Which is more efficient and requires less power?

Regardless of the speaker nominal impedance, impedance curves, or phase angles, what you will need at your listening position of 8 feet for an SPL level of 89 db is 90 watts for the 94 db/W-M speakers and 285 watts for the 89 db/W-M speakers. This is assuming 15 db peak headroom which is reasonable for classical and jazz.

But which speaker requires more power? Can't tell. If the 89 db speaker stays flat, then all it needs is 285 WPC. But if the 94 dB speaker drops down to 1-ohm at some frequencies, then it may need 360 WPC or more to prevent clipping while delivering the same SPL as the 89 db speaker.

Nominal ratings are a guide, but minimum and maximum power requirements are spec'd out by the manufacturer for these reasons.

Tvad wrote:

"It seems 91db, 8 ohm speakers would be a more significant change from what I presently own in terms of efficiency and driving ease than would 94db, 4 ohm speakers."

Efficiency no; driving ease probably.

I assume you're quoting the 1-watt efficiency figures instead of "sensitivity" figures (sensitivity is referenced to 2.83 volts input; 2.83 volts into 8 ohms = 1 watt but 2.83 volts into 4 ohms = 2 watts. Sometimes manufacturers give the 2.83 volt sensitivity and call it "efficiency". Read closely to see if this is the case, especially with speakers whose nominal impedance is below 8 ohms). Assuming your numbers are efficiency and not sensitivity, then the 94 dB/watt, 4 ohm speaker would be 3 dB more efficient than the 91 dB/watt, 8 ohm speaker.

Now if the 4 ohm speaker manufacturer is actually quoting the 2.83 volt sensitivity instead of the 1 watt efficiency, then its actual 1 watt efficiency is only 91 dB, in which case both speakers have the same efficiency.

In either case, the 8 ohm speaker will most likely be the easier load. I say "most likely" because I sell a supposedly "8 ohm" speaker that's a more difficult load than most 4 ohm speakers due to the unusual nature of the load (it's a fullrange electrostat).

Now assuming that your tube amp is a transformer coupled push-pull type, as long as the manufacturer doesn't caution against 4 ohm loads you probably wouldn't have any problems. I would think that at a given SPL your amp would be less taxed by the 94 dB, 4 ohm speaker than by your present 89 dB, 6 ohm speaker. But I'm not sure about the comparison between the 94 dB, 4 ohm speaker and the 91 dB, 8 ohm speaker.

As you can see there are so many exceptions and caveats and what-if's that a generic answer probably won't suffice (see post by Gs5556 above as an example of how far off my generic answer could be). We might be better able to give useful information if you let us know the specific speakers and amp.

Duke

05-04-06: Gs5556
Regardless of the speaker nominal impedance, impedance curves, or phase angles, what you will need at your listening position of 8 feet for an SPL level of 89 db is 90 watts for the 94 db/W-M speakers and 285 watts for the 89 db/W-M speakers. This is assuming 15 db peak headroom which is reasonable for classical and jazz.

I don't follow.

I have measured the SPL from my listening position, and according to the chart Elevick provides, I am using 1 watt to produce 89db and 36 watts to produce 101db...far below your 90watt/285watt requirement. Furthermore, I reach 101db with the volume control on my preamp at about 2 o'clock. My speakers are 89db/1w/1m 6 ohm nominal. My tube amp is 110 wpc.

Tvad, you have to take into account that as the distance from the speaker increases the SPL decreases. That one watt of power is at a distance of one meter, not the listening position (about 3 meters). Doubling the distance decreases SPL by 3db. Also, the headroom required to reproduce the type of music listened to is a factor. The SPL db at a given distance is (desired SPL - sensitivity) + 20 LOG(Distance to speaker/1 watt) + Headroom dB. The power required is 10^(SPL dB/10).

Your numbers are correct - but only at one meter with a 3 dB headroom. At 3 meters with 15 db headroom, it's 90/285.

The position of the volume pot is indicative of gain, not power. Power consumption is only done by the speaker as it draws current from the source amplifier which has a constant voltage, power supply permitting. The draw is determined by the impedance at any given frequency and the amp has to be rated for the power the speaker calls for.

My real world experience does not agree with your numbers, Gs5556,
because when I'm listening to Metallica measured at 101db (not at peak)
at 8 feet from the drivers with an SPL meter, I have plenty of headroom
remaining, and my PP tube amp is 110 wpc. Your numbers would
indicate this is an impossibility, since according to your calculations, I
require 285 watts to produce only 89db at 8 feet. Yet not only is it
possible, it's real well beyond what your numbers tell you.

Perhaps it's the 15db headroom allowance that is the difference. I have
never experienced a peak of 104db when the average listening level was
89db. I can say for certain that with an average reading of 101db on the
SPL meter, peaks are generally no higher than 105db. With readings
around 90db, peaks are no higher than about 95 or 96db.