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General speaker questions for audio and home theater.

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RE: Correct- and it is measured 1 watt/1 meter nt

Posted by Presto on May 9, 2017 at 10:37:58:
Nope.
Sensitivity is, and always has been, expressed in 1W/1m.
"Efficiency" is often used to describe sensitivity while taking into account impedance. (Sensitivity compensates for impedance). This is not really what actual efficiency is... efficiency is really an expression of electrical power in compared to acoustic power out.
A tweeter that is 90db 1W/1m sensitive is 0.6% efficient
A prosound woofer that is 97db 1W/1m sensitive is 3% efficient.
The rest goes into heat (I^2*R coil losses and overcoming frictional forces of the mechanical system, which also results in heat).
I always refer to this calculator page for efficiency.
http://www.sengpielaudio.com/calculator-efficiency.htm
Although 32 and 16 ohm speakers were once very common, 8 ohms became a common "nominal" impedance for speakers for many decades. As such, the voltage used for efficiency is 2.83V, as this voltage when applied to an 8 ohm nominal load gives 1 Watt.
P = (V^2) / R
P = 2.83^2 / 8
P = 8 / 8
P = 1W
Interestingly, this means an 8 ohm speakers will have the same value for efficiency and sensitivity!
When you apply 2.83 volts to a 16 ohm speaker you get
P = (V^2) / R
P = 2.83^2 / 16
P = 8 / 16
P = 0.5W
When you apply 2.83 volts to a 4 ohm speaker you get
P = (V^2) / R
P = 2.83^2 / 4
P = 8 / 4
P = 2W
Efficiency takes into account the speakers impedance in so far as how it impacts power. Sensitivity compensates for this by adjusting the input voltage to keep input power constant.
I could be wrong, but I think this is a common usage of the word "efficiency" when it's technically incorrect. In reality, it's really sensitivity with and without using the 8 ohm reference for power. Sensitivity 1W/1m and sensitivity 2.83V/1m.
1W/1m = let's adjust voltage to have a common 1W input.
2.83V/1m = let's apply 2.83V and allow power to vary with impedance.
Since amplifiers are voltage sources, the 2.83V/1m has real-world applications, where sensitivity really speaks more to the speakers actual *efficiency*.
As such, a 2 ohm speaker can be less efficient than an 8 ohm speaker but still produce higher SPL at 2.83V. This is because you get 4W at 2 ohms and 1W at 8 ohms with 2.83V applied.
Cheers,
Presto