A.
Audio feedback (also known as the Larsen effect after the Danish scientist, Søren Larsen, who first discovered its principles) is a special kind of feedback which occurs when a sound loop exists between an audio input (for example, a microphone or guitar pickup) and an audio output (for example, a loudspeaker). In this example, a signal received by the microphone is amplified and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again. This is a good example of positive feedback. The frequency of the resulting sound is determined by resonant frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them.

More specifically, the conditions for feedback follow the Barkhausen criterion, namely that an oscillation occurs in a feedback loop whose delay is an integer multiple of 360 degrees and the gain is equal to or greater than 1 (both at the given feedback frequency). If the gain is greater than 1, then the system can start to oscillate out of noise, that is to say: sound without anyone actually playing. If the gain is large, but less than 1 then the high pitched feedback tones will only be created with some input sound and will slowly decay.
B.
Optical feedback is the optical equivalent of acoustic feedback. The feedback occurs when a loop exists between an optical input, for example, a videocamera and a television screen or monitor. (A simple example of optical feedback is an image cast between mirrors.)

In this GIF movie, and the JPG still image examples (right), light from a candle is received by a videocamera, amplified and then sent by cable to a monitor projecting electron beams on the inside of the monitor screen. The image on the monitor is then captured by the videocamera again, and fed back to the monitor in a continuous loop.

The original light source, in this case from the candle, can then be extinguished, while the feedback loop continues. For each loop the image is doubled and the image interferes with itself. The electronic loop moves with near light speed, but as the resulting image is projected onto the phosphor dots on the inside of the screen the speed is stopped for as long as allowed by the time the phosphor points glow, and thus creating a "queue" of illuminated dots on the screen.

The resulting images depend on different camera and monitor settings, such as light amplification, contrast, distance, angle and physical vibrations. Optical feedback can be combined with music, or other sound sources, to influence the image loop
C.
Electronic noise
Main article: Electronic noise
Electronic noise exists in all circuits and devices as a result of thermal noise, also referred to as Johnson Noise. Semiconductor devices can also contribute flicker noise and generation-recombination noise. In any electronic circuit, there exist random variations in current or voltage caused by the random movement of the electrons carrying the current as they are jolted around by thermal energy. Lower temperature results in lower thermal noise. This same phenomenon limits the minimum signal level that any radio receiver can usefully respond to, because there will always be a small but significant amount of thermal noise arising in its input circuits. This is why radio telescopes, which search for very low levels of signal from stars, use front-end low-noise amplifier circuits, usually mounted on the aerial dish, and cooled with liquid nitrogen.
D.
Visual noise
Main article: Visual noise
Noise is also present in images. Electronic noise will be present in camera sensors, and the physical size of the grains of film emulsion creates visual noise. This kind of noise is referred to as "grain."

Noise is also used in the creation of 2d and 3d images by computer. Sometimes noise is added to images to hide the sudden transitions inherent in digital representation of color, known as "banding." This adding of noise is referred to as "dithering." Sometimes noise is used to create the subject matter itself. Procedural noise (such as Perlin noise) is often used to create natural-looking variation in computer generated images.