# quick question

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#### Etocynned

using Hornresp, loving it!!!

on a given driver size(8,10,12) how do you go bout calculating sd?

ive heard 1/3 surround, but with some drivers having oversize surrounds?

Can i relate it to mounting hole diameter, no drivers surround extends outside the mounting hole?

Does it make a huge difference?

Will aliens come from space and vaporize my cerebral cortex if i get this wrong?

Do i have a cerebral cortex?

thx!

#### AndrewT

If the inner edge of the surround moves with the cone and the outer edge stays stationary with the chassis/baffle board, then there is some volume of displacement due to the surround movement.
A good approximation is halfway across the surround on both sides of a diameter.

#### Etocynned

that makes sense, how do i infer sd from the specs of a given diameter driver on a website?

i often noodle with drivers rom the part express site.

#### Zero D

@ Etocynned

Hi, yeah HR is great, & about to get even better with the soon to be added Filters

I just now tinkered around in HR to try & get it to Auto derive Sd by wiping out the TS entries, & then entering as many as i could, in various different combinations. Hoping Sd would be Auto calculated. Unfortunately it "seems" to require more TS parameters to able to do that It can however Auto calcaulate quite a few of them, after entering certain ones

WinISD though will Auto calculate Sd after entering the other parameters. The drivers in the screenies are not the same, i only posted them to show the Sd issue

Also these links might prove useful

Given in sq cm or sq inches. Basically means how much area the cone has to move air with. Larger cones will have bigger Sd?s and smaller cones will have smaller Sd?s. An average Sd for an 18? cone would be 1150 sq cm and a 15? driver would have an average Sd of around 890 sq cm. But the depth of the cone also has to be taken into account. A deeper cone will give you a higher Sd for the same diameter. So that?s why you see different Sd?s for same size drivers. The ones with the higher Sd?s have deeper cones or have less surround material or both.

Speakerplans.com

3.4.1 Sd

We shall start with an easy measurement; all you need is a ruler! Measure the diameter
straight across of your loudspeaker cone including one-third of the surround at both
ends. This is Diameter (D). To calculate Effective Projected Surface Area of Driver
Diaphragm (Sd), simply divide D by two, square the result, then multiply that by Pi
(3.142). However, this is one of the few parameters that you can trust the manufacturers
data on, if it's available.

#### Attachments

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• winisd.png
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#### AndrewT

Calculate Sd from Area (Sd) = Pi * D^2 / 4 = Pir² from your school days.

#### OscarS

Calculate Sd from Area (Sd) = Pi * D^2 / 4 = Pir² from your school days.

that is so not the issue at hand, it ain't even funny.

For the OP, I always use half the surround, because at high excursion, the middle to outer half will move, but at low excursion (will still move) but will move very little. Split the difference--halfway across the surround.

#### AndrewT

I think that is what he is asking.
Initially I thought his question was only about whether to include 1/3 or 1/2 of the surround in the moving area.
He came back and confirmed that the "area" is what he also wants to know.
The area formula is far more accurate than a number generated by the other t/s parameters.
And it was not meant as a joke.
It was meant as a reminder of where Pi*d^2/4 comes from.
Many forget that school arithmetic is a life long tool that is useful for a lifetime.

The instructions that come with DATS say to use the cone and between 1/3 and 1/2 of the surround and calculate the area from that.
FWIW

Dave

#### fastbike1

You also need to subtract for a phase plug if used.

#### OscarS

I think that is what he is asking.
Initially I thought his question was only about whether to include 1/3 or 1/2 of the surround in the moving area.
He came back and confirmed that the "area" is what he also wants to know.
The area formula is far more accurate than a number generated by the other t/s parameters.
And it was not meant as a joke.
It was meant as a reminder of where Pi*d^2/4 comes from.
Many forget that school arithmetic is a life long tool that is useful for a lifetime.

I agree with you there. I thought he already knew the formula for the area of a circle. I like to use that formula myself, since there is no need to "half" anything to get radius.

Yup, all too often people wanna learn all about audio, enclosures, acoustics, without having a firm grasp of at least high school physics and not having ready any good books like Dickason's Loudspeaker Cookbook.

#### Etocynned

I know the are of a circle, pi(r^2).

I am wondering if theres a way people are assuming the diameter of a cone when the driver isnt in front of them to calculate sd.

perhaps its just to vague a question.

I tend to use the cutout diameter/2, ill continue.

Loudspeaker design cookbook IV was the most dog eared book on my wall. I had commited to memory the calculations required to design a qb4 alignment for any set of TS parameters i came across.

When i designed towers for a pair of radio shack 6 1/2 inch drivers i used the formulae from that book for the design, predicion of pipe resonances(standing waves) and also vent area that would fit in the tower i designed without bends( 2 3" pvc pipe 5 1/4" long each tower).

They were wonderful excepting the fact that i used polarized caps in the crossover not thinking of how that thrashed the high end( wah wah waaaahhh).

If i could find a copy at my local library id take it home with me today. Books dont Crash!

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