As mentioned earlier, geometric acoustics and statistical acoustics seem to be powerless in the analysis of acoustic staining. In this case, I have to resort to the theory of volatility.
The theory of wave motion in closed spaces tells us that for small rooms, especially small rooms with regular shapes, the degeneracy of the normal frequency at low frequencies is difficult to avoid. The result of the simplified frequency degeneracy will greatly enhance the corresponding simplification. If we think of the room's normal way as a collection of many resonators, some of the stimuli are violently fired, and the effect is like some resonators being fired at the same time, and these resonators are at the same frequency. Resonance is generated, thus greatly enhancing the sound of this frequency. In order to obtain good frequency transmission characteristics, the normal frequency distribution of the room is required to be as uniform as possible, and the normal frequency cylinder is avoided. If the auditory characteristics of the human ear are taken into account, the interval of the normal frequency distribution should be greater than 2 Hz and less than 20 Hz. Less than 2 Hz, the hearing is difficult to distinguish; the effect is equivalent to the corresponding normal frequency degeneracy; greater than 20 Hz, because there is no corresponding frequency in the corresponding frequency band corresponding to it, the sound source is unlikely to stimulate the corresponding frequency Simple way. The presence of such conditions is detrimental to the uniform transmission of the frequencies of the sound source.
According to the theory of closed space fluctuations, in order to make the normal frequency distribution uniform, two basic conditions must be met: first, the volume of the room should be large enough; second, the size of the room should be irregular or have appropriate length, width and height. The ratio between the two. The first condition cannot be satisfied. Even if the size of the small room is designed to be irregular, it is very difficult to achieve a normal frequency interval of 2-20 Hz.Studies have shown that an effective and simple way to eliminate acoustic staining is to increase the average sound absorption coefficient of the room and reduce the energy of the corresponding mode of the dyeing frequency. In general, when the average sound absorption coefficient of the room is greater than 0.3, the sound staining phenomenon in the small room no longer exists. In fact, this is to limit the low frequency reverberation time. This result has been confirmed by the BBC and the Japan Broadcasting Association. In recent years, the results of the application of film recording workers in China have successfully solved the problem of acoustic dyeing in their own work. For example, for a room having a volume of less than 196 cubic meters, when the average sound absorption coefficient in the room is 0.3, the corresponding reverberation time is not more than 0.3 seconds. If the room is large, the reverberation time will be lengthened. For example, when the volume of the room is larger than 500 cubic meters, the reverberation time can exceed 0.5 seconds. If a low frequency hum is present, after finding the frequency at which the acoustic stimuli are generated, it can be eliminated by the resonant sound absorbing structure. This is not difficult to understand. In the previous discussion, since the indoor boundary surface is assumed to be rigid, the simple vibration mode corresponding to a certain normal frequency is not only narrow but also strong, which is equivalent to the maximum value of Q in a simple oscillation circuit. Happening. Increasing the damping, that is, increasing the absorption of the room, widens the width of the formant and weakens the height of the peaks, which makes the sound modes with a certain width overlap each other to cover the entire frequency range. This is the basic basis for eliminating acoustic staining by increasing the room sound absorption.
As for the influence of the position of the sound source and the microphone on the occurrence of acoustic dyeing, it should be considered from the degree of vibration excitation and its sound pressure distribution, and they are not likely to change the original characteristics of the room. In theory, the state in which the stimuli mode is excited is related to the position of the excitation source. In the low frequency band, the influence of the sound source position is correspondingly increased due to the small number of simple vibration modes. In general, when the sound source is in the sound pressure belly point of the simple mode, the normal mode is easily excited. On the contrary, if the sound source is placed in the sound pressure node of the simple mode, it is more difficult to excite. This is the reason why the sound source at the "corner" has the best excitation condition. The so-called "wall corner" refers to the range of 1/4 wavelength from the intersection of the three boundary surfaces in the room. In the same way, when acoustic dyeing occurs, the receiving point should avoid the abdominal point of the simple vibration mode corresponding to the dyeing frequency. If the sound source is placed on a "wall" within a quarter of the wavelength of the highest frequency considered, the full mode of vibration considered can be strongly excited. In fact, this is not possible with actual recording. A well-proven position is 1/3 of the diagonal of the rectangular plane; if acoustic dyeing occurs, the position of the sound source and the receiving point is appropriately changed, that is, the excitation state of the room vibration mode or the simple state is changed. The sound pressure value of the vibration mode is expected to reduce the intensity of the dyeing frequency.
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