View Full Version : Sensitivity Spec in Car Audio : Hoffmans Iron Law Discussion

01-28-2012, 03:09 PM
Originally posted our other forums in December 2010 :


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.

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

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 :

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 copper coil driver is!

The CCAW coil doesn't pull ahead in real efficiency (output per watt) 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... the answer is YES but not as much 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.

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 on the copper coil... for a HUGE gain down low.

Is the higher sensitivity driver a better choice for peak SPL at higher bass frequencies ? Generally, yes -- that extra 1.5 dB will win the show... but is it worth the massive loss on the lowest octaves in a daily system ? In the minds of most car audio enthusiasts the answer would certainly be a resounding "NO!"

01-29-2012, 03:46 AM
so if I'm not in a completion then go aqhdc3 regular but if tryin to win a comp go hdc3 and pay the extra for the aluminium but this is
not just a straight alum coil ccaw is copper clad alum right?

01-29-2012, 04:03 AM
he's sayingwith the higher sensitivity driver you would need a bigger box to achieve the same performance in the 40 on down area than the same driver with a lower sensitivity would. which you choose would be dependent on your box, listening preference, and power on tap.