Reference tone (test tone) is theoretically a constant pure wave of the desired waveform. ... In practice, many test tones are not perfectly shaped, and the quality of the test tone produced is generally directly proportional to the quality of the device (usually some type of oscillator) being used to produce it. A reference tone is a pure tone corresponding to a known frequency, and produced at a stable sound pressure level (volume), usually by specialized equipment. Many electronic tuners used by musicians emit a tone of 440Hz, corresponding to a pitch of A above Middle C (A4). More sophisticated tuners offer a choice of other reference pitches to account for differences in tuning. Some specialized tuners offer pitches used commonly on a particular instrument (standard guitar tuning, fifth intervals for string instruments, the open tones for various brass instruments