If you are considering using your indoor speakers for outdoor use, there are a number of pros and cons to consider before completing your installation. Indoor speakers, as you may have guessed, are not designed for extended use outdoors, and thus can be seriously damaged if used improperly. Likewise, outdoor speakers have many flaws that detract speaker users from buying them.
The outdoor speaker is specifically designed to be used in wide-open spaces, without the traditional indoor speaker's use of a subwoofer. In order to achieve this, many outdoor speakers have balances that favor treble frequencies more than indoor speakers, oftentimes to favor the use of the speaker as a PA or intercom. If you choose to install your indoor speakers outdoors, be aware that you may not be getting the same sound quality that you would from an outdoor speaker.
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Weatherproofing is probably one of the most important benefits of the outdoor speaker. Since outdoor speakers are specifically designed for long-term use in the elements, the box that encases the speaker is usually made of a metal or hard plastic that resists dirt and rain damage. If you use an indoor speaker outdoors, be aware that the box that encases your speaker is probably made out of some sort of compressed sawdust, and may or may not stand up to the elements as well as an outdoor speaker's box would.
The actual components of the outdoor speaker are also specifically designed to withstand the elements. While many cheaply made indoor speakers are designed with paper cones, most outdoor speakers are made of rubber or kevlar lining to resist the weather. Heavy rain on an indoor speaker used outside can cause permanent damage.
Since the outdoor speaker is designed to project across wide-open spaces, its capacity for volume is usually much higher. This means that, if you choose to use an indoor speaker outside, it may lack the same volume capacity that an outdoor speaker has. However, although it may lack high volume output, the indoor speaker produces a richer and more resonant sound quality than the outdoor speaker, so the choice is really about compromising.
Many indoor speakers are designed to be mounted on a stand or, in some cases, not designed for mounting at all. In contrast, almost all outdoor speakers come equipped with brackets that allow the speaker to be mounted and hung from a wall. If you want to use your indoor speakers outdoors, you will need to find your own way of mounting them, as they may or may not be equipped for this type of use.