Speaker's efficiency, sensitivity and impedance

Well, my answer is a little bit different - so I guess you get to take your pick!

Unfortunately the relevant terms get used inconsistently or imprecisely, leading many people to think that efficiency and sensitivity are the same thing.

Here is the convention that I follow. Some manufacturers also follow this, but not all.

As commonly used in consumer audio, "efficiency" is the 1 meter on-axis broadband anechoic SPL that a speaker delivers with a 1 watt input. Some manufacturers use a non-anechoic measurement (including room contribution), and some use a peak somewhere in the response curve instead of using a broadband average - so comparing two manufacturer's numbers may not really be apples-to-apples.

"Sensitivity" is the 1 meter SPL the speaker delivers from a 2.83 volt input. Sometimes called "voltage sentitivity" to make that clear. The same opportunities for optimistic specs enter the picture - namely, using non-anechoic conditions and/or taking the measurement at a peak rather than using a broadband average.

So, to recap: Efficiency is related to Watts, and Sensitivity is related to Volts.

(Some manufacturers use these terms interchangeably or backwards from this convention, so it's up to you to read carefully and figure out what they're really saying. Look and see whether they are talking about 1 watt input, or 2.83 volts input.)

Now, here is how the two are related: 2.83 volts into 8 ohms is one watt; 2.83 volts into 4 ohms is 2 watts; and 2.83 volts into 16 ohms is one-half watt.

So if you have a 4-ohm, 91-dB/2.83 volt sensitive speaker, it's an 88-dB/1 watt efficient speaker. If you have a 16-ohm, 85 dB/2.83 volt speaker, it is also an 88 dB/1 watt efficient speaker. I prefer to convert back to watts, because you can usually figure out the amplifier's maximum wattage output into a given load more easily than you can figure out its maximum voltage output (which may or may not vary with load, depending on the amplifier), and because if you are interested in power handling then you want to know the wattage going into the speaker.

Hope this helps.

Duke

That was quite elaborative Duke, thanks. So in fact sensitivity is closely related to efficiency and in fact they are expressions of the same thing but differs from one to the other in approach? Mlsst: instead of picking on me and telling me that I have an impedance obsession disorder (in fact I'm a bit neurotic, but that's another story :) ) it is better to explain things the way Duke did. I do appreciate your precious input though because I know you are a very knowledgeable person.

Chris

Chris

Efficiency is related to Watts, and Sensitivity is related to Volts.

To supplement Duke, if I may,here are some semantics. Basically,
* efficiency is a measure of how much electrical energy the spkr converts into acoustic energy (very little). It can be expressed in %: this speaker's efficiency is 5% (that high!)

* Sensitivity relates to how much acoustic energy a spkr produces given a specific amount of electrical (potential) energy.

I.e. for a "sensitive" spkr: you give it a little bit of electricity and, being very sensitive, it starts singing

So, the more sensitive it is, the more efficient it will be in the long run. I.e., if it is "sensitive" and plays very loud with very few volts, it is more "efficient" converting electricity into sound.

As you know, impedance is the resistance the spkr "system" has at 1kHz usually.(the actual res changes with frequency, hence the name "impedance" rather than "resistance"). Paraphrasing Duke above, and as a rough rule of the thumb, the lower the impedance quoted, the less "efficient" the speaker.

Chris, my ribbing was meant to be good naturedly humorous, but I think my underlying point was serious. You've spent a lot of energy trying to reduce a rather complex issue to a single parameter. Impedance is only meaningful when it is considered in combination with all the other factors.

Simply put, there is no one "audiophile" impedance that people should shoot for. It is only one of many factors to be juggled depending on the overall design goals for the speaker in question.