Sensitivity Spec in Car Audio : Hoffmans Iron Law Discussion

[sundownz]

After getting several e-mails and calls about Sensitivity of subwoofers and seeing quite a bit of forum action regarding sensitivity I've decided to post this up...

Sensitivity does not = Efficiency on a subwoofer on LOW bass.

This is regarding industry standard sensitivity measured at 1khz -- this is where the spec is measure on 99.9% of speakers in the world.

Hoffman's Iron Law dictates that out of sensitivity, small box size, and low-end extension you can pick two and the other suffers.

In car audio, especially, you want small box and good low-end so sensitivity will typically be lower than a home audio woofer -- where you can use a large box and then have both high sensitivity and good low-end.

Since we have done quite a bit of SA-8 testing recently I'll use it as an example for the rest of the discussion.

Some recent SA-8 testing results to demonstrate this... measured in Sean's CRX in a box tuned in the mid 30s:

SA-8 / CCAW Coil (83.42 dB 1w/1m) D2 :

Frequency / Power / SPL

25 - 430 - 123.9

30 - 400 - 127.4

35 - 300 - 129.0

40 - 210 - 129.6

45 - 120 - 128.7

50 - 90 - 128.5

SA-8 / Copper Coil Stock (78.6 dB 1w/1m) D2 :

25 - 470 - 125.0

30 - 240 - 131.1

35 - 140 - 132.9

40 - 100 - 132.6

45 - 170 - 130.7

50 - 270 - 129.3

Take note low-bass fans... at 30 Hz the copper driver with nearly 5 dB less sensitivity is using 60% of the power and is 3.7 dB louder.

So... which one is more EFFICIENT on sub-bass by the real definition... output per watt.

The CCAW coil doesn't pull ahead in real efficiency until out of the really low sub-bass region @ 50 Hz.

So... sensitivity should at least pan out in maximum SPL, right ? We put in the SPL box tuned very high and still... not as much effect as you might imagine... the CCAW coil achieved 151.0 dB @ 1310 watts and the copper 149.9 dB @ 1600 watts -- not nearly as much as one would imagine by the 5 dB sensitivity different, huh ?

Even at lower power... both drivers at 250-watts we have 144.9 on the CCAW and 143.3 on the Copper -- just 1.4 dB down at 68 Hz... for a HUGE gain down low.

Is the higher sensitivity driver a better choice for peak SPL ? Yep -- that extra 1.5 dB will win the show... but is it worth a nearly 5 dB loss at 30 Hz (adjusted for power difference the actual loss is about 5dB) for a daily system ? In the minds of most car audio enthusiasts that would be a HUGE "NO SIR!"

-----

This testing was done in-house for my own purposes of product development and not to simply prove a point... but since the point needs to be made I have decided to share it.

Product design is a balancing act... with an 8" woofer on a standard diameter spider landing you need a certain amount of stiffness to hold up to what people do to drivers in the real world. On a driver like the SA-8 by the time you get to that level of stiffness a bit of additional mass is needed to drop the FS to a reasonable level so we used a copper coil to get there.

Can I soften up the CCAW driver spider a bit and help the low-end ? Sure can -- and we'll be testing that too. The sacrifice is you end up with a less mechanically rugged driver that, in terms of VAS, wants a bigger box while at the same time the bigger box further compromises mechanical power handling. In terms of the SA-8 we are working on different spider tools and so forth to try and get around some of these things. So far the combination we have is the most effective for what most people need / want. If we end up being able to improve on it without sacrificing durability then a v.2 SA-8 will be born.

[OrionStang]

Nice info. I've wondered about this at times.

[TheBetterMethod]

Great info! I'm ready to see the new SA-8.

[LT.Smoke]

Thanks for that information Jacob it was very helpful in letting me understand more clearly about how sensitivity works

[meade916]

a good read.......nice one Jacob

[kirill007]

Finally a thread about it from a company, now i can show this link to people

EDIT: Finkster what u mean?

I didn't see anything that looked hard to comprehend. :s

Edited December 30, 2010 by kirill007

[finkster]

This is probably more of a take notes session than a discussion, as there's probably only a handful of people on the site that know wtf all of this just meant, truly

[VOPA]

So, can we attribute the performance difference to the difference in MMS?