Noise Gate

The purpose of this machine is to clean the audio signal, while passing through the various circuits of the guitar, amplifier and effects, it even collects noise generated by the electrical current inside the equipment. This noise from the electrical current is an annoying humming, that in some cases can be louder than the actual sound.
This defect is mainly due to the nature of electricity. For instance, this noise could be caused by the incorrect voltage in the electrical system from where the current is drawn (it is always better to have a current stabilizer), from audio cables, or from the pedal transformers. Even equipment that doesn't have anything to do with the system can interfere (for instance, in places with an inadequate electric system, you can hear noises from the freezers, dishwashers, etc.). You should try to discover these problems before you begin playing.
In any case, there will always be a small amount of background noise. This is where the noise gate or noise suppressor comes in. It basically has the opposite function of the compressor, which compresses the sound within assigned values. The noise gate closes an audio line when there is no signal, expanding the sound above or below a certain set threshold.
The treshold control determines the level at which the signal is cut. The decay or release control establishes the natural decay of the note before the noise gate intervenes.
The settings are very important. Too high a threshold would "strangle" the sound, and too low of a treshold would be insufficient in a place with a poor electric system. Therefore check the various controls of the noise gate during the soundcheck (sound test before the concert), to verify any particular inconveniences and act accordingly.
A noise gate should be placed after the preamplifer, especially if it's a valve preamp. In this way, you can control any possible source of humming from the beginning. If, before the preamp or the amplifier, there are some pedals that cause disturbances (compressors, distorters, overdrive), the noise gate should be placed after them. The noise gate should always be placed before any digital processor effects or pedal effects like the chorus, the delay or the reverb.

Phaser and Flanger

These two types of effects are very similar to each other. The sound created by them deals mainly with the phase ratio between the two signals (electronic or acoustic) thats initially derived from the same source. Combining two signals that have the same wavelength, frequency and volume, but are out of phase by 180° will cancel each other out. This is why two loudspeakers connected to the same amplifier are in phase with each other, otherwise some of the frequencies would be cancelled (particularly those at the low end of the sound spectrum). In practice, however, the total cancelation of all the frequencies won't occur, since the physical separation between the loudspeakers will alter the phase relationship. You will therefore have a slight reduction of the low frequencies.
The phaser and the flanger divide the signal into two paths, introducing a slidding phase variable to one signal before the two are rejoined.
When it was created in the early 1970's, the phaser, also called the phase shifter, which tried to emulate the sound of an offset recording tape, but without interfering with the delay, which is what the flanger does. The principle is similar to the Doppler effect, that consists of a modification of the frequency to the relative motion of the source with respect to an observer who instead hears the sound and vice versa. The Doppler effect is the famous effect that sounds like a siren, where you hear an acute sound nearby that degrades in tone as it gets further away.
This phenomenon, also called the siren effect, occurs when the arriving sound waves are followed by greater frequency that lowers as the sound moves further away. This effect is electronically reproduced by a circuit called the L.F.O. (Low Frequency Oscillator), which can produce frequency signals, amplitude and programmable pre-arranged forms.
As already stated, two wave forms that are out of phase by 180° cancel each other. But if they are out of phase by 90°, you will get the phaser effect.
The typical controls found on the phaser are:

The flanger is different only in the fact that it acts on the delay in which a wave enters in phase with another. In the 1960's, if you wanted to get some particular effects during a recording, one way was to record the same sound on two different tapes. During mixing one of the two tapes would be slowed down by holding back the flange of the spool (therefore the name "flanger"). In this way the sound was out of phase by means of the delay.
Besides the controls that we have already seen for the phaser, there is also the manual delay control that influences the delay of the sound.