Sensitivity Spec in Car Audio : Hoffmans Iron Law Discussion

[sundownz 197]

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.

[jcarter1885 995]

Food for thought, thanks Jacob

[ford302redneck 15]

Interesting...

[Kanaworm$ 292]

Great read!

Thank you

[BigBodyLexus 3]

Knowledge is power

[edouble101 314]

[sefugi 166]

Great info!! Thanks for taking the time to share.

[xxjakedeuelxx 0]

I could only imagine actually hanging at the warehouse for a day.... that's my version of heaven.

[ford302redneck 15]

I could only imagine actually hanging at the warehouse for a day.... that's my version of heaven.

Wouldn't do any good. You would have to hang out with people who actually know what they are talking about.

Lol. I kid I kid. Jake is very knowledgeable, and people can really learn a lot by just hanging/talking with him. Well if they actually understand what he trying to explain to you. Sometimes he can go way over most peoples head, and leave you dumbfounded on what he saying.

I've been wanting to visit Jake again for the last 2 years almost. And I still haven't made it up there. I hope to before summer is up though. I actually have quiet a bit to discuss with him.

[xxjakedeuelxx 0]

I do know quite a bit about car audio. But when it comes to speakers and the stats and specs and details... Im pretty much lost... I spend most of my time reading this forum throughout my day thanks to the new mobile version.... but im still kind of lost, however if I did/could hang with jake for a day, I would not pass it up, that's for sure. But who knows... maybe it will happen sooner than I expect.

Edited March 14, 2011 by JDeuel20

[sundownz 197]

Lol. I kid I kid. Jake is very knowledgeable, and people can really learn a lot by just hanging/talking with him. Well if they actually understand what he trying to explain to you. Sometimes he can go way over most peoples head, and leave you dumbfounded on what he saying.

I've been wanting to visit Jake again for the last 2 years almost. And I still haven't made it up there. I hope to before summer is up though. I actually have quiet a bit to discuss with him.

Come on by

[ford302redneck 15]

Lol. I kid I kid. Jake is very knowledgeable, and people can really learn a lot by just hanging/talking with him. Well if they actually understand what he trying to explain to you. Sometimes he can go way over most peoples head, and leave you dumbfounded on what he saying.

I've been wanting to visit Jake again for the last 2 years almost. And I still haven't made it up there. I hope to before summer is up though. I actually have quiet a bit to discuss with him.

Come on by

I will someday hopefully lol.

I keep saying in a few months I will, then don't. But this time I hope to make it happen.