"Acoustic" and "acoustical" are both used as adjectives, and both are used often in combinations such as "acoustical engineering", "acoustic energy", "acoustic model", etcetera. Some of these combinations sound better than others, e.g. "acoustical energy" sounds wrong to me, but that may be purely subjective.
Is there any difference in meaning between the words? Or might it just be a UK/US English difference?
The Acousical Society of America pondered this question for years before including definitions and usage rules in a 1955 update to their glossary of acoustical terms.
A report of the discussion is here. http://scitation.aip.org/content/asa/journal/jasa/27/5/10.1121/1.1908102
Their work was not 100 percent satisfactory, but it has remained generally in effect in America. They admit they cribbed their definitions from the electric/electrical definitions in American Standard Definitions of Electrical Terms (1941). The current Glossary costs $150 to buy, Maybe someone with access to an engineering library can check it out.
The only material differences that I can see are that "acoustic" can be used as a noun as well as an adjective, whereas "acoustical" is only adjectival. The usage of "acoustical" is also more common in N America than other English speaking countries.
Unfortunately, the predominant use in a particular location wins this argument as there is no logical reason for using one form over the other. For instance, we say electric guitar but electrical engineering. Who's to say that electric engineering and electrical guitar are grammatically wrong.
Unlike Spanish and French, English does not have an academy of scholars who meet to decide on these things, based on grammar or etymology or whatever.
Another example is heart transplant and heart transplantation. Strictly speaking heart transplantation would seem to be more correct. But when Christian Bernard, a South African, performed the worlds first successful one in 1967 (I think), it was reported as a heart transplant and that stuck.
Another example is where British people say London Transport as the title of a public body, whereas Americans might say London Transportation which would seem to describe in the name, what the said public body does.
The best advice, in cases like these where there is no logical right reason, is to say whichever is most widely used in your cultural background and be true to who you are.
