Xref: utzoo sci.electronics:15183 rec.boats:4801
Path: utzoo!attcan!uunet!ogicse!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics,rec.boats
Subject: Re: non-magnetic speakers
Summary: Shielding won't work, but speaker selection might
Message-ID: <9878@milton.u.washington.edu>
Date: 25 Oct 90 00:39:34 GMT
References: <1990Oct16.140417.15202@phri.nyu.edu> <1990Oct24.170623.6664@tc.fluke.COM>
Organization: University of Washington, Seattle
Lines: 28


In article <1990Oct16.140417.15202@phri.nyu.edu> roy@alanine.phri.nyu.edu (Roy Smith) writes:
>
>	I want to put a pair of speakers on my boat, for a stereo.
>Unfortunately, the place where the speakers should go is within a couple of
>feet of the compass, so anything magnetic is bad.  Are there speakers which
>do not generate any magnetic fields? 

	There are piezoelectric tweeters (which have no magnet), but
basically all woofers are magnet-based.  So what?  You only care about
the field far from the speaker, right?  Most magnet designs for speakers
are self-shielding.  The leaking magnetic field at 2ft distance is
typically a third to a tenth of the Earth's field (I'm not guessing;
I got a gaussmeter and checked a couple of speakers).  If you find
a well-designed woofer, it could have even less field at a distance.
	Try visiting a store with a compass in hand.  Find a patient
salesman and orient a speaker various directions (twist it on a 
wooden stool or chair) while you watch the compass (preferably 2 ft. 
due East or West of the speaker) for fluctuations.  If you find a 
speaker which doesn't move the needle, buy it.
	If you find a speaker which only moves the needle a small amount 
(a degree?), buy it and BOLT IT DOWN before you install and trim 
the boat compass.  Most good compasses have enough trim capability
for such minor variations.  The magnet is a PERMANENT magnet, so
the field shouldn't change while the speaker is kept in place.

		John Whitmore
		whit@milton.u.washington.edu