If you find yourself listening to audio on your notebook computer for more than the briefest time, you'll soon appreciate the need for outboard speakers.
A typical laptop sounds tinny, because it has little speakers in little space. Usually, the upper frequencies will be enhanced, and the lower ones are totally absent. It is a miracle of the human brain that stuff we hear on our computers is even recognisable, given that so much is missing; typically, four of the 10 audible octaves aren't present.
We've limited ourselves to USB-powered speakers here, since these are ideal for notebooks. There are basically two types: those that use USB only for power, and those that also use it for the signal.
The first kind are more common, and generally lower in cost. They have a standard 3.5mm audio plug that goes into the headphone output of the computer. Their USB plug is simply for drawing power from the system. In those that we collected for review, all are two-speaker units. That is, you have separate left and right speakers that sit on your desk. They all have a power switch and volume control.
The other kind is USB Audio devices. Plug them into a computer, and it will recognise them as such. The main difference here is that this completely bypasses the audio side of your computer. The speaker itself takes USB digital data and decodes it into something with which it can drive the speaker cones. One nice side effect is that there is only one connection cable required.
But there are marked limitations on what can be done with a single USB connection supplying the power. USB 2.0 has an official current supply of 500mA at 5 volts. At best, you've got just 2.5 watts available, so don't expect party levels of sound.
Likewise, don't use an unpowered USB hub. And, if your computer is a bit old, you might run into trouble. Up until at least 2008, USB devices were typically required to perform a handshake before they'd be permitted to draw more than 100mA (limiting total power to just half a watt). That shouldn't affect those that are USB Audio devices, but those that use USB simply as a wall socket will suffer.