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Harmonic distortion is the deformation of a wave form by unwanted multiples of an original frequency, causing interruptions to the way the wave form behaves in electrical circuits, or sounds, in the case of music and sound waves. Not all multiples, known as harmonics, are bad; for example, the rich, complex sound of musical instruments is the result of harmonics, where they produce sound waves at multiples in addition to the original frequency. In distortion, the multiples interrupt the signal and can cause problems.
With electrical circuits, harmonic distortion causes the waves to change shape and deform as they move through the system. This results in voltage fluctuations. Sensitive equipment can be damaged by such fluctuations, and if they spike high enough, a system may shut down to protect itself. In music, harmonic distortion can interfere with sound quality, causing something to sound wrong or creating a lot of background noise.
There are a number of tools people can use to control this phenomenon. Building systems out of appropriate materials, including things like wiring with excellent conductivity, is an important step. Sometimes, people may use a filter to prevent interference and problems with the signal. It may also be possible to make adjustments to the way the system processes wave forms, and the equipment can require shielding to protect it from interference.
A measurement known as total harmonic distortion (THD) is used to express the degree of distortion happening with a single percentage measurement. People want to keep this number low, typically below one percent. High fidelity stereo systems use a variety of technology to control the amount of harmonic distortion, keeping it below levels the human ear can detect. Even a very high quality system will experience some sound wave deformation, but often people do not notice because it occurs at a low level.
Measuring harmonic distortion is sometimes challenging. In electrical systems, special equipment is needed to spot the unwanted harmonics, while with stereo equipment, sensitive tools are made specifically to check on harmonic distortion. Once a technician can identify it, the problem may be very frustrating to resolve. Fine tuning and small adjustments are a critical part of the process, as is exploring the possibility of changing equipment or components to see if that gets to the root of the problem. Particularly when equipment is in a highly sensitive environment with a low tolerance for distortion, technicians have to be very careful about installation, maintenance, and repair to avoid creating problems.