For the measurement of cone surface area i.e. Sd, does it matter particularly that the outer parts of the cone on the roll surround move less and less the further they get toward the edge? That is, the part of the surround that joins the cone travels the full stroke, the halfway point of the surround moves about half the distance the cone does, and of course the very edge of the surround doesn't really move at all.
This being the case I imagine the right way to measure the active diameter of the cone is from the halfway point of the surround on one side of the cone to the same point on the other side. This would approximate the diameter of a cone that moved the full stroke over it's full diameter, therefore would make sense in calculating the amount of air that gets displaced etc etc. So then, what is the right way(s) to measure the cone area / diameter?
This being the case I imagine the right way to measure the active diameter of the cone is from the halfway point of the surround on one side of the cone to the same point on the other side. This would approximate the diameter of a cone that moved the full stroke over it's full diameter, therefore would make sense in calculating the amount of air that gets displaced etc etc. So then, what is the right way(s) to measure the cone area / diameter?
Hi Circlotron
I would just use the dimensions of the cone, and ignore the surround, after all, the surround is not moving in a pistonic way, if you look at it, it is rolling as it moves in and out, and not contributing much back and forward movement.
What are these mysterious speakers you have bought and are testing to death?
I would just use the dimensions of the cone, and ignore the surround, after all, the surround is not moving in a pistonic way, if you look at it, it is rolling as it moves in and out, and not contributing much back and forward movement.
What are these mysterious speakers you have bought and are testing to death?
This is always tricky. The surround does ad someway to the radiating area. But how much? In general I measure the cone diameter at halfway the surround as a good compromise.
This is exactly what I am doing. If you take a few different drivers and measure the cone diameter, you will find that the Sd of the spec is always a little bit bigger. Half way the suround is a better approx.
Audio writer David Weems, in his book Designing, Building and Testing Loudspeakers, recommends including half the surround in the measurements, and so that is what I do.
However, I must mention that when I do this, I tend to end up with measurements that are slightly larger than the manufacturer's specs. I think that most manufacturers measure just the cone.
It seems to make only about a 10% difference anyway, not really enough to affect anything. I guess the choice is basically yours.
However, I must mention that when I do this, I tend to end up with measurements that are slightly larger than the manufacturer's specs. I think that most manufacturers measure just the cone.
It seems to make only about a 10% difference anyway, not really enough to affect anything. I guess the choice is basically yours.
It's silly to worry too much about this (as the French would say, "enculler une mouche"). Sd is used to calculate Vas. Vas is a parameter that varies all over the place- if the room changes temperature by a few degrees or you change the drive level for the measurement slightly or the relative humidity or barometric pressure changes, you'll have just as big a change in the Vas as the error introduced by where you decide the cone ends.
Where the rubber meets the road, so to speak, is in the impedance curves of the actual driver/box combo. That's the stuff to sweat.
Where the rubber meets the road, so to speak, is in the impedance curves of the actual driver/box combo. That's the stuff to sweat.
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