NOISE in Audio Systems

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NOISE in Audio Systems Understanding the nature and the characteristics of noise is very important for the study of Electroacoustic systems.

NOISE in Audio Systems So... What is noise…

NOISE in Audio Systems So... What is noise… The definition of noise is similar to the definition of a weed…!!! What's a weed?...It's the flower you didn't plant and don't want to grow. What's noise?... It's the sound you didn't intentionally create, and don't want to hear.

NOISE in Audio Systems Types of Noise in Audio Systems: White noise……. Pink noise………. Hum……… Buzz…….. Static……. Blue, Brown, Violet, Gray

NOISE in Audio Systems Thermal/Random noise…. Is generated as a result of thermal agitation of the charge carriers which are typically electrons within an electrical conductor. This thermal noise actually occurs regardless of the applied voltage because the charge carriers vibrate as a result of the temperature. Thermal noise appears regardless of the quality of component used. The noise level is dependent only upon the temperature and the value of the resistance.

NOISE in Audio Systems So… What am I going to measure across this resistor with a voltmeter?

NOISE in Audio Systems So… What am I going to measure across this resistor with a voltmeter? A Noise voltage “Vn”

NOISE in Audio Systems So… What am I going to measure across this resistor with a voltmeter? A Noise voltage “Vn” With constant energy across all frequencies

NOISE in Audio Systems Assuming that the value of the resistance remain constant across all frequencies in a given bandwidth, the voltage of the thermal noise will be given by: Where: Vn = integrated RMS voltage between two frequencies. R = resistive component of the impedance (or resistance) Ω. T = temperature in degrees Kelvin. k = Boltzmann constant ( ^-23 J/K ⁰ )

NOISE in Audio Systems Assuming that the value of the resistance remain constant across all frequencies in a given bandwidth, the voltage of the thermal noise will be given by: Where: Vn = integrated RMS voltage between two frequencies. R = resistive component of the impedance (or resistance) Ω. T = temperature in degrees Kelvin. k = Boltzmann constant ( ^-23 J/K ⁰ ) Example: Calculate the thermal noise levels for room temperature, 20°C or 290°K, for a 1 Hz bandwidth, for an impedance of 50 Ω.

NOISE in Audio Systems White noise…. “White noise is unfiltered, unaltered thermal noise”. When the energy content of a white noise signal is averaged (integrated over time), it will be found to contain equal energy per Hz.

NOISE in Audio Systems White noise…. When the energy content of a white noise signal is averaged (integrated over time), it will be found to contain equal energy per Hz. “White noise is unfiltered, unaltered thermal noise”.

NOISE in Audio Systems White noise…. When the energy content of a white noise signal is averaged (integrated over time), it will be found to contain equal energy per Hz. “White noise is unfiltered, unaltered thermal noise”.

NOISE in Audio Systems A B “White noise is unfiltered, unaltered thermal noise”. White noise…. Energy (A) = Energy (B)

NOISE in Audio Systems Plotting the power of white noise, with respect to frequency in a logarithmic scale, will result in a 3 dB per octave buildup in level, as the frequency rises.

But…. What is OCTAVE …

Octave is a music/frequency interval defined by the ratio 2:1 regardless of the starting frequency. The frequency space from 100 Hz to 200 Hz is an octave, as is the interval from 2000 Hz to 4000 Hz.

NOISE in Audio Systems The 3dB-slope concept: For the octave 100->200 we have a frequency window of 100Hz For the octave 200->400 we have a frequency window of 200Hz If the power per Hz is “A” Watt, the total power for the octave 100->200 will be 100A Watt In a similar way the total power for the octave 200->400 will be 200A Watt The increase in dB will be then:

NOISE in Audio Systems Pink noise is white noise that has been modified with a filter that gives a 3 dB per octave roll- off. Pink noise: (Hz) dB

NOISE in Audio Systems Plotting the power of Pink noise, with respect to frequency in a logarithmic scale, will result in a straight line in level, as the frequency rises.

NOISE in Audio Systems As we can see now, pink noise has equal energy per octave, and therefore is more useful as a loudspeaker test and calibration signal.

NOISE in Audio Systems As we can see now, pink noise has equal energy per octave, and therefore is more useful as a loudspeaker test and calibration signal. Pink noise is balanced to more closely simulate the kind of signals the sound system will actually be required to reproduce.

NOISE in Audio Systems As we can see now, pink noise has equal energy per octave, and therefore is more useful as a loudspeaker test and calibration signal. Pink noise is balanced to more closely simulate the kind of signals the sound system will actually be required to reproduce. It is also closer to the way the human hearing performs.

NOISE in Audio Systems All other "coloured" types of noise are actually white noise with a different type of filtering….

NOISE in Audio Systems Noise caused by electric network.... Hum and Buzz……. Hum is generally the result of leakage of AC power line energy into the audio circuit. It can be caused by a poorly isolated power transformer, a power supply problem, ground loops or indirectly by electromagnetic coupling of AC magnetic fields into cables or other components. It is usually sine wave components at 120 Hz, 180 Hz, and other harmonics of 60 Hz. Buzz is similar to hum, but contains harmonically related noise energy across the audio spectrum, including high frequency energy. Static noise can be caused by distant lightning, by intermittent sparking of power lines, generators or electric motors, or by radio frequency energy that enters the sound equipment and is rectified.

NOISE in Audio Systems …Using noise as a test signal is one thing. Finding it at the output of a mixing console, a signal processor or a power amplifier that is supposed to be quiet is quite a problem…. …..So the next session will be, the measurement and the specification of noise in audio systems…