What is driver reference efficiency?

[neonblack 10+ year member]

I read that this is a good parameter to look at for comparing subwoofers but I haven't found the definition for it.

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[Papermaker85 5,000+ posts]

not really, its a good figure, but there are manny factors..

 

[squeak9798 5,000+ posts]

The reference efficiency, η0, "is the theoretical fraction of electrical input power that is converted to acoustic output power with the driver mounted in an infinite baffle."

It is expressed as a percentage, and not many manufacturers (in car audio) even list it as a parameter. Though if the other parameters are accurate it can be calculated out.

η0 = 9.64 * 10^(-10) * Fs^3 * Vas/Qes

(Vas in L)

From this, the sensitivity can then be calculated;

SPL = 112 + 10*log(η0)

It's not a very good parameter to compare speakers by individually, and still not overly important when considering the drivers as a whole. About the only thing I really use efficiency/sensitivity for in car audio is 1) to call BS on manufacturers fudging the numbers (which happens unfortunately quite frequently), and 2) a quick reference based on Hoffmans Law (if a driver does have a high true efficiency/sensitivity, then it's going to sacrifice either enclosure size or low end output)

 

[Papermaker85 5,000+ posts]

1w1m shouldn't be used as a efficency rating as its not measured.. again having the correct stifness and motor strength as well as excursion/powerhandling is the most important factors..

 

[audioholic 5,000+ posts]

squeak hit the nail on the head. Considering Hoffman's Iron Law, high sensitivity isn't always desirable in a subwoofer.