The noise floor of any measurement system can be defined as the sum of all unwanted audio signals outside of the signal that is being monitored by an experiment or test. The noise floor represents a wide variety of signal origin points, including atmospheric noise, and more. The higher the noise floor, the more difficult it is for a measurement device to isolate an individual signal. Noise floors that are high enough can drastically reduce the viability of experimental results. It is always in the best interests of the experimenter to lower the noise floor as much as possible either through artificial means or by seeking out conditions that correspond to the lowest noise floor possible.
The noise floor began to matter to humans once sound recordings became a reality in 1857. Édouard-Léon Scott de Martinville recorded the first sound on a device called the "phonautograph," a machine with the ability to transcribe a wave of sound into a line that was drawn on glass or paper. This preceded Alexander Graham Bell and the first iteration of the telephone in 1875. The first sound transmitted through radio came in 1900, a feat accomplished by Reginald Aubrey Fessenden. All of these signals included a noise floor.
Wherever an audio signal is measured or transcribed, noise is also introduced into the measurement. Acoustic geologist Gordon Hempton has concluded after years of experimentation that there is literally no place on earth that is not affected by noise,
meaning that every unaffected captured audio signal will contain some measure of unwanted background noise. This finding was repeated by Bernie Krause, a bioacoustics expert.
Noise originates from the beginning of the universe and is known as cosmic noise or cosmic background noise. This cosmic noise is generated through the constant movement of weighted bodies in the universe. Contrary to popular belief, sound does not actually need air to travel - when the universe was smaller and denser, sound easily traveled through the "vacuum" of space.
Atmospheric noise is relatively high-frequency background radio noise that comes from natural processes within the atmosphere. The most common process causing this noise is the lightning flash - 3.5 million of these events occur in a single day (40 per second). Atmospheric noise usually occurs within the 90-110 kHz range and may cause sound recording instruments to vibrate so much that they cannot properly record the experiment signal.
Incidental noise is an umbrella term that represents all of the background noise that comes from far off events such as a plane flying by. Many of these events are so far off and the sound waves so small that humans cannot perceive them, but they do affect the sound waves that occur in measurements.
Finally, the noise that comes from the measurement system itself comprises a major component of the noise floor of sound measurements. As systems measure objects, they give off heat, radiation and cause friction between moving parts, all of which
generates noise and adds to the noise floor. Depending on the measurement device that you use, you may be able to hear this noise as it is created.