atom12v, post: 39836, member: 1423 wrote:
Nah, I didn’t mean loudness.
For one, no serious PA system has a loudness button, so pretty hard to switch something on that isn’t there.
It has to do (and I didn’t want to get into that earlier because we have to get all into physics and such) with the inertia of a moving object. In a subwoofer the membrane makes a relatively large and slow motion. It will travel to the end of it’s physical reach and then settle back in neutral before the next push comes. Clearly this happens between 35-125 times per second (Hz). Now as the membranes get bigger, they have to contend with more air to overcome both pushing and retreating and the overal weight of the membrane (and yes the attached voicecoil and it’s resistance within the magnet) moving become a bigger factor. The whole thing is just a tad slower switching from forward to backward movement, or vice versa.
At lower energy levels (lower volume) it can still do it pretty accurately since it doesn’t travel to its edges. But once it really has to work hard, the problem becomes audible. The membrane is a bit confused and can’t keep up.
The other bit is that when it finished a “push->return to neutral”-cycle, it won’t stop immediately but will swing backwards PAST neutral and then back to neutral. Depending on the amount of energy put into the original movement this may even happen more than once. If a new impuls to move in the other direction comes as it is actually behind the neutral point going backward and it now needs to go forward again, it has a disadvantage. Instead of going from stand still forward, it has to stop the backward motion, turn around and THEN start moving forward.
While all of this happens really quickly, at high energy sound levels, with lots of different information in that 40-100Hz area, the membrane really does get a bit confused and you can hear that.
High frequency speakers have it MUCH easier. The travel is very short (very little air needs to be moved, even if it is up to 20.000 times per second) and the membrane/voicecoil mass is very very low.
For the purist amongst us: I know there is intentional backward travel of the membrame past neutral too, since an audiowave has a positive and a negative half, but for the intent of the above explanation, I figured it was enough to just consider one half of the equation, the neutral-forward-neutral (or push) travel of the membrane.
I hope I have confused some of you completely, but that most of you get the idea of what I mean.
With audio reproduction, in all of it’s apparent physical simplicity = a driver in a wooden or plastic cabinet going boom boom boom, the actual physics behind it can become pretty complicated.
Greetinx,
C.
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