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There is a difference between speaker recognition (recognizing who is speaking) and speech recognition (recognizing what is being said). These two terms are frequently confused, and "voice recognition" can be used for both. In addition, there is a difference between the act of authentication (commonly referred to as speaker verification or speaker authentication) and identification. Finally, there is a difference between speaker recognition (recognizing who is speaking) and speaker diarisation (recognizing when the same speaker is speaking). Recognizing the speaker can simplify the task of translating speech in systems that have been trained on specific person's voices or it can be used to authenticate or verify the identity of a speaker as part of a security process.
Speaker recognition has a history dating back some four decades and uses the acoustic features of speech that have been found to differ between individuals. These acoustic patterns reflect both anatomy (e.g., size and shape of the throat and mouth) and learned behavioral patterns (e.g., voice pitch, speaking style). Speaker verification has earned speaker recognition its classification as a "behavioral biometric".
Verification versus identification 
There are two major applications of speaker recognition technologies and methodologies. If the speaker claims to be of a certain identity and the voice is used to verify this claim, this is called verification or authentication. On the other hand, identification is the task of determining an unknown speaker's identity. In a sense speaker verification is a 1:1 match where one speaker's voice is matched to one template (also called a "voice print" or "voice model") whereas speaker identification is a 1:N match where the voice is compared against N templates.
From a security perspective, identification is different from verification. For example, presenting your passport at border control is a verification process - the agent compares your face to the picture in the document. Conversely, a police officer comparing a sketch of an assailant against a database of previously documented criminals to find the closest match(es) is an identification process.
Speaker verification is usually employed as a "gatekeeper" in order to provide access to a secure system (e.g.: telephone banking). These systems operate with the user's knowledge and typically requires their cooperation. Speaker identification systems can also be implemented covertly without the user's knowledge to identify talkers in a discussion, alert automated systems of speaker changes, check if a user is already enrolled in a system, etc.
In forensic applications, it is common to first perform a speaker identification process to create a list of "best matches" and then perform a series of verification processes to determine a conclusive match.
Variants of speaker recognition 
Each speaker recognition system has two phases: Enrollment and verification. During enrollment, the speaker's voice is recorded and typically a number of features are extracted to form a voice print, template, or model. In the verification phase, a speech sample or "utterance" is compared against a previously created voice print. For identification systems, the utterance is compared against multiple voice prints in order to determine the best match(es) while verification systems compare an utterance against a single voice print. Because of the process involved, verification is faster than identification.
Speaker recognition systems fall into two categories: text-dependent and text-independent.
If the text must be the same for enrollment and verification this is called text-dependent recognition. In a text-dependent system, prompts can either be common across all speakers (e.g.: a common pass phrase) or unique. In addition, the use of shared-secrets (e.g.: passwords and PINs) or knowledge-based information can be employed in order to create a multi-factor authentication scenario.
Text-independent systems are most often used for speaker identification as they require very little if any cooperation by the speaker. In this case the text during enrollment and test is different. In fact, the enrollment may happen without the user's knowledge, as in the case for many forensic applications. As text-independent technologies do not compare what was said at enrollment and verification, verification applications tend to also employ speech recognition to determine what the user is saying at the point of authentication.
The various technologies used to process and store voice prints include frequency estimation, hidden Markov models, Gaussian mixture models, pattern matching algorithms, neural networks, matrix representation,Vector Quantization and decision trees. Some systems also use "anti-speaker" techniques, such as cohort models, and world models.
Ambient noise levels can impede both collection of the initial and subsequent voice samples. Noise reduction algorithms can be employed to improve accuracy, but incorrect application can have the opposite effect. Performance degradation can result from changes in behavioural attributes of the voice and from enrolment using one telephone and verification on another telephone ("cross channel"). Integration with two-factor authentication products is expected to increase. Voice changes due to ageing may impact system performance over time. Some systems adapt the speaker models after each successful verification to capture such long-term changes in the voice, though there is debate regarding the overall security impact imposed by automated adaptation.
Capture of the biometric is seen as non-invasive. The technology traditionally uses existing microphones and voice transmission technology allowing recognition over long distances via ordinary telephones (wired or wireless).
Digitally recorded audio voice identification and analogue recorded voice identification uses electronic measurements as well as critical listening skills that must be applied by a forensic expert in order for the identification to be accurate.
See also 
- Kinnunen, Tomi; Li, Haizhou (1 January 2010). "An overview of text-independent speaker recognition: From features to supervectors". Speech Communication 52 (1): 12–40. doi:10.1016/j.specom.2009.08.009.
- Jean-Francois Bonastre (1), Fr�d�ric Bimbot (2), Louis-Jean Boe (3), Joseph P. Campbell (4), Douglas A. Reynolds (4), Ivan Magrin-Chagnolleau (5) (September 2003). ["http://www.isca-speech.org/archive/eurospeech_2003/e03_0033.html" "http://www.isca-speech.org/archive/archive_papers/eurospeech_2003/e03_0033.pdf" "Person Authentication by Voice: A Need for Caution"]. Person Authentication by Voice: A Need for Caution. 8th European Conference on Speech Communication and Technology. Geneva, Switzerland: isca-speech.org. "http://www.isca-speech.org/archive/eurospeech_2003/e03_0033.html" "http://www.isca-speech.org/archive/archive_papers/eurospeech_2003/e03_0033.pdf". Retrieved February 21, 2012.
- Pollack, Pickett, Sumby (1974). Experimental phonetics. MSS Information Corporation. pp. 251–258. ISBN 0-8422-5149-9.
- Van Lancker and Kreiman (July 3, 1984). "Familiar voice recognition: Patterns and parameters. Part I: Recognition of backward voices". Journal of Phonetics. pp. 19–38. Retrieved February 21, 2012.
- "British English definition of voice recognition". Macmillan Publishers Limited. Retrieved February 21, 2012.
- "voice recognition, definition of". WebFinance, Inc. Retrieved February 21, 2012.
- "Linux Gazette 114". Linux Gazette. Retrieved February 21, 2012.
- Lisa Myers (April 19, 2004). "An Exploration of Voice Biometrics".
- National Institute of Standards and Technology
- Elisabeth Zetterholm, Voice Imitation. A Phonetic Study of Perceptual Illusions and Acoustic Success. Phd thesis, Lund University. (2003)
- Homayoon Beigi, ``Speaker Recognition, Biometrics / Book 1, Jucheng Yang (ed.), Intech Open Access Publisher, 2011, pp. 3-28, ISBN 978-953-307-618-8.
- Beigi, Homayoon (2011). Fundamentals of Speaker Recognition. New York: Springer. ISBN 978-0-387-77591-3.
- Speaker Identification and Verification
- Circumventing Voice Authentication The PLA Radio podcast recently featured a simple way to fool rudimentary voice authentication systems.
- Speaker recognition - Scholarpedia
- - Voice: Technologies and Algorithms for Biometrics Applications, IEEE eLearning Library (formerly IEEE Expert Now eLearning) Tutuorial, Sep. 2010
- Voice recognition benefits and challenges in access control