8ohm vs. 4ohm vs. 2ohm High Efficiency

[wushuliu]

I'm contemplating future builds down the road and was wondering, if I have a speaker that is:

97db at 2ohms
94db at 4ohms
91db at 8ohm

Which would be considered 'highest sensitivity'/efficient?
(And yes, assume there is an amp that can power the 2ohm just fine. Yes, using the same woofers/tweeters)

 

[18Hurts]

I'm contemplating future builds down the road and was wondering, if I have a speaker that is:

97db at 2ohms
94db at 4ohms
91db at 8ohm

Which would be considered 'highest sensitivity'/efficient?
(And yes, assume there is an amp that can power the 2ohm just fine. Yes, using the same woofers/tweeters)

All those speakers are the same efficiency assuming something like a 2.83V input. Put 2.83 volts into 8 ohms is one watt of input. 2.83V into 4 ohms is 2 watts of input which increases the sensitivity to 94dB at 2.83V. Throw 2.83 volts into 2 ohms and that is 4 watts so output increases to 97dB at 2.83 volts but the efficiency is still 91dB.

Why two different ways to do it? Well, say you have a midrange that is 8 ohms and 90dB of efficiency and have a tweeter at 93dB--you can pad down the tweeter with power resistors to cut the power in half to 90dB so they would be the same efficiency. Or, you could use a 4 ohm version of the exact same midrange which will run at twice the wattage as your 8 ohm tweeter--twice the wattage boost sensitivity up 3dB so it would "match" the more efficient tweeter.

The other direction is also used--saw a PA speaker that had a 16 ohm compression driver horn coupled with an 8 ohm woofer. The tweeter was so efficient that they dropped the sensitivity down 3dB by using 16 ohms but the efficiency remained the same. The 8 ohm woofer was driven naturally twice as hard with the same signal as the 16 ohm tweeter.

The reason for doing this is the most efficient way to use amplifier power.

 

[LineSource]

Many high quality digital recordings generate HUGE transient current demands. High SPL Bass down to 20Hz. Transient current demands need consideration.

Most high-quality solid state amplifiers produce 2x the power into a 4-ohm load than into an 8-ohm load. Very few amps can generate 4x the power into a 2-ohm load than into an 8-ohm load.

Active crossovers allow the amplifier to be direct connected to the speaker to utilize the maximum transient current. Direct connect of the amp-to-woofer is the most important for optimum performance.

 

[Brett]

Ditto 18Hz and LS responses above.
I generally design for the highest nominal impedance.

 

[wushuliu]

All those speakers are the same efficiency assuming something like a 2.83V input. Put 2.83 volts into 8 ohms is one watt of input. 2.83V into 4 ohms is 2 watts of input which increases the sensitivity to 94dB at 2.83V. Throw 2.83 volts into 2 ohms and that is 4 watts so output increases to 97dB at 2.83 volts but the efficiency is still 91dB.

Thanks, that's what I thought but wanted to be sure.

 

[head_unit]

Most high-quality solid state amplifiers produce 2x the power into a 4-ohm load than into an 8-ohm load...

And not even that, not at clipping. Some amps are RATED to double their power, but that is a marketing gimmick. Heck, when I worked at a large consumer electronics company I did the same thing, as competitors were rating their power to double and beating us up about it. I've measured a lot of amps and nothing truly doubled its power-the chief engineer explained losses are proportional to the square of the current. That's why I'm not fond of driving amplifiers hard into low impedances.