It is a rather odd value, and leads one to wonder how the post office derived it.  Id be far more worried about near in fields and things sticking to the machines.

The first thing is to convert to Teslas or Gauss, but being educated SI I will use Teslas in prefernce to Gauss, in the air around the magnet, the H and B will be parallel and in the ratio of  u0 -1.27E-6

so  1.27 micro Tesla per amp/metre,  (The unit is equivalent to inductance of free space in uH per metre or micro newtons per square amp depending ehat you want to calculate, but here the Tesla per amp per metre is fine.)

So 0.4A/m is approx  0.6 microtesla.

This figure  is just crackers as it cannot easily be measured, as in fact it is far is lower than the field of the earth itself - that being around the 30-60 microtesla mark, depending how close the poles (higher) or equator (lower) you are.

Now the field in the gap between the poles of a saturated hard steel steel magnet of the kind we grew up with, is about 0.8 to 1,0 tesla, and in a modern rare earth magnet, a touch higher but not by a factor of more than 2. In the pole gap of your speaker this will be the sort of exposed field level.

So your question is how big a pole gap can be exposed for a saturated magnet, that will have fallen away to below 1 part in a million by a distance of 4.6m.

Here we need to recall that magnets are dipoles, and the field from a simple dipole falls as cube law of the ratio of the distance to the pole gap.

So to with a geometric factor K (K = pi, pi/2 or similar depending on detail of magnet shape) if the pole gap is say 1mm, then by 1m away, the field is down by 1000^3, or 10^9

Similarly by 4m away, the field from a 4mm pole gap is down by the same amount.

I suspect that any magnet with an open pole gap of less than about 10mm will be fine, regardless of geometric details.
Anything stronger should be shipped with a keeper.

It is a very silly threshold.

Mike.

,

It is a rather odd value, and leads one to wonder how the post office derived it.  Id be far more worried about near in fields and things sticking to the machines.

The first thing is to convert to Teslas or Gauss, but being educated SI I will use Teslas in prefernce to Gauss, in the air around the magnet, the H and B will be parallel and in the ratio of  u0 -1.27E-6

so  1.27 micro Tesla per amp/metre,  (The unit is equivalent to inductance of free space in uH per metre or micro newtons per square amp depending ehat you want to calculate, but here the Tesla per amp per metre is fine.)

So 0.4A/m is approx  0.6 microtesla.

This figure  is just crackers as it cannot easily be measured, as in fact it is far is lower than the field of the earth itself - that being around the 30-60 microtesla mark, depending how close the poles (higher) or equator (lower) you are.

Now the field in the gap between the poles of a saturated hard steel steel magnet of the kind we grew up with, is about 0.8 to 1,0 tesla, and in a modern rare earth magnet, a touch higher but not by a factor of more than 2. In the pole gap of your speaker this will be the sort of exposed field level.

So your question is how big a pole gap can be exposed for a saturated magnet, that will have fallen away to below 1 part in a million by a distance of 4.6m.

Here we need to recall that magnets are dipoles, and the field from a simple dipole falls as cube law of the ratio of the distance to the pole gap.

So to with a geometric factor K (K = pi, pi/2 or similar depending on detail of magnet shape) if the pole gap is say 1mm, then by 1m away, the field is down by 1000^3, or 10^9

Similarly by 4m away, the field from a 4mm pole gap is down by the same amount.

I suspect that any magnet with an open pole gap of less than about 10mm will be fine, regardless of geometric details.
Anything stronger should be shipped with a keeper.

It is a very silly threshold.

Mike.

,