Firstly, having been immersed in music since childhood, Beethoven could no doubt put together music in his mind and on paper without being able to aurally hear. By the time he was deaf, hearing music from his inner ear was innate to the great composer.
Secondly, it is said that Beethoven used techniques to physically feel music and notes in his body. This included holding a pencil in his mouth and resting it against the piano so he could feel the vibrations of the notes against his lips. The lower notes emulated a deeper vibration, therefore, pieces written in the second half of his career included fewer high notes and more low ones. There is evidence that Beethoven also cut the legs of his pianos, so that the sound vibrations resonated through the ground and into his body as he wrote his compositions! Beethoven apparently ruined many pianos in his later career from hitting the keys so aggressively in order for him to ‘hear’ what he was playing.
While the cause of Beethoven’s deafness remains unknown and debated, tests undergone on a salvaged lock of Beethoven’s hair indicated an abnormally high amount of lead. One theory is that Beethoven acquired chronic lead poisoning from the lead used in wine as a sweetener, or from the goblet he drank from, which ultimately affected his hearing.
A mere three years after the debut of his most famous work, Beethoven passed away due to the plethora of health problems he possessed, but what he left behind was an immortal abundance of rich, vibrant pieces of music, including the iconic Symphony No.9.