[chris3377]

I am in the process of writng everything I have learned in beginner terms into a book so I can show it to my friends when they have questions, and there are quite a few things I don't understand, so I can't explain them either. Here's a typical one:
 
 Joe: let me see if I have this right, if I double the power to the driver I get +3dB.
  Bob: right.
 Joe: So if I double it again I get another 3, so a total of 6 with 4 watts of power? Or if I cut the power in half I lose 3dB.
 Bob: right again.
 Joe: Now to get a doubling of sound, I need to 10X the power to get a 10dB increase.
 Bob: Right, 10dB is what the human ear hears as double the volume.
 Joe: Why couldn't I just add another speaker, double the drivers, double the SPL right?
 Bob: uhhhh.....no, you get +3 with another driver.....
 Joe: Why is that? Is it because they are each using 1/2 the power as one by itself? So if I add a driver and double the power I get double the sound then, right?
 Bob: No, you get +6dB.
 Joe: But that's where we are with 4 watts and 1 driver.
 Bob: yep.

If adding another driver only gives you +3, then doubling the power gives you another 3, you have spent twice the money on drivers just to get the same SPL you could get with 4 watts of power. And since 4 watts is no where near enough to blow a driver rated at 100 watts let's say, why bother?
  If it is for the surface area increase, there is another can of worms I don't get. If I am doubling the surface area, but halving the power, how do I get a 3dB increase? I would think that double drivers=double sound, but then you have to take away some because each driver is only getting half the power, so it loses 3 each, for a net gain of 4. ((double sound=10dB and 10dB-3dB(2drivers)=4db))
  And what does surface area do to the sound? If you have multiple small drivers, they act as thought they are one larger one, but with the same Xmax as the smaller ones, what does that accomplish? How is having the equivalent of a 10" driver with an Xmax of 1.5-3mm going to help you?

 I know this is a lot to put in one post, but I am too confused to try to be able to expalin it to myself, nevermind trying to help someone else. Thanks in advance for any help, or links.

[fabaudio]

[NealH]

Often the number of drivers a given design incorporates has more to do attaining other virtues that mere loudness per watt inputed.

For instance, here are some reasons a designer may add drivers:    

1. To effect a certain radiation pattern/characteristic.  Or, to effect a certain reverberant field effect.  
2. To enable a higher ultimate SPL (doubling the number of drivers, or speakers, effectively doubles the SPL capability).
3.  To provide a more linear response and thus potentially less distortion/compression.  A driver's linearity declines as it's movement withing the gap increases.  Therefore, all things being equal, there is more distortion/compression from one driver than two.

[_scotty_]

chris3377, If you have two 8 ohm drivers connected in parallel the amp will
see a 4 ohm load and deliver twice the power.  You have gained 3dB by doubling the drivers cone area and another 3db because each driver has the
same amount of power as when you had only one. The power was not cut in half when the drivers were doubled. If you have two 4ohm drivers connected in series you have doubled the cone area and halved the power each driver sees so the net gain is only 3dB.    A general rule of thumb governing driver distortion is that when you cut the cone excursion in half to reach a given SPL
by doubling the number of drivers you have reduced the distortion by 75%
over that produced by a single driver at the same SPL. It's a good thing.
Recent studies suggest that as little as a 6 to 8db increase in loudness may be perceived by most people as being twice as loud.

Scotty

[chris3377]

Good point Scotty, I had totally forgotten the power would double when the ohms were halved.
 So that answers part of the question. What about the way they say the SPL is increased:
  "A doubling of perceived loudness is equivalent to a 10dB change in acoustic output. A 3dB increase in acoustic output requires double the amplifier power, while a 10dB increase in acoustic output requires a one-hundred-fold increase in amplifier power. "
 If I do the math, double power=+3dB, so if I double again=6dB, double again=9dB. So I went from 1 to 2 to 4 to 8watts to get +9dB, not 100watts as the quote says.
 Or if we start with 5 watts, doubled 3 times we get to 40 watts, not 500.

 Anyway, this is confusing to me because when I was at Good Guys one day long ago, the guy was showing me Monster's amps saying how powerful they were..blah blah blah, and showed me the meter on the front that showed it was putting out like 600 watts during a scene from Behind Enemy Lines. Now if the speakers are rated at say 90dB with 1 watt, to get to 600 watts is like doubling the power 9 times, (only 512watts), which would make the final SPL like 117dB. If 120 is the limit human ears can handle why would you ever need that much power? I hear the term "headroom" thrown around all the time, but if I can't understand it, how can I explain it to someone else?

[ctviggen]

The formula is db = 10 log p1/p2.  So, 3 db = 10 log p1/p2; 0.3 = log p1/p2; 10^0.3 = p1/p2; 2 = p1/p2, such that p1 = 2p2 (and p1 > p2 because it's +3db; p1 < p2 if -3db).  For 10 db, 10 = 10 log p1/p2; 1 = log p1/p2; 10^1 = p1/p2, such that p1 = 10p2.  I get 10 times, not 100 times the power, unless my math is wrong and I can't see an error right now.  So, if you have 1 watt and you want to get a + 3db increase, you'd need 2 watts; to get a +10 db increase, you'd need 10 watts.  

I think that if you're listening at Dolby Digital's 0.0 db, you should hit 115 db or so at the listening position during peaks -- and only peaks -- of a movie.   That's why I'm typically around -10db to -20db (depending on movie), as 0.0db is too loud for me.

[_scotty_]

chris3377, If the loudspeakers were an eight ohm resistive load some
of what you saw the meter do might be closer to being accurate.
Power meters on the front of power amps are notoriously inaccurate and are on the equipement for marketing reasons and are not present to serve as an accurate gauge of the power consumed by the loudspeaker.  This is not to say that large amounts of power were not being used during the demonstration the exact quantity is what is in question.  At a live concert of classical music a person setting five rows back from the proscenium may frequently experience peak SPLs of over 115dB and never think twice about it.
This is transisent event in the middle of a loud passage which may average 90dB or less.   Peak SPLs encountered during the course of watching a movie
played back conforming to Dolby Digital reference levels are another matter.
I brought a Rat Shack SPL meter with me to Revenge of the Sith and the opening battle sequence had peaks of around 116dB and continous levels of 110dB. Those high SPL continous levels are the real problem from a listening standpoint, they will cause hearing loss. I routinely bring hearing protection with me to watch a movie.
Back to the question of why a lot of continuous power may be necessary.
Distortion is the reason. It sounds bad and can damage your loudspeakers.
How much power you need is dependent on four things,how loudly you listen to music,how efficient your speakers are, the recorded dynamic range of your music, and how big the room is that you are listening in. A key thing to remember is that if you are listening
to a live unamplified performance of a piece of music distortion is not part of the experience and unless you are setting very close to the performers
neither is listening fatigue. Another reason for more power is that amplifiers frequently start to come apart sonically and their distortion spectrum is elevated  before they exhibit hard clipping. If you can stay away from this region of elevated distortion and hard clipping your music will be easier to listen to long term and your speakers will thank you.  
Scotty