The phase in the sound system is very abstract and involves many related problems. Here we mainly discuss some of the more easily encountered "phase" problems often encountered in engineering.
In physics, the concept of phase is a physical quantity that reflects the state of an AC signal at any time. The size and direction of the AC signal change with time.
In the sound system, the problem of phase is often encountered. Usually, the phase in the sound is relative to the change of the volume in the sound field, so there is the problem of the in-phase and anti-relative sound system.
For example: two audio signals with the same frequency are output from the power amplifier. If a speaker line is inverted, the phase difference is called the phase difference, or phase difference. In physics, the phase difference between the two is exactly 180 °. This situation is called reverse phase, or reverse phase.
In actual projects, we often encounter these phase-related problems:
1. The first is the power phase of the audio system. If the power used by a set of system equipment is not in the same phase (for example: the power amplifier is powered by phase A, the mixer or the surrounding is powered by another phase), when the power consumption of the equipment is uneven , The difference between the high and low phases will cause some equipment to work abnormally due to high or low voltage, which will cause some interference to the overall system effects, such as hum, current sound, etc .; it may also be due to phase differences. It will cause damage to the equipment.
2. The phase of the speaker unit. After the audio signal of the same frequency is output from the power amplifier, if a pair of speakers are connected in reverse phase (usually referred to as "+" and "-" poles connected in reverse), the unit of the pair of speakers will produce an AC phase Poor, the audio signals cancel each other out, the low-frequency signal after sound reinforcement is obviously weak, the sound becomes hard and dry, and the middle and high frequencies will float back and forth, affecting the debugging and overall effect of the entire sound reinforcement system.
3. In the sound source pickup part, the microphone also has the same phase. If multiple microphones are used at the same time, one of the microphones will be in reverse phase, which will cause an overall sense of imbalance in the overall pickup. If the pickup microphone is close to it, it will cancel the signal. When two microphones in phase pick up together at close range, it is easy to cause signal overlap, increase the generation of positive feedback, and generate self-excitation. This phenomenon is more common in karaoke. At the same time, an experiment can be made. Two moving coil microphones of the same phase are used to pick up the sound relative to 180 °, and the sound pressure will be obviously low, or even no signal output.
4. The power amplifier also has the problem of phase. Fortunately, the output terminal is easier to understand the positive and negative poles after using the four-core plug. The banana output plug also has obvious red and black ports for identification, but when the power amplifier is bridged, pay attention to the signal The red end of the input control (usually A extension) is positive, and the red end of the other input control (usually B channel) is negative.
5. A mixer with a better grade is generally equipped with a phase conversion switch. When using several microphones of different models to pick up in parallel, if the sound is abnormal or weak, you can press the phase conversion switch of one of the microphone channels. Determine whether the phase problem is caused. When the two singers sing together "affectively", if the microphone in hand is close to a relatively horizontal position, you will find that the pickup signal is reduced. At this time, pressing one of the microphone phase shift switches can solve this problem.
As a technician engaged in professional sound reinforcement engineering, if there is a reverse phase in the system, this is simply unacceptable, because there is only one explanation. Your speaker cable is reversed (or the signal cable is reversed), except for inferior products. In addition, the possibility of reverse phase connection between the speaker itself and the power amplifier is very low.
In our work, the main problem related to "phase" is often inverted, and the phase shift is rarely encountered unless we design the frequency divider or audio signal processing circuit ourselves. For example, if the tweeter is connected with a capacitor, the phase difference is 90 degrees. While inversion is easy to find in practical applications, there are two simple ways to solve this problem:
1. For the phase problem in the circuit, the domestic-made phase detector is now very cheap, less than 300 yuan, with various interfaces such as Canon, RCA, 6.3 in-line, etc., which can directly detect whether there is a line Reverse problem. But you can't connect them all for the next test, because if there are two serially connected lines in the middle, if they are reversed, they will be negative and positive again. It should be measured section by section. This should be a procedure that must go through in the project installation.
Second, whether the power amplifier is inverted or not can be judged directly by the sense of hearing. If you feel that you are not sure, you can send a mono signal into the two channels of the mixer at the same time, and then locate the left and right channels through the PAN (sound image positioning knob), and then pass the mixer. The left and right main outputs are sent to the left and right speaker groups. Then keep the phase of one of the two input channels unchanged, and the other channel uses the phase switch to switch in reverse phase. The low frequency is obviously full and powerful, and the middle frequency is full, which means that there is no problem with the phase.
Technical Forum Comments: In the sound reinforcement system, due to the reverse polarity of the microphone signal output line or speaker power signal input line and the phase distortion existing in the system, it will cause a variety of sound reverse phase or phase shift problems. The correctness of the sound phase relationship (especially the reverse phase) will directly affect the quality of sound restoration. However, at present, many domestic small and medium-sized audio engineering technicians do not seem to attach great importance to the reverse phase of the system. Most sound workers do not consider the phase of the microphone and speaker at all after connecting the system; when adjusting the equipment and system, they do not consider the series of phase distortions that may be caused by the adjustment. This is for modern sound systems. Is undoubtedly a shortcoming. I hope this article can attract more friends who are engaged in the application of small and medium-sized engineering technology.
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