Tinnitus is the perception of sound in the absence of sound. Described another way, tinnitus is hearing an intermittent or constant ringing, buzzing, hissing, or heartbeat sound when there is no such sound.
In order to understand what causes tinnitus, and eventually how to cure tinnitus, we have to understand how the ear works. The ear is a fairly complex organ that converts soundwaves into brain signals by passing the waves through the ear canal, onto the ear drum and then through the ear bones into the cochlea (inner ear hearing organ), a small shell-like structure that picks up individual frequencies of sound. The cochlea works by transmitting different frequencies from the point where it receives the frequency to the corresponding point in the brain. The brain and cochlea are arranged exactly alike, with the frequencies (pitches) that are next to each other on the cochlea being exactly next to each other in the brain as well.
Tinnitus is caused when a particular frequency range in the cochlea is damaged and no longer sends information to the brain. This may seem strange at first as tinnitus is caused by the brain NOT receiving signals rather than receiving too much. To explain this, we need to understand one more piece of the puzzle of hearing: what do you hear when you hear nothing? Your brain is actually hearing a steady state of sounds, however soft, at every frequency simultaneously. Your brain tells you that you hear sounds only when it detects a pattern in the consistent noise it’s receiving. So, when you stop receiving signals from a particular frequency, it can no longer detect any patterns so you will at first hear nothing at that frequency. If you don’t hear anything at a particular frequency at first, why do you eventually hear too much at that frequency?
Your brain is incredible and is very, very good at reusing space. For example, if you are blind your brain will often convert the areas traditionally used for image processing to instead process extra audio, smell, and taste sensations. In the case of tinnitus, your brain quickly recognizes that you no longer have a use for the space reserved for the frequencies that are damaged in your cochlea, so it reuses the space for other frequencies. The frequencies near the damaged frequencies are given extra processing power. So they become better at detecting patterns and also tend to spontaneously fire more often. Unfortunately, that means they detect sounds much more regularly than other frequencies are detected, prompting the brain to create ringing or tinnitus. That extra spontaneous activity (firing of neurons) in the brain is interpreted by the rest of the brain as sound. So the conscious brain sees activity in the hearing part of the brain and interprets it as ringing.