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Voice activated switch used in noisy conditions

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I'm needing a inexpensive Voice Activated Switch / Mic to turn a mini 12VDC motor on for 2 to 3 seconds at a time. Problem is it must operate in noisy conditions such as dirt bike motorcycle noise conditions. The mic only is mounted inside a full faced helmet. This will be used in a product going to market so price is a concern.
Thanks for your advise,

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I had a somewhat similar problem where a click from a plastic coupling were to stop a test. The test machine was located in a production hall.
The solution was to rectify the signal from the microphone to get an "average noise level" the rectified signal was "stored" in a capacitor with a resistor in parallel. As i recal the time constant was around 250msec.
That signal was then fed to a differentiator, with a trigger circuit on it's tail.

The idea being that a sudden increase in noise level would set off the trigger, not the general noise level itself.

After quite a bit of fiddling with amplification and timing, the circuit worked really well.

Hope that helps,

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