Signal-to-noise ratio, or SNR, is a measurement that describes how much noise is in the output of a device, in relation to the signal level.
Every device has some amount of noise at its output. The question is, “How much is too much?” The answer is, “it’s all relative”. A small amount of noise may not objectionable if the output signal is very strong. In many cases, the noise may not be audible at all. But if the signal level is very small, even a very low noise level can have an adverse effect.
To make the determination objectively, we need to look at the relative strengths of the output signal and the noise level. Or to put it another way, the Signal-to-Noise Ratio.
SNR is actually two level measurements, followed by a simple calculation. First, we measure the output level of the device under test with no input signal.
Then we apply a signal to the device and take another level measurement. Then we divide.
SNR measurements are usually expressed in decibels. As we saw with THD+N ratio, using dB makes it much easier to manage large ranges in numbers, the kind of range you’d get when dealing with microvolts of noise compared to potentially a hundred volts or more with power amps. There is one difference between THD+N ratio and Signal to Noise ratio. THD ratios look at distortion and noise divided by signal, which results in a very small number like 0.001%, which is -100dB, whereas SNR looks at signal divided by noise, which results in a large number, like 100,000, or 100 dB.
Most noise sources are broadband, meaning that the noise extends across a wide frequency range. This means that a noise measurement of a device made across a wide frequency range will measure more noise than a measurement made across a more narrow range.
Bandpass filtering and weighting are often used with noise measurements, to ensure that the measurements are both appropriate and comparable.
Bandpass filtering limits both the low and high frequency range being measured, defining a measurement bandpass. A typical measurement bandpass for audio devices is 20 hertz to 20 kilohertz.
A weighting filter may also be used, either alone or in conjunction with a bandpass filter. Weighting filters apply one of a number of standard weighting equalization curves to the measurement, typically representing the response of human hearing.
Any filtering used in a noise measurement must be stated as a component of the measurement conditions.
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