A sound post is a small wooden dowel located inside string instruments like the violin and cello. It is positioned vertically between the front and back plates of the instrument, helping to transmit vibrations from the strings to the body, which enhances the instrument's sound quality. The sound post plays a crucial role in the instrument's acoustics by balancing the tone and volume. Its placement can affect the instrument's responsiveness and overall sound, making it an essential component for musicians and luthiers when setting up or repairing string instruments.