Hi, I want to passively amplify a specific audio frequency range of a microphone. When I say passively I mean by non-electronic means - no op-amps or transistors. Like a speaker can be designed for specific frequency range - a woofer/tweeter/etc... I presume this works in the other direction - If a make a large membrane it will respond more to a lower frequency range relative to a small membrane. If I place a microphone in the other side of the membrane(opposite to audio source) I will accomplish this goal. I am looking for something not to just filter, as in a band-pass filter, a specific frequency range, but to amplify it so I get maximum power conversion from audio power to electronic signal - maybe some structure that will resonate in that frequency range. If you know a good book/website on the subject or an advance method you've encountered, as in adding ridges to that membrane or using a loudspeaker as a means of focusing sound onto a microphone instead of radiating sound or other ideas that are clever, I will be glad to hear. By the way I want to use a piezo transducer as a microphone and it's resonance freq. is 180Hz... Oh and one more thing - reduced size is an advantage!