PSN-L Email List Message

Subject: Re: Speaker Like Sensor ?
From: ChrisAtUpw@.......
Date: Wed, 10 Mar 2010 07:14:31 EST


 
In a message dated 10/03/2010, gmvoeth@........... writes:

Hello  instrument folks,

Is it possible to get a speaker company to build the  mechanism like a 
speaker without a cone that would be like 500 to 10K OHMS or  more instead of 
the typical 8 ohms ?

I think it may be the best of all  sensors for a velocity type anything 
geophone ?

Comments  ?



Hi Geoff,
 
    If you wanted 10,000 off, maybe.
 
    Speakers mostly use ferrite magnets which are weak  and temperature 
sensitive. The coils are designed with tiny clearances to suit  an in/out 
motion and don't work too well with coils moving in the arc of a  circle. You can 
get small speakers with 1000 ohm coils.
 
    You definitely DON'T want coils much above 500  Ohms, or the resistance 
noise will be more than the background signal. Resistor  noise is broad 
band. 
    
    Using quad NdFeB magnets on mild steel backing  plates and flat 
rectangular coils is the easiest construction for amateurs. See  
_http://jclahr.com/science/psn/chapman/lehman/index.html_ 
(http://jclahr.com/science/psn/chapman/lehman/index.html)  for a  coil + magnet drawing.  
 
    Regards,
 
    Chris Chapman





In a message dated 10/03/2010, gmvoeth@........... writes:
Hello=20 instrument folks,

Is it possible to get a speaker company to buil= d the=20 mechanism like a speaker without a cone that would be like 500 to 10K OH= MS or=20 more instead of the typical 8 ohms ?

I think it may be the best= of all=20 sensors for a velocity type anything geophone ?

Comments=20 ?
Hi Geoff,
 
    If you wanted 10,000 off, maybe.
 
    Speakers mostly use ferrite magnets which are= weak=20 and temperature sensitive. The coils are designed with tiny clearances to= suit=20 an in/out motion and don't work too well with coils moving in the arc of= a=20 circle. You can get small speakers with 1000 ohm coils.
 
    You definitely DON'T want coils much above 50= 0=20 Ohms, or the resistance noise will be more than the background signal. Res= istor=20 noise is broad band.
    
    Using quad NdFeB magnets on mild steel backin= g=20 plates and flat rectangular coils is the easiest construction for amateurs= .. See=20 http:= //jclahr.com/science/psn/chapman/lehman/index.html for a=20 coil + magnet drawing.  
 
    Regards,
 
    Chris Chapman

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