Speaker Sensitivity Question...

In theory, you're correct. In practice, though, its very difficult to do things that way. Power ratings on amps are not consistent at all. Every manufacturer has their own standards for how they measure power. For example, one company could rate an amp at 100 watts/8ohms. A different company that has higher standards may rate the exact same amp at 75 watts. And another may rate the same amp at 150 watts. If you are new and are having trouble selecting an amp, you'll need to gain some experience through experimentation. Also, talk to the people that make your speakers. They should be able to recommend something that will work for you.

For most music most of the time, a considerable majority of the power that is required will be required at frequencies greater than the crossover frequency that is likely to be used with a powered sub, e.g., 40 or 50 or 60 Hz or thereabouts. (I'm assuming you are referring to a powered sub). As a rough approximation, I believe that 300 Hz is a frequency at which equal amounts of power will often be required at higher and lower frequencies. So in typical circumstances I wouldn't count on a significant reduction of the amplifier power requirement as a result of using a powered sub, crossed over or not.

Also, if the crossover function is being performed "after" the power amplifier, as would be the case if it is performed by the sub itself (with the power amp being connected to the sub, and a high pass filtered output of the sub being connected to the main speakers), the power amp will still have to output a voltage range corresponding to the full frequency range of the signal. So if you were to go to a lower powered amp based on the expectation that it would not have to supply as much power as the full range signal would otherwise require, you still might find yourself clipping the amp if its voltage swing capability is less than what a higher powered amp might be capable of.

ZD makes good points as well. Regards,
-- Al

Zd542 is right about power ratings of amplifiers, they're not always a reliable indicator of true performance.Some amps rated at 50 watts per channel will clearly out drive some amps that claim 100-150 watts per channel.There are numerous factors that determine actual speaker load drive ability.Power supply "quality"(and output transformer quality) isn't always reflected in the measurements. Yet these two variables are very important.
Charles,

Per Al's remark, my understanding is that the sensitivity specification is usually for a 300hz or 1khz input signal, so a sub wouldn't affect that (either way).

Moreover, even if that weren't the case, I'd think that a properly designed speaker shouldn't be affected by filtering bass from its input signal. (Whether you filter by crossing to a subwoofer or by filtering the output of the source, for example, shouldn't be relevant. Either way, from the main speaker's point of view, you're just removing bass.)

A "flat" speaker system would take a wide band input signal and produce a flat output across its bandwidth. In a three way system, for example, it's probably true that the woofer will require more power to produce a given SPL than the mid-driver. However, the speaker system should be tuned so that both drivers produce the same SPL when fed the same signal. The output of the mid driver may need some attenuation (possibly within the crossover) to achieve that result, but flat system response requires it.

If that holds true, then adding a sub will increase overall system sensitivity if that sub is more sensitive than the main speaker or decrease system sensitivity if the main speaker is more sensitive (provided the same input signal is used). The biggest difference in the combined sub-speaker should be in maximum clean output capability, not increased sensitivity.

Also, bear in mind that the sensitivity spec is probably quoted for an anechoic environment and you will get room gain in your listening room.

Most powered subs don't have an active high pass for the main speakers although they do have an active low pass for the subs. That means that the main speakers still get the full range signal and the sub supplements the low frequencies.

If you do add an active high pass for the main speakers the benefit has more to do with intermodulation and other distortions. End result... you'll be able to play louder with less distortion. Without the high pass and just a sub, some of those distortions get masked.