The mechanism of generating standing waves in home theater, the countermeasures and the significance of EQ equalizer

EQ is the abbreviation of Equalizer. The mainland China is called equalizer. The Hong Kong and Taiwan area is called equalizer. Its function is to adjust the gain value of each frequency band signal. Ordinary people initially contacted the equalizer on the advanced recorders of the 1980s. The high-end recorders of the year were equipped with N-band equalization adjustment, and the regulator was an equalizer.

EQ modifies a certain type of music by modifying the composition of each frequency in the sound to enhance the sense of people. Common include: normal, rock, pop, dance, classical, soft, jazz, metal, bass and custom. Customization is self-adjustment. There is no fixed mode. The real EQ according to personal preference can satisfy different personal listening preferences.

The above is just one of the purposes of using EQ initially, which is to change the tone.

We are now playing EQ for AV (Home Theater), in addition to one of the above purposes, it is used to calibrate the frequency response characteristics of the room. Audio manufacturers have worked hard to make speakers, often telling how the frequency response curve is flat (that is, the frequency of different frequencies can be consistent), how the sound is so good, but, in different people's rooms, it often reflects Not one.

Here, in addition to the different listening habits of each person, the more important factor is that the sound reflection of the room destroys the originally very flat frequency response curve. A point sound source, the sound that spreads around, will reflect when it hits an obstacle, and the reflected wave will overlap with the original wave. The result of the superposition is that some frequency points are constructive (sound pressure increases) In some frequency bands, the cancellation (sound pressure reduction), the reflection of different distances in all directions of the room makes the situation very complicated.

So, what is the relationship between different frequencies and the distance between the room and the occurrence of standing waves?

According to the fluctuation formula: v=λf

Where v is the wave propagation velocity in m/s and the acoustic wave is 340m/s

λ is the wavelength, that is, the distance unit between two peaks m

f is the frequency unit Hz, the audio is 20Hz-20kHz

It can be seen that v=λf and λ=340/f. At this time, the wavelength corresponding to 20Hz is 17m, and the 20kHz is 0.017m.

When the distance between a reflecting surface and a point source is exactly an integer multiple of a half wavelength, a standing wave is generated! For high frequencies, when the sound source reaches the reflecting surface, it has already experienced countless wavelengths, and the signal attenuation is severe, and the wall surface is also It is easy to absorb high frequency (the pores are absorbed more than half wavelength), and it is not easy to generate standing waves. For a frequency of 20 Hz, the wavelength is up to 17 m. Therefore, it is very difficult to absorb low frequencies, and the geometry of the absorber should be equivalent to half wavelength!

If the distance between the muzzle of the subwoofer and the back wall of the room is 5 m, that is, the half wavelength is 5 m, then f = 340/5 × 2 = 34 Hz, which is a standing wave frequency generated by one of the reflecting surfaces. 68Hz, 136Hz will be the standing wave of its harmonics. The other reflective surfaces in the room also produce standing waves of different frequencies.

Therefore, the subwoofers in two different positions have different distances from the reflecting surface, and the standing wave frequency is different. It is impossible to adjust the 2-door gun with an EQ. We generally choose the largest one or two standing waves for processing. There are many ways to handle standing waves:

1. Find the best gun method

Dragging the cannon, looking for the best point in the room, including the method of turning the muzzle.

In fact, according to the above principle, unless there is a large space, the standing wave is always present. How can there be the best shot point without standing wave? In fact, it is necessary to find a point that just uses the A wall. The length of the face is offset from the B wall to achieve balance. For small spaces, it is too difficult, and it does not necessarily exist. Especially the square room.

2. Using low frequency trap absorption method

In the most severely reflective areas, place a soft body (such as a cylindrical soft body) whose body shape is equivalent to a half-wavelength of the standing wave frequency to absorb a part of the energy at this frequency. Theoretically feasible, technically difficult.

There are two main problems. First, the frequency of absorption is exactly the standing wave frequency. This can be calculated by half-wavelength to obtain the height of the cylinder. The second is to absorb the energy just right, this is related to the diameter of the cylinder, etc., not very good control.

3. Signal level gain control method

The above two methods of overcoming standing waves are actually a passive way. In fact, there is a more convenient and simple active method of controlling from a signal source. For the time being, we will not discuss the advantages and disadvantages. This is also a controversial topic. Controlling from the signal source is to control the level gain of a certain frequency entering the gun. For example, if the loudness of a 40Hz frequency is higher than the reference loudness of 10db, we first reduce the level of the 40Hz frequency on the signal source, so that The final output of 40Hz is basically the same as the reference loudness. The device used to control the gain of a certain frequency is the EQ equalizer mentioned above.

However, in fact, the effect of using EQ is sometimes unsatisfactory.

One of the reasons is that the frequency of EQ is often a fixed number of points, not necessarily the one that you just want to control, often only accommodated.

The second reason is that although the gain of a certain frequency point is controllable (such as -10db~+10db), the width of the two sides of the center frequency point cannot be controlled (Q value), which often causes the center frequency to be pressed down. The pressure on the two sides can not be pressed.

To avoid this, a better parametric equalizer PEQ (paragraphiceq) should be used. Ordinary EQ frequencies are fixed (the center frequency in Figure 1 is a fixed number of frequencies), the shape of the waveform can not be changed, only the control gain can be changed. The parameter equalizer PEQ not only adjusts the frequency fo continuously within the design range (easy to correspond to the frequency you need), but also affects the bandwidth of the two sides of the center frequency (Q value is variable, the controllable range is large, see figure Three to six), plus the adjustable gain, is very flexible.

For example, the old Qin YC-1EQ is the low frequency three-point parametric equalizer PEQ.

In the figure above, 9 knobs are divided into 3 groups, each group forming an independent PEQ. Note that the print positioning on the knob is rough and can only be approximate.

On the left, FREQ ---- control center frequency 18HZ--80HZ continuously adjustable. This is the center frequency to be processed. If you want to cut the peak, the knob should be adjusted to the highest peak frequency. If you want to improve the dive, you can adjust it to the frequency of the low-band -6DB of the gun test curve.

In the middle, BANDWIDTH --- This is actually the reciprocal of the Q value. Understanding the bandwidth is determining the left and right width of the center frequency. 0.1--1.0 is continuously adjustable, that is, the frequency range affected by the control point is up and down. (For example, if your center control frequency is 40HZ, do you want to adjust 38HZ--42HZ or want to adjust 36HZ--44HZ? You can use this knob to control, see Figure 3 to Figure 6).

On the right, LEVEL -- this is the gain of the control point, -15-- +6DB is continuously adjustable, this is actually the peak clipping or valley filling control.

More home theater acoustic processing issues, welcome to pay attention to China's home theater network official WeChat: cnhifi, free for you to answer.