Please briefly talk about the phase of sound, and by the way, the meaning of 180 phase difference.

The phase is relative to the listening position, and a single speaker can also be said to be relative to its central axis; There are many factors that cause phase shift, only for the speaker: (1) its wiring will be reversed 180 degrees; The reverse installation of the horn (magnetic steel facing outwards) is also 180 degrees. The first frequency division is 90 degrees behind, the second frequency division is 180 degrees behind and the third frequency division is 270 degrees behind. . . And so on. (2) In addition, because the sensitivity of the horn causes the difference in time difference between the high bass and the low bass, it will also lead to phase distortion, so why the filters between the high bass and the low bass in the same frequency divider are often inconsistent. Usually, the sensitivity of treble driver is much greater than that of bass unit, so we often find that there are few components for bass frequency division, while the treble part is often more resistance, capacitance and inductance. . . . . This processing uses the filter betweenness to balance the sensitivity between them, so that the time when the high and low tones reach the human ear is synchronized and their phases are corrected. (3) The signal will lose when it passes through any equipment, and it will lag when it passes through the filter in any circuit, which will also lead to its phase shift. (4) When two speakers with correct phase sound at the same time, the audio-visual drift caused by pointing angle, placement position, distance from the audience and other factors can actually be classified as "phase", but the sound of the two boxes does not reach the human ear synchronously and the sound of a single speaker does not reach the human ear synchronously. There is a special audio-visual potentiometer on the mixer to balance this problem. (5) When multiple loudspeakers are used for on-site sound reinforcement, engineers need to measure the phase of each loudspeaker, because you don't necessarily use the same brand and model products; The phase difference of these speakers has a great influence on sound reinforcement. If you are not careful, the sound pressure of the whole system will deviate from the target design value. For example, one or two boxes are upside down, haha! If you haven't noticed, this is a disaster. Even if the phase difference between speakers is not enough to reverse 180 degrees, too much phase difference will still cancel out a lot of effective power, so when choosing speakers, we should not only pay attention to whether the speakers are reversed, but also pay attention to their phase difference. (6) On the frequency response curve, the influence of phase difference on sound pressure is extremely obvious: the frequency bands that cancel each other will form deep valleys on the measured frequency response curve. This is why when there is a phase inversion, the sound of two speakers is not loud enough for one speaker.