The human voice is a fascinating instrument. It has the ability to not only produce sound, but also to mimic sound. One of the most incredible abilities of it is in the case of singers. If you just stop and think about what is actually going on physiologically, you will be amazed. Air is being pushed upward from the lunges at a steady rate. The vocal folds contract, and the air passes through the vocal folds. As this happens, The vocal folds begin to vibrate, which in turn causes phonation, or sound. To make the sound higher or lower, or to alter the pitch that is produced, the vocal folds lengthen or shorten to produce higher or lower sounds. The sound that is now pitched correctly can now be refined even more by the muscles and ligaments in the neck, above the larynx. These muscles in combination with other muscles of the mouth, and nasal passages help to create a congruent sound that can be perceived as either "Nasally" or "Deep," depending on what the singer desires. A happy medium is usually where most singers should "place" their sound. Furthermore, the sound is "articulated" by diction, consonants and vowels formed by the tongue, teeth, and lastly, the lips and nose. At this point the sound can not be altered naturally except for placing the hands in various positions around the mouth and nose to cause alterations. One other thing to note, singers can produce different 'tones,' depending on the style of song. They are able to produce a full tone by fully adducting (Closing) their vocal folds. An extreme example of this is a voice that mimics Kermit the Frog. The opposite would be a breathy, or hollow sound caused by abducting (opening) the vocal folds which in turn allows more air to pass through then is being used to cause vibration of the vocal folds.