Need help designing a sensitive mic and preamp circuit

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I'm trying to put together an art project, an interactive sculpture, that listens to the environment around it, and responds only to a narrow frequency range. I've already gotten some good ideas for designing the bandpass filter (it will take much more experimentation, though), but I'm totally failing to put together a microphone sensitive enough to pick up ambient background sound (as in people mulling around and talking in a large room). I've tried to use an electret condenser mic, but my signal is just way too weak. I suppose I could amplify the crap out of it, but I'm beginning to wonder if there is a better way to go about it. I have very little experience in working with actual audio circuits (poor me, a digit head).

The basic design is this:
mic-->preamp-->bandpass-->analogue input pin (Arduino AVR processor)

(There might also need to be an amp or compressor either built into the mic preamp or inserted in between the bandpass filter and the AtoD pin on the Arduino.)

What would be the best combination of microphone type and preamp design to be able to easily pick up human voice, say, from 30 to 40 feet away? Note that in no way do I require hi fidelity (it's not for recording, only "sensing" the signal strength of a narrow band of human voice).

Can anyone point me in the right direction?

Many thanks! :)
Thank you for the replies. :)

Actually, non-directional is a good thing - the goal of the project is for an interactive sculpture to count how often it can pick up a narrow frequency range of human voice (over, say a 25 to 50 Hz range). I'd like the project to react whether the audience is directly participating with it or not. Its only goal is to register the sound, not record it, so the receiving quality can be horribly poor. :) I want it to react sort of like a dog perking up its ears when it hears something interesting in the background.
Wow - I didn't realize this would be such an issue. I could make use of a directional mic, but size is an issue. I would need a smallish directional mic, nothing like a huge shotgun mic or a parabolic dish no larger than, say 3 inches in diameter. I suppose there are other form factors of directional mics out there used in the audio recording industry.

I will have to re-design the sculpture a bit to incorporate a directional "zone" where people can interact, but it's totally doable. This is why I'm asking in the first place, to sort of nail down the design by gathering what's feasible with what I have to work with! Many thanks for the input!

So, with that in mind, where can I find some microphones that might work for this project? I would need the specs / whitepapers or circuits to drive them. Also, there's still the matter of amplifying, filtering, and conversion to analog signal level (0 to +5V).
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