Question about speaker efficiency. (1 Viewer)

[Justin Ward]

I'm doing a physics paper on some aspects of sound. I'm sure I heard somewhere that most speakers only use like 1%(or something else really low)of energy as sound energy. I haven't been able to track down that spec again. I realize that it will vary by the type of speaker, but how much energy in general is wasted in a speaker?

 

[Mark Seaton]

Hi Justin,

Indeed 1% is in the vecinity, and in fact most home speakers are closer to 0.5%. Especially since you are talking physics and real science, be sure to remember and maintain the distinction between efficiency and sensitivity. Efficiency is a percentage. Always. It is the percentage of the input power which is transformed to work, or in this case, acoustic power. Sensitivity is a quantification of output produced for a given input stimulus(signal/power/voltage). Typically specifications are given for 1W into the *nominal* load, measured at 1m from the loudspeaker.

We can relate efficiency to a 1W sensitivity for a theoretical point source operating in 1/2 space (ie on the ground) where 1% is about 92dB @ 1W/1m, and 0.25% is about 86dB @ 1W/1m. Some horn loaded system do in fact reach MUCH higher efficiency, where an optimally loaded horn can reach 25-50% efficiency as seen in some of our professional bass horns. As you can imagine, there is the additional benefit in this application where 1/4 to 1/2 of the power delivered by the amplifier is not heating up the woofer, but instead is transformed to acoustic output. As such, a driver only needs to "handle" the waste power, ie 50-75%.

Here is a good website to get some information form a very reliable source:
True Audio Tech Topics

More directly, this spreadsheet will help with the conversion I referred to above:
SPL to % Efficiency Spreadsheet

Another great link:
HyperPhysics
and more specifically:
HyperPhysics - Sound and Hearing

That should keep your head spinning for a while... :rolleyes

 

[Chu Gai]

It can also be estimated from the Thiele-Small parameters from the following equation.

n0 = 9.64 * 10-10 * (Fs3 * Vas/Qes)

Fs is the free air resonance in Hertz
Vas is the equivalent volume of compliance in leters
Qes is the Q of the driver at resonance (electrical losses only)

The result will give you efficiency i.e the ratio of power in to power out, as a percentage.

 

[Justin Ward]

Thanks alot guys. Its amazing how much energy is wasted. This should definetley help me considerably.

 

[Chu Gai]

Also for what it's worth the sensitivity can be calculated from n0. So depending upon whether we're looking at radiating that sound into a sphere or 1/2 sphere we have respectively...

sensitivity = 109 + 10*log(n0)

sensitivity = 112 + 10*log(n0)

Of course this implies that if you know only the sensitivity, you can back calculate the efficiency, right?

Why don't you crank some #'s and see what you get?