by Ron Kurtus (revised 8 March 2006)
Waveforms such as sound waves, alternating current (AC) electrical cycles, and electromagnetic waves—including visible light waves—can include noise. Extra or unwanted waveforms and static are considered noise for the original waveform.
The noise can be irritating and interfere with communication. The signal-to-noise ratio determines how much noise there is.
Questions you may have include:
- What is sound noise?
- What is electronic noise?
- How is the signal-to-noise ratio used?
This lesson will answer those questions. Useful tool: Units Conversion
Useful tools: Units Conversion.
Sound and noise
Any type of sound that interferes with your normal hearing can be considered noise. It is often loud and irritating. Some noise can also be quiet sounds that you can barely hear but can affect your desire for quiet.
Loud noises can interfere in being able to hear conversation and other desired sounds. Noise can inhibit the ability to concentrate or think and can even cause health problems. Some people have to work in noisy environments.
Examples of this type of noise includes loud conversations, TV or radio that is too loud, and loud vehicles.
Another example is wind noise in an automobile. The shape of some cars will create the rushing sound of air or even a whistling sound when the auto is traveling fast, even with the windows closed. This noise can be irritating, as well as to inhibit listening to the radio or CD while on the highway. Automobile engineers try to eliminate that noise.
There are times when a person wants complete quiet. Subtle noises can be heard, which can disturb the person. A TV in another room, a clock ticking or someone talking quietly can be disturbing noise to some people.
White noise is a type of random noise similar to static. It is called white noise because it consists of a full spectrum of waveform frequencies, similar to how white light is a combination of a full spectrum of visible colors.
Random waveform of white noise
Surprisingly, white noise can help people fall asleep by masking other sounds, such as the sound of a television in a neighboring room. It is used in noise reduction.
(See Noise Reduction for more information.)
Electronic noise can be either noise in your electrical lines or static in electromagnetic communications.
Your alternating current (AC) electricity cycles at either 60 cycles per second (60Hz) or 50Hz, depending on your country. Although this signal starts off as a pure sine wave, it can pick up stray waveforms from various electrical devices.
One example of electrical noise that is added to your home current can be caused by a nearby electric motor that may create an additional regular signal, which can add to your normal AC signal.
AC waveform with noise added from electric motor
Turning on a light will create a spike in the waveform. Most electrical systems have filters to protect the electronics from such small spikes. But if lightning hits a power line to your house, there will be a huge spike in the voltage. It can pop the circuit breakers in your house. Such a large spike can also fry the insides of your computer. That is why it is good to have a surge protector in the electrical line to your PC.
Thermal fluctuations in conductors can create what is called Johnson Noise that can affect the operation of sensitive electronics.
Electromagnetic waves include radio waves, TV, and visible light among others. Electronic noise is when outside signals interferes with signals you want to receive. It is also called electromagnetic noise.
Static on radio
One example of this is the static you can hear on an AM radio during a thunderstorm. Every time a lightning bolt flashes, static is heard on the radio. FM stations are immune to this type of electronic noise.
Static on TV
Another situation is the static you can see on your television when someone is using an electric hair dryer or mixer. The static noise seen on the screen can be irritating.
A way to determine how much noise there is can be determined by measuring the signal-to-noise ratio (S/N).
The average amplitude of sound you want to hear divided by the average noise amplitude will give you the signal-to-noise ratio. Typically the noise is not regular, thus you take its average to determine the effect on the listener. When the S/N is 2 or less, it starts to get difficult or unpleasant to hear the sounds.
With AC electricity, your signal could be 110 volts. In the picture above of the AC waveform, the voltage of the noise is about 11 volts. Thus the signal-to-noise ratio (S/N) is 10 (110/11), which is very good.
Noise consists of unwanted waveforms that can interfere with communication. It is often irritating. Sound noise can be loud or even very subtle. Electronic noise is often experienced in static in AM radio and television. The signal-to-noise ratio determines how much noise there is.
Resources and references
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