I want to buy a powerful computer speaker , How can I know the extend of the sound is loud or weak .
Is it check the output watt of the speaker ???
Any other suggestion to jusify the loudness of the speaker ??
The standard industry way of measuring output of an amplifier is in Watts RMS (which means average) per channel. The speaker is rated at how many watts it can handle. So you might match a 30 watt amplifier to a speaker which could handle more than 30 watts safely.
Unfortunately the power of cheap plastic powered loudspeakers such as those sold for use with computers is not usually expressed in standard industry terms (it's the same problem with car stereos where highly unlikely figures are claimed on the box). So the figures mentioned are essentially meaningless.
Buy a speaker system from a reputable brand -- Creative, Yamaha, Logitech -- and you can probably judge the power output most accurately by size and price.
If you are more serious about sound you might (like me) find an old used Japanese hifi amplifier and a pair of good but small hifi speakers and use this system with your computer. The results are far better than you'll get from anything sold as computer speakers -- and you'll probably spend less.
most computer speakers of major brands are labeled with a watts rating. i'm not sure if this is watts rms but it definitely makes sense with the pricing. the watts rating is for the entire system not individual components.
5-30w total = low end, basic speakers, typically 2.0 or 2.1
30-350 total = average, 2.1 to 5.1 setup. typically good value for the money.
350w+ total = loud 5.1-10.1 setup. if you buy a quality brand they sound great.
keep in mind that either you will want a good high quality integrated sound chip... or more likely a nice pci-slot sound card if you want the best sound (with systems greater than 2.1).
unless you're a complete audiophile (and even if you are..) a $100-200 set of computer speakers is more than enough for what most people use a pc for. i personally prefer logitech 5.1 systems. great sound, good price. can shake the room with them if you so desire.
any recommendations will come down to what you will need the speakers for and your budget.
The best way to tell is to go into a store that has several of them set up and listen. You and you alone know what you value: Total volume, accurate placement of sound, deep bass, crystal clear treble, or what. If you care about anything more than sheer volume, listening is worth your time.
Of course, I spent two weeks listening to speakers for my hi-fi before I settled on a pair-plus-woofer.