There are two main factors which makes a person's voice unique. Firstly, it is the physiological component which is known as the voice tract. Secondly, it is a behavioral component which is known as the voice accent. By combining both of these factors, it is almost impossible to imitate another person's voice exactly. Taking advantages of these characteristics, biometrics technology created voice recognition systems in order to verify each person's identification using only their voice. Mainly, voice recognition will focus on the vocal tract because it is a unique characteristic of a physiological trait. It works perfectly in physical access control for users.

Voice recognition systems are easy to install and it requires a minimal amount of equipment. This equipment includes microphones, telephone and/or even PC microphones. However, there are still some factors which can affect the quality of the system. Firstly, performance of users when they record their voice to database is important. For that reason, users are asked to repeat a short passphrase or a sequence of numbers and/or sentences so that the system can analyze the users' voice more accurately. On the other hand, unauthorized users can record authorized users' voices and run it through the verification process in order to get user access control to system. To prevent the risk of unauthorized access via recording devices, voice recognition systems will ask users to repeat random phases which are provided by the system during verification state.