Most outdoor speakers use plastic enclosures, which vibrate and create their own booming sounds interfering with your music. Expensive outdoor speakers deal with this by having larger enclosures for the boom. 

So, basically, the difference between one company’s relatively inexpensive outdoor speaker and a more expensive model is usually just the size of the woofer and the enclosure. 

The best outdoor speaker: Dayton Audio IO655 (sealed, almost water-resistant design, great price)

The outdoor speaker with the best bass sound: OSD Audio AP670WHT (but not water-resistant)

The best looking outdoor speaker: Boston Acoustics SOUNDWARE 4.5-Inch (affordable, compact)