4ohm, 8ohm speakers and output power

Nope. First of all, there is no such thing as an 8ohm or 4 ohm speaker. What you have is a speaker with a nominal impedence of 4 or 8 ohms. The actual impedences varies considerably throughout its range. Some nominal 8ohm speakers have droops in the impedence curve to 2 ohms or less, usually in the bass, and rises to 16ohms or more in the mids/highs. Ditto speakers with a nominal rating of 4 ohms. Such speakers present major difficulities for many amps. Impedences droops require more power. An amp that doesn't double its available power to a 4ohm load will clip much earlier when called upon to support the droop in impedence than will an amp which doubles its power into 4 ohms. Hope this description makes sense to you.

Yeah, what Newbee sez : )

A speaker that presents a nominal 4 ohm load will draw twice as much power as a speaker with a nominal 8 ohm impedance. Since the amplifier has to work harder i.e. generates more heat to deliver twice the power, you aren't gaining anything in terms of system efficiency when you go to a lower impedance load with the same sensitivity.

Other than that, you really have to look at how they are rating the sensitivity of the speaker. An 8 ohm speaker with 2.83 volts fed into it that is 88 dB's is measurably more efficient than a 4 ohm speaker that is rated at 88 dB's with the same 2.83 volts. The only time that you can compare sensitivity ratings between different speakers is if they specifically state that they are tested with 1 watt of input. Using a "standardized" voltage like 2.83 is NOT "universal" due to variances in impedance. The 1 watt method takes the impedance into consideration and compensates accordingly.

Another factor to think about is damping factor, which is a HIGHLY misunderstood term. That's something different though, so i don't want to get into that here. I'll just say that a lower impedance speaker has more potential to distort the output of the amplifier as the loading conditions vary dynamically. This is why "beefy" amplifiers typically work best into lower impedance speakers i.e. they typically have a lower output impedance than a smaller amp. The greater the difference in output impedance of the amp and the input impedance of the speaker, the more stable the circuit will become. Since tube amps can sometimes have output impedances as high as 2 - 4 ohms and are typically of a very limited current design, you can see why they would work best with a speaker of higher impedance. Otherwise, you run into problems with the speaker "modulating" the output of the amp. Not only will this change tonal balance and frequency response, but it will also alter distortion and transient response characteristics. This is yet another reason why some components are VERY system dependent i.e. they aren't nearly as stable and require a phenomenal amount of attention to avoid aggravating this problem. The ASL amps and some Cary's come to mind here.

All things being equal, one is typically better off with a high efficiency, higher impedance speaker that isn't real reactive. Having said that, i don't know of any speakers that are made like that which i really like. While this is obviously a matter of personal preference, it just goes to show how theory and reality sometimes don't jive. Sean
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Sean: Let's say that the sensitivity is rated using 1W as a reference which says that given an 1W input, both speaker can generate both 88db of sound at 1meter distance. And of course as you said, speaker impedance was taken in account which means that the amp has to apply a higher voltage to generate the same amount of power to a 4ohm speker as opposed to a 8ohm.
(Assume also that the 4ohm speaker always has its impedance lower than the 8ohm speaker in all frequency band from 0 - 22khz)

I guess we can agree that there can be no argue about the rating of the speaker and how it was measured. So let's ignore how efficiently the amp can generate power, and assume that somehow we manage to double the power when we drive the 4ohm speakers, then I don't see how the 4ohm speaker cannot be played louder than the 8ohm speaker.

The only factor that we don't know is that sensitivity may not be linear which means that higher Wattage the 4ohm speaker somehow is less sensitive than the 8ohm speakers but that is another set of specs all together.

Sean, FWIW to others who like tubes but are not aware, your point on amp/speaker matching is right on. One of the big priorities I set when I was matching speakers and amps was getting speakers with a minimum impedence of 5 ohms and 8 ohms nominal - a fairly flat curve I think. I also paid attention to the speakers impedence curves and the amps output impedence curves to make sure they complimented each other. Did I do good?