Editor's note:

The acoustic characteristics of the listening room are an extremely important part of the hi-fi system, which seems to have not received enough attention among the majority of audio enthusiasts. Perhaps some people have already learned the importance of it in theory, but they don't know how to do it, or they have not achieved the desired effect because of the wrong approach, and even have become indifferent to the interest in room acoustics. To this end, the special correspondent of this journal interviewed Professor Cha Xueqin, a 57-year-old famous indoor acoustic expert in China. She has worked in the design department of the Ministry of Radio and Television for many years in interior acoustics design and is currently employed by the Fraunhofer Institute of Building Materials in Germany as the head of the room acoustics department. Her research results are many and she is also very familiar with and concerned about the family listening environment.趁 She recently returned to China for a vacation, and this special reporter visited her and asked her to make an in-depth analysis of the importance of the listening environment from both theoretical and practical aspects, and made the basic principles for the acoustic characteristics of small rooms. Detailed description. The following is an interview record based on the recording, and the professor has been asked to review the correction.

In Germany, a sound reinforcement system barrier caused almost a political storm.

Deaf student: I heard that the German Federal Parliament Building had a problem with acoustic design. You also participated in and contributed to the solution. Can you talk about the details?

Professor Cha: In 1992, Germany built a new federal parliament building in Bonn. When it was launched in December, who knows that the speaker has just said a few words and there is no sound. Originally thought that the sound reinforcement system had a breakdown, I had to ask all the members to withdraw, and urgently recruited the technicians of the factory to repair. After a few hours, the technicians said that there was no problem and they could continue to meet, but in less than two minutes there was no sound. Since the conference was broadcasted nationwide through television, the audience all over the country saw this situation, which led to public opinion. On the second day, the national newspapers reported that this was a scandal.

After-the-fact analysis showed that the cause of the accident was not equipment failure, but room acoustic design mistakes. Originally, the designer wanted to make the building an unprecedented and unique building, so the design was very special: it was round and 4l in diameter. The walls are all glass to symbolize "political transparency." The roof is translucent for natural daylighting. On the wall in front of it is the German national emblem (eagle), called the "Eagle Wall". The speaker's position is roughly in the center of the hall, close to the center of the circle. In addition, in order to avoid the appearance of a speaker in the TV lens, the speaker is hung very high. The sound reinforcement system is computer controlled and automatically trips once feedback occurs to avoid unpleasant whistling.

It is reasonable to analyze each of these design requirements separately for analysis. However, since no one put them together for consideration, there was a problem: the horn was hung high, the radiation surface was large, and the reflection coefficient of the glass was large, and the indoor sound reflection was very serious. Since the walls are circular, all of the reflected sounds are precisely focused on the position of the speaker (i.e., the microphone), creating strong feedback that causes the sound reinforcement system to trip. Afterwards, experts from all walks of life were convened to sit down and discuss the reasons and solutions. Interestingly, the Germans also "kick the ball" and blame each other when analyzing the reasons: the person who is engaged in the room acoustics said that it is a problem of the sound reinforcement system, and the person who is engaged in the sound reinforcement system said that it is a problem of room acoustics, and later joined forces to say It is a problem of architectural design. The person who engaged in architectural design immediately published an article in the newspaper saying that he had never done a good job of building a sound reinforcement system before building a house. He always built a house first, and then engaged in a sound reinforcement system. It means to say: Your sound reinforcement system should adapt to my house and not my house to adapt to your sound reinforcement system.

Fighting for it, the issue of responsibility can’t be done. The next step is how to solve this problem. Experts have adopted a variety of methods: the speakers are all replaced with more directional products to reduce the radiation surface, drilling holes in the "Eagle Wall", adding glass wool to increase the sound absorption coefficient; Some sound absorbing materials are added to the sound absorbing material; two layers of glass reflectors are added to the upper part of the wall, and the upper reflecting plate faces upwards, and the sound is reflected to the sound absorbing material area of ​​the roof and sucked up, and the lower reflecting plate faces downward, The sound is reflected to the audience to avoid causing the sound focus near the microphone, changing the tilt angle of the audience; the walkway is carpeted. . . . . .

However, there is a problem that makes them embarrassed: the entire building has 24 glass doors with a total area of ​​more than 100 square meters. The acoustic reflection produced by these glass doors cannot be ignored, but it is impossible to add glass wool to the door or to add a reflecting plate, which cannot be solved by conventional means.

A small Chinese woman drilled a million holes in the German Federal Parliament building.

Hey: How is this problem finally solved?

Check: I just arrived in Germany at the time. The director of the institute asked me if I could do it. I said: The theory of the "micro-perforated plate sound-absorbing structure" by the famous Chinese scientist Professor Ma Dazhao can be solved. The basic method is to perforate the glass plate by a certain pitch and aperture according to the required absorption frequency and absorption coefficient, so that the glass surface changes from the reflective surface to the absorption surface.

Another advantage of this construction is that it does not require mineral fibers. The use of glass wool as a sound absorbing material has many problems: whether glass wool is carcinogenic, and its recycling and recycling are also problematic. In recent years, many people are studying sound-absorbing materials without mineral fibers, and Professor Ma's theory is the most complete. Unfortunately: for such a theory, the Germans have not heard of it.

In order to verify Professor Ma's theory, a set of samples was first made from steel plates. Take the test in the laboratory. The measurement results are in good agreement with the previous calculations. I was surprised to see all the acoustics people, because I have never seen the calculations and experimental results match. This shows that Mr. Ma's theory is very complete and accurate. According to the theory of the past, the resonance frequency of the resonance type sound absorbing structure can be calculated relatively accurately. It is difficult to absorb the sound coefficient and the width of the absorption band. It is difficult to calculate due to a series of complicated problems such as acoustic damping. . The perforated plate is also a resonant type of sound absorbing material. Using Mr. Ma's theory can be very accurate, and even the measurement results can be used to proofread the factory's machining accuracy. Once we requested the perforated plate to have a hole pitch of 3.5 mm and a hole diameter of O. 6 mm. After making the measurements, it was found that the acoustic characteristics did not match the original design. The geometry of the workpiece was measured and found to be very inaccurate: the hole pitch was actually 3.32 mm and the aperture was small. Substituting these two dimensions into the formula is recalculated and the result is exactly the same as the measured value.

After the initial test was successful, the director asked me: Can you use glass? I said: Of course! Because this theory has a characteristic, the acoustic characteristics are basically independent of the materials used. Of course, due to the different thermal conductivity of metal and non-metallic materials, there are some differences in sound absorption characteristics, but this difference can also be calculated.

For verification, another sample is required. The Germans' control over funds is very strict: you have no money without a project, and you can't spend money without money. I just arrived in Germany, of course, there is no project, so I have no money to order from the factory, I can only do it myself. I said to the other two Chinese students in Germany: We must make the test on schedule, and this weekend is no longer a break.

For ease of processing, we used plexiglass as a sample and used tens of thousands of holes in an area of ​​20 x 20 cm. My hands and feet were fast, I sat up in front of the drill floor and did not move for eight hours. I finally finished. The next step is to perform acoustic measurements and write experimental reports... The tasks accepted on Thursday were all completed by Monday. On Wednesday, samples and reports were all delivered to Bonn on schedule. Among the many competitors, this program was finally adopted and applied to the transformation of the parliament building and achieved success.

Afterwards, the German "Engineer's Newspaper" said: "The Chinese have made the German Federal Parliament Building." "Pictorial" said: "A small Chinese woman has drilled millions of holes in our federal parliament building." I personally feel that I cannot accept this statement. Because the acoustic transformation of the parliament building has taken many measures, my measures are only one of them. However, other measures are conventional, and the "drilling" measures are unconventional.

German: Some people are not convinced, but they are not convinced.

Hey: I feel very enjoyable after listening to this incident. You really gave the Chinese a sigh of relief.

Check: I thought about it when I did this. Someone said afterwards: We measured it on the spot, and this thing seems to be useless. Our director replied very well. He said: Now to prove that it is useful, there is only one way: to remove all these measures and see if the same will happen. However, this is not possible. No one can say in advance that the sound reflection of the door can be neglected, so all these measures can not be lacking.

Some people called the director to protest, saying that this theory book has already existed, and it is not Chinese at all. I was so angry that I was terrible. The director said: Don't be angry, prepare the papers in your academic report.

When I first wrote the thesis, I wanted to introduce the principle of this sound absorption structure. Now that these arguments have appeared, I decided to change the content of the paper and change it into a argument why the theory is Chinese. I started from the study of the sound absorption of microtubules by the British people more than 100 years ago, and discussed how to gradually develop into the theory of Mr. Ma. It is also demonstrated that some seemingly similar theories of other people are completely different in mechanism, thus proving that this theory is Chinese.

The German is still pretty cute in obeying the truth. My report was arranged in the last post of the day, and the hall was full of seats, and even the steps were full of people. At the end of the report, many people came to the stage to congratulate me and said that we are convinced. Since then, no one has ever refuted this problem.

Where is the real Hi-End?

渝: This example just shows the acoustic characteristics of the room is very important. However, for the listening room of the average family, the acoustic feedback problem does not seem to be very prominent. So, where is the importance of indoor acoustic design?

Check: It is also very important. The entire sound system is like a chain. It consists of many links: first, recording, then CD player, amplifier, speaker system, and should also include the human ear. The sound from the speaker system must pass through the room to reach the human ear, so the room is also a part of the chain of the sound system. If the acoustic characteristics of the room are not considered, then there is a lack of a ring in the whole chain.

渝: What happens if the acoustic conditions of the room are not good?

Check: First of all, the frequency response, it is not difficult to achieve the frequency response of the CD player, amplifier and speaker system in the range of 20 to 200O0. But the room is also a resonator and has its resonant frequency. Due to the different three-dimensional geometrical dimensions of the room, the resonance frequency is not one, but many, distributed in the entire frequency band. If the density of the vibration frequency is relatively large and evenly distributed, the frequency response of the whole room is relatively straight, otherwise There will be many peaks and valleys. If there is no frequency in the room's simple vibration spectrum, even if the speaker system emits a sound of this frequency, it will not stimulate the room's air to vibrate accordingly, and our ears will not hear it.

I have experimented in a room with poor acoustic properties: using a sweeper to emit a continuously varying 20-20,000 Hz audio signal, which was then played back through the amplifier and speakers, and found that some of the frequencies were gone. That is to say: the sound that should have been continuous has become intermittent, like a person is gasping. Is it the frequency response of the amplifier? No. Because the output of the power amplifier indicates that the surface of the power amplifier does not move during the entire playback process, the power sent from the power amplifier to the speaker system does not change. Is it a problem with the speaker system? We immediately removed the speaker and sent it to the anechoic chamber for measurement. The result is no problem. The final conclusion is that there are many peaks and valleys in the frequency response curve of the room, and the frequency at the valley point is not heard. After proper acoustic treatment of the house, the problem is solved.

It can be seen that if the frequency response of the room is not good, even if the frequency response of your equipment is good, it will become a very bad curve when superimposed with the characteristics of the room.

The room is a very important part of the sound system and is currently the weakest link. The strength of a chain depends on the lowest strength of the ring: if one of the rings breaks, then the other links are no better. In this sense, it can be said that the room is the most important part of the current sound system.

渝: I remember that Professor Guan Shanqun once said a paragraph that was similar to yours when interviewed by this reporter. He said: Today's enthusiasts pay special attention to equipment. Always working on one component and one line, it only shows that he is still in the early stages. The hallmark of entering the advanced stage is to focus on the sound field, which is the acoustic characteristics of the room.

Check: I have also discussed this issue with Professor Guan. I remember seeing an article in a magazine saying that the real Hi-End is my feeling. I think this is not true, because everyone's feelings are different. If the feeling is Hi-End, then there is no standard to follow. In terms of the level at which current equipment production can be achieved, the defects of the equipment are very weak compared to the unevenness of the room. So I think: the real Hi-End is not a device, but a room. If you listen to the acoustic characteristics of the sound room and ignore the equipment and only work on the equipment to find Hi-End, it will never be found.

渝: In other words, do you think that compared with the defects of the room, some modifications to the equipment, that is, the so-called motor, is really insignificant?

Check: Of course, the impact of the room is much greater.

What is a good listening room?

Hey: Can you give some examples to illustrate the importance of the acoustic properties of the room?

Check: Ok, let me talk about a listening room I have in Germany. It is actually not a dedicated listening room, but the office of the director of the institute. The area is 35 square meters and the volume is about 100 cubic meters. It is probably for the purpose of publicity. It is required to keep the original appearance and function of the office. It is acoustically processed and can double as a high quality listening room. I only made some simple acoustic treatments on this house and I got good results. The measurement results show that the frequency response curve is very straight and not at all worse than the professional listening room. The head of a private recording company with a long history and a large scale, and two sound engineers came here to listen and said that the voices of the various places are the same in this room. In fact, this is the same. It is the result of proper acoustic processing. In some rooms with poor acoustic characteristics, the best listening position (that is, the “Emperor’s position” that enthusiasts often say) is very small, and it’s not as good as a head squat. How can people tie their heads while listening? I can see from this example that correct acoustic processing can not only improve the frequency response, but also expand the range of the best listening position. The requirements can also be loose.

Later, they released their tapes and heard that the bass was too heavy. I told him: It is not the bass of this house is too heavy, because the measurement results show that the frequency response of the house is very straight. This only shows that the bass in your production is too wrong. You feel that the bass is not enough in that house, and you try to raise the bass upwards. Now you get the right room and the bass is much more. He countered: You made the frequency response of the room so flat, but in fact no one room's frequency response can be so straight. I told him: You are right, our room is handled very high, and the actual room is rarely done. What is the frequency response of the actual room? If you only measure one room, there must be many peaks and valleys, but if you measure a thousand questions, 10,000, and then average the measurement results, the average value must be a straight line. Curve. The frequency response between productions can only be done according to the average room situation, that is, the frequency response must be straight.

Hey: Is your handling of this room complicated? Can you do it in amateur conditions?

Check: It can be done completely. This is called a difficult person, and it is not difficult for the meeting. The method I used was mainly to add a ceiling of ten centimeters thick. The material of the ceiling was a "composite plate resonance absorber" I developed in Germany. Of course, some other measures have also been taken. Such as carpeting, etc., but the most important is this ceiling.

When the development of this thing began, the Germans demanded that the thickness should not exceed ten centimeters. At first I thought it was impossible, because I was working on a recording studio in China, and I added the sound-absorbing material to 1.3 meters to solve the problem. But the Germans are very rigid: say ten centimeters is ten centimeters, and one centimeter is not allowed to exceed. As a result, a bite was also made, and the effect was very good in many places, and the patent was also declared.

Hey: This kind of sound absorbing material is really good, but it is a German product, and it has been patented. It is difficult for domestic amateurs to get it, and the price will not be too cheap. Under our existing conditions, use What can I do to improve the acoustic characteristics of the room?

Check: Ok, let me introduce some basic principles.

The rooms we listen to are acoustically "small rooms", and the so-called small rooms are compared to wavelengths. For example, a 50 Hz sound wave with a wavelength of 6.8 meters makes the room's geometry smaller. The sound field in the small room is definitely uneven, and it is necessary to deal with it. The basic principles of processing are:

1. The volume of the room must not be wasted. Because the density of the vibration frequency depends on the volume, the density of the vibration vibration frequency is small, and the sound field is not uniform. For example, there is a valley at 50 Hz on the frequency response curve of the room. It means that there is no resonance frequency near 50 Hz, so it can't be stimulated at all, and we can't hear the sound of this frequency. Therefore, don't put things that are unnecessarily wasted. For example, some people make two low-frequency diffusers in the house in order to increase the low-frequency diffusion. This is completely useless. Because the diffuser has to work, its geometry must be equal to the wavelength to be diffused. The diffuser is impossible to make it so big. Putting in a diffusing body that doesn't work, isn't it a waste of volume? There are people who make the roof and wall into a zigzag shape, which is also useless to solve the problem of low-frequency diffusion. Of course, it is still good for solving the echo problem.

2. Adjusting the geometry of the room to a suitable ratio improves the uniformity of the vibration frequency and solves the standing wave problem.

3. If the size and size of the room have been determined and cannot be modified, then try to increase the damping and absorption of the low frequency. This is probably the most realistic way for most people. How much is the absorption material added? To be more extreme: it can never be too much. I am afraid that it is not enough, you can't think too much. I have done a test: a small room with a volume of about 100 cubic meters, playing music signals inside the speaker, receiving it with a dummy head, and people returning to the outside of the room to listen with headphones. At the same time, a signal is directly connected from the sound source. This signal is not passed through the room, and the two signals are compared with A and B. I found that as more sound absorbing materials are added, more and more low frequencies are heard. Some low-frequency signals that were not heard at first, such as drum sounds. I can hear it after adding more sound-absorbing materials. Moreover, the more the sound absorbing materials, the more consistent the signals of the two groups of AB, that is, the closer the signal passing through the room is to the original sound in the sound source.

In order to measure the data, I removed all the sound absorbing materials, moved them into the instrument, and added the sound absorbing materials a little bit. The result is exactly the same: as the sound-absorbing material increases, the frequency response of the room becomes more and more straight.

In short, there must be enough absorption. There is a misconception: When the bass is insufficient, it is considered that the bass reverberation time is not enough, the bass is sucked too much, in fact, the opposite is true.

Hey: Can you more specifically say: What materials should be used and how? How have you seen people in the past: Putting foam on the wall, hanging tapestries, etc. Are these methods useful?

Check: For low frequencies, these practices are useless. The conventional method is to add a corner sound absorber to the corner of the wall and the edge of the wall. The sound absorber can also be used in an amateur condition, for example, a cardboard box is used to make a long box, and a hole is placed therein, and a glass wool (packed with a plastic film) is placed inside. Reluctant to use glass wool, you can also put foam or other fabrics, and you can put clothes. The key is that the size must be large enough, generally required to be 1/4 wavelength of the frequency to be absorbed, and must be placed in the corner. Some rooms have a row of hanging cabinets on the top corner. They can also be used. Just punch holes in the door or replace them with a screen door to make it breathable. You can still put clothes inside. Of course, if you only use the wall cabinet, it is not enough. Then you can find another way to add something else. Some people put the sound absorber into the shape of a flower stand, put it in the corner of the wall, and still put a pot of flowers on it, which is very beautiful and has good acoustic effect.

Reverberation time - is it better or shorter?

渝: I talked a lot about the frequency response of the room. Another important indicator of the acoustic characteristics of the room is the reverberation time. How much should the reverberation time of the listening room be adjusted?

Check: What we need is the original reverb in the source, not the reverb generated by the room. But no room can be reverberant. The sound we hear is the result of the reverberation of the room and the reverberation of the source. If the reverberation time of the room is too long, the listening effect will of course be affected.

Hey: Since the reverberation of the room is harmful, does it mean that the shorter the reverberation time should be?

Check: Can't say that. Although in principle, the shorter the reverberation time, the more realistic the signal is heard, for example, in the anechoic chamber, there is no reverberation at all, and the signal heard is the most realistic. At present, the international demand for ideal listening is "neutral", what is "neutral"? That is to ask for listening in the room is equal to no room, in other words, there is no reverberation. But people can't listen in the anechoic room, and if the room is too "dry", it will cause a series of other problems that are difficult to handle. For example, the reflection problem of the equipment surface: in a very "dry" room, the reflection of the equipment surface will be very prominent, and the room that is too dry is not in line with people's habits. When you first walk into the anechoic room, you will feel "press your ears", but after ten minutes, you will feel the voice is very clear. In the acoustically-treated office mentioned above, people will feel a little different when they walk in. But after staying in this room for a while and then going to another room, you will feel the sound is mixed, not as good as this one. The house is good.

Another problem is that it is easier to make the medium and high frequency reverberation time shorter, and the low frequency reverberation time cannot be made very short. For example, if the reverberation time of the medium and high frequency is 0.1 or 0.15 seconds, and the low frequency is 0.5 second, the sound will be very unpleasant and very stuffy. Some people have encountered this situation and think that the mid-high frequency reverberation time is too short. I always tell him: This is not because the high frequency is too short, but because the low frequency is too long. If you pull the low-frequency mixing time to 0.1 to 0.15, or even pull it to 0.2 to 0.3, you will feel a little boring. We can think about it: when listening to the sound outdoors, there is no reverberation, but we never feel bored. Why is the medium-high frequency reverberation time inside the house short-staying? The problem is low-frequency mixing. When asked, it was too loud.

Hey: What can I do to shorten the low frequency reverberation time?

Check: Still absorbed. Although it can't be done very short, but try to make it short, but also pay attention to the linear balance across the entire frequency band.

Hey: I talked a lot today, although there are still many questions I would like to ask you. But you will travel to Germany to go to Germany tomorrow, I am really sorry to disturb you again. I hope that we can introduce some specific methods of indoor acoustic processing and design examples of listening rooms in the future. Thank you very much for accepting our interview.
Check: Thank you too. Whenever there is an opportunity in the future, we must try our best to meet your requirements.

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