Analysis of indoor frequency response, reverberation and standing wave in home theater

Section 2 Indoor Frequency Response and Reverberation

The indoor environment reflects sound and absorbs sound. Generally speaking, the smoother the hard surface, the more reflective the sound, while the softer and more porous objects absorb the sound. High-frequency sounds absorb more, and low-frequency sounds are hard to absorb. So, indoors, even if you use millions of top-level monitors, the frequency response will be absolutely straight, the indoors will reflect more low frequencies, the sound you hear will appear cloudy, and at the same time, the general family white The sound absorption coefficient of the wall is usually quite low (the wallpaper is slightly higher), the sound will be reflected for a long time, and it will be absorbed until it is inaudible. Although the sound pressure of the sound is reduced, it will become inconspicuous, but the sound still exists. You still have some faint feelings, this is reverberation.

In general, the reverberation time indicator is accustomed to using the RT60. The so-called RT60 is the time after the sound is emitted from the horn until it is attenuated by 60 decibels. The reverberation is too high, too high, the sound will not be scattered for a long time, it will affect the sound behind, causing the overall sound to be turbid, awkward and chaotic, and the sound field is not clear. The reverberation is too low, and the sound will disappear immediately, which will make the current sound look very dry and thin. Appropriate reverberation generally depends on the individual's listening habits. Actually, my personal listening, 30 square meters environment, the low frequency is more comfortable within about 0.8 seconds, the medium and high frequency should be less than 0.5 seconds, and the high frequency is close to 0.3 seconds. The reverberation time of the low frequency will be longer because it is difficult to be absorbed.

Some larger ones like the concert hall, pursuing some sound effects, the overall reverberation time can reach 1.5-2 seconds

figure 1:

Indoor test frequency response test equipment, WM61A electret measurement microphone, uncalibrated, the overall sound pressure is somewhat high, but the accuracy is sufficient, (subsequent posts have the use of high-quality voice program and calibration test after the sound pressure level). Onboard HD-AUDIO MIC input, 5K or more high frequency is not accurate, please ignore high frequency. REW software.

Green line: bookshelf: treble: equal magnetic field aluminum with treble, mid-bass, 5-inch paper-based composite basin. The frequency response diagram of the anechoic chamber given by the manufacturer is very straight.

Blue line: AV main box: treble: 2 inch silk enamel dome treble, medium bass: double 6.5 inch carbon fiber basin.

Brown line: AV main box + 15 inch subwoofer.

The amplifier is the Marantz SR5008, pass-through mode.

It can be seen that there is a very abnormal peak around 40 Hz, which is caused by the ambient standing wave. The frequency response is very fluctuating in the middle frequency band, and the peak-to-valley difference is even close to 10 decibels. This is usually caused by the indoor environment. These boxes do not have such dramatic fluctuations in the anechoic room, and the frequency response is quite flat.

figure 2

This is the frequency response of the green line bookshelf in the muffler room:

image 3

This is the AV main box, the frequency response attenuation map.

The red part of the background color is the sound directly emitted by the speaker, and the blue frequency response is the residual sound of the room after 160 milliseconds.

It can be seen that although the high-frequency part does not dare to say the frequency response, it is significantly attenuated a lot, while the low-frequency part, especially the standing wave, has almost no attenuation.

Figure 4

This is the flame map of the AV main box. It is easy to understand that the energy emitted by the main box accumulates in the indoor time. The simple basis is that the lower the flame, the better, the colder the better.

It can be seen that there is a standing wave near 40 Hz, the peak is very high, and the red color is very deep, indicating that the standing wave has high energy and is difficult to dissipate, and the accumulated portion has turned red, which is poor data.

Figure 5

This is even worse, the entire low-frequency part of the energy is piled up a lot, but note that this is the configuration to turn on the subwoofer for watching movies, so it is tolerable. Because the movie is just a shocking effect, there is no need to be as strict as HIFI.

Figure 6

The waterfall map is a trend graph in which the sound pressure level of each frequency is gradually attenuated by sweeping the frequency response every three seconds from the sound until 300 milliseconds.

Still consistent, the fastest high frequency, the second lowest frequency, the slowest trend of standing wave. And some frequencies will decay for a certain period of time but suddenly increase because the reflected sound is coming back.

About standing wave and resonance

When the sound wave propagates forward and encounters the obstacle, it is reflected back, and the direction of the original wave is opposite. The two columns are opposite in direction, and the waves with the same frequency and amplitude will form a standing wave.

The characteristics of the standing wave, when you walk through an area with standing waves, the sound heard will be big and small

E.g:

The sound in the picture plays pink noise, and there is a significant standing wave near the wall. When you lean against the wall, from one end of the room to the other, the sound heard will be louder and smaller.

The standing wave dissipates very slowly. When the bass stops, the standing wave will take a long time to be significantly attenuated. This will cause low frequency turbidity, slow speed, weakened hitting, etc., and if your listening position is in standing wave. On the nodes of the wave, the frequency response heard will change significantly, or the amount of low frequency will increase significantly or decrease.

In addition to standing waves, the mid-low frequency sound will resonate with the large area of ​​the room. Try to use thick plates and glass. Some thin plates and the like must be fixed firmly and sealed with silicone glue.