EP0688142A2 - Acoustic resistance, especially for electroacoustical transducer - Google Patents

Abstract

The acoustic resistance uses a pair of relatively spaced aperture plates (5,7) provided with a high number of apertures (8,10), which are relatively offset from one another, so that the sound waves pass through them via the intermediate gap (9) between the plates. The inside surfaces of the aperture plates have raised projections (6) bridging the intermediate gap between the plates. The height of the raised projections is such that the frequency-independent component of the acoustic resistance is greater than the frequency-dependent component of the acoustic resistance in the working frequency range. <IMAGE>

Description

The invention relates to an acoustic resistor, in particular for electroacoustic transducers, consisting of two or more perforated plates spaced apart from one another and provided with a large number of through holes, so that the sound passes through the holes and through the cavities between the perforated plates.

Electroacoustic transducers, regardless of whether they are designed as sound transmitters or sound receivers, require acoustic frictional resistances, be it to attenuate a peak in the frequency response or to produce a phase-shifting link, to form a directional characteristic or the like.

Known materials for acoustic resistors are felt, fiber fleece, filter paper, open-cell foam, metal or textile fabric, and finely perforated metal or plastic films are also used. Many of these materials have the disadvantage that they were not designed for acoustic purposes and therefore often have very large tolerances in their acoustic values. This applies in particular to felts and non-woven fabrics. Metal mesh and perforated metal or plastic films, in turn, are generally very expensive.

It has therefore been tried for a long time to realize acoustic friction in the injection molding process from plastic, since this is a simple and inexpensive manufacturing method. DE-AS 2 206 093 has already described an annular slot in a plastic part which dampens the membrane movement of a telephone converter. Disk-shaped slots of 0.01-0.025 mm have been proposed in DE-PS 1 041 082. An adjustable plastic injection mold was proposed in AT-PS 337 793, with the aid of which plates or transducer overmouldings with a large number of holes can be produced. These holes then give the necessary acoustic friction.

AT-PS 378 306 also describes the production of hole friction using a laser. In GB-PS 670 868 an acoustic filter is proposed which consists of two or more perforated disks which can be rotated against one another and changed at a distance from one another. With this arrangement, the filter characteristic can be changed by rotating the two perforated plates against one another and by changing the distance between the plates.

In AT-PS 378 306 "Acoustical Engineering", van Nostrand Comp. (1957) quotes that each acoustic resistance formed by holes or gaps consists of a frequency-independent and a frequency-dependent component. For a cylindrical hole, the formula specifies that the frequency-independent component is inversely proportional to the 4th power, but the frequency-dependent component is inversely proportional to the 2nd power of the hole radius.

A reduction in the hole diameter will therefore lead to an increase in the overall acoustic resistance, but the frequency-independent portion will increase disproportionately.

To influence the frequency response of an electroacoustic transducer, attachments which are designed in the form of a Helmholtz resonator are very often used. These resonators consist of at least one air volume and at least one air mass and have the general purpose of improving the frequency response of the transducer, as is the case is described in DE-A 36 30 951. However, undesired resonances of the transducer element can also be shifted or cut off. A special design of the space in front of the vibrating element is described in DE-A 30 07 773. A suitable choice of the sound openings reduces undesired elevations. Where transducers have a clear sensitivity peak in a frequency range that should not be transmitted at all, these prefixes are designed as a low-pass filter, as is described, for example, in DE-A 28 31 401 and DE-A 31 07 344. Very often, frequency-independent attenuations, for example in the form of porous foams, are also inserted into these attachments. Arrangements of this type are protected in DE-A 32 07 695 and DE-A 30 07 808. DE-A 28 03 885 describes a protective cap with no special acoustic effect except that it holds a damping disc in position.

All of these prefixes are always devices which, according to the invention, have a non-flat frequency response. Additional parts then often have to be used for uniform attenuation over a larger frequency range.

Very often, however, frequency-independent resistors are desired in acoustics. The invention therefore has set itself the goal of providing an acoustic resistor which is frequency-independent over a wide range and can be produced cheaply.

In the simplest case, the acoustic resistance is realized according to the invention by two perforated plates lying on top of one another, which are provided with elevations, so that a cavity is formed between them. The perforated plates must be placed on top of each other in such a way that their holes do not overlap.

If this resistance is now sonicated from one side, the sound passes through the holes of the first perforated plate, then continues between the plates and emerges again at the holes of the second perforated plate.

µ

Viskositätskoeffizient der Luft, 1,86.10⁻⁵ [kg/m.sec]

w

Tiefe des Spaltes [m]

l

Länge des Spaltquerschnitts [m]

d

Dicke des Spaltquerschnitts [m]

g

Dichte der Luft, 1,2 [kg/m³]

f

Frequenz der Schallwelle, Hz [s⁻¹]

The height of the elevations is chosen so low that in the frequency range of interest the frequency-independent portion of the acoustic resistance is significantly higher than the frequency-dependent portion. The height of the elevations can be estimated from the formula for the acoustic resistance of a crevice friction, according to Olson, "Acoustical Engineering", Van Nostrand Comp, New York, (1957): $$\text{Z = (12 · µ · w) / (d³ · l) + i · (12 · g · w · π · f) / (5 · l · d)}$$

µ

Air viscosity coefficient, 1,86.106 [kg / m.sec]

w

Depth of the gap [m]

l

Length of the gap cross section [m]

d

Thickness of the gap cross section [m]

G

Density of air, 1.2 [kg / m³]

f

Frequency of the sound wave, Hz [s⁻¹]

Assuming that the frequency-independent part of the friction should always be at least 5 times larger than the frequency-dependent part, this results in e.g. for the hearing frequency range of 20 - 20000 Hz a maximum thickness of the gap of 16 µm. If you restrict yourself to the speech frequency range up to 4000 Hz, a gap thickness of 35 µm is sufficient under the same conditions.

As can be seen from the formula, the value of the acoustic resistance increases as the gap thickness is reduced. If very thin gaps have to be used, particularly high values of acoustic friction can then occur.

In order to prevent the acoustic resistance from becoming too high, the number of entry and exit holes is selected to be very large, as a result of which many parallel sound paths are formed. Their individual resistances add up to the total resistance according to the known rules of resistance addition. The more entry holes there are, the lower the value of the acoustic resistance according to the invention will be.

Together with the diameter of the entry and exit holes and the height of the elevations, the number of effective entry and exit holes gives an easy way to determine the resistance range.

In order to achieve a good statistical distribution of the individual resistances, the holes and the spacing elevations should be evenly distributed over the entire perforated plate.

The holes can preferably be arranged in such a way that there is a significantly shorter path from each entry hole to a single exit hole than to the other holes in the following plate. As a result, most of the incoming sound runs along this path.

By moving the perforated plates against each other, the acoustic resistance can be changed in a first approximation proportional to the change in path between the entry and exit holes.

The surveys themselves can be implemented in different ways. In the case of plastic parts, they can already be contained in the mold.

Another way of realizing the distance between the perforated plates is to involve those involved To provide surfaces with a defined roughness, as can be done, for example, by sandblasting. With plastic parts, the desired surface roughness can be created directly during spraying. In this case, particularly good results can be expected, since all parts look the same, and because of the statistical distribution of the elevations, the acoustic resistances will show very little scatter.

According to DIN 4766, the surface roughness is divided into classes N1 ... N12. By specifying this roughness class, the effective distance between the perforated plates can be stated in a technically reproducible manner, since the mean roughness value R _a roughly corresponds to the effective distance between the perforated plates.

• Fig. 1 zeigt einen elektroakustischen Wandler mit einem erfindungsgemäßen akustischen Widerstand, in
• Fig. 2 ist dieser ausschnittsweise im Querschnitt dargestellt,
• Fig. 3 zeigt ein Ausführungsbeispiel mit definierter Oberflächenrauhigkeit, die
• Fig. 4 zeigt eine Ausführung mit kreisrunden Lochplatten, die
• Fig. 5 die gleiche Anordnung, jedoch mit einer Verdrehung der Lochplatten um 22,5°, die
• Fig. 6 zeigt die Abhängigkeit des Widerstandswertes vom Drehwinkel, die
• Fig. 7 und 8 zeigen Ausführungen mit drei Lochplatten mit verschieden langen Schallwegen und die
• Fig. 9 zeigt ein Beispiel, bei dem eine Lochplatte in das Wandlergehäuse integriert ist.

The invention is explained in more detail in the following description of exemplary embodiments with reference to the drawing. The

• 1 shows an electroacoustic transducer with an acoustic resistance according to the invention, in
• 2 this is shown in detail in cross section,
• Fig. 3 shows an embodiment with a defined surface roughness, the
• Fig. 4 shows an embodiment with circular perforated plates, the
• Fig. 5 shows the same arrangement, but with a rotation of the perforated plates by 22.5 °, the
• Fig. 6 shows the dependence of the resistance value on the angle of rotation, the
• 7 and 8 show versions with three perforated plates with sound paths of different lengths and the
• Fig. 9 shows an example in which a perforated plate is integrated in the converter housing.

1 schematically shows a sound transducer consisting of membrane 1, electrode 2, housing 3 and the invention acoustic resistance 4 shown. The movement of the membrane 1 is damped by the acoustic resistance 4 according to the invention.

A section of a simple embodiment of this resistor is shown in FIG. 2. The perforated plate 5 facing the sound is provided with elevations 6, on which the second perforated plate 7 lies in contact. The sound enters through holes 8 on the perforated plate 5 facing the sound. The sound will propagate through the cavity 9 between the first and second perforated plates in the direction of the nearest hole 10 of the second perforated plate and leave the second perforated plate 7 through this hole. A sound path is indicated by arrow 11.

The acoustic resistance of an individual sound path is the sum of the individual resistances of the following three acoustic elements arranged in series: entrance hole 8, cavity 9 between entrance and exit hole and exit hole 10.

An embodiment with two perforated plates 5, 7, in which their spacing takes place by means of a defined surface roughness, is shown schematically in FIG. 3.

The use of a large number of through holes in the perforated plates is a further feature of the invention. Fig. 4 shows an embodiment with circular perforated plates in plan view. Holes 8 of the upper perforated plate are marked black, holes 10 of the lower perforated plate are marked white.

An acoustic resistance can of course also be realized by several perforated plates, the through holes of each plate being arranged offset in relation to the previous plate. Fig. 7 shows a Embodiment with three perforated plates 12, 13, 14. The sound path is shown schematically by an arrow 15.

The change in the resistance value can be achieved in all embodiments by changing the sound path between the entrance hole and the exit hole. This is best done by moving one or more perforated plates against each other.

In the case of circular plates, this can advantageously be done by twisting, as shown in FIG. 5. Fig. 6 shows the value of the acoustic resistance as a function of the angle of rotation.

In the case of plates of a different shape, a displacement is conceivable in an analogous manner. In a multi-plate system, one or more perforated plates can be moved against each other in order to change the resistance value. Using the example of the three-plate system of FIG. 5, FIG. 8 shows how the sound path changes when the middle perforated plate is moved.

Fig. 9 shows a further embodiment in which a perforated plate is designed as part of the housing 16. The subsequent perforated plate 17 can then e.g. by snapping, gluing or by ultrasonic welding can be easily assembled in a known manner.

The proposed measures make it easy to set the resistance according to the invention to a desired value. This is particularly necessary in the case of high-quality converters, where tight tolerances are required, but cannot be achieved using conventional materials.

Claims (6)

Acoustic resistance, preferably for electroacoustic transducers, consisting of two or more perforated plates (5, 7), spaced apart from one another and provided with a large number of through holes, so that the sound passes through the holes (8) and through the between the perforated plates (5, 7) lying cavities (9) passes through, characterized in that the perforated plates (5, 7) are provided with elevations (6) on which they lie in contact with one another, the height of the elevations (6) being chosen so low that in frequency range of interest, the frequency-independent portion of the acoustic resistance is significantly higher than the frequency-dependent portion (Fig. 2). Acoustic resistance according to claim 1, characterized in that the perforated plates (5, 7) instead of elevations (6) have a defined surface roughness (Fig. 3). Acoustic resistance according to claim 1, characterized in that the number of contacting elevations (6) of adjacent perforated plates (5, 7) is at least as large as the number of holes (8, 10) in a plate. Acoustic resistance according to one or more of claims 1 to 3, characterized in that the holes are arranged on the perforated plates, preferably evenly distributed. Acoustic resistance according to one or more of claims 1 to 4, characterized in that the perforated plates are made of plastic, at least one perforated plate being an integral part of the housing (16) of the electroacoustic transducer (Fig. 9). Acoustic resistance according to Claim 1, characterized in that the resistance value can be changed by mutually displacing one or more perforated plates, or, in the case of circular perforated plates, the resistance value can be changed by mutually rotating the perforated plates (FIGS. 4 to 8).

Images