built it but never tested it because I had an idea the coil and magnet were just too undersized.  It probably works but would require some strong amplification, further more, I have no idea what the period would be.
I wonder if one could use a "Slinky"  a toy spring used by kids? They are about 3" in dia. and collapsed about 5" tall.  I think I will get one and suspend it vertically, to check its suspended length and its natural resonance.  I have no idea how long the spring would expand under its own weight?   But let's say under 8 feet. If so, maybe it could be hung from the ceiling and the coil and mass would be near the floor?

 A sensitive seismic system must be covered to prevent air currents from causing vibrations much larger than those generated by earthquakes.  A vertical system will have to be very tall, and thus difficult to enclose.

It just might be something kids would enjoy.  

Yes, indeed.  This would demonstrate the concept of a seismic sensor but would not be very good for detecting earthquakes.

Q. 1.  If I pull down on a spring and release it, it will move up and down, until it come to rest, if each cycle is 1 sec, (say a very long spring), is that its natural period? Yes, one full cycle is the natural period.
Q. 2. If I put a mass on the end of the spring, will it move slower?  
Yes.  If so, is that, now its period?     Maybe 2 or 3 seconds?
Q. 3. If I could work out the construction details, what period should I target for a vertical sensor? 
The AS-1 seismic systems have a natural period of 1.6 seconds.  To see surface waves the electronics with a period of 20 seconds, the electronics have to greatly boost the low frequencies.
Q. 4. Are there any websites or information on using long vertical springs?

 I've posted various designs on my web site:

Keep us posted what you come up with!


   Many thanks, Ted

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