Lena:

The effect I was talking about is really about the efficiency of the speaker verses the power output of an amplifier. Picture if you will an amplifier that is putting out a constant level of hiss. Place speaker "A" on it with an efficiency of 86db/w, typical of a speaker with low to medium efficiency. Lets say that, just for sake of comparason it produces, with your ear next to the tweeter, 50db SPL of hiss. Now place speaker "B" on the same amplifier, not changing anything else except that speaker "B" has an efficiency of 96db/w, which is not unheard of in HT speakers. If you were to put your ear to the tweeter as before, the noise would measure 60db SPL : a 10 db increase, or subjectively twice the level of hiss. Now hang an efficient horn speaker on this amplifier with a sensitivity of 106db/w, as mine are. The hiss you would measure would be 70db SPL - twice again as much!.

Now, every solid state amplifier creates a certain amount of electronic noise (hiss), distortion, and other undesirable artifacts such as crossover distortion. These are usually a small percentage of their total power output . Some of these get worse as the power is increased (distortion), some are worse at the very lowest levels of an amplifier's output. Crossover distortion and hiss are the ones that are worst at the lowest levels.

We don't want an amplifier to operate near either of those extremes: full power and beyond with it's increased distortion, and clipping of it's output, and at the lowest end, with it's noise and crossover distortion. Generally, we want the amplifier to operate most of the time somewhere in the middle, well away from the detrimental effects at either extreme. It's that lowest end of the amplifier's output that I am concerned with here.

Now, power the speakers in the first paragraph with a 200 watt amplifier. Which speaker do you think will be using the least of this power? If you chose the 106db/w one, you were right. If you put that 200 watt amplifier on the 106db/w horn speaker, the amplifier would be asked to deliver a tiny fraction of it's available power almost all the time. In other words, the amplifier would be operating at at the lowest extreme of the power amplifier's output where the noise and crossover distortion is at it's worst.


Now, if you put a 50 watt amplifier on that horn speaker with it's 106db/w efficiency, you would see that relatively speaking, more of that 50 watt amplifier's power is used. In other words, the amplifier is operating more in the middle area of the amplifiers total output. It is farther away from the lowest extreme where the noise and crossover distortion is at it's worst. And since the speaker is very efficient, it is way less likely to need the full power output of the 50 watt amplifier.

Now if you put the 86db/w speaker on the 200 watt amplifier, you can see that more power would be asked of the amplifier most of the time and as a result, it would be operating more in the middle area of the amplifier's total output. And since the speaker is less efficient, it would be more likely to need 200 watts in order to avoid clipping. If you used the 50 watt amplifier with this speaker, you would likely run into clipping.

In the days of 2 channel (back when dinosaurs roamed the land, and soundhound was a young lad with a cow lick in his hair) almost all speakers were of low efficiency. Those speakers required amplifiers with large amounts of output power in order to drive them effectively, so that the amplifier would not clip when relatively high sound pressure levels were asked for (like when cranking Led Zeppelin ) In those days, a big amplifier was better than a small one.

But something happened during the morphing of 2 channel stereo into Home Theatre. It was found that speakers needed to have higher efficiency in order to deal with the loud explosions, gunfire, and car chases (and that's just in romantic movies - blockbusters are worse). It was also necessary to raise efficiency of the speakers so that multi-channel amplifiers could be built that had practical power outputs (a 7 X 1000 watt amplifier would be impractical!) However, the "bigger is better" mentality remained. To paraphraise Porgy and Bess: that ain't necessairly so.

Another consideration is that in a typical home theater setup, the really low bass is re-directed to the subwoofer. That low bass is where the amplifier is asked to deliver it's most power. If the amplifier is relieved of this requirement by bass management, then the amplifier is going to be working again nearer that lower end of it's power range, relatively speaking.

As you can see, it is not as simple as you would think. There are a lot of variables to take into consideration. Generally speaking, if you have speakers that are more in the typical range of efficiency, in the 80's to mid 90's, you really need a more powerful amplifier. If you have really efficient speakers, in the high 90's and above, I would look at what I've said here, and re-consider your power requirements.

I hope this explains the situation better!

[This message has been edited by soundhound (edited January 24, 2003).]