Public address system and Speaker unit

Public address system noun - An electronic amplification system used as a communication system in public areas.

Speaker unit and public address system are semantically related. In some cases you can use "Speaker unit" instead a noun phrase "Public address system".

Speaker unit noun - Electro-acoustic transducer that converts electrical signals into sounds loud enough to be heard at a distance.

Public address system and speaker unit are semantically related. You can use "Public address system" instead a noun phrase "Speaker unit".