The qualifying adjectives acoustic and acoustical have the following meanings: arising from, actuated by, containing, producing, or related to sound. In general, acoustic is used when the term being qualified designates something that has the properties, dimensions, or physical characteristics associated with sound waves; acoustical is used when the term being qualified does not explicitly designate something that has the properties, dimensions, or physical characteristics of sound (e.g., acoustical engineering). However, sometimes these two terms are used interchangeably.