Hi,

also better is to bias the coil with a small battery.
A simple AA battery (also one with just a residual of charge).
Connect an oscilloscope or other suitable device to the sensor output.
Atthe same time connect the battery - to ground of oscilloscope and the 
use the + of battery to excite the geophone.
Do this all with the geophone in its natural position (horizontal if it 
is horizontal sensitivity, or vertical if it is a vertical sensitivity one).
You will see a nice swing when you release the + bias from the sensor.
The time between of the top of two sequential wave crest tell you the 
period, and its reciprocal is the freqency.

best regards
Mauro

Il 16/04/2012 02:23, Tiago Agrelos ha scritto:
> Hi.
>
> I´ve small geophone almost a year, but i never knew its natural
> frequency, how can i "find" its approximate natural frequency?
>
> The geophone doesnt have anything written on it, its smaller than 4.5Hz
> L15B geophone sold by Larry, generate a lot more output than the 4.5Hz
> one and i read 380ohm from it.
>
> I appreciate any help.
>
> Regards,
> Tiago.

-- 
Mauro Mariotti
http://www.infoeq.it
http://www.sara.pg.it

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