Surface Acoustic Waves (SAW) and Bulk Acoustic Waves (BAW) are two types of filters that convert electrical energy to mechanical acoustic waves, and then back to electrical energy.
Mechanical Acoustic Wave: waves that result from an oscillation/change in pressure when traveling through a liquid, solid, or gas.
That sounds a little confusing, but imagine it as you're sending a wave through a medium, and the internal pressure of that medium is changing while the wave goes through.
Piezoelectric Materials: Materials that can produce electrical energy when undergoing mechanical stress. Conversely, they can generate mechanical stresses when supplied with electrical energy.
Now you can start to see how the Mechanical Acoustic Waves are generated. We send an electrical signal through the Piezoelectric medium, which causes mechanical stress on it. Then, this creates a change (or oscillation) in pressure (mechanical stress from all directions). By definition, a mechanical acoustic wave is created.
The main difference between SAW and BAW is what happens while the signal is going through this piezoelectric medium. In SAW filters, the acoustic waves travel across the surface of the piezoelectric material. However, for BAW filters, they travel through the piezoelectric.
However, they should be seen as complementary towards each other. SAW Filters can usually handle frequencies up to 2GHz, while BAW filters handle those up to about 2.7GHz. Note that these max frequencies are determined by the point at which the components inside the filters would be too small.
So, you might be wondering, what's the point in using SAW and BAW filters? Why even use a piezoelectric when typical components work fine.
By using these piezoelectric, the filters become way more precise and efficient. Typically, the bandpass filters are much more selective and have significantly less insertion loss. Additionally, they are a lot more compact, and typically cheaper.
It’s fairly complicated why that’s the case, and it probably requires lots of material science knowledge (ew), so I’ll leave that to you if you want to do additional research.