Propeller Head Plaza

Posted by tomservo (M) on June 02, 2007 at 09:47:23

In Reply to: RE: Not DO, But WHY Speakers Require A "Break-In" Period posted by thetubeguy1954 on June 01, 2007 at 14:19:17

Hi Tubeguy

Well mechanically speaking it is as others have suggested, flexible elements like edge suspensions and spiders have properties, which change a little after use.

A clue to what it is.
A pure “spring” has no losses associated with it and if combined with mass, forms a high Q resonance. For a woofer, this resonance (conjugate of mass and spring) is also normally the low corner F as well.
To make a happy woofer, one wants the Q that describes this low corner to be in the range of about .5 to .7 with .63 supposedly being a magic value (a trade off of delay and response).
This Qt can be dominated by the electrical damping (Qe) or Qm which is mechanical damping.

Now, in general (a wild arm waving generalization follows) drivers which have a lower Qm, tend to also have properties which change more over time AND with changes in temperature. The reason is it not easy to make a mechanical resistance which does not age or change with temperature.

Not so obvious would be drivers like soft dome tweeters, these depend entirely on mechanical damping. I have not measured these to “see” but what I have seen with others would make one suspect.
Things with mechanical resistance (like damping goo, “soft materials” etc) also tend to have hysteresis, a level dependent distortion of the ideal pure resistance action.

For the woofer, the “post break in” response is ideally unchanged although I have see some pretty large shifts in Fs after break in.
When in a cabinet, the drivers “spring” in parallel with the box (the springiness of the air in trapped in the box).
This is why the drivers Fs is lower than when in the box Fb, the spring force is higher but the mass is the same.
In a cabinet, the drivers mass is unchanged with break in, the motor strength is unchanged with break in, the Qm usually goes up some, the Fs usually goes down some on break in.

On top of all that is that loudspeaker drivers in general have an expected life of about 10 years if not abused. Things like ozone, the continuous drying out of plastic parts and such means they usually will become inoperable, fragile or change greatly eventually even if unused (like most modern stuff).
I suppose this is an up side of the old way of doing things.
For example, I have an old engine that was made in 1913, it runs fine and has had very little done to it in its whole life. I wonder how many modern devices will work as intended when they are 90 odd years old.
Best,

Tom Danley