Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Having electrostatic element
Patent
1998-03-19
2000-06-13
Kuntz, Curtis A.
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Having electrostatic element
381398, 181171, H04R 2500
Patent
active
060758678
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a micromechanical microphone with a housing in which a transducer element is placed, and which has a sound inlet on one side of the transducer element and a pressure compensation hole on the other side. Primarily the pressure compensation hole has a high acoustic impedance at audio frequencies, and is placed in a, in other respects, closed rear chamber.
The transducer element normally consists of a membrane which deflects due to the sound pressure, and an arrangement to convert this deflection into an electrical signal.
Commonly known microphones of small dimensions, as of the magnitude 3.5 mm.times.3.5.times.2 mm, for example for use in hearing aids, are traditionally manufactured by assembling a number of individual parts, such as plastic foils, metal parts, hybrid pre-amplifiers etc., in total 12-15 parts.
In the past, many different prototype microphones have been fabricated using micromechanics which is a technology based on advanced silicon integrated circuit manufacturing concepts but used for the fabrication of mechanical components. The advantage of this technology is that microphones with improved characteristics can be obtained and it is possible to realise batch fabrication where hundreds of thousands of devices are processed at the same time implying that production cost can be reduced significantly.
Up till now the micromechanical microphones have not been able to fulfil the demands for use in hearing aids, especially because they have been far too sensitive to humidity, dust and dirt which partly or totally has been able to damage the performance of the microphone. Of prior art within the area, which can be used to overcome some of the above mentioned disadvantages, is disclosed in U.S. Pat. No. 2,086,107 which describe a condenser microphone of conventional (i.e. not micromechanical) design, where the transducer element on the sound inlet side is sealed by the microphone membrane itself, and where the back side is closed by a rubber membrane which can expand and contract by changes in the barometric pressure as a chamber on the outer side of the rubber membrane is connected with the environment via a pressure compensation hole.
This technical solution gives a sealing which is adequate for traditional microphones as the condenser microphone referred to above, but it has a number of disadvantages when used in small micromechanical microphones. This is because the membranes exhibit very large static deflections when the atmospheric pressure and/or the temperature changes.
The membranes centre deflection is for example more than twice as large as the height of the encapsulated volume multiplied by the relative pressure change and even bigger if the area of the membrane is smaller than the rear chambers sectional area. Static pressure variations of .+-.10% are not unrealistic, meaning that the membrane's static deflection can be in the range of 0,5 mm at a height of 2 mm. In a micromechanicel microphone this is unacceptable. Firstly, deflections of this magnitude consumes far too much space, meaning that the microphone gets significant bigger than necessary and desirable. Secondly, it requires a very soft membrane material to keep the membrane acoustical transparent under such large static deflections. It may not be impossible to find a material that meets these requirements, but if it should be compatible with a micromechanical production process, it limits the possibilities drastically, meaning a far more complicated production process is needed.
One solution would be to make the sealed membranes of a material with microscopic pores allowing a pressure compensation as described in U.S. Pat. No. 5,222,050 and WO 95/21512. It could e.g. be a porous PTFE-film ("Teflon"), which among others is sold under the trade name "GORE-TEX". This material does not allow water and dust particles bigger than the pores in the material to pass, while gasses diffuses freely through. This solution, however, is not appropriate as the pores will clog-up, and further, the material is di
REFERENCES:
patent: 3980838 (1976-09-01), Yakushiji et al.
patent: 4232205 (1980-11-01), Ribeyre
Bay Jesper
Bouwstra Siebe
Hansen Ole
Kuntz Curtis A.
Microtronic A/S
Ni Suhan
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