RU2196206C2 - Sound-measuring echo-free acoustic chamber ( variants ) - Google Patents

Abstract

FIELD: acoustic measurements, sound wave analysis, sound-absorbing structures. SUBSTANCE: in correspondence with first variant there is proposed sound-measuring echo-free acoustic chamber that includes space limited by wall surfaces, measuring microphone and sound absorber comprising layer of glass wool, electromagnetic screen and layer of sound-absorbing wedges separated by air gap. Layer of glass wool has thickness 150.0- 900.0 mm and is pressed against walls with the aid of steel sheet or metal grid with mesh spacing between 0.05-0.5 mm, air gap has thickness of 10.0- 15.0 mm. In accordance with second variant layer of glass wool is pressed against walls by means of resonance sound absorber produced from thick cloth. Electromagnetic screen can come in the form of two layers made of metal sheet and metal grid parallel to sheet or in the form of two parallel layers made of metal grid which divide thickness of layer of glass wool in 1:2 ratio. Resonance sound absorber can include two parallel layers of thick cloth placed at some distance from wall that does not exceed thickness of layer of glass wool and which divides thickness of layer of glass wool in 1:2 ratio. EFFECT: raised absorption factor. 8 cl, 3 dwg, 1 tbl

Description

 The invention relates to acoustic measurements, analysis of sound waves, sound-absorbing structures and can find application in the design, construction, certification and operation of acoustic, damped, sound-measuring chambers.

 There is an acoustic chamber, muffled, rhythmic, designed to study and measure the acoustic characteristics of sound emitters (1).

 The chamber consists of surfaces forming a closed volume, an electromagnetic screen of a steel 3 mm sheet, and a sound absorber sequentially covering the surface. The sound absorber consists of successively deposited on all bounding surfaces of a layer of mineral glass wool 100 mm thick and through an air gap of 50-70 mm a layer of sound-absorbing wedges with a height of 500 to 1250 mm and higher, depending on the class of chamber.

 The sound source under study is located in the center of the muffled space of the camera on a special table, and the sounds emitted by it are recorded by a measuring microphone and, when they fall on internal surfaces, are absorbed by the sound absorber.

 The disadvantage of this camera is that if at high and medium frequencies the sound waves are well absorbed by the sound absorber, then at frequencies below 100 Hz the sound absorption drops sharply and some of the sound waves reflected from the bounding surfaces fall on the measuring microphone and distort the frequency response of the sound emitter under study. To combat this drawback, OST 4.275 000-79 divides acoustic, sound-measuring chambers into 5 classes, in which chambers differ in size and length of sound-absorbing wedges, which allows lowering the lower cut-off frequency of the camera from 100 Hz (4 class) to 31.5 Hz (0 "upper class).

 Another disadvantage of the camera is that sound absorbing wedges made of staple fiberglass grade 150 (specific weight 150 kg / m) are used as the main sound absorber, the sound absorption of which is low at frequencies below 100 Hz. In accordance with (1), sound damped chambers are characterized by an internal volume having the shape of a parallelepiped. The inner surfaces of the chamber are sequentially covered with an electromagnetic screen of 3 mm steel sheet, a 100 mm layer of glass wool of grade 150 made of staple fiberglass, and through a 50 mm air gap a layer of sound-absorbing wedges with a base of 200 mm x 200 mm and a height of 500 to 1250 mm, depending from the camera class. In accordance with (1), sound damped cameras are divided into 5 classes from 4 to 0 (higher).

 Their main characteristics are given in the table.

The disadvantage of the acoustic sound chambers listed in (1) is that at frequencies below 100 Hz the sound absorption coefficient of the materials that are facing the chamber walls decreases, and in order to obtain an anechoic space at frequencies below 100 Hz in accordance with (1), the volume, the inner surface and the thickness of the sound-absorbing layer are increased (see table) and, according to metrological characteristics, the damped sound-measuring chambers are divided into 5 classes in order to reduce the lower cut-off frequency from 100 to 31.5 Hz. Another disadvantage of an acoustic sound chamber is that (1) that sheet steel with a thickness of at least 3 mm is used as an electromagnetic screen, which covers internal bounding surfaces with a material density of 7.9 g / cm ³ higher than the density of all building materials used in the construction of the chamber . There is a sound-absorbing design for acoustic damped cameras (2). To improve the design, sound-absorbing wedges of the same length of 500 mm are used in the design of the chamber, and the glass wool layer changes in the design of the chamber from 150 to 900 mm and is pressed against the bounding surfaces by a layer of metal mesh or sheet steel with holes over the entire area. There is a broadband sound absorber (3), which can be used in the design and manufacture of acoustic sound level cameras. To ensure uniform sound absorption in a wide range of frequencies, including low ones, the sound absorber consists of adjacent sections having a triangle shape in cross section with a base coinciding with the face plane of the absorber, and the space between the side walls of each such section is divided by geometric partitions having the shape of parallelograms in the cross section, chambers of a set of air resonators consisting of several families of identical resonators, so that in each new family the dimensions of the chambers are half as much, and their number is twice as large as that of the previous family of larger resonators and the same chambers are always separated from each other by larger ones, with each set chamber having one of its own angles in which the cavity of the resonator is made, for example, a slotted one, faces the front plane sound absorber. The disadvantage of this sound absorber is that an air volume is used to excite an energy-consuming vibration, and in order to provide sound absorption at lower frequencies, it is necessary to increase the sound absorber volume, which is unacceptably constructive, and also increase the complexity of tuning the lower cut-off frequency. There is an anechoic chamber (4), characterized primarily by the fact that the ferrite plates used in coating the chamber walls perform the functions of both a sound absorber and an electromagnetic screen. The disadvantage of this camera is its low absorption coefficient at low frequencies.

 Of all the listed analogues (1-4), the acoustic camera described in analogue (1) is the closest in its characteristics to the proposed acoustic camera, since it allows you to use your design features to eliminate the main disadvantage - low sound absorption coefficient at low frequencies. To eliminate this drawback, resonant sound absorbers are placed behind a layer of sound-absorbing wedges in a damped sound chamber, which increase the sound absorption coefficient at low frequencies. In order for the wedge gap to work as a resonant sound absorber, it must be filled with 150 mm mineral glass wool to the entire depth of 150 mm and pressed against the inner surfaces with a dense tarpaulin fabric or metal mesh with a mesh pitch of 0.05-0.5 mm. The camera device is shown in figure 1, where a layer of glass wool 5 with a thickness of 150 mm is pressed against an electromagnetic 3-mm sheet 8, covering all the inner surfaces 6, and a layer of sound-absorbing wedges 4 of KZK-0.5 with a height of 500 mm is placed. The air gap 9 between the layer of wedges 4 and the canvas 7, covering the layer of glass wool, is reduced to 10-15 mm

 Figure 2 shows a sound absorber for all classes of chambers, wedges 4 of the same length 0.5 mm, and the wedge space of the air gap 9 and the layer of glass wool 5 make a variable thickness depending on the class of the chamber from 150 to 900 mm and a dense cloth or metal mesh 7, the glass wool is pressed against the inner surfaces of the chamber 6. The glass wool layer is pressed against the bounding surfaces by two parallel sheets of dense fabric or metal grids with a cell pitch of 0.05 to 0.5 mm and separating the glass wool layer in a 1: 2 ratio. When using two metal grids, they can serve as an electromagnetic screen. The second variant of the sound absorber is shown in Fig. 3, where the glass wool layers are pressed against the surfaces by an electromagnetic screen of a steel 3-mm sheet with holes over the entire area, placed with equal pitch, the diameter of which is calculated based on the lower cut-off frequency of the acoustic chamber.

Used Books
1. OST 4.275.008-79.

 2. Invention 2064559 “Sound-Absorbing Design for Muffled Acoustic Chambers”, cl. 6 E 04 B 1/82.

 3. Copyright certificate 446898 "Resonant sound absorber", cl. 6 G 10 K 11/00.

 4. Invention 2113040, class G 01 To 17/00.

Claims (8)

 1. The acoustic chamber is muffled, sound-measuring, having a volume limited by the surface of the walls, a measuring microphone and a sound absorber, consisting of a glass wool layer, an electromagnetic screen and through the air gap of a layer of sound-absorbing wedges, characterized in that the glass wool layer has a thickness of 150-900 mm and is pressed against steel walls a sheet or metal mesh with a cell pitch of 0.05-0.5 mm, while the air gap has a thickness of 10-15 mm  2. The acoustic muffled sound chamber according to claim 1, characterized in that the electromagnetic screen is made of two parallel layers - a metal sheet with a thickness of 3 mm, with openings over the entire area, placed with equal pitch, and parallel to it of a metal mesh with a cell pitch 0.05-0.5 mm, placed at a distance from the walls not exceeding the thickness of the glass wool layer, and dividing the thickness of the glass wool layer in a ratio of 1: 2.  3. The acoustic muffled sound chamber according to claim 1, characterized in that the electromagnetic screen consists of two parallel layers made of metal mesh with a cell pitch of 0.05-0.5 mm, placed at a distance from the walls not exceeding the thickness of the glass wool layer , and dividing the thickness of the glass wool layer in a ratio of 1: 2.  4. The acoustic drowned sound chamber according to any one of the previous paragraphs. 1-3, characterized in that the wedges have a constant length equal to 500 mm  5. The acoustic chamber is muffled, sound-measuring, having a volume limited by the wall surfaces, a measuring microphone and a sound absorber, consisting of a glass wool layer and through the air gap of a layer of sound-absorbing wedges, characterized in that the glass-wool layer has a thickness of 150-900 mm and is pressed against the walls by a resonant sound absorber made of canvas dense fabric, while the air gap has a thickness of 10-15 mm  6. The acoustic damped acoustic chamber according to claim 5, characterized in that the resonant sound absorber consists of two parallel layers of dense fabric located at a distance from the walls not exceeding the thickness of the glass wool layer and dividing the thickness of the glass wool layer in a ratio of 1: 2.  7. The acoustic silenced acoustic chamber according to claim 5 or 6, characterized in that a tarpaulin is used as the material of the resonant sound absorber.  8. The acoustic drowned sound chamber according to any one of the previous paragraphs. 6 and 7, characterized in that the wedges have a constant length equal to 500 mm

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