The majority of accelerometers for our applications are piezoelectric devices. A small piezoceramic crystal is sandwiched between the base and a seismic mass, so when the base is accelerated, the crystal is stressed, causing a proportional charge output. Because it is a simple mass/spring system, it will have a fundamental resonance – the crystal is very stiff, so this will be high, some kilohertz for most devices. Below that resonance, the response is virtually flat and linear, making an excellent transducer.
To make a sensitive accelerometer, make the mass and/or crystal bigger – but, this brings the resonance down, so there’s a trade-off to be made. Thankfully, most requirements for sensitive accelerometers are at low frequencies!