The sound quality model measurement of the hall is an important means of architectural acoustic design. It has a history of nearly 100 years of application and has a research history of more than 40 years in China. The model is applied with the principle of similarity. The model is made at a ratio of 1:10. The scale of the room is reduced by 10 times. The wavelength of the sound wave is also shortened by 10 times, that is, the frequency is increased by 10 times. The decoration material in the model is selected at 10 times the frequency. The sound absorption coefficient is the same as the actual decoration material. Because of the constant sound velocity, the sound quality indexes such as reverberation time, sound field uniformity, sharpness and diffusivity in the model are shortened by 10 times on the 3D scale and time scale. By measuring the sound quality parameters of the model and reducing the length and time by 10 times, the sound quality parameters of the actual hall can be obtained. Model measurement is the only practical method known at this stage that can better simulate the fluctuation characteristics of indoor sound. It is characterized by accurate simulation of acoustic fluctuations such as diffraction, scattering and edge reflection of complex shapes in the room, and can perform subjective evaluation of audible subject. The acoustic defects that appear in some architectural designs can be directly found, and the sound quality parameters such as reverberation time and sound field diffusivity can be accurately predicted. Many major projects in China have applied sound quality model testing methods to assist acoustic design, such as Shanghai Grand Theatre, Beijing Tianqiao Theater, Fujian Grand Theatre, National Grand Theatre, Dongguan Grand Theatre, Daqing Grand Theatre, etc. Some ordinary halls are needed due to engineering needs. It has also gradually begun to use this method for sound quality design, such as the Central Conservatory of Music High School Music Hall, Shenzhen Children's Theater.
Since the 1960s, Tsinghua University has begun research on model measurement, and has completed more than a dozen model measurement projects. It has rich experience in model making and measurement, and has the best model measuring instrument in the country, which can undertake model measurement at a high level. task. The work includes 1:10 scale model making, analog sound absorbing material selection, sound quality parameter measurement, data analysis, scheme acoustic effect evaluation, and improved program measurement.
In the 1960s, the hall sound quality simulation theory and testing technology were gradually developed and perfected. After a lot of research and practice in the world, the model measurement predicted that the sound quality of the hall had basically reached practical use. At present, the sound source, microphone, and analog acoustic material can already correspond to the real object, and the frequency band of the instrument is also expanded. The practical precision has been achieved in the common indicators such as the simulation of reverberation time, sound pressure level distribution, and distribution of short delay reflection sound. . With the development of software technology, the use of computers for simulation of sound fields has become a reality. From a mathematical point of view, the propagation of sound is described by the wave equation, which is described by the Helmholtz equation. In theory, the acoustic impulse response from the sound source to the receiving point can be obtained by solving the wave equation. However, when the indoor geometry and interface acoustic properties are very complex, people cannot obtain accurate equation forms and boundary conditions at all, nor can they obtain valuable analytical solutions. If the equation is completely simplified, the results obtained are extremely inaccurate and impractical. It is not feasible to use the wave equation to solve the indoor sound field by computer. From a practical point of view, using the acoustic acoustic line tracking method and the mirror virtual source method, room acoustic parameters with a certain degree of reference can be obtained by a computer program. However, due to the simplification of the fluctuation characteristics of the sound, the high frequency sound and the near reflection sound are better, and the information of the simulated sound field is still insufficient. In recent years, using the method based on finite element theory to simulate the high-order wave characteristics of sound, some progress has been made in low-frequency simulation. Welcome to Home Theater Network
The distribution of short-delay reflection sounds in the hall, that is, the intensity, number of reflections, and the arrangement on the time axis within a few hundred ms (mainly 200 ms) after the direct sound are important factors in determining the sound quality. It depends on the size of the hall, its size and the arrangement of the internal materials. In the scale model, the short delay reflection sound distribution measured by the electric spark as the pulse sound source has a good correspondence with the short delay reflection sound distribution of the actual hall, and the size and body shape of the hall are determined at the design stage. Important reference. This is one of the important uses of hall sound quality model testing. Reverberation time is a well-recognized sound quality parameter that can be used to predict the reverberation time of the hall to be built. The purpose of this standard is to predict the sound quality of the newly designed hall through the hall sound quality model test, and adjust the volume and shape of the hall and the number and location of the sound absorbing material arrangement so that the future hall meets the requirements for use. Sound field inhomogeneity is also an important sound quality parameter. The natural sound-based hall has not yet established a quantitative standard for this parameter. The hall with sound reinforcement system and the GYJ25 hall hall sound reinforcement system developed by the Ministry of Radio, Film and Television. The Characteristic Index specifies the standard of sound field inhomogeneity and can be used for model test reference. [page]
The measurement system, measurement method and result of the model test are expressed in the same way as the actual hall, but the measurement frequency needs to be changed according to the scale ratio s of the hall model when measuring the reverberation time and the sound field unevenness measurement. Namely: in the model, the measurement frequency = actual hall measurement frequency × 1 / s. In the hall sound quality model test, other sound quality parameters, directional diffusion, subjective evaluation and other tests can also be carried out, but the above research is still in the research stage as a whole, and it is necessary to have a unit with rich engineering and experimental experience to complete.
Sound waves of different frequencies propagate in the air medium, especially high-frequency sound waves. The attenuation caused by air absorption varies greatly under different temperature and humidity conditions, and generally has no linear relationship with the scale ratio s of the model test. This leads to a large error between the test results and the actual hall. Some people in foreign countries have tried to eliminate this effect by using dry air in model tests, or by using moisture to reduce the humidity in the model, or by using nitrogen to remove the air in the model. However, the equipment using these measures is relatively large and the model is not easy to seal. The sound propagation medium in the model can be normal temperature and humidity air. For the measurement of reverberation time, the influence of air absorption should be corrected accordingly, and the accuracy is sufficient.
For the measurement of short-delay reflection sound distribution, the scale ratio of the hall sound quality model is generally 1/5 or 1/10, and l/20 is also used. However, due to the limitation of the test equipment and the frequency, the accuracy is affected. . For the measurement of the reverberation time, when the scale ratio s is 1/20, it can only correspond to the frequency of 1000 Hz or 2000 Hz in the actual hall. The recommended scale ratio s is not less than 1/10, and the measurement of reverberation time and sound field unevenness can be extended to 4000 Hz in the actual hall. The accuracy of the short-delay reflected sound distribution measurement is also high.
The shape of the inner surface of the model, which has a small undulation size, has little effect on the reflection and diffusion of sound waves, and can be simplified in the production of the model. However, fluctuations equal to or greater than the wavelength of the sound wave of 2000 Hz in the actual hall must be retained and cannot be omitted. Because these parts have a greater impact on the unevenness of the sound field.
It is necessary to make the sound absorption coefficient of each part of the inner surface of the hall sound quality model, including the auditorium, completely match the sound absorption coefficient of the corresponding inner surface of the hall and the auditorium in the measured frequency range. Difficulty, thus allowing ±10% error. The absorption of the auditorium in the general hall accounts for a large proportion of the total absorption of the inner surface of the hall (about 1/3 to 1/2), so the sound absorption simulation of this part should be as accurate as possible.
In order to avoid the background noise in the model being too high and the dynamic range is not up to the requirements and affecting the accuracy, the enclosure of the hall sound quality model must have sufficient sound insulation. Because the signal frequency of the model test is relatively high, it is not difficult to make the outer casing of the model have a certain amount of sound insulation in the high frequency band. Therefore, it is easy to achieve that the background noise level inside the model is lower than the sound level of the test signal by more than 45 dB.
The size, shape and sound absorption of the stage space have a great influence on the short-delay reflection sound distribution, reverberation time and sound pressure level distribution of the audience hall. This part should be included in the model test. The sound absorption of the stage space should also be simulated accordingly.
The sound source signal used for the measurement of the short-delay reflected sound distribution is the pulse sound generated when the capacitor is discharged, that is, the electric spark pulse sound. When the discharge voltage is 4000-6000V, the pulse width is about 0.2ms, the directivity is approximately spherical, and the sound power is large enough, which is suitable for the pulse sound source signal in the model test. The center of the sound source is defined as the center of the general performance area, and the height is equivalent to the height of the population. The sound source position and height measured by the sound field unevenness are the same as the reverberation time measurement. Welcome to Home Theater Network
A commonly used method for short-delay reflection sound distribution measurement is to amplify the received direct sound and reflected sound signals on the oscilloscope with time as the horizontal axis, that is, the impulse response sound map (echo map). The map can be recorded by a digital device, and the pulse signal is amplified, stored in a computer by analog/digital conversion, and then output to a plotter to depict an impulse response map. In order to ensure that the graphics drawn by the computer have sufficient accuracy, the resolution (word length) of the A/D converter should be no less than 16 bits, and the sampling frequency is not less than 100 KHz.
For the microphone for reception, a condenser microphone or a spherical piezoelectric crystal microphone with high sensitivity can be used. The diameter of the microphone should not be too large to prevent the cylindrical shape of the microphone from affecting the sound field at the receiving position. The measurement of the temperature and relative humidity of the air within the model is required to correct for deviations from the actual hall reverberation time due to excessive air absorption in the model due to high frequency sound.
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At the central main building of Tsinghua University 104
Review of the early determination of the sound quality model in October 2004
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