Speaker Sensitivity Question...

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.

Adding a sub will help with power needs, especially if you use the sub crossover.
Keep in mind that doubling the amp's power will only get you 3db increase. You need to increase db by 10db to get a perceived doubling in volume. ie 1 watt=90db, 2 watts get you 93db, 4 watts=96db,8 watts=99db and so on. It gets tough when you have 84 db Maggies and need to go from 200wpc to 400 wpc just to gain 3db volume....

So, I take it that even though my processor is crossing over at 80hz to the powered subs, I gain nothing as far as watts that I would still need for a given db level.

I was hoping it would help.

On a side note. For rear speakers is surround sound... do I need to follow the same amp needs as a front speaker? Or can I get away with a lower watt amp for them?

Bill