Noise floor
The noise floor is the sum of all the noise sources present in a system. These sources can be considered intrinsic (inherent to the system), or extrinsic (dependant on exterior influences).
In the intrinsic noise category, we find thermal noise (Johnson-Nyquist noise), which is due to the motion of electrons inside an electrical conductor. The noise type is usually white, with a frequency independent magnitude. However the magnitude is dependant on the impedance value. The formula for the RMS voltage of this noise in a resistor is
,where R is the resistance in ohms, T the absolute temperature in Kelvin,
Boltzmans constant, and
the frequency bandwidth.The
noise due to current fluctuations created by loose contacts, inhomogeneous parts, etc. has a power density which varies aproximatly at
. It is therefore particularly intrusive at low frequencies.Some of the extrinsic noise is due to external electromagnetic phenomena like radio waves, RADAR, the mains current, unoptimised switched mode power supplies, etc. Another part can be the product of different mechanical vibrations, such as footsteps, traffic, the drumming fingers of the sound engineer. In the case of acoustical measurements, the background noise created by traffic and equipment situated in and around the building is also picked up along with the signal of interest.
