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Speaker Magnets and Age
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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Update: June 2012
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