Actually they are not the same thing though they have come to be used as meaning essentially the same thing in an audio-slang terminology. Sensitivity is the measure of SPL (sound pressure level) output from the loudspeaker averaged over a broad frequency range when 1 watt of power is applied to the speaker terminals. This doesn't take into account the variance in voltage (due to Ohm's Law) which a change in impedance will cause. Thus the more common, and correct, value of 2.83 volts has been used as a constant that takes impedance into the equation. In its simplest terms sensitivity is the amount of voltage input need to obtain a certain voltage output. This is a common manner of measuring tuners, microphones and so forth. When applied to loudspeakers, the formula gets turned around to measure how much output results from a fixed input level.
Efficiency is a percentage of 100%. Most loudspeakers are less than 5% efficient in their ability to turn electrical power (wattage) into acoustic power (SPL). If you interchange the terms sensitivity and efficiency, no one is likely to complain; but the two are different animals. Here's some defintions to show how the terminology varies.
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Efficiency rating - the loudspeaker parameter that shows the level of sound output when measured at a prescribed distance with a standard level of electrical energy fed into the speaker. Note, however, that a driver with a high efficiency rating needs a larger box to play a lower frequency than a driver with a lower efficiency rating. This means that a low efficiency driver in a small box will actually have higher sub bass SPL's than a high efficiency driver in a similar small box!
Efficiency The ratio (expressed as a percentage) of the output energy in the required form to the total input energy. In a speaker, the ratio of power applied to input terminals expressed as a percentage.
Sensitivity -- The minimum value of input signal that is required by an electronic unit, such as a tuner, to deliver a specified output signal level.
Sensitivity: A measure of the efficiency of a loudspeaker. A typical sensitivity figure for a loudspeaker is 87dB. A high sensitivity 94dB or more. A low sensitivity is 80dB or less. (See Efficiency). Low sensitivity loudspeakers require a high amplifier power output to obtain realistic volume levels. High sensitivity loudspeakers will be happy working with low power output amplifiers (20W per channel maximum or lower).
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The typical tradeoffs in a loudspeaker are efficiency, size and bass extension. If you change one of those three, you will affect the other two.
As a very general rule, high efficiency loudspeakers are likely to throw away some accuracy and bass extension for the sake of SPL. An inefficient speaker will not, however, be accurate just because it takes more power to play at the same SPL. Most accurate loudspeakers often require more power to drive, but that generalization does not operate in reverse.
Efficiency is a percentage of 100%. Most loudspeakers are less than 5% efficient in their ability to turn electrical power (wattage) into acoustic power (SPL)
Sounds technically right, but I've never heard of anyone using the term that way. It's usually interchangeable with sensitivity.
Most people talking about efficiency usually mean sensitivity since that is more meaningful. Why is this? Well, give or take a very few models, almost all speakers vary between 3% and 5% efficient. Crazy isn't it? 95% of the power your amp delivers to your speakers is wasted in heat. So it doesn't really do to talk in efficiency terms since you'd be comparing between, say, 4.5% efficient and 4.7% efficient.
However, speaker sensitivity varies between 82db/w/m and 112db/w/m, the average being 87db/w/m for a 1khz signal. Now for every 3db rise in volume, a human notes a doubling in volume. Therefore, for a given power with a 1khz signal, a speaker that produces 87db is twice to three times as loud as a speaker in the 83db sensitivity range for the same input. An efficient speaker (say 94db/w/m) would be twice to three times louder than the average speaker (87db/w/m) for the same given input.
This is assuming all the speakers are rated at a similar nominal impedance of 8 ohms (which is typical). It's worth noting that a speaker quoted at 4 ohms is typically going to benefit from a 3db lift in sensitivity. I'm not quite sure why that works out like that, but it does. So a 4ohm 90db/w/m is about as sensitive as an 8 ohm 87db/w/m speaker. A lot of speakers are being quoted at 4-ohms nowadays because they have somewhat strange impedance characteristics. This boosts their sensitivity rating, so be careful when you see a sensitivity rating without a nominal impedance quote (or sometimes you'll see 4-8ohms which is really annoying).
Now for every 3db rise in volume, a human notes a doubling in volume.
Actually, for every 3 dB there is a doubling or power, but I've heard it said that it takes a full 10 dB for ears to quantify that as a doubling of sound. But let's leave that aside since it's subjective.
Therefore, for a given power with a 1khz signal, a speaker that produces 87db is twice to three times as loud as a speaker in the 83db sensitivity range for the same input.
4 dB increase is 2.5 times the power.
An efficient speaker (say 94db/w/m) would be twice to three times louder than the average speaker (87db/w/m) for the same given input.
It's worth noting that a speaker quoted at 4 ohms is typically going to benefit from a 3db lift in sensitivity. I'm not quite sure why that works out like that, but it does.