Singing is the act of proucing musical sounds with the voice, which is often contrasted with speech. During passing breathing, air is inhaled with the diaphragm while exhalation occurs without any effort. Exhalation may be aided by the abdominal, internal intercostal and lower pelvic muscles. Inhalation may aided by use of external intercostals, scalenes and sternocleidomastoid muscles. The pitch is altered with the vocal cords. With the lips closed, this is called humming. A piece of music with a singing part, either a cappella (without accompaniment) or accompanied, is called a song; someone who sings is called a singer. Nearly anyone who can speak can sing, since in many respects singing is merely a form of sustained speech. It can be informal and just for pleasure, for example, singing in the shower; or it can be very formal, such as singing done professionally as a performance or in a recording studio. Singing at a high amateur or professional level usually requires a great deal of regular practice, and/or instruction. Top-quality singers will have instruction and training from coaches throughout their career.