New Speakers and Old Compass

[mcerdos]

Okay, this is stupid of me so, please hold off on the "what were you thinking" comments. We are all entitled to a little bit of stupid once and a while...

The outside speakers in my cockpit were shot. I purchased a replacement pair based on size and square shape. I think the original speakers were custom built for Amel and are no longer available. I purchased a set of DC Gold N4C speakers. The magnets on the new speakers are much more powerful than the old speakers. I did not realize the DC Gold speakers would have such huge magnets. I had sent an email to the manufacturer and they assured me they would not interfere with the compass if they had 3 feet of space, which it does. This turned out to be total load of crap. I can move the compass with the speakers when they are 6 feet away. I now think the new DC Gold N4C speakers are going to be a lesson hard learned. When the speakers we in the cut-outs my compass was off by about 90 degrees.

I would still like to replace the speakers. I am now looking at PolyPlaner MA-4054 is a 4” cone size two-way coaxial speakers. PolyPlaner claims "ultra-low magnetic field speakers eliminates the problem of speakers magnets affecting navigation equipment such as compasses and autopilots".

The instructions for the Plastimo Offshore 105 Compass simply reads:

Deviation correction :
If deviation is no more than 7° : simply draw a deviation table assessing the error, and keep it for future reference.

If deviation is between 7 and 20° : we recommend that you have your compass compensated as a start, and then draw a deviation table. The compensation procedure is a delicate operation and should ideally be carried out by a professional compass adjuster.

I have a Raythenon fluxgate compass and know the normal variance between my magnetic compass and the fluxgate. My question is: Once I install the new speakers, how do I adjust the helm magnetic compass to compensate for the change in the magnetic field surrounding the compass?

[sy_gilana]

Anything you do that will affect deviation must be accounted for by re-swinging your compass.

Why not get some kick ass big JBL's and mount them on the spreaders and pump up the jam? That way it wont affect your compass, and you will have the anchorage to yourself.

[a64pilot]

Usually there are two magnets in a compass just for compensating, that are moved by one of two screws, marked EW and NS. If compensating on a easterly or westerly heading adjust the EW screw. From compensating many, many aircraft, I've never had luck adjusting more than say 10 deg, 90 is way off, I doubt that can be made to work.

I'd look for two plugs, or a removable bezel or something that these screws would be under if yu chose to try to "swing" your compass.

[mcerdos]

I am not familiar with the term "swinging the compass". Does this mean adjusting the screws?

The compass is flush mounted and I can access the bottom of it via a cabinet. I have two screws at the base of the compass. They are unmarked. I am assuming these are the screws for adjustment.

I plan to sell the DC Gold speakers and try my luck with the lower magnetic field PolyPlaner speakers. I am hoping this would leave me with something way less than 90 degrees. Do you know of a procedure or link somewhere to help with the adjustment?

[Albro359]

Adjusting the screws is compensating the compass for magnetic interference, but I would think the magnets in those speakers are having too much affect on the compass to be able to be compensated for.

Swinging the compass is turning the boat in a large slow circle and noting the difference between the compass heading and the true heading on all points of the compass to compile a deviation table.

The book that came with your autopilot shows you how to do this for the fluxgate compass - it tells you to drive the boat in a large circle and it automatically takes deviation into account.

Cheers
Alan

[mcerdos]

Originally Posted by Albro359

Adjusting the screws is compensating the compass for magnetic interference, but I would think the magnets in those speakers are having too much affect on the compass to be able to be compensated for.

Swinging the compass is turning the boat in a large slow circle and noting the difference between the compass heading and the true heading on all points of the compass to compile a deviation table.

The book that came with your autopilot shows you how to do this for the fluxgate compass - it tells you to drive the boat in a large circle and it automatically takes deviation into account.

Cheers
Alan

Cheers Alan,

Yep! Those new speakers are going on eBay

Thanks for clearing up the terminology. I will get out the instructions for the fluxgate once I get new (and less magnetic) speakers installed.

Many thanks!

[Ex-Calif]

Hold the phone here!

The compensating magnets in the compass don't magically eliminate the interference. All you can do is spread the variation out on a NS and EW direction.

Let's say the compass magnets were strong enough to counter a 90 degree shift (they are not I can assure you) - So when pointed "real magnetic north" you adjust the screw until you get a 45 degree error on north. You will then have a 45 degree error on south.

You then do the same with EW "balancing" the error. Let's say that also has a 45 degree error at each of E and W heading.

So to drive north you need to steer 45 for example. To drive south you'd drive 225. This is a 90 degree right turning error - the error is still there. You just steer to compensate.

To "swing" the compass you would aim the boat at as many headings as you see fit. Say each 30 degrees. You write down the heading to steer to achieve each real magnetic heading.

Then when sailing the boat you add or subtract the error to the compass reading to get the real magnetic heading you want.

This is what is called a compass deviation card. The plastimo instruction indicate that you can't compensate (successfully anything over 20 degrees and even if you did the compass is still always going to be reading off (10 degrees at north and 10 degrees at south for example)

You need to move the compass or move the speakers. I would suggest the original speaker were like 4W or something. You may get away with finding a replacement speaker with the same wattage as the old ones. If you have them you may be able to open them up and read the power rating right off the magnets.

[frank_f]

Could something like a mu metal covering or a Faraday cage be used around the speaker magnets?

[Ex-Calif]

You could also get magnetically shielded speakers...

Amazon.com: magnetically shielded boat speakers: Electronics

Not sure how well they work...

[nigel1]

Another problem with having a large magnetic influence near the compass, and then trying to remove the deviation with the correcting magnets is that the magnetic field around the compass can be severely weakened, and the compass card will tend to swing wildly.

[Saltyhog]

Originally Posted by Ex-Calif

You could also get magnetically shielded speakers...

Amazon.com: magnetically shielded boat speakers: Electronics

Not sure how well they work...

This is likely the best solution. Magnetically shielded speakers generally will have a second "bucking" magnet attached to the driver's magnet. This effectively reduces the strength of the field outside of the area of the voice coil. I know that Kicker did, and from the link from Ex-Calif it appears still does, manufacture these. I'm pretty sure Rockford Fosgate also makes them. They are not common since the magnet structure is one of the most expensive components of a loud-speaker, and a bucking magnet can double that cost. Magnetically shielded speakers used to be more common for home entertainment speaker in the days of CRT televisions, as a standard speaker near the CRT would distort the image.

[MBWhite]

Here's an informative book on compass adjusting, with plenty of stuff you may find interesting but unnecessary unless you own a battleship:

https://www1.nga.mil/ProductsService...ents/HoMCA.pdf

Way back in the stone age of computers, when they first started making sound cards there were problems when people started putting speakers on the desk next to the monitors. First issue is they can warp the picture and if left there for a long time it could be permanent (this was before monitors had a degauss button, don't know if they put that in to deal with this problem or something else). Second issue is if you stacked your floppy disks on a speaker it could erase the data.

Anyway, back then there was a certain amount of homebrewing effort put into magnetically shielding the speakers. Out of curiosity I did a search that brought up some info, so you might find searching the term "magnetically shielding speakers" to be helpful just be aware this will also bring up actual tinfoil hat websites as well, so use some common sense.

As a64pilot and ex-calif mention, there will always be deviation so a person needs to make a deviation card up. Here is an example:


With a calibrated fluxgate this would be easey peasey, just travel on a course with the fluxgate long enough to let the magnetic compass settle down, then write the mag compass heading down on the card.

[mcerdos]

Thanks for all the suggestions. Since I now "own" the speakers, I am looking into making some sort of shielding case. I believe if I can wrap them with some stainless steel sheeting, this might be enough to lower the amount of interference.

[MBWhite]

You can't use stainless steel, as most grades are non-magnetic (well, most all of the 300 series is IIRC, which is the corrosion resistant stuff) so you won't get any magnetic damping from it.

Try finding some tin cans that are big enough to fit around the magnets and then maybe a larger can to fit over the first can. If that does not reduce it enough wrap the smaller can with iron "mechanics wire" or "electric fence wire" until the larger can just fits over it.

[mcerdos]

Originally Posted by MBWhite

You can't use stainless steel, as most grades are non-magnetic (well, most all of the 300 series is IIRC, which is the corrosion resistant stuff) so you won't get any magnetic damping from it.

Try finding some tin cans that are big enough to fit around the magnets and then maybe a larger can to fit over the first can. If that does not reduce it enough wrap the smaller can with iron "mechanics wire" or "electric fence wire" until the larger can just fits over it.

Hmm? I thought since 304 stainless sheeting was non-magnetic (unless heated), it would serve nicely as a shield. Am I wrong?

The obvious choice would be to use iron but this is going to disintegrate and will be flush mounted in an area where it will be difficult to evaluate.