well my main point is that 3db isnt twice as loud... its only a threashold increment of loudness. like if you turned the volume knob up to make soemthing twice as loud your db would be 8-10 more, not 3. thats why i quoted from your website:


"If a sound has 10 times the power of a reference (10dB) we hear it as twice as loud. If we merely double the power (3dB), the difference will be just noticeable"


this is not meant to be personal i was just pointing out that you were using a power equation to make a point about db and loudness so i just saw a problem in that. thats all really : )

reading back through my response i can see how it can be taken personally and i appologize for that...probably should have worded it better; i just get caught up in the terseness of forum lingo.

hehe, you're right, it's my mistake. Sometimes I think that I'm better in physics than I actually am I didn't take offense, I just thought you didn't understand what I was saying.

Ok, Thy to the rescue (tadaaaa)

I'll try to keep it simple, most things have been explained already, but I'll try to be complete.

The wattage of a speaker is basically how much power it can output. You should consider two kinds of values. There is the peak power (PMPO, max power output, ...) which says how much the speaker can output at most for very short periods. Try to run them at that power continuously and you'll fry them in a couple of seconds.
It's more important you take note of the RMS power (Root Mean Square) which says how much the speaker can output continuously, without breaking apart. If for example you get a 100 W RMS speaker, you'll be sure it can handle a continuous 100 W signal (be aware that usually it's measured at 1 KHz, the output also depends on the frequency caracteristics of the speaker) for a long period. RMS is always lower than the max power (at best, it's half of the max power). I hear you ask, why do manufacturers give max power ratings then? Well, just think what would sell best? A 1000 W speaker that costs at much as a 500 W (RMS this time) speaker... It's just a marketing trick, the consumer that doesn't understand the difference gets fooled.

Like said before, you need to take the sensitivity in account also. It's measured in dB SPL, the speaker is driven by 1 W, and the measurement is taken at 1 m distance. Why is that important? Well you will need a much less powerful amp for a more sensitive speaker to achieve the same sound pressure as a less sensitive one.

A little word about dB's. A decibel doesn't mean anything on it's own. A decibel is a relative (and logaritmic) value, so theoretically you could compare prices of bread in dB also. That's why you always see extra information next to dB (dB SPL, dBu, dBm, dBV, dBv, ... all different meanings). The most important value for speakers is dB SPL though. SPL means sound pressure level. You can say it's a definition of loudness. The dB SPL scale has been chosen that way that 0 dB SPL is the absolute perception threshold of human hearing (it has some kind of relation with wattage : 0 dB SPL = 10^-12 W/m²). A sound lower than that will never be perceived (doesn't mean other species can't hear it). Theoretically, there is no maximum dB SPL, but usually 120 dB SPL (for some people 130) is defined as the instant pain threshold. There you'll have hearing damage right away. It doesn't mean that lesser levels won't do damage to your hearing. For example a prolonged exposure to 90 dB SPL will cause damage in long term. The "safe" pressure is said to be 80 dBA (a weighted pressure level). It's also relative to the ear frequency response. Our ears aren't equally sensitive to the whole frequency range. Some guys called Fletcher and Munson have made graphical curves on how we hear. They have found out that around 500 to 1500 Hz we perceive the best, and lower and higher than that we need more pressure to perceive that frenquency as loud. These curves change according to the sound pressure also. If the overall sound gets louder, the curves will compress. Decibel meters take that in account. There are different measure weights that take the Fletcher-Munson curves in account at a set SPL standard. dBA is for moderately quiet signals, B for louder, C even louder and D for extreme loud signals (jets and stuff like that). Theoretically if one makes a measurement in a loud club, one has to use the dBC scale, but in practice it's accepted that dBA gives a good enough result, so that scale is usually applied for law decreits.

A 3 dB SPL increase means a doubling in sound pressure (as you are based on a 10 log base, for voltages, a doubling is 6 dB, as you are based on 20 log Vout/Vref). But this doesn't mean we perceive it as twice the sound level (again this has mainly to do with how we perceive sound), psychoaccoustically a 7-10 dB SPL increase will mean we'll perceive it twice as loud.

PS : for measurements, 1 kHz is usually used as the frequency for testing/measurement/setup purposes. When you see/hear the test beep on a tv channel, that's 1 kHz, it's used to set the ideal levels (adjusting VU meters to analog and digital peakmeters and so on, but that's another story).