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Name: Matt
Status: Student
Grade: 9-12
Location: CA
Country: United States
Date: May 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 Matt,

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,

ob Wilson



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