# Relative Driver Size - These vs Those

#### BlueWizard

##### Distinguished Member
I posted something similar to this a very long time ago. But since then I have revised the numbers to create a more accurate chart.

This compares the Relative Size of Speaker Drivers with a 4" driver being a 1, and each drive adjusted relative to a 4". For example, as you will see, a 6" driver is 2.25 times larger than a 4". This will make more sense when you see the Charts and the Examples.

Let's say you are looking at two speakers the Focal 936 (3x6.5", £2500) and the Focal 946 (2x8", £2600), and you are wonder which is the better value. It would be nice to know the relative difference between the sizes of 3x6.5" and 2x8". We will come back to that.

In my earlier charts, I simple took the rated size of the Driver, and found the surface area. But a 10" driver does not have a 10" Piston, so I subtracted about 1.5" (0.75 on each side) to get a closer approximation of the actual Piston Size.

This is the chart that resulted from this-

A = (pi) ((D - 1.5) / 2)²

Adjusted True Piston Size of the Driver
-

4.00" = 4.91 sq.in. = 1.00
5.00" = 9.62 sq.in. = 1.96
5.25" = 11.05 sq.in. = 2.25
6.00" = 15.90 sq.in. = 3.24
6.50" = 19.64 sq.in. = 4.00
7.00" = 23.76 sq.in. = 4.84
8.00" = 33.18 sq.in. = 6.76
10.0" = 56.75 sq.in. = 11.56
12.0" = 86.59 sq.in. = 17.64
15.0" = 143.14 sq.in. = 29.16
18.0" = 213.83 sq.in. = 43.55

Let's redo the 3x6.5" vs 2x8" calculation and see if it made a difference.

3x6.5" = 3 x 4 = 12

2x8"
= 2 x 6.76 = 13.52

13.52 / 12 = 1.1266 = 1.13

Meaning using a closer approximation of the Piston size of the drivers, 2x8" are 13% larger than 3x6.5".

In a home cinema room, 13% is probably not that significant, so it becomes a question of whether the person considering these speakers wanted to spend the extra money to gain 13%.

Now, it is not as simple as size vs size. We would need to take the speaker's rated Sensitivity into consideration, the cabinet size, and any change in the bass depth. But as a place to start, relative size has some value.

For maximum accuracy, there are lot of other helpful parameters, that likely there would be no way to find out. So, while knowing them would increase the accuracy, if you can get the information, it is not really helpful.

You many not need this information often, but when you do need it, it will come in very handy.

Here is another example. You are trying to decide between a speaker with a 5" Driver and one with a 6.5" Driver, what is the difference? -

5" = 1.96
6.5" = 4.0

4.0 / 1.96 = 2.04

In short, a 6.5" speaker is TWICE AS BIG as a 5" driver.

Let's try the same with a 5.25" Driver -

5.25" = 2.25
6.5" = 4

4 / 2.25 = 1.78

A 6.5" is 1.78 TIMES LARGER than a 5.25"

That's got to be worth knowing.

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

Here were my original calculation, for whatever value you might find in them. In the case below I used the rated size of the speaker, so for a 10" driver I used 10 inches. While this gives a bit nicer numbers, I don't feel it is as accurate.

The applied formula is -

A = (pi) R²

Area is PI (3.14159) times the Radius Squared.

Here are the results based on the -

Rated Size of the Driver -

4.00" = 12.57 sq.in. = 1.00
5.00" = 19.64 sq.in. = 1.56
5.25" = 21.65 sq.in. = 1.72
6.00" = 28.27 sq.in. = 2.25
6.50" = 33.18 sq.in. = 2.64
7.00" = 38.49 sq.in. = 3.06
8.00" = 50.27 sq.in. = 3.999 = 4.0
10.0" = 78.54 sq.in. = 6.25
12.0" = 113.1 sq.in. = 8.997 = 9.0
15.0" = 176.7 sq.in. = 14.06

If we want to compare 3x6.5" to 2x8", we divide the relative numbers. A single 6.5" is 2.64. A single 8" is 4.

3x6.5" = 3 x 2.64 = 7.92

2x8"
= 2 x 4 = 8

Now we divide -

8 / 7.92 = 1.01

That means, using the Rated Size, two 8" are only 1% larger than three 6.5" speakers.

Again best to use the number at the top - A = (pi) ((D - 1.5) / 2)²

I think being able to compare various size and number of drivers has some value. If you don't see the value, that's fine.

For what it is worth.

Steve/bluewizard

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#### BlueWizard

##### Distinguished Member

How many 10" subwoofers does it take to move the same air as a 15" subwoofer?

10" = 11.56
15" = 29.16

29.16 / 11.56 = 2.5225

So, it takes a bit more than 2.5x10" Subs to equal 1x15" Sub.

Seems like something worth knowing.

How about 10" vs 12" Subs

10" = 11.56
12" = 17.64

17.64 / 11.56 = 1.53

Roughly 1.5x10" equal 1x12".

Less helpful since you can't have HALF a Subwoofer, but it gives you a sense of the relative size.

You can work this backward too. How much smaller is a 10" Sub than a 12" Sub?

1 - (11.56 / 17.64) = 0.3447

A 10" is 35% smaller than a 12".

Again, just a For What it is Worth post, make of it what you will.

Steve/bluewizard

##### Distinguished Member
What about excursion and other parameters

A 10 inch with high excursion fed with plenty of power may move more air than a low powered 12 inch with small excursion...

#### BlueWizard

##### Distinguished Member
What about excursion and other parameters

A 10 inch with high excursion fed with plenty of power may move more air than a low powered 12 inch with small excursion...
This always comes up. No problem, tell me the excursion and other Parameters of the Focal 936 and the Focal 948. I suspect you can't and so those parameters don't matter if you can't know them.

These chart tells you what the tells you, but they don't what the don't.

This is the Chart of RELATIVE SIZE, and no more than that.

One would not buy a speaker based solely on this chart, but this chart could be helpful in deciding which speaker or combination of speakers to buy, that along with Frequency Response, Output Sensitivity, Power Rating, Dimensions, and other factors.

But ... you can't know what you can't know.

Steve/bluewizard

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