Piezo electric transducer-Vibration to Electrical Energy
I guess to put all of my comments into an answer to be more organized.
That's basically what a lot of cheaper microphones are, just a piezoelectric material that directly converts sound waves to physical compression and thus to electrical signals. You just to need to have a basic amplifier circuit to amplify the signal, depending on your application of course. Some sensors can provide digital output, otherwise most are going to be analog based and will probably need to be passed through an ADC. The frequency sensitivity will vary and you have to keep in mind that the sensitivity is based on resonance. Thus, a lower frequency vibration sensor is generally going to be larger than a higher frequency sensor.
Digikey and others have a nice selection of vibration sensors.