Is guitar the only instrument used for acoustic music?

No. Pretty much any stringed instrument, percussion instrument, brass, woodwinds, etcetera can be acoustic. The list includes harps, violins, guitars, saxophones, clarinets, and other instruments. An acoustic instrument is worked entirely by human effort, and in most instances by the energy of the player alone.

Acoustic music is played without any electrical instrumentation. While all music was once acoustic, electronic instruments, such as the electric guitar and the synthesizer became popular. Acoustic string instrumentations had long been a subset of favorite music, especially folk.

No! No! No! This is a very bland misconception. The reason most acoustic music is considered to be guitar-oriented is that the guitar is a very popular instrument and one of the few that sounds excellent on its own.

Acoustic music implies music made without the influence of electronic equipment like electric guitars, electric organs, synthesizers etc. Acoustic music may sometimes allow an amplifier if being performed on a stage but that’s it!

Synthesizer

So basically, any music made on an instrument is acoustic. Be it a drum solo, a violinist concert etc, everything is acoustic music. The term didn’t even exist before the advent of electronic music in the 1960’s.

There are those who personally hate other forms of music and tend to consider acoustic music to be the most ‘pure’ form of music but they are wrong.