How much Power do speakers really use ???????

Twl's response of "it depends" is about as accurate as you can get. Given the massive amount of variables involved ( speakers being used, their radiation and compression characteristics, the type of music being listened to, the individual recording being used, the size and acoustics of the room, your listening distance, etc...), it would be way too hard to be specific.

Having said all of that, i will make one generalization: The more efficient your speakers are, the smaller your room is and the closer you sit to the speakers, the less power that you will need. Kind of makes sense, huh ??? : )

If we look at this strictly from an amplifier to speaker power transfer point of view and use Classical music as a source, we find that the standard for peak to average listening ratios as recorded is appr 15 dB's. Using "old school" sealed speakers of the low efficiency variety, average power levels of 5 watts or less would be required for all but the largest rooms on an average basis. The power required to generate full peaks without compressing the amplifier would be ( theoretically speaking under ideal conditions ) about 160 watts to achieve 15 dB's of dynamic range. If we reduced the peak to average ratio to appr 10 dB's, which is probably more common in most recordings, 50 to 55 watts would be all that you would need. That is, so long as the amp was fast enough to respond to the demands placed upon it. Otherwise, you would run into problems with slewing induced distortion ( SID, which is a form of "clipping" ) from an amp with a slow slew rate.

Bare in mind that these figures are based on a speaker that is both steady in impedance and presents minimal amounts of reactance. In plain English, we are counting on having an 8 ohm load ( for example ) that stays very close to 8 ohms and does not produce a lot of reflected EMF or sharp phase angles. This would be considered "heaven" as far as a speaker load goes and very few speakers of this nature tend to exist.

In other words, the above "theory" only works on paper as the dynamic conditions of a speaker / amplifier are continually changing as power and spl are varied. In most cases, the changes are NOT very linear and can change quite a bit from low spl's to high spl's. In effect, i would count on needing quite a bit more "muscle" than the above figured 160 watts to deal with the "less than perfect" situations that one encounters in real world operation with less than "ultra high" efficiency speakers. How much more one needs would depend on ALL of the above criteria that i mentioned in the 1st paragraph.

Obviously, Twl's situation is a little different due to using highly efficient speakers. While his figures are all correct for a "theoretical" installation under ideal conditions, i've yet to see a speaker that actually transfers input power to output SPL's in a completely linear fashion. This is due to thermal losses within the driver itself and compression that takes place on longer excursions. Once again, that brings us back to needing ( or should i say "wanting" ) more power on hand so that one could more easily overcome deficiencies / lack of linearity in the system.

To throw a BIG wrench into the above works, let's try looking at the same system with the same peak SPL level using hard rock as a music source. The accepted norm of dynamic range for a rock recording is appr 5 dB's. Obviously, this is due to the use of large amounts of compression and the much more consistent "drive" or playing of notes associated with "harder & faster" music.

On the surface, it would appear that we would need a lot LESS power to play rock due to the greatly reduced amount of dynamic range. That is simply not the case when you look at the big picture. Remember, we are comparing apples to apples here i.e. peak SPL's with Classical to the same peak SPL with "rock". We are not comparing the average listening levels between the two, otherwise rock would require a lot less power. After all, how many people do you know that like to "jam" to rock music at the same level that one listens to Classical music at ??? Most would consider listening to "rock" music at that level as being "elevator music".

Since we used 15 dB's as a reference for peak to average ratios on Classical music and "rock" music typically only displays appr 5 dB's of dynamic range, the average listening level would have to be appr 10 dB's higher for rock than that of Classical music in order to obtain the same appr peak readings. With that in mind, the 5 watts that sufficed to power the speakers for Classical music is now required to sustain 50 watts on an average basis for rock music. While the peaks would still be hovering around 160 watts, the continual heat dissipated by the amp and speakers on an average basis would be 10 times as high. Needless to say, this is MUCH harder on the speakers and the amplifier and explains why "rock" music is much more demanding ( in terms of ruggedness and reliability ) when building a system.

To further confuse the issue as to how much power one needs, different speakers "spray" sound or pressurize the room in different manners. Danner touched on this subject in his post. While the sound coming out of all speakers "falls off" as it gets further away from the speakers, this effects some designs more than others. If listening at 10' from the speakers, a 90 dB 8 ohm "line array" ( like the Pipe Dreams ) will need less power to produce the same volume that a 90 dB 8 ohm "conventional" woofer / mid / tweeter type system would require at the same distance. This has to do with the dispersion pattern i.e. far and near-field radiation. As such, a 90 dB speaker can actually play louder into the distance than a speaker with slightly higher sensitivity due to differences in radiation patterns. SPL's from line arrays do not fall off as fast as a "conventional" design as listening distance is increased.

While there are formulas to figure out the exact distances that spl's will start to diminish at a given frequency, the typical results are that line arrays and other "acoustically coupled" designs will produce more sound per watt in a large room than a conventionally designed speaker would. Needless to say, comparing speaker sensitivities at face value is only useful if listening at 1 meter from the speakers. Otherwise, you have to take a lot of other factors into consideration.

My personal thoughts are that the quality of wattage used is more important than the quantity ( under most conditions ). As such, one would want to strive for a high level of linearity ( Class A ) for as high of a wattage as possible. This amp should also be fast enough to respond to large changes in amplitude ( fast rise time and slew rate ) to accomodate the pulsed and highly dynamic changes found in some types of music. That is, if you want to retain all of the speed and attack that instruments have when listening to them live. On top of this, if one had a huge reserve of power available, the system would tend to sound much cleaner when driven hard due to the system never being "pushed" AND easily control the speaker even if it were considered a "highly reactive" design. Both Bigtee and Gs5556 touched on this in their posts from slightly different perspectives but with the same goal in mind. As far as DrDiamond goes, i'm with him. So long as quality does not suffer, i'd rather have a LOT more power than not enough.

Obviously, not everyone has the same listening tastes, speakers and rooms. As such, what one "needs" and what one "wants" can be very different things. Each situation is independent of another and needs to be assessed individually. That is the reason that most of us stress the importance of in-home auditions when possible. After all, what works "good" for you might seem "dynamically challenged" for someone else in their system and vice-versa due to all of the variables mentioned.

I hope that this helps explain a few different aspects of what we hear and why in an understandable manner. I'd also like to say thanks to Pbb for having faith in me to be able to possibly explain such things. I'll try to extend the same amount of courtesy back to him in the future : )

As to JCB's comments, i basically agree with what he had to say, especially about the wealth of info that we have available to us here and on a few select other forums. I learn from others every day. As proven by this thread, everyone can contribute a little bit with it all ending up to make for a well rounded yet diverse perspective on any given subject. Sean
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Personally, i think that a system built for Classical music should be able to sustain a peak of at least 110 dB's at your seated listening position. Obviously, different radiation patterns from various speakers, how far you sit from them and the size of the room will vary how much power you need.

For a "jammin" system that is built for hard rock / metal, the system should be capable of sustaining 110 dB's average at the seated listening position. All of the variables above apply here just like they did above.

As you can see, there is a BIG difference as to what "loud" is in both circumstances. While both systems should be capable of doing 110 dB's at your seated listening position, one of those figures is peak while the other is average. Obviously, a "rock & roll" system that someone tries to achieve "concert level" spl's with will take a HELLUVA lot more of a beating on an average basis. This is NOT to say that everyone listens at these levels, but that the systems should be capable of doing so if one desired. At lesser levels, the systems will be coasting and should sound like it.

I would think that average listening levels at the seated listening position would be somewhere between 70 and 95 dB's on an average for most all types of music. It is only when one really wants to "let loose" or has highly in-efficient speakers that one needs a gob of power. Most of the time, an amp will be producing under 10 watts RMS with peaks below 50 - 100 watts ( depends on the type of music and amount of compression ). Some large scale bass peaks may require hundreds upon hundreds of watts though, so system requirements and what you want to achieve with your specific system and its' limitations will dictate how much power you really need. Look for quality first and "big numbers" later. Sean
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I would beg to differ with the conclusion that Pragmatist came up with regarding the B&W's. The figures that he quotes are for a listening distance of one meter from the speakers AND do not take into account speaker compression. I don't know of a speaker made that is linear in terms of input vs output, especially when one starts hitting them with some juice. This has to do with thermal losses in the drivers and crossover components and is pretty much unavoidable to one extent or another.

As such, one should not expect to hear anywhere near the spl's quoted at their seated listening position unless one sits very near their speakers and has a very small room. To achieve the figures quoted at a typical listening distance, i would count on needing at least twice the power ( and more realistically four times that amount ) in order to keep the amp from getting nasty due to being pushed too hard. This is especially true if running a typical SS design.

My experience with speakers rated at 96 dB's and an amplifier rated at 120 wpc in a similarly sized room supports the above statements that i made. I kept popping tweeter diaphragms from driving the amp into hard clipping. Given that these speakers are four times as sensitive as the B&W's in question, i would not count on having enough muscle and doing it cleanly unless you had quite a bit of power. Sean
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Pragmatist: I hope you understand the point that i was trying to make as my post was not meant as an attack on you personally or the information that you provided. I thought that the effort that you put forth in responding was very kind and covered the basics quite well. What i was getting at is that math and various formulas can be very helpful in predicting accurate results, but only when you factor all of the variables into the equation.

Busaganashi: I think that most people would find a quality 250 wpc amp with 90 dB speakers to be plenty of power and capable of attaining SPL levels that are more than enough in the size room that you mentioned. On top of being a bit of a nut, i also tend to push things noticeably harder than most folks. As such, i try to figure for worst case scenario, which pretty has one covered under any circumstances.

If you have the opportunity to audition the amps that you were considering in the confines of your system in your listening room, i would by all means do so. Otherwise, i would be relatively confident that a well built 250 wpc SS amp was up to the task at hand for someone that was slightly more sane than myself. You might be able to get away with a slightly smaller tube amp due to the differences in sound quality that they produce under stress. Most SS amps tend to get hard and grainy if pushing them whereas many tube products remain musical, but get a little sloppier. The fact that your speakers are of reasonable impedance and not highly reactive also helps things out a bit : ) Sean
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