US3459902A

US3459902A - Pressure gradient microphone with two diaphragms

Info

Publication number: US3459902A
Application number: US588875A
Authority: US
Inventors: Paul-Friedrich Warning
Original assignee: Sennheiser Electronic GmbH and Co KG
Current assignee: Sennheiser Electronic GmbH and Co KG
Priority date: 1965-10-25
Filing date: 1966-10-24
Publication date: 1969-08-05
Anticipated expiration: 1986-08-05

Description

g- 1969 PAUL-FRIEDRICH WARNING 3,

v PRESSURE GRADIENT MICROPHONE WITH TWO DIAPHRAGMS Filed Dist. 24, 1966 2 Sheets-Sheet 1 aknvm FIG.

PAUL-FRlE DRiCH WARNIING 3,459,902 r PRESSURE GRADIENT MICROPHONE WITH TWO DIAPBRAGMS FiledOct. 24. 196

United States Patent US. Cl. 179-121 17 Claims ABSTRACT OF THE DISCLOSURE A pressure gradient microphone having both a high and low frequency system. The microphone has two diaphragms facing in opposite directions. Each diaphragm has its own voice coil and is activated through main and auxiliary sound inlets.

The invention concerns a pressure gradient microphone with two diaphragms for sound in different frequency ranges.

If a broader frequency range is embraced two microphones can be used: one adjusted to low frequencies, the other to high frequencies. As it is cumbersome to handle two microphones an attempt has been made to house several microphone systems in one casing.

The object of the present invention is to connect both microphone systems in such a manner that the size of the microphone is kept to a minimum.

Another object of the present invention is to create a single magnetic circuit for two microphones.

A further object of the present invention is to provide the magnetic circuit with a chamber serving as an acoustical RC system to revolve accoustical phase.

A further object of the present invention is to contrive an RC system which can be regulated.

A further object of the present invention is to fasten the microphone with springs.

Details of the invention are given in the following description referring to the drawings.

FIGURE 1 shows a drawing in perspective of a model of the microphone;

FIGURE 2 shows a longitudinal section through the microphone drawn in FIGURE 1;

FIGURE 2a is an enlarged section of the encircled portion of FIGURE 2;

FIGURE 3 shows another longitudinal section through a part of the microphone perpendicular to the first longitudinal.

FIGURE 1 shows a revolvable casing 3 attached to a stand 1 in which a tubular shell 2 is housed on springs. At one end of the casing 3 there is a sound-permeable removable cap 3a, which may be of wire screen. The shell 2 and the casing 3 with its sound permeable cap 3a are tubular, i.e. cylindrical, so that the microphone has the appearance of a rod. The microphone has a first main sound inlet opening 5 and a second main sound inlet opening 6. Furthermore, there are a first plurality of auxiliary sound inlet openings 7 and a second plurality of auxiliary sound inlet openings 8. The first plurality of auxiliary sound inlet openings 7 is provided with a first plurality of resistances 9, the second plurality of auxiliary sound inlet openings 8 with the second plurality of resistances 4.

The shell 2 has a front shell piece 10 and a back shell piece 11 (see FIGURE 2). The shell piece 10 has an essentially cylindrical chamber 12, one base of which is the first main sound inlet opening 5 and the other base of which is a first diaphragm 13. In the wall of the shell piece 10 there are

additional openings

14 and 15 which are covered by an acoustical resistance sheet 16. Chamber 12 represents a first main sound channel on the sound path through the main sound inlet opening 5 and the covered

openings

14 and 15 into this chamber 12 and to the diaphragm 13.

The front shell piece 10 also contains the first plurality of auxiliary sound inlet openings 7. The first plurality of auxiliary sound inlet openings 7 are covered by an acoustical resistance sheet 17. Between the shell piece 10 and the inside of the microphone there are air channels 18 which belong to a first auxiliary sound pathway. Starting from one of the auxiliary sound inlet openings 7 the first auxiliary sound pathway communicates acoustically with the diaphragm 13. The first main sound pathway passing through chamber 12 reaches the first diaphragm 13 from one side, the first auxiliary sound pathway the first diaphragm 13 from the other side.

This first diaphragm 13 is fixed in its position relative to the first main sound pathway and to the first auxiliary sound pathway by the first fastening means 20.

Between the front piece 10 and the back piece 11 of the shell 2 there is a closed magnetic circuit 21. The closed magnetic circuit 21 has a first ring-shaped magnet 22, a second ring-shaped magnet 23, a first ring-shaped pole piece 24, a second ring-shaped pole piece 25, a rodshaped core 26 and a cylindrical connecting piece 27; between the first ring-shaped pole piece 24 and the foremost end of the core 26 there is a first air slit 28 and between the second ring-shaped pole piece 25 and the hindmost end of the core 26 there is a second air slit 29. The position of the core 26 relative to the ring-shaped pole piece 24 is ensured by a punched out positioning piece 39, its position with respect to the ring-shaped pole piece 25 by a positioning piece 40 serving as an acoustical absorber. Positioning

pieces

39, 40 are non-magnetic. The individual parts of the magnetic circuit 21 are glued together by a dual-action material so that they constitute a compact unit.

Between the first diaphragm 13 and the first ring-shaped pole piece 24, respectively the front piece of the core 26 there is a chamber 30. A cylindrical chamber 31 is surrounded by the closed magnetic circuit. A third acoustical resistance 32 is located in this chamber 31 near first air slit which is prolonged by the holes left in the positioning piece 39. Chamber 30 communicates through the first auxiliary sound pathway with the outer chamber and through air slit 28 and the third acoustical resistance 32 with chamber 31.

For adjusting resistance 32 accurately there are a nonmagnetic, cylindrical holder 33 drawn over the core 26 and onto which resistance 32 is pushed, and a mechanical force distribution means. These press resistance 32 between them and a collar 33a of the holder 33. The collar 33a has round holes through which air slit 28 is prolonged up to acoustical resistance 32. The mechanical force distribution means has a hollow rod 34 with a flange 34a and an eccentric. The eccentric comprises a cam 36, a screw 37 and a nut 38. The core 26, the connecting piece 27 and the cam 36 have bored holes. The screw 37 fits into these holes. Before the ring-shaped magnet 23 is assembled, the cam 36 is fastened to the shaft of the screw 37 by means of a worm screw 36a. After the closed magnetic circuit 21 has been assembled, the screw 37 together with the cam 36 can be turned and made to exert a rather slight pressure on resistance 32, thereby regulating the acoustical resistance. The desired position can be held by tightening the nut (friction between cam 36 and holder 33 as well as between connecting piece 27 and nut 38 prevents undesired slipping). Sheets 35 are pasted on to the hole in connecting piece 27 and act as sound absorbers. As shown in FIGURE 3 a passage 41 is formed by spaces left in ring-shaped magnet 22, connecting piece 27, ringshaped magnet 23 and positioning piece 40. Passage 41 is acoustically insulated from chamber 31 by positioning piece 40 and closed off from the outside by a plate 42 so that the second main sound inlet opening 6 remains free. The second main sound inlet opening 6, passage 41 and the second air slit 29 form a second main sound pathway.

'Second fastening means 44 hold the second diaphragm 43 in position relative to the second air slit 29 in the same manner as the first fastening means 20" hold the first diaphragm 13 in position relative to air slit 28. There is a chamber 45 between the second diaphragm and ringshaped pole piece 25, respectively core 26. The mass of the diaphragm and of the attached coil and the coupled acoustical masses as well as the reduced tension of the diaphragm allow the second diaphragm to accommodate lower frequencies than the first diaphragm.

The back piece 11 of the shell 2 comprises a pipe piece 46, an anterior partition 47, a posterior partition 48 and a thin pipe 49. The second plurality of auxiliary sound inlet openings 8 is located in the posterior partition 48. These are covered by a resistance sheet 50. The second acoustical resistance 4 is formed in this manner. The thin pipe 49 is fastened near an opening 51 of the anterior partition in such a manner that a sound passage is formed between the auxiliary sound inlet openings 8, the thin pipe 49 and opening 51. This sound passage is the second auxiliary sound pathway. The dimensions of the thin pipe 49 are such that the mass moved through the second auxiliary sound pathway corresponds approximately to the mass moved through the second main sound pathway. The anterior partition 47 has an additional opening 52 with a corresponding fourth acoustical resistance 53. There is a chamber 54 between the anterior partition 47 and the second diaphragm 43 and a chamber 55 between the anterior partition -47 and the posterior partition 48.

Chamber 54 communicates on the one hand through the auxiliary sound pathway with chamber 55 and through the auxiliary sound inlet openings 8 with the outer chamber and, on the other hand, through opening 52 and the fourth acoustical resistance 53 with chamber 55.

The casing 3 fastened to the posterior partition 48 and thus the back of the microphone by means of a soundpermeable spider spring 56 or another sound-permeable connecting piece and a screw 57. The casing 3 has a sliding ring 58 so that the microphone, which lies inside, can move freely with respect to the casing 3. The sliding ring 58 is located on the middle part of the microphone but so far back that the second plurality of main sound inlet openings 6 are not covered by it.

Near the first air slit 28 the first diaphragm 13 has a first voice coil 60, near the second air slit 29 the second diaphragm 43 has a second voice coil 61. The electrical potentials of these voice coils are conducted by double

electrical leads

63 and 64 to filters 65 and 66. Filters 65 and 66 are connected by a lead 67 and terminals 68 over leads 69 and 70. One of the circuit clamps is connected by another lead 71 with shell 46 and thereby with the mass. The double leads '63 and 64 pass out of chamber 31 through an acoustical absorber 72 and into chamber 55 though another acoustical absorber 73. Leads 69 to 71 pass out of chamber '55 through another acoustical absorber 74. The

acoustical absorbers

72, 73, 74 are made of synthetic glue which closes the openings through which the leads pass.

In practice the sound pathway is 22 mm. wide and 18 mm. long in order to adjust diaphragm 13' to a relatively high frequency range. The diameter of openings 14 is 8 mm., of openings --4- mm. Openings 14 are located 14 mm. from the first diaphragm 13 and openings 15 7.5 mm. from diaphragm 13. The distance of the first auxiliary sound inlet openings 7 from diaphragm 13 is 9 mm. (The distances indicated are reckoned to the plane of the diaphragm 13, which is given by fastening means 20.) The form of the main sound pathway can deviate .4 from the suggested dimensions. Independently thereof the distance from the first auxiliary sound inlet openings 7 to the first diaphragm 13 should not exceed 20 mm. and the distance from the second auxiliary sound inlet openings 8 to the second diaphragm 43 should not be less than 50 mm. In this manner diaphragm 13 is adjusted to relatively high frequencies and diaphragm 43 to relatively low frequencies. The first auxiliary sound pathway need not be constituted by several first auxiliary sound inlet openings 7; a single opening is sufficient. The second main sound pathway need not be constituted by a single opening 6 and corresponding sound passage 41; it may consist of several parallel openings and sound passages. The second auxiliary sound pathway need not be formed as indicated by several openings *8, a chamber 55 and a mass passage (inside of pipe 49); the second auxiliary sound pathway may also be otherwise constructed. In particular, the second auxiliary sound pathway may be identical to the first auxiliary sound pathway. This means that chamber 55 may be eliminated from the second auxiliary sound pathway led through the acoustical resistance or resistances 4, sound passages, as formed by pipe 49, directly into chamber 54.

In the described model the closed magnetic circuit is glued together and to the front and back shell parts 10 and 11. It is also possible to join the front and back shell parts 10 and 11 so that the closed magnetic circuit lying between them is held together mechanically. It is also possible to use a one-piece shell to hold the closed magnetic circuit together mechanically.

It is further possible, inlieu of a single magnetic circuit, to use two separate magnetic circuits which touch on their diaphragm-free sides. Whereas the anterior diaphragm faces the direction in which the sound travels, the posterior diaphragm is turned by and faces away from the direction of the sound. With this arrangement it is possible to eliminate the base plate of the magnetic circuit whereby every magnetic circuit presents an opening on the side facing the diaphragm. These openings are joined together, forming a corresponding chamber 31. This chamber can be divided by a sound absorber (similar to positioning piece 40 in FIGURE 2). Each sub-chamber created in this manner can be acoustically related to the neighboring diaphragm. This acoustical relating occurs through sound inlet means such as air slits similar to air slits 28 and 29. Moreover, a chamber corresponding to chamber 55 can be provided by the shell for the posterior microphone; the variations of the model shown in FIG- URE 2 can be made.

I claim:

1. A pressure gradient microphone comprising the following items:

a closed magnetic circuit; this closed magnetic circuit has at least one magnet and a first and second air slit; a first diaphragm and first fastening means for this diaphragm; the first fastening means hold the diaphragm in position relative to the first air slit of the closed magnetic circuit;

a second diaphragm and second fastening means; the

second fastening means hold the diaphragm in position relative to the second air slit of the closed magnetic circuit;

a second voice coil; this second voice coil is attached to thle second diaphragm and extends into the second an s it;

a first main sound pathway; this item has at least one first main sound inlet opening and communicates with the first diaphragm;

a first auxiliary sound pathway; this item has at least one first auxiliary sound inlet opening and communicates with the first diaphragm;

the first main sound pathway reaches the first diaphragm from one side while the first auxiliary sound pathway reaches the first diaphragm from the other side;

a second main sound pathway; this item has at least one second main sound inlet opening and communicates with the second diaphragm;

a second auxiliary sound pathway; this item has at least one second auxiliary sound inlet opening and communicate-s with the second diaphragm;

the second main sound pathway reaches the second diaphragm from one side while the second auxiliary sound pathway reaches the second diaphragm from the other side;

electrical connection means for the first and second voice coil;

a shell; the shell surrounds the closed magnetic circuit; the first and second diaphragm, the first and second fastening means, the first and second voice coil, the first and second main sound pathway and at least the first and second auxiliary sound pathway;

whereby the closed magnetic circuit constitutes a single unit.

2. Microphone comprising the items of claim 1 and ineluding a first acoustical resistance; this first acoustical resistance is related to the first auxiliary sound pathway; a second acoustical resistance; this item is related to the second auxiliary sound pathway;

whereby the first and second auxiliary sound pathway constitutes a channel;

whereby the first auxiliary sound pathway is shorter than the second auxiliary sound pathway.

3. Microphone comprising the items of claim 1 and a sound absorber;

whereby the closed magnetic circuit contains an inner first chamber communicating with the first auxiliary sound pathway and separated from the second air slit by the second absorber;

whereby the shell has an inner second chamber communicating with the second auxiliary sound pathy;

4. Microphone comprising the items of claim 3 and a second chamber related to the second diaphragm as well as third and fourth acoustical resistances;

whereby the third acoustical resistances separate the first auxiliary sound pathway from the first chamber; the sound absorber separates the first chamber from the second main sound pathway and the fourth acoustical resistance separates the second space from the second chamber.

5. Microphone comprising the items of claim 1, whereby the first main sound inlet opening, the shell and the first diaphragm delimit the first main sound pathway and the shell has openings leading to the main sound pathway.

6. Microphone comprising the items of claim 1, whereby the closed magnetic circuit has at least one air channel constituting the second main sound pathway.

7. Microphone comprising the items of claim 1, whereby the air masses of the second main sound pathway and the second auxiliary sound pathway are about equal.

8. Microphone comprising the items of claim 1, whereby the distance between the first auxiliary sound inlet opening and the first diaphragm is not greater than 20 mm.

9. Microphone comprising the items of claim 1, whereby the distance between the second auxiliary sound inlet opening and the second diaphragm is not less than 5 cm.

10. Microphone comprising the items of claim 5, whereby the first main sound pathway is a maximum of 20 mm. in length.

11. Microphone comprising the items of claim 4 with a mechanical force distribution means; this mechanical force distribution means includes a rod located inside the first space and an eccentric which articulates with one end of the rod by means of an adjustable cam whereas the other end of the rod articulates with the third acoustical resistance.

12. Microphone comprising the items of claim 1 as well as a casing and a sound porous, spring-like connecting piece, whereby the casing surrounds at least the middle and posterior portion of the microphone and the springlike connecting piece is fastened to the casing and to the posterior portion of the microphone.

13. Microphone comprising the items of claim 12, whereby a sliding-ring bearing rides between the casing and the middle portion of the microphone.

14. Microphone com-prising the items of claim 12 as well as a sound porous, removable cap for the tip of the casmg.

15. A pressure gradient microphone containing a first and second microphone system; the first microphone is adjusted to a higher frequency range than the second microphone; the first microphone system has a first diaphragm facing the direction of the sound; the second microphone system has a second diaphragm facing away from the direction of the sound i.e. turned at an angle of main and auxiliary sound pathways are related to the first dia phragm which they adapt to a high frequency range; main and auxiliary sound pathways are related to the second diaphragm which they adapt to a low frequency range; the first and second diaphragm have voice coils; these first and second voice coils are connected to the terminals by electrical connecting means.

16. Pressure gradient microphone according to claim 15, whereby the first closed magnetic circuit has an opening on the side facing away from the first diaphragm and the second closed magnetic circuit has a second opening on the side facing away from the second diaphragm; the first opening and the second opening are joined; the space thus formed has sound inlet means.

17. Pressure gradient microphone according to claim 16, whereby the inner space formed by the first closed magnetic circuit and the second closed magnetic circuit is partitioned by an acoustical insulator, so that part of the space communicates acoustically with the first diaphragm and the other sub-space communicates acoustically with the second diaphragm.

No references cited.

KATHLEEN F. CLAFFY, Primary Examiner ARTHUR A. MCGILL, Assistant Examiner US. Cl. X.R.