Acoustic Additive Synthesizer aims to create fully acoustic device that could resynthesize any sound by the means of Fourier analysis and resynthesis, thus utilising basic principles of spectral music in realtime. It is in fact small pipe organ, with pitch and volume controlled by a computer. Each of the seven pipes has a motorized slider, which changes the pitch continuously, and a dedicated motorized air valve, which effectively changes the volume of the sound. As the sound of organ pipes is very close to sinusoid, this setup, together with purpose-written Pure Data patch, allows for resynthesis of voice or creating quasi-synthetic sounds. Alternatively, the instrument may be used as a regular pipe organ, but with ability to play glissandos or any off-the-12TET microtonal pitches.First version of AAS was commisioned by Instalakcje Festival in 2015, as an interactive installation. The work reached the semi-finals of the Guthman Instrument Competition (2016) and won an honourable mention at the Prix Ars Electronica (2017).