QUESTION: Is 90db twice as loud as 80db? How does that work and what speakers are appropriate for an outdoor movie for maybe 15 people. It needs to take an RCA or HDMI connection. Preferably the former.
ANSWER: It is 10x louder actually.
Here are some articles that can explain decibels more eloquently than I can:
As far as the speakers, it depends on the size of the area the people are sitting at, and how loud you want the movie to be. There are hundreds of speakers that could fit that description... I suggest going to a local hi-fi shop or music store and start listening to systems until you find something you like.
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QUESTION: I might be looking more for computer type speakers. Less than $150 if possible because it might rain and I might not get everything moved quickly enough. I need them loud enough for a group of 15 people outdoors. The movie screen is 96" across. What minimums should I be looking for?
Again, it depends on the area the group is sitting in, and how loud you need it. Since that is a subjective thing, it would be best for you to go try some systems out in person. I can't tell you if something is going to be loud enough for your satisfaction.
Also, I do not make any brand recommendations... I offer technical assistance only, and do not endorse any products.