EP0085194A1

EP0085194A1 - Electro-acoustical converter

Info

Publication number: EP0085194A1
Application number: EP82200060A
Authority: EP
Inventors: Hugo R. Michiels
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-01-19
Filing date: 1982-01-19
Publication date: 1983-08-10
Also published as: JPS58130699A

Abstract

Electro-acoustical converter with a closed vibration space, one wall of which is formed by a vibrating plate consisting of a metal diaphragm, clamped along its circumferential edge, on which is fixed a disk made of a piezo-electric material, characterized in that the vibrating plate (TP) is acoustically coupled with an acoustical filter (AF) which is formed by at least two successive chambers (K₁, K₂), which are communicating the one with the other through at least one aperture (O₁) in an intermediate wall (TW), the first (K,) of these chambers being separated from the vibration space by this vibrating plate and the second (K₂) of these chambers communicating with the ambient air through at least one second aperture (0₂,0₃) in an outer wall (BW).

Description

The present invention relates to an electro-acoustical converter with a closed vibration space, one wall.of which is formed by a vibrating plate consisting of a metallic diaphragm clamped at its peripheral edge, on which diaphragm is fixed a disk made of a piezo-electric material.
Such an electro-acoustical converter is well known in engineering and the invention has in view to provide a suchlike converter with a nearly flat sound intensity/frequency characteristic in the range of the lower frequencies from about 800 Hz to about 4000 Hz.
According to the invention, this objective is attained through the fact that this vibrating plate is acoustically coupled with an acoustical filter which is formed by at least two successive chambers that are communicating via at least one first aperture in an intermediate wall, the first of these chambers of this vibration space being separated by this vibrating plate and the second of these chambers communicating with the ambient air via at least one second aperture in an outside wall.
The acoustical filter ameliorates the acoustical adaption between the vibration space and ambient air, whereby the aforesaid lower frequency range is reproduced more strongly and about uniformly.
At the same time, the reproduction of the higher frequency range is weakened.
The invention will be described hereinafter, reference being made to the attached drawings, wherein :

figure 1 shows a schematical longitudinal section of an electro-acoustical converter according to the invention, the relative dimensions of the components and distances between these components, however, not having their real values;
figure 2 shows an equivalent electric diagram of the acoustical filter AF of figure 1.

This converter comprises a cylindrical housing B with a diameter D and length L and provided with a vibration space TR with the length L₁ and with an acoustical filter AF, acoustically coupled therewith, with the length L₂ + L₃. This filter comprises two successive chambers K₁ and K₂ with the respective lengths L₂ and L₃. The ratios
,
and
are respectively comprised between 4 and 10; 10 and 60; and 9 and 30.
The vibration chamber TR is completely closed and separated from the filter chamber K₁ by a vibrating plate TP. This vibrating plate TP consists of a circular metal diaphragm M which at its circumferential edge is fixed to the housing B and which, in its central zone, is firmly assembled, for instance by means of glue, with a circular disk S which is made of a piezo-electric material, for instance piezo-electric ceramics. The disk S is connected with two electric connecting cables E₁ and E₂. The metal of this diaphragm has a modulus of elasticity comprised between 6,5.₁0³ N/mm² and ₂₁₀.₁0³ N/mm² and its density is comprised between 1,5.10 kg/m and 10.10³ kg_/m³. The thickness of this diaphragm M is comprised between 0,5 and 1,2 times the thickness of the disk S and the diameter of this disk is comprised between 0,3 and 0,9 times the diameter of the diaphragm M.
The filter chamber K₁ with a volume V₁ communicates with the filter chamber K₂ with a volume V₂ via a circular aperture O₁ with a diameter D₁ in a intermediate wall TW with a tickness d_l, the product d₁.D₁ being comprised between 2 mm2 and ₁2 mm2.
The filter chamber K₂ communicates with the ambient air via a multiplicity of circular apertures as are 0₂ and 0₃, in the outer wall BW with a tickness d_l. These apertures have respectively diameters D₂ and D₃ and the product of the sum of the diameters of all the apertures, that is to say D₂ ⁺ D₃ + ..., and the thickness d₂ is comprised between 10 mm² and 20 mm2. The intermediate wall TW and outer wall BW are both made of a vibration damping plastic material, for instance polyamide, in order that these walls should not form parasi- tary sources of vibration.
Due to the presence of the closed vibration space, the sound intensity/frequency characteristic is being ameliorated. As a matter of fact, due to this, the own frequency of the mechanical system is being heightened. The frequency range extending between the first and second resonance frequencies of the vibrating plate TP is, however, being reproduced too weakly.
Through the application of the acoustical filter AF, this drawback is being suppressed, because this filter extending within the frequency band that extends between the aforesaid first and second frequencies has an impedance-transforming action, whereby the adaption between the vibrating plate and air becomes ameliorated. Furthermore, this filter acts as a low pass filter, the tipping over frequency being chosen so that the higher frequency band is being strongly weakened. This is a consequence of the values chosen of the aforesaid ratios
,
and
, thicknesses d₁ and d₂ and products D₁.d₁ and (D₂+D₃+...).d₂.
The equivalent electric diagram of the acoustical filter is shown in figure 2 and comprises :

- the capacity CK₁ and self-induction LK₁ due to the chamber K₁:
- the capacity CK₂, self-induction LK₂ and resistance RK of the apertures as are 0₂ and O₃ in the wall of BW;
- the radiation resistance Z. the values of CK₁, CK₂, LK₁, LK₂ and RK are given by the following formulas, if one supposes that the outside wall BW is provided with n apertures with a radius a₂ and the intermediate wall is provided with one aperture with a radius a₁. The radius a₁ of the aperture O₁ in the intermediate wall TW is chosen so great that the resistance of this aperture may be so low as to be neglected.
- ρ_o = density of the air
- y = 1,4 for air
- ρ_o = static pressure
- ω = 2πf with f = frequency
= cinematic viscosity coefficient of air = ₁,₅₆ x 10^-5 m²/sec (20° - 0,76 mhg)
a₁= radius aperture in TW
a₂= radius aperture in BW
V₁= volume of chamber K₁
V₂= volume of chamber K₂
d₁= thickness of wall TW
d₂₌ thickness of wall _BW
_RK = resistance of holes in BW (0₂- 0₃ etc.) η = number of holes in BW.

Claims

1.- Electro-acoustical converter with a closed vibration space, one wall of which is formed by a vibrating plate consisting of a metallic diaphragm clamped along its circumferential edge, on which is fixed a disk of piezo-electric material, characterized in that this vibration plate (TP) is acoustically coupled with an acoustical filter (AF), which is formed by at least two successive chambers (K₁, K₂), which are communicating by at least one first aperture (0₁) in an intermediate wall (TW), the first (K₁) of these chambers being separated from the vibration space.(TR) by this vibrating plate (TP) and the second (K₂) of these chambers communicating with the ambient air through at least one second aperture (0₂, 0₃) in an other wall.

2.- Electro-acoustical converter according to claim 1, characterized in that this intermediate wall is provided with a first aperture (0₁), whilst this outer wall (BW) is provided with a multiplicity of second apertures (0₂, 0₃).

3.- Electro-acoustical converter according to claim 1 or 2, characterized in that the disk (B) is made of a piezo-electric material and the diaphragm (M) and disk (S) are circular, the thickness of this diaphragm (M) being comprised between 0,5 and 1,2 times the thickness of the disk (S) and the diameter of the disk (S) being comprised between 0,3 and 0,9 times the diameter (D) of the diaphragm.

4.- Electro-acoustical converter according to claim 1, 2 or 3, characterized in that the metal of the diaphragm (M) has a modulus of elasticity comprised between 65.10 3 N/mm2 and 2₁0.10³ N/mm², whilst the density is comprised between 1,5.10³ kg/m³ and ₁₀.₁₀ ³ _kg_/m³ _.

5.- Electro-acoustical converter according to one of the preceding claims, characterized in that the vibration space (TR) is cylindrical and has a ratio diameter/length (

) that is comprised between 4 and 10.

6.- Electro-acoustical converter according to one of the preceding claims, characterized in that the acoustical filter is cylindrical, the ratios diameter/length (

,

of the first chamber (K₁) and second chamber (K₂) being respectively comprised between 10 and 60 and 9 and 30.

7.- Electro-acoustical converter according to claim 2,characterized in that the first aperture (0₁) in this intermediate wall (TW) is circular, the product of the diameter (D₁) of this aperture and thickness (d1) of this intermediate wall being comprised between 2 mm and ₁2 _mm ².

8.- Electro-acoustical converter according to claim 2, characterized in that the second apertures (0₂, 0₃) in this outer wall (BW) are circular, the product of the sum of the diame- te_rs (_D2, _D3) of these apertures and thickness (d₂) of this outer wall (BW) being comprised between ₁0 mm² and ₂0 mm2.

9.- Electro-acoustical converter according to one of the preceding claims, characterized in that the intermediate wall (TW) and outer wall (BW) are made of a plastic material, as is polyamide, which has vibration damping properties.