Which is more efficient and requires less power?

Assuming the numbers given are accurate, the 94 dB-watt 4 ohm nominal speaker is 5 dB more efficient than the 89 dB-watt 6 ohm speaker. In this case, the 4 ohm speaker will also have a higher 2.83 volt sensitivity, at 97 dB for a 2.83 volt input vs roughly 90.5 dB for a 2.83 volt input for the 6 ohm nominal speaker.

I have heard that the convention is to quote a "nominal impedance" value roughly 1.25 times the minimum impedance, rounded to the nearest ohm. There's no guarantee that a given manufacturer follows this convention, though.

Note that a few amplifiers might still be happier with the lower efficiency 6 ohm speaker.

Duke

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

After reading Tvad's question in the post just above this one, I ran my own estimates of the power required at an 8 foot listening distance. Assumptions are a semi-reverberant room, and we're listening to stereo (both channels driven).

Anechoically, sound pressure level falls off by 6 dB for each doubling of distance from a point source. Eight feet is about 2 1/2 meters, so we could expect a falloff of about 8 dB relative to the 1 meter SPL. But we have two speakers, each driven by its own separate amplifier channel. Double the total cone area equals +3 dB, and double the total amplifier power equals another +3 dB, so we gain back 6 dB right there. And this is a semi-reverberant room, so we can expect a dB or two of room contribution. Let's be conservative and estimate 1 dB additional from room contribution at 8 feet (at greater distances, the relative room contribution would be more). Plugging in the numbers, we have -8 dB distance falloff, +6 dB for double the cone area and double the amp power, and +1 dB for reverberant field contribution, for a net change of -1 dB compared to the single-speaker 1 meter SPL. So back at 8 feet, from a stereo pair of 89 dB/watt efficient speakers we could expect 88 dB SPL at the listening position for a 1 watt input. Similarly, with 94 dB efficient speakers, we could expect 93 dB at the listening position.

Elevick's chart is for a single speaker at one meter, but as you can see it's certainly in the ballpark for what you can expect from a stereo pair at your approximate listening distance. One thing the chart necessarily leaves out is the effects of power compression, but that's a whole nother can of worms and can't be generalized in a such a chart as it varies significantly from one speaker to another.

Now Gs5556 brings in the issue of peak-to-average SPL, and he's right in estimating 15 dB of headroom being required to avoid clipping. On some recordings the peaks can be as much as 25 dB above the average level! Fortunately, the ear is relatively forgiving of modest amounts of clipping. In fact, many popular recordings have numerous "flat tops" (clipped waveforms) already. The extra power requirments figured into his numbers are to avoid clipping on instantaneous peaks, but if you don't have that much power available doesn't mean you can't still enjoy the music. You're just missing a little bit of the liveliness and sense of effortlessness that having no amplifier-induced clipping would give you.

At least, that's my take.

Tvad, the fact that you're running a tube amp helps a great deal if you're pushing the amp into clipping sometimes. Tube amps "soft clip" (relatively little high order harmonic distortion produced), while solid state amps "hard clip" (lots of high order harmonic distortion produced). The ear is vastly more sensitive to high order harmonic distortion than to low order harmonic distortion. You've perhaps heard something like "one tube watt equals two solid state watts"? Well, the reason has to do with the audibility of the type of distortion they produce when driven into clipping.

Duke

Tvad, well maybe and maybe not on that 200 watt amp. I'll admit that I probably overgeneralized in my last post, and that many exceptions exist. And to a certain extent it depends on how happy the amp is with the load the speakers present, and solid state amps tend to be happier with difficult loads.

Also, the short-term power available for transients often exceeds the rated power by a considerable margin.

And from what I've been told by amplifier manufacturers (and supported by my own experience) there are different ways to arrive at the rated power figure - some optimistic, and some conservative. If that's 200 watts per channel reproducing broadband pink noise with both channels driven, it's an extremely powerful amplifier. If that's 200 watts per channel reproducing a sine wave a 1 kHz, that's not nearly as impressive and real-world it will probably clip long before the other amp.

Just as a matter of interesting trivia, FTC rules require stereo amplifier to be rated with both channels driven. But those same rules do not apply ot home theater amplifiers - the other channels only have to be on, not driven to the same power levels. This often results in highly inflated power specifications for home theater amps. Let me illustrate:

You walk into Bust Buy and there's the dimunitive Zony X-1,000,00 home theater system on sale for $299, featuring an 800 watt 7-channel amplifier. They achieve this amazing engineering feat by skimping on the power supply. You see, they only have to run one channel up to 115 watts with a 1 kHz sine wave, with the other channels barely on. They are then permitted to multiply that 115 watts by the number of channels to arrive at their rated power. And never mind that a 1 kHz sine wave is much easier to reproduce than a broadband music signal, and that the actual broadband power output before clipping may be as little as 1/8 that figure (this based on in-house testing described to me by an amplifier manufacturer).

As an extreme exammple, a decent 10 watt per channel tube amp could conceivably play louder than the "800 watt" home theater amp before audible distortion sets in.

The point of that tangent was just to illustrate one of the ways that the published specs often don't tell the whole story when it comes to amplification. It's not that measurements are inherently untrustworthy, it's that inadequate measurements provide inadequte (and often misleading) information.

Duke

Tvad, I think it's very unlikely the Zeros would help with a solid state amp. With a push-pull tube amp I'd guess somewhat unlikely but still possible. I have customers using Zeros with their push-pull tube amps, but the speaker load is an unusually difficult one (fullrange electrostats).

Feel free to e-mail me. I've had intermittent e-mail trouble lately so if you don't hear back from me in a reasonable amount of time try again or post a message here.

Duke