Outdoors, things are much clearer sounding, but bass gets lost sometimes. It takes a much more powerful speaker to drive bass outdoors than it does indoors.

Anechoic chambers are attempts to make an echo-free environment. It's not too hard to do at high frequencies, but gets much more difficult down low. Most anechoic chambers are only truly anechoic down to the midrange. That's why it's better to test subwoofers outdoors - It's a truly anechoic environment.