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Name: Steve
Status: Other
Grade: Other
Location: NC
Country: United States
Date: August 2007


Question:
Do speaker magnets "age"? A guitarist was explaining his preference for older speakers because he said that the field changes over time (like 20-30 years) and provides a more compressed, rolled-off output. He described this as a subtle, yet noticeable effect. What is the science behind this?



Replies:
Hi Steve,

The only thing a magnet may do as it ages, is to get a little weaker. If the speaker sounds better with a slightly weaker magnet, then all the manufacturer needs to do is to use a weaker magnet in the first place. There is no magic here!

In a sealed box speaker, "Hoffman's Iron Law" states that there are only four things that affect the speaker's output. They are efficiency, "motor strength" (strength of the magnet and coil assembly), box volume, and low frequency response. These four things can be played off against each other for a desired result. For example, you can get good low frequency response (that is, good bass) in a small volume, but you may need to sacrifice efficiency and therefore require a lot of power to drive it. If you want good bass, and high efficiency, you may need to employ a larger cabinet.

One interesting point is increasing the strength of the "motor" by using a stronger magnet will certainly increase mid range and high frequency efficiency (the speaker will, at these frequencies, be louder for a given power input), but the low frequency response will in fact suffer as a result of increased electromagnetic damping.

Conversely, if the magnet becomes weaker, low frequency damping will be less, and there will be an increase in low frequency output. Unfortunately this increase is as a result of reduced damping and a more undamped, undesirable "boomy" sound will result. For those who like this unnatural sound, there are faster ways to achieve it than waiting for a magnet to age, such as using a too-large cabinet than the speaker requires, or, for a "ported" bass reflex speaker, you can use an excessively large port. Personally, I prefer a speaker to be as accurate a reproducer as possible; it should not add phoney "coloring" to the sound that was not there in the first place.

Mostly, though, I suspect the supposedly better sound is pure imagination. Magnets simply do not age all that much.

Regards,

Bob Wilson


To answer this question, it may help to understand how the magnets in the speakers are made.

Typically, an iron ore powder is sintered (heated and compressed together) into a solid 'magnet'. However, the 'magnet' is more like a blank canvas at this stage -- it is not yet 'magnetic' until you put it into a strong magnetic field. The strong magnetic field aligns the magnetic particles in the magnet all in the same direction. After you do this, the magnet retains a permanent magnetic field, and is now 'magnetic'.

The same process can be repeated to demagnetize or change the magnet's field. If you apply a new external field, the magnet will respond accordingly and change its own magnetic field. Adding to the complexity, there is a hysteresis effect where you cannot just apply an equal-but-opposite field to undo a magnet's field. Magnets have a 'maximum' magnetization that they can hold. If you want to completely demagnetize something, you usually have to completely magnetize it first, and then take it back to zero. If you start somewhere between zero and max, it is usually to hard to figure the exact external field to apply to get it back to exactly zero. (look up "magnetic hysteresis" for more info)

So, getting to your question, any time you place an external magnetic field around a permanent magnet like the one in speakers, the speaker magnet will change. The stronger the field, the more it will change. Most fields are too weak to make much of a difference, but over time, it certainly is possible to made differences that are perceptible.

Since it is the permanent magnet interacting with an electromagnet in the speaker that moves the sound cone and produces the sound, a change in the permanent magnet would affect the sound somehow. So I think it is very plausible that older speakers 'mature' over time, especially if you're stacking your speaker around other big speakers with large permanent magnets themselves. I have known many audiophiles (people who are really, really into perfect sounding audio) who can hear tiny, tiny differences in equipment. I would guess they care more about this effect, and notice it more as well.

Hope this helps,
Burr Zimmerman



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