Bias in a study means that certain responses or outcomes are favored or encouraged over other responses or outcomes.
Unfortunately, not all bias is obvious, and researchers can accidentally include variables or features that lead to response bias.
Random selection of study subjects is one way of avoiding bias. All subjects within a population have the same chance of being selected for the study. Limiting the stimuli in a study setting is another way to avoid bias.
Sound can limit bias so planning thoroughly, running pilot tests, and going back to revise the design of an experiment are very important steps to follow.