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Signal-to-noise ratio (SNR), however, is the ratio between the noise floor and an arbitrary reference level or alignment level. In "professional" recording equipment, this reference level is usually +4 dBu (IEC 60268-17), though sometimes 0 dBu (UK and Europe – EBU standard Alignment level).
Noise measurement can also be part of a test procedure using white noise, or some other specialized form of test signal.In audio systems and broadcasting, specific methods are used to obtain subjectively valid results in order that different devices and signal paths may be compared regardless of the inconsistent spectral distribution and temporal properties of the noise that they generate.
The objective aspect limits the acceptable sound level in the local community. Illinois (Environmental Protection Act 415.25) exempts certain stadiums and exempts festivals, parades, or street fairs. Colorado Spring, CO (Section 9.8.101) has similar exemptions, but limits the sound levels to 80 dB(A) at residential locations.
Microphones, amplifiers and recording systems all add some electronic noise to the signals passing through them, generally described as hum, buzz or hiss. All buildings have low-level magnetic and electrostatic fields in and around them emanating from mains supply wiring, and these can induce hum into signal paths, typically 50 Hz or 60 Hz (depending on the country's electrical supply standard ...
In other cases, special noise may also be tolerated or even introduced at higher levels, for example to create acoustic "privacy", or to help mask other more irritating noise sources. The process of determining the single value for a particular room is first to measure the sound frequency spectrum created by the background noise in the room ...
Dynamic range in analog audio is the difference between low-level thermal noise in the electronic circuitry and high-level signal saturation resulting in increased distortion and, if pushed higher, clipping. [23] Multiple noise processes determine the noise floor of a system.
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"Mic level" is sometimes defined as −60 dBV, though levels from microphones vary widely. [5] In video systems, nominal levels are 1 V P-P for synched systems, such as baseband composite video, and 0.7 V P-P for systems without sync. [5] [6] [7] Note that these levels are measured peak-to-peak, while audio levels are time averages.