Properties Research Of Small Reverberation Room Based On Statistical Energy Analysis Method

The sound absorption coefficient is an important acoustical property of a kind of material, and this attribution indicates significance in engineering industry. The measurement of absorption coefficient usually adopts the standard reverberation room methods to the corresponding ISO standards and national GBJ47-83 standards. Reverberation room methodologies requires that the material should be made from 10 to 12 square meters test pieces. Additionally some small materials can also measured by standing wave tube for its absorption coefficient.However in many fields, especially in the automotive industry, some tested materials, such as automotive interiors, seats, dashboard, carpet, cannot be made for the requirements of the standard chamber test piece 10-12 square meters, or the test standard of standing wave tube, cut into small specimens, so the test piece must be measured for absorption coefficient as a whole. This needs us to develop a way to meet this measurement, so in the 80s of last century a new way has been developed to make this kind of measurement, and the small reverberation chamber is called Alpha Cabin. It is based on the standard measurement method for small chamber measuring instruments. However, a small reverberation chamber on related technical parameters and there is no uniform ISO or national standards. So, if we study this small chamber to meet the relevant requirements of diffuse sound field, the full spreadness of the sound field, and the uniform of energy density, this small reverberation chamber can be made as a measurement tool to determine the absorption coefficient of the material.In this paper due to the study for enclosed space of the sound field characteristics, I deviate the sound field pressure expression and the normal vibration frequency formula. Then I study a number of the normal vibration modes and frequency and volume, total surface area and total edge length relationship, and do the relevant fitting analysis. In the analysis we know that the higher the frequency, the sound field consists of the more normal modes, so much the normal mode of a standing wave where the wave section, to another may happen to be standing wave antinodes. This superposition of a large number of the normal mode of the standing wave sound field can effect the "average", and tend to make uniform the sound field enclosed space. In addition, if placed in the sound source sound field is not given but with a certain single-frequency wave acoustic bandwidth, such as 1/3 octave pink noise, the higher the center frequency of the sound source may be stirred in a large number of sound field The normal wave, which makes the enclosed space sound field is approximately diffuse sound field, so in this sense, we usually refer to the diffuse sound field can be regarded as enclosed space standing wave high-frequency approximation. But We found that if the enclosed space of the geometric shape is very symmetrical, and even simple integer ratio, the internal sound field will be serious degeneracy, which also is a lot of the normal mode corresponding to the same frequency, so that we by the formula obtained by normal mode actual results will differ materially. Especially in the low frequency, probably not in a frequency range of the normal frequency, while in another frequency range of more memory in the normal frequency, making the uneven distribution of the normal frequency. So we designed a small chamber to the surface as possible when not parallel to each other, not equal edge length, the length and breadth was the best harmonic series. In addition to the acoustic field in a closed space installed diffusers to improve the sound field diffusion, and for high-frequency pure tone or narrowband noise tests, the proliferation of the body also has a role. Diffusers in the choice of a fixed component in the spread of the wall, suspended and rotating diffuser diffuser, etc., for our small study size limited the reverberation chamber, and the smaller sound field, we generally use a fixed spread on the wall components. Finally, we studied theoretically sound source in the sound field center only way to stimulate the standing wave sound field angle for position in the one-eighth. This means that the angle position of the sound source placed elsewhere than on the sound field of standing waves stirred up more, rather more standing wave field tend to make a sound more evenly. So on the angle on the sound source will help to produce diffuse sound field, especially in the low frequency is more to be the case. So by the previous way we study for the small chamber, we will make a certain design concept of the small reverberation chamber.Then we introduce the statistical energy method for small chamber study further. I made a simulation model of the system and establish a good system in various subsystems. Then I defined the model requires a variety of materials related to physical properties. And the modal density of the system, the loss factor, coupling loss factor has made a theoretical analysis, combined with our model calculation shows the relevant results. After the introduction of sound source excitation model we analyze the various parameters related to, and analyzes the model number of tunes and tune the modal internal energy, sound pressure level, etc., and divided the cavity into 8 small way by adding the sound source, sound source of energy on the cavity. By comparison we found that a single sound source, adding the energy source of the whisper chamber above the source of the whispers not to join the chamber, and the farther away the tune of energy, sound pressure level are smaller, which is not conducive to the generation and diffuse sound field measured absorption coefficient. When we add more sound source (usually three), and whispered in the cavity energy and sound pressure level has been significantly improved, at low frequencies, non-cavity sound pressure level is lift by an average of about 5dB. This makes a better spreadness of sound field of a small reverberation chamber, and of course will lead to a practical accurate measurement.Finally, we analyze the sound insulation capability of small chamber walls. I adopted theoretical analysis, got the mass law of the single wall -- when the material is constant, sound insulation wall thickness and the frequency with the increase of Thickness or frequency for each doubling of capacity to improve sound insulation 6dB. However, environmental requirements for the higher acoustic acoustic instruments, such as our small chamber, by increasing the wall thickness to achieve higher sound transmission loss is obviously not the way the economy is also very heavy, so we introduce the double-wall insulation Problems, the analysis we found that when we use double-wall insulation, you can greatly improve the small reverberation chamber and off the acoustic sound environment, and allows us to design a small chamber within the sound field will not receive significant outside interference . Ensure the accuracy of measurement. Same article also noted that the existing theory of the double wall insulation there is a large theoretical error, making the application and the actual access to them is very large. Based on the analysis found that for double-wall, we should consider its sound insulation between the double walls connected together and double the amount of noise when the single-wall and wall of sound insulation between independent. When the air layer thickness is small, it should be seen as connected to a small double-wall spring. Because of its existence, the ratio of double-wall insulation is much lower than the separate one, but still larger than the case that they are connected together.From the above argument and simulation, we have got a clear mind and an intergate method to constuct an Alpha Cabin, and apply it into the practical engineering...