In a horn loudspeaker, the term acoustic transformer or acoustical transformer may refer to either of two components:
- Horn (acoustic), which attaches to the compression driver unit
- Phase plug, a component within the compression driver, the interface between the diaphragm and the horn
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/75/Horn_loudspeaker%2C_compression_loading_with_phase_plug.jpg/220px-Horn_loudspeaker%2C_compression_loading_with_phase_plug.jpg)