32 results about "Circular diaphragm" patented technology

An assembled diaphragm for steam turbine and a production device thereof are provided, which relate to the technical field of steam turbine manufacturing. The diaphragm includes diaphragm inner rings and diaphragm outer rings, the diaphragm inner rings and the diaphragm outer rings are semicircular, and blades are arranged between the diaphragm inner rings and the diaphragm outer rings, two the diaphragm inner rings are symmetrically connected through a connecting assembly, and two the diaphragm outer rings are symmetrically connected through multiple connecting assemblies. Two semicircular diaphragm inner rings are connected into a complete circular diaphragm inner ring through multiple connecting assemblies, and two semicircular diaphragm outer rings are connected into a complete circular diaphragm outer ring through multiple connecting assemblies, and then the blade is welded and fixed between the complete circular diaphragm inner ring and the circular diaphragm outer ring.

The invention discloses a total internal reflection wafer detection device, which comprises a camera, laser, a reflecting prism, a circular diaphragm, a support column and a refraction cone, and is characterized in that the direction of the laser entering the reflecting prism is changed to be vertically upward, and the circular diaphragm which shields a laser center part to form annular laser is arranged on a vertically upward laser light path; the annular laser is refracted by the refraction cone to enter a wafer to be detected and is totally reflected in the wafer to be detected, when internal defects and surface dirt of the wafer are encountered, the internal defects and the surface dirt are scattered out of the wafer and detected by the camera, the supporting column is located between the refraction cone and the circular diaphragm, and the wafer to be detected is arranged on the refraction cone; the camera is arranged above the wafer to be detected; the cone angle # imgabs0 # of the refraction cone enables the reflection angle # imgabs1 # refracted into the wafer to be measured to be larger than the total reflection angle. The device has the advantages that laser enters the wafer, total reflection detection is carried out from the center to the outer ring of the wafer, and compared with a method for carrying out detection from the edge of the wafer, the efficiency is higher, and the detection precision is higher.

The invention discloses an all-quartz double-film structure optical microphone prepared from a microstructure optical fiber and a preparation method thereof, and the all-quartz double-film structure optical microphone is formed by welding a first single-mode optical fiber, a waveguide diaphragm and a sensing circular diaphragm in sequence. The welding surface of the first single-mode fiber and the waveguide diaphragm is a first reflecting surface of Fabry-Perot interference; and the welding surface of the sensing circular diaphragm and the waveguide diaphragm is a second reflecting surface of the Fabry-Perot interference so as to form a Fabry-Perot interference effect. The all-quartz double-film structure optical microphone can keep stability and reliability under extremely harsh environmental conditions, and is suitable for various high-temperature tests and other high-voltage electromagnetic monitoring occasions.

The invention relates to a motor vehicle seat headrest (1), comprising two loudspeakers (5), each with a circular diaphragm (6), mounted on a support (7) and housed in two compartments (8). The support comprises two box-shaped parts (9) in which each loudspeaker is mounted. Each loudspeaker is covered by a front panel (19) having a multi-perforated area (10) extending opposite its diaphragm (6). A single cylindrical duct (11) is formed in a housing (3) opposite the multi-perforated area. The duct has a diameter of less than 35 mm and is free of filler material. A cap (4) covers the duct and has, opposite the duct, a circular air-permeable area (12) with a diameter similar to that of the duct. Figure 2

In order to provide a diaphragm rupture signalling device for a diaphragm pump, which ensures a more functionally reliable rupture signalling compared to the prior art, a diaphragm to be arranged between the drive unit and the delivery unit is disclosed, with at least a first and a second essentially circular diaphragm layer, wherein the diaphragm has a clamping area and an operating area and a sensor area arranged in the section of the clamping area in which at least one of the diaphragm layers is designed such that, when the pressure between this diaphragm layer and the adjacent diaphragm layer increases, it moves further away from the adjacent diaphragm layer in the section of the sensor area than in the other sections of the diaphragm, with an increase in the distance between the diaphragm layers, thereby triggering a diaphragm rupture signalling signal in the diaphragm pump.

The invention discloses a bidirectional extrusion circular plate diaphragm filter press with annular reinforcing ribs, and relates to the technical field of filter presses, the bidirectional extrusion circular plate diaphragm filter press comprises a mounting rack, a pressing seat, a circular plate diaphragm plate, a mounting box, a control mechanism, a transmission part, a knocking mechanism, an extrusion mechanism and a timer, a circular diaphragm plate is slidably connected into the mounting frame, a mounting box is fixedly connected to the front end of the mounting frame, empty grooves are formed in the left side and the right side of the rear end in the mounting box, a control mechanism is fixedly connected to the right rear end of the mounting box, a transmission part is arranged on the right side of the mounting frame, and knocking mechanisms are fixedly connected to the left end and the right end of the bottom in the mounting box. An extrusion mechanism is fixedly connected to the rear end in the mounting box, a timer is fixedly connected to the left end of the bottom in the mounting box, a circular diaphragm plate is arranged, the circular diaphragm plate is prevented from being locally and excessively stretched or torn through an annular reinforcing rib, and the deformation amplitude and the damage speed of the circular diaphragm plate are reduced.

The variable area flowmeter based on the H-shaped opening and the arc parting line is characterized in that the variable area flowmeter comprises a pipeline assembly, a flexible thin film assembly and a pressure tapping connector; the pipeline assembly comprises a flowmeter upstream pipeline and a flowmeter downstream pipeline. The two ends of the pipeline assembly are the air inlet end and the air outlet end respectively. A circular diaphragm is placed in an upstream pipeline of the flow meter and is mounted in a direction perpendicular to the air flow direction; gas can only flow through the throttling opening area of the diaphragm; the flexible thin film assembly comprises a circular diaphragm; the pressure tapping interface comprises a flowmeter upstream pressure tapping hole and a flowmeter downstream pressure tapping hole; a flowmeter upstream pressure tapping hole and a flowmeter downstream pressure tapping hole are formed in the upstream position and the downstream position of the pipeline assembly respectively and matched with a pressure sensor to measure the static pressure difference before and after gas passes through the circular film. The gas flow sensor is simple in structure and convenient to process, and can realize low-pressure-loss, high-sensitivity and wide-range gas flow measurement.

This invention discloses a highly sensitive detection device and method for immersion-type ultrasonic signal detection in liquids. The invention provides a circular diaphragm with concentric corrugations and a center-thinned structure, and uses this diaphragm as a reflective surface of an optical interference cavity to construct a highly sensitive detection device for immersion-type ultrasonic signal detection in liquids. The diaphragm, used to sense ultrasonic signals, has identical concentric corrugations and a center-thinned structure on both sides, enabling the diaphragm to achieve higher mechanical sensitivity without significantly reducing its natural frequency. The concentric corrugations effectively release initial residual stress introduced during processing and packaging, reducing surface warping and baseline drift. Simultaneously, a replaceable acoustic membrane is constructed in front of the diaphragm, achieving high sound transmission and anti-contamination under long-term immersion in liquid conditions. This detection device can be directly placed in the liquid for detection, improving the sensitivity of ultrasonic signal detection while maintaining a wide frequency range, preventing contamination of the detection element, and ensuring the long-term stability of the detection device.

The invention discloses an optical fiber acoustic atomic force probe structure and a preparation method, and belongs to the technical field of optical micro-nano sensor preparation. One end face of the connecting diaphragm is fixed at one end of the single-mode optical fiber, the acoustic grating array is connected with the other end face of the connecting diaphragm through a plurality of supporting columns, the connecting diaphragm, the supporting columns and the acoustic grating array form a Fabry-Perot cavity, the Fabry-Perot cavity is integrated on the end face of the optical fiber and is obtained through 3D printing, and an environment medium is arranged in the cavity; the acoustic grating array is a cone-shaped body formed by arranging a series of circular diaphragms with gradually increased radiuses and the same thickness along a central shaft at equal intervals. External sound pressure acts on the conical acoustic grating array, when the audio frequency is high, the inherent structural characteristic frequency of part of acoustic gratings is in a high-frequency range, the cavity drives the diaphragm to generate large deformation along with the acoustic gratings, ultrasonic signal detection is achieved by detecting interference light intensity changes, and the atomic force probe structure can achieve broadband high-frequency ultrasonic signal detection. The sensitivity at the resonant frequency is high, and high-frequency weak ultrasonic signals can be accurately captured.

The present invention relates to the fields of fiber optic sensing and deep learning, and particularly to an ethereal drum structure, a low-frequency acoustic signal detection device and method based on this structure. By combining the advantages of an F-P cavity formed by two fiber Bragg gratings, tapered optical fiber, and the ethereal drum structure, the simple structure, high sensitivity, and easy multiplexing of the optical F-P cavity are fully utilized. Considering the characteristics that the fiber Bragg grating only reflects specific wavelengths and the natural high sensitivity of the tapered optical fiber, and combining the narrowband filtering advantage of the circular diaphragm, circular diaphragms of different sizes are prepared, with corresponding resonant frequencies being different from each other, enabling the ethereal drum structure to detect sound waves of different frequencies. Without the need for additional Fourier transform and filtering operations, the frequency of the external sound wave can be directly obtained, realizing the intelligent and autonomous recognition of the target sound signal. It is more sensitive, compact, and efficient than traditional low-frequency acoustic signal sensing systems, and at the same time has advantages such as narrowband filtering, real-time spectrum extraction, and autonomous target recognition.

The invention discloses a circular diaphragm wall anchorage design method and system based on a wall side soil resistance space, and the method comprises the steps: taking a circular diaphragm wall anchorage foundation as a core, integrating a related structure and a surrounding soil body to form an integral calculation system, and determining boundary conditions, load types and mechanical parameters through combining investigation, test and numerical analysis results, and establishing a space stress calculation sketch. Vertical distribution of horizontal resistance coefficients is defined in a buried depth related mode, annular soil resistance distribution of front wall non-uniformity and rear wall attenuation type non-uniformity is quantified, and three-dimensional space characteristics are defined. A mud cake reduction coefficient determined according to a soil body type, an anti-arch reduction coefficient determined according to rock body characteristics and the like are fused, a soil pressure calculation formula is deduced based on a static balance principle and an elastic foundation beam theory, deformation and internal force distribution are solved, structural parameters are adjusted, and a safe and economical design scheme is completed. And technical support is provided for anchorage foundation engineering under complex geological conditions.

The utility model provides a shape memory alloy diaphragm type compartment device. The shape memory alloy diaphragm type compartment device comprises a supporting piece, a shape memory alloy diaphragm and a heat-proof layer, wherein the shape memory alloy diaphragm is installed on the supporting piece, and a heat-proof layer is arranged on the surface of the side, away from the supporting piece, of the shape memory alloy diaphragm; and the shape memory alloy diaphragm is a circular diaphragm formed by splicing a plurality of fan-shaped shape memory alloy single sheets. The utility model further provides an engine. The shape memory alloy diaphragm type compartment device is simple in structure, light in weight, high in forward pressure bearing intensity, capable of being opened in the reverse direction, reliable in sealing, good in heat insulation performance and capable of reliably achieving the intermittent working function of a solid engine.

This invention relates to a method, system, equipment, and medium for calculating the vertical bearing capacity of a hollow circular diaphragm wall. The method includes: obtaining basic parameters of the circular diaphragm wall, including the perimeter of its inner and outer sides, the wall end area, the standard value of its ultimate end resistance, and the thickness and ultimate lateral resistance standard value of each soil layer within the depth range of the circular diaphragm wall; based on these basic parameters, calculating the vertical ultimate bearing capacity of the circular diaphragm wall using the bearing capacity calculation formula: where is the vertical ultimate bearing capacity of the circular diaphragm wall, is the perimeter of the outer side of the circular diaphragm wall, is the perimeter of the inner side of the circular diaphragm wall, is the standard value of the ultimate lateral resistance of the i-th soil layer on the wall side, is the standard value of the ultimate end resistance, is the thickness of the i-th soil layer around the wall, is the wall end area, is the inner side friction coefficient, and is the wall end resistance correction coefficient. This invention can calculate the vertical bearing capacity of a circular diaphragm wall with a hollow interior, better reflecting actual working conditions.

Herein disclosed is a membrane (12) for a pressure measurement device (10) comprising an elastic circular diaphragm (120) for sealing one side of the pressure measurement device (10) so as to define a space between the interior and exterior of the pressure measuring device; wherein, the elastic circular diaphragm (120) is fixed horizontally on the surface of the chamber of the pressure measurement device (10) through a peripheral portion of the membrane (12) away from its center; and the elastic circular diaphragm (120), when there is no pressure difference between the pressure acting respectively on the inner surface (121) and on the outer surface (122), is flat. Also disclosed are a pressure measurement device and an extracorporeal blood circuit.

The invention provides a construction method for adjusting supporting force in real time aiming at a circular diaphragm wall enclosure structure, which comprises the following steps: step 1, a supporting ring is spliced in advance on a flat site, the size of the supporting ring is adjusted through a hydraulic oil cylinder, the spliced supporting ring is in a contraction state, and the radius of the supporting ring is R2; 2, a gap liner layer is laid along the circumferential side face in the circular foundation pit; 3, the spliced supporting circular rings are integrally hoisted and placed at the set depth position of the circular foundation pit, then the supporting circular rings are adjusted to enable the radius of the supporting circular rings to be increased to R1, and the supporting circular rings generate jacking force on the circular diaphragm wall; 4, in the construction process, when the radius of the supporting circular ring is reduced to R2, the supporting circular ring is moved downwards to another elevation, the hydraulic oil cylinder is stretched again to enable the radius of the supporting circular ring to be increased to R1, and jacking force is generated on the circular diaphragm wall; and 5, the step 3 and the step 4 are repeated through the multiple supporting circular rings, and finally the multiple supporting circular rings are formed in the digging depth direction of the circular foundation pit.

The invention discloses a piezoelectric MEMS loudspeaker with a clamped center, which comprises a substrate, a vibrating diaphragm, a fixed column and a rib plate, a circular through hole is formed in the middle of the substrate, and a vibrating diaphragm unit of the piezoelectric MEMS loudspeaker is composed of a circular vibrating diaphragm formed by one or more fan-shaped cantilever vibrating diaphragms. The cantilever diaphragm units are isolated by rib plates beside the air gaps and fixing columns, and all the cantilever diaphragms vibrate with the central areas connected with the fixing columns as fixed supporting ends. The beneficial effects are that the clamped end is located at the central position of the device, thereby obtaining a larger out-of-plane displacement, reducing the overall rigidity of the vibrating diaphragm, and reducing the limitation of the supporting region in the vibrating process of the vibrating diaphragm; in addition, the flexible air isolation layer is deposited on the surface of the substrate, the acoustic resistance of the MEMS loudspeaker is increased, and the sound pressure level output of the loudspeaker is effectively improved. Finally, high sound pressure level output of the loudspeaker is realized under the conditions of larger bandwidth and smaller device size.

The invention relates to a diaphragm rupture signaling device. In order to provide a diaphragm rupture signaling device for a diaphragm pump which ensures functionally more reliable rupture signaling compared to the prior art, a diaphragm arranged between a drive unit and a delivery unit is disclosed, having at least a first and a second substantially circular diaphragm layer, the diaphragm having a clamping region and a working region and a sensor region, at least one of the diaphragm layers is designed such that the distance between the diaphragm layers increases when the pressure between the diaphragm layer and an adjacent diaphragm layer increases, the diaphragm layer moving further away from the adjacent diaphragm layer in the section of the sensor region than in other sections of the diaphragm, and the diaphragm layer moving further away from the adjacent diaphragm layer when the pressure between the diaphragm layer and the adjacent diaphragm layer increases. A diaphragm rupture signaling signal in the diaphragm pump is thereby triggered.

The invention relates to a headrest (1) for a motor vehicle seat, said headrest comprising an auxiliary module having two lateral parts (7), each lying in a reference plane (P), said lateral parts being fitted with two lateral cushions (9) comprising a padding block (10) and an air-permeable covering layer (11), and a sound transmission device comprising two loudspeakers (12) mounted respectively on each of said lateral parts, said loudspeakers each having a circular diaphragm (13), the loudspeaker (12) being mounted in a housing (14) defining a rear acoustic cavity (15) and being provided with a circular opening (25) opening forward, the periphery of said diaphragm being sealed to the periphery of said opening, the housing (14) extending forward from the reference plane (P), said padding block being provided opposite said opening with a conduit (16). Figure 1b

An all-silicon carbide micro-electro-mechanical system (MEMS) acceleration-pressure integrated sensor chip includes a first patterned silicon carbide plate, a MEMS acceleration sensor chip, a second patterned silicon carbide plate, a MEMS pressure sensor chip and a third patterned silicon carbide plate fixedly connected in sequence. The MEMS acceleration sensor chip has an eight-beam and five-mass-block structure. The MEMS pressure sensor chip includes an arc-shaped cross beam and four circular diaphragms. A method for preparing the integrated sensor chip is also provided.

A loudspeaker driver or tweeter (e.g., 100, 200 or 300) having a voice coil connected with a loudspeaker diaphragm (e.g., 106, 206 or 306), having a substantially circular diaphragm portion which is connected with or rests upon a voice coil attachment segment (e.g., 104, 204, 304). The diaphragm also includes a first central elliptical (non-circular-shaped) inner roll portion (e.g., 112, 212, 312) defining a central recessed area and a second, outer elliptical (non-circular-shaped) roll portion (e.g., 108, 208, 308) so the central portion and the outer roll portion that have substantially elliptical edge circumferences with substantially circular central peripheral edges to define diaphragm segments with a varying radial Chord lengths having non-uniform diameter dimensions (inside diameter to outside diameter) and when in use, the excursion of the diaphragm is controlled such that any breakup modes are minimized and the associated resonances are minimized.

A loudspeaker driver or tweeter (e.g., 100, 200 or 300) having a voice coil connected with a loudspeaker diaphragm (e.g., 106, 206 or 306), having a substantially circular diaphragm portion which is connected with or rests upon a voice coil attachment segment (e.g., 104, 204, 304). The diaphragm also includes a first central elliptical (non-circular-shaped) inner roll portion (e.g., 112, 212, 312) defining a central recessed area and a second, outer elliptical (non-circular-shaped) roll portion (e.g., 108, 208, 308) so the central portion and the outer roll portion that have substantially elliptical edge circumferences with substantially circular central peripheral edges to define diaphragm segments with a varying radial Chord lengths having non-uniform diameter dimensions (inside diameter to outside diameter) and when in use, the excursion of the diaphragm is controlled such that any breakup modes are minimized and the associated resonances are minimized.

The utility model relates to a sensing assembly used for a pressure sensor, the sensing assembly (1) comprises a channel housing (2) which is used for introducing a fluid medium and is provided with a hollow cylindrical cavity (3), one end of the channel housing (2) is sealed by a circular diaphragm (4) which is integrally formed with the channel housing (2); the diaphragm (4) can be deformed under the action of the fluid pressure of the fluid medium, the diaphragm (4) is provided with at least one strain gauge (8) in the radial direction on the side (4b) facing away from the fluid medium, and each strain gauge (8) comprises at least two varistors (8a, 8b) extending in the radial direction. In the utility model, in order to overcome a series of process problems caused by the thinning of the diaphragm, a circular bulge (5) is arranged in the middle on one side (4a) of the diaphragm, which faces a fluid medium. In addition, the utility model relates to a pressure sensor comprising the sensing assembly.

The application discloses a kind of diagnostic system and method for measuring low wave number turbulence of magnetic confinement fusion device, including gold film plane mirror, for changing the direction of mixed light formed by main laser and scattered light coupling;Zinc sulfide lens, for changing the proportion of main laser and scattered light in mixed light to beam separation;Aluminum alloy baffle, for shielding most of the high-power main laser emitted;First circular diaphragm, for selecting low wave number range mixed light and controlling power size;Second circular diaphragm and third circular diaphragm, for shielding stray light, collimating optical path and matching mixed light into detector;Focusing lens, for focusing mixed light emitted and propagated along a particular direction;Tellurium cadmium mercury detector, for receiving plasma scattered light signal in a particular direction.The application can realize the measurement of low wave number turbulence in magnetic confinement fusion device by using special coated zinc sulfide lens to control the ratio of light reflection and transmission to separate main laser and scattered light.

The invention relates to a headrest (1) for a motor vehicle seat, said headrest comprising an auxiliary module having two lateral parts (7), each lying in a reference plane (P), said lateral parts being fitted with two lateral cushions (9) comprising a padding block (10) and an air-permeable covering layer (11), and a sound transmission device comprising two loudspeakers (12) mounted respectively on each of said lateral parts, said loudspeakers each having a circular diaphragm (13), the loudspeaker (12) being mounted in a housing (14) defining a rear acoustic cavity (15) and being provided with a circular opening (25) opening forward, the periphery of said diaphragm being sealed to the periphery of said opening, the housing (14) extending forward from the reference plane (P), said padding block being provided opposite said opening with a conduit (16). Figure 1b

In order to provide a diaphragm rupture detection device for a diaphragm pump which ensures a more reliable rupture detection than the prior art, the invention proposes a diaphragm to be arranged between the drive unit and the delivery unit, with at least a first and a second substantially circular diaphragm layer, wherein the diaphragm has a clamping region and a working region and a sensor region arranged in the section of the clamping region, in which at least one of the diaphragm layers is designed such that when the pressure between this diaphragm layer and the diaphragm layer adjoining it increases, the sensor region moves further away from the diaphragm layer adjoining it in the section of the sensor region, thereby increasing the distance between the diaphragm layers than in the other sections of the diaphragm, whereby a diaphragm rupture detection signal is triggered in the diaphragm pump.

The application discloses a diaphragm valve diaphragm fixing mechanism and a diaphragm valve. The diaphragm valve diaphragm fixing mechanism comprises a diaphragm, a first pressing part and a second pressing part. The diaphragm has a circular shape. The first pressing part and the second pressing part are arranged around the edge of the diaphragm, and the second pressing part is located on the side of the first pressing part away from the diaphragm. The circular diaphragm of the diaphragm valve diaphragm fixing mechanism is fixed through the first pressing part and the second pressing part, thereby ensuring that the diaphragm is uniformly stressed in the circumference and avoiding the problem that the diaphragm is unevenly pressed due to screwing. Moreover, the elastic pressing force provided by the second pressing part can play a damping and buffering role, thereby ensuring the sealing property of the diaphragm and being beneficial to prolonging the service life of the diaphragm.

This invention relates to a diaphragm (12) for a pressure measuring device, comprising a resilient circular diaphragm (120) for sealing one side of the pressure measuring device (10), thereby forming a gap between the interior and exterior of the pressure measuring device; wherein the resilient circular diaphragm (120) is horizontally fixed to the surface of the chamber of the pressure measuring device (10) via a peripheral portion of the diaphragm (12) away from the center; and the resilient circular diaphragm (120) is flat when there is no pressure difference between the pressures acting on the inner surface (121) and the outer surface (122), respectively. The invention also relates to a pressure measuring device and an extracorporeal circulation tubing, which are manufactured using simple and reliable technology, saving materials and manufacturing costs while maintaining accurate pressure measurement.

This invention discloses a wave-fin robot based on a ring oscillator logic pneumatic circuit, belonging to the field of soft robot technology. The robot includes an outer shell, a connecting bracket, a ring oscillator composed of an odd number of fluid valves connected end-to-end, fins that drive correspondingly to each fluid valve, and flexible wave-fin surfaces fixed to the fins. Each fluid valve adopts a rigid-soft hybrid structure, utilizing the synergistic deformation of a semi-circular diaphragm and a flexible hose to form a fluid inverter, whose output state is logically opposite to the input control state. Under pressure from a single continuous pressure fluid source, the ring oscillator automatically generates periodic oscillations with a phase difference without external electronic timing control signals. The drive connecting rods of each valve, via limiters, drive the fins to swing sequentially, thereby driving the flexible fin surfaces to generate continuous traveling waves, achieving propulsion.

The invention provides a supporting system for a circular foundation pit, comprising: a plurality of arc-shaped supporting beams which are arc-shaped section steel, the arc-shaped supporting beams forming a supporting ring, the supporting ring being located on the inner side of a circular diaphragm wall; the multiple hydraulic connector boxes are correspondingly arranged at the joints of the adjacent arc supporting beams, and each hydraulic connector box is correspondingly connected with the arc supporting beams on the two sides; the gap liner layer is arranged between the supporting circular ring and the circular diaphragm wall; and the hydraulic pump station is in communication connection with the hydraulic joint box and is communicated through a hydraulic pipeline. The supporting system for the circular foundation pit can provide stable and reliable support for the circular foundation pit, has the characteristic of intelligence, can achieve real-time remote monitoring and unattended automatic adjustment, and further has the advantages of being short in construction period and easy to disassemble.

The invention relates to a motor vehicle seat headrest (1), comprising two loudspeakers (5), each with a circular diaphragm (6), mounted on a support (7) and housed in two compartments (8). The support comprises two housings (9) in which each loudspeaker is mounted. Each housing has a front wall with a multi-perforated area (10) extending opposite the corresponding diaphragm. The block (3) extends in front of the housing. A single cylindrical duct (11) is formed in the block opposite the multi-perforated area. The duct has a diameter of less than 35 mm and is free of filler material. The cover (4) covers the duct and has, opposite the duct, a circular air-permeable area (12) with a diameter similar to that of the duct. Figure 2