Speech analysis requires substantial computation. It is desirable to run this analysis only when needed and at other times to go to a low power state. Here we propose a self-biased low power speech detection wake up circuit which interfaces directly to standard electret microphones. The speech detector includes a microphone preamplifier, a power extraction squaring circuit, a bandpass filter passing power of the modulation spectrum in the speech band from 2-12 Hz, a half rectifier which extracts this phoneme band power, and a PFM silicon neuron which emits spikes indicating phoneme-rate modulation of the audio spectrum. The output of the speech detector circuit is an asynchronous stream of digital spikes at a rate of 1Hz to 20Hz whose temporal structure indicates the presence of speech. A subsequent conventional processor will go to sleep between spikes and only wake up for full power speech analysis when the temporal structure indicates speech. The circuit is built in 1.6um 2P-2M CMOS and consumes 500uW with a 3V supply when attached to a standard electret microphone.