I was thinking of purchasing a speaker for my keyboard. I was looking at the Behringer Eurolive B115D . Based on the specs, would I be able to connect my keyboard directly to the speaker OR would I have to purchase a mixer also? Thanks for your help!!
It is unclear from the question whether the keyboard has a mono or stereo output so it's hard to say if you would need one or two of these. They do have 2 inputs but it will just mix them together. For true stereo you would need two and you would need them placed so that they form an equilateral triangle with the listener (you).
It is also unclear whether the keyboard gives out one, two or more outputs and what kind of connector is used. I assume that you have 2x 1/4" jacks which is the most common. If so, one of them, the left, is usually marked as the mono output and you could connect it directly to the speaker's input. If you decide to connect both outputs, you might get cancellations with wide stereophonic sounds (ones where the phase between the left and right channel varies greatly). Otherwise you should use two of them.
Finally, these are PA speakers. They are not for monitoring in a studio setting. This means they are designed to be loud, not flat. If that's what you're after, they are absolutely fine.