A study in 2000 tested 1156 patients, who experienced a significant improvement in speech intelligibility when changing from a device with 1 channel to 2 channels. Significant improvements were also experienced when moving from 2 to 3 channels and 3 to 4 channels. However, there were diminishing returns above 4 channels, where no statistically significant improvements were discovered in the test subjects.
It’s important to keep in mind that how channels benefit the end user is up to the design of the hearing device. Channels can be used to change shape, frequency, etc. In fact, there is actually a downside to having too many channels: it increases the required processing power, which can slow down response time as well as decrease battery life. This delayed processing time can lead to an undesirable echo or “reverb” effect.