De-magnetisation of Loudspeaker Magnets

[PRYML]

I read the online literature of a Class D (or was it "Tripath") amplifier manufacturer mentioning that the clicks and pops produced by the act of turning on/off a "noisy" amplifier de-magnetises the loudspeaker magnets :hmm:

Is there any merit to such claims, and if so, I wonder whether or not regularly playing CDs that claim to de-magnetise a hi-fi system (to improve the sound quality) will eventually affect the loudspeaker magnets as well... :scratch:

[Reid Malenfant]

Some loudspeaker magnets tend to de-magnetise even if the driver is driven pretty normally though at a highish level ;) The worst offenders here are speakers with Alnico magnets :rolleyes:

Ferrite magnets & i should imagine neodymium will be far more if not totally immune to this problem..

The CD won't demagnetise your speakers but i'm sure it'd sound pretty vile :eyebrows:

[Ali Tait]

Never heard that one before.

[DSJR]

All this affection for AlNiCo magnets makes me smile, as these magnets are not too hot on retaining their "charge" over the decades. Nowt wrong with ceramic and similar magnets IF the magnetic circuit has been properly researched (gap widths and lengths etc). I know Tannoy have returned to these magnets in their top speakers, but i bet that if you asked the designers, it would be purely for marketing reasons to the far eastern audiophools, rather than them "being better," especially as the DC driver concept was completely re-vamped and improved in the 80's, along with the crossovers.. :)

[YNWaN]

Is there any merit to such claims

No, it's utter bollocks (sorry to be blunt, but in this case it can't be helped).

[Barry]

I read the online literature of a Class D (or was it "Tripath") amplifier manufacturer mentioning that the clicks and pops produced by the act of turning on/off a "noisy" amplifier de-magnetises the loudspeaker magnets.

It is not impossible but extremely unlikely, and in doing so it would probably burn out the voice coil of the speaker.

Let us do a little research – and a very good place to start is by looking in the AoS Library itself, under the heading Technical, Basics & General, “Permanent Magnets”: http://theartofsound.net/forum/showthread.php?t=3845.

As others have pointed out, AlNiCo magnets have the lowest coercivity. This is the size of the magnetic field necessary to demagnetise the magnet. For AlNiCo magnets the coercivity is around 80kA(turns)/meter. If an AlNiCo magnet were placed inside a (long) solenoid where the product of the number of coil turns and the current flowing through the coil is 80,000 then the magnetic field created would just be sufficient to demagnetise the magnet.

Now such a coil exists within your speaker: the voice coil. It is not long so the magnetic field we calculate will be an overestimate, however let us proceed. Let us assume the coil is 40mm long and is wound with wire having a diameter of 0.1mm. The number of turns in the voice coil will be 400. If extended over a metre length the number of turns would be 10,000, so to achieve the demagnetising field strength of 80,000 ampere turns per meter, a current of 8A would have to flow.

This might suggest that an amplifier of some 500W rating could demagnetise the magnets. Not so, there are a couple of additional points to bear in mind. First the demagnetising field has to be created with a direct current: unless operating at very low audio frequencies, the alternating current will just re-magnetise the magnet every half cycle. Unless the amplitude of the (alternating) demagnetising current is slowly reduced, there will be no overall effect. An example can be given concerning tape head de-fluxing. These de-fluxing devices consist of a coil of wire wrapped around a steel rod, shaped to be placed close to the tape head. They run off the mains frequency of 50 or 60Hz. To demagnetise, or de-flux, the head the device is slowly moved away from the head, so taking the permeable material of the head around smaller and smaller loops of the B-H hysteresis loop, until any residual magnetism has vanished.

The second point is that the magnetic circuit formed by the voice coil and loudspeaker pole piece is not particularly efficient; it is not closed or complete and will have what is called high reluctance (the magnetic equivalent of resistance). So what field is generated will not be very effective as regards demagnetising effects.

It might also be thought that with time there could be some cumulative effect. Well this comes down to the actual shape of the B-H hysteresis loop of the material. Permanent magnets have ‘fat’ loops, material used for transformers have thin ‘skinny’ looking loops. The area of the loop is a measure of the stored magnetic energy, so clearly it is desirable for permanent magnets to be made of a material where the area of the loop is large. Furthermore the B-H loops for permanent magnets are not only large in area, they are also nearly ‘square shaped’ with a flat top. This means that to demagnetise the material the full coercive field strength has to be applied. A lesser field will not take you down the ‘shoulder’ of the loop and effect a partial demagnetisation.

So unless you play electronic music consisting of some very loud slow-changing notes of low frequency, I wouldn’t worry at all. I have to confess I have never heard of speakers becoming de-magnetised with use, and I’m sure in a domestic environment it has never happened. There might be examples of speakers used as monitors in recording studios that have done so: these monitors are played at very loud levels and get a lot of use. Perhaps someone might know of cases where this has happened?

Regards

[keiths]

So unless you play electronic music consisting of some very loud slow-changing notes of low frequency, I wouldn’t worry at all.
Regards

You listening Mark (Reid Malenfant)? :eyebrows:

[Reid Malenfant]

You listening Mark (Reid Malenfant)? :eyebrows:
:lol: & i want to get realistic levels down to 12.5Hz :eek: @ 112Db 4356

It'll be ok Keith, the magnets are monstrous ferrite jobs ;)