Electrodynamic acoustic transducer with magnetic gap sealing

Details Electrodynamic acoustic transducer with magnetic gap sealing Electrodynamic acoustic transducer with magnetic gap sealing

1997-03-18

2001-03-27

Kuntz, Curtis A. (Department: 2643)

Electrical audio signal processing systems and devices

Electro-acoustic audio transducer

Electromagnetic

C381S430000, C381S412000

Reexamination Certificate

active

06208743

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an electrodynamic acoustic transducer with a coil which projects into an air gap subject to an electromagnetic field.
BACKGROUND OF THE INVENTION
Such an electrodynamic acoustic transducer is known in many cases for example from German Patent No. P 43 29 982.2.
Such an electrodynamic acoustic transducer comprises a magnetic system and a vibration system which includes a membrane and a wire-wound coil supported by said membrane. The membrane being divided into two differently profiled portions, which have to perform different functions. Firstly, the membrane comprises an acoustically effective central portion with a convexity in the form of a cap, which is generally called a “spherical cap”. This central portion is bordered by a coil seat which holds the electrical coil of the transducer. A ring portion for elastic support extends from the connection having for instance an arc shaped profile and being generally called “bead”. If the bead extends in an outward direction, it is an electrodynamic acoustic transducer with an outward bead, if the bead is located beneath the central portion in an inward direction, it is a so called “transducer with inner bead” as described in P 43 29 982, wherein the coil seat not only forms the outer border of the central portion but also the border of the ring portion and defines the maximum outer diameter of the entire membrane.
Such a transducer with inner bead has the problem that there is an air connection between the volume which is located beneath the spherical cap—hereinafter called “rear volume”—and the area which is located in front of the spherical cap—hereinafter called “front volume”. This can result in an acoustic short circuit with the consequence that the effective sound emission is substantially reduced. The sound being projected from the front side of the spherical cap is of opposition phase to the sound being projected from the rear side of the membrane. Across the air gap the two sounds add up and therefore mutually cancel each other out.
SUMMARY OF THE INVENTION
The object of the invention is to form an acoustic transducer of the type mentioned at the beginning wherein an acoustic short circuit is prevented.
According to the invention the problem is solved by means of an electrodynamic acoustic transducer with a coil which projects into an air gap subject to an electromagnetic field, whereby the air gap is sealed by means of a viscous or solid medium, for example a ferrofluid. The subclaims give a description of advantageous modifications of the invention.
Preferably the coil forms an inner and an outer air gap when projecting into the air gap, wherein the sealing medium is located in both air gaps, preferably however only in the outer air gap. The sealing of the air gap prevents the above mentioned addition which is particularly effectively achieved in the low frequency range.


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Melillo and Raj “Ferrofluids as a Means of Controlling Woofer Design Parameters”, May 15, 1978.

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