US3053339A

US3053339A - Pipe microphone

Info

Publication number: US3053339A
Application number: US861262A
Authority: US
Inventors: Kishi Kanesuke; Nakatsuru Kaname; Kawakami Hirotake
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1959-09-07
Filing date: 1959-12-22
Publication date: 1962-09-11
Anticipated expiration: 1979-09-11

Description

sept. 11, 1962 KANESUKE KlSHl ETAL.

PIPE MICROPHONE 2 Sheets-Sheet 1 Filed Dec. 22, 1959 Sept 11, 1952 KANESUKE Klsx-u ETAL 3,053,339

PIPE MICROPHONE 2 Sheets-Sheet 2 Filed Dec. 22, 1959 J1 mman mum vvd- Lima 6,1500 mono {reparle} Xg aou 40o 60u 1pm: 2.01m 4,0m Gpno 10.000

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w wwmm 3,953,339 Patented Sept. 11, 1962 ice 3,053,339 PIPE MCROPHNE Kanesukc Kishi, Kawasaki-shi, and Kanarne Nalratsuru and Hirotake lawalrami, Tokyo, llapan, assignors to Sony Kabashiiriiraisha (Sony Corporation), Tokyo, Japan, a corporation of Japan Filed Dec. 2'2, 1959, Ser. No. 861,262 Claims priority, application .tapan Sept. '7, 1959 2 Claims. (Cl. ISL-31) This invention relates to a pipe microphone, and more particularly to a stand microphone in which an acoustic receiving cap is attached on the upper end of an acoustic transmission line which is composed of telescopically fitted pipes and a resistance controlled type transducer tor microphone unit is coupled to the lower end of the acoustic transmission line.

It has been well known that an acoustic transmission pipe having a uniform cross sectional area at every part thereof is considered to be equivalent to an electrical transmission line having a distributed electrical constant. Such an acoustic transmission pipe has an improved transmission characteristic since standing Waves in the pipe are suppressed, if the input terminal and the output terminal of the pipe are terminated with adequate acoustical resistances. In the case of making a pipe microphone by combining the acoustic pipe and a microphone unit, it is necessary to bring a sound source to the neighbourhood of the sending end of the acoustic pipe in order to obtain a sound pick up effect.

In such a case it is convenient that the acoustic pipe is composed of a plurality of pipes which are telescopically fitted one to another to adjust the length thereof.

In general such a telescopic pipe, however, is composed of an outer pipe of comparatively large diameter and an inner pipe of comparatively small `diameter so that the whole pipe has a discontinuity in diameter at the assembled end.

The discontinuity changes acoustic transmission characteristics for the worse owing to the reilection of sound from 4the discontinuity end and a microphone having superior characteristics can not be expected with this type of arrangement.

The basic idea of this invention is that in the acoustic transmission line which is composed of a plurality of pipes telescopically fitted to one another, the upper pipe has a larger diameter than a lower one so that the latter is received telescopically sliding into the former, whereby the sound reflection from the assembled end of the pipes can be minimized to improve the transmission characteristic-s.

One object of this invention is to provide a pipe microphone having superior transmission characteristics in which an acoustic transmission pipe which is composed of a plurality of telescopic pipes has a minimum reection of sound waves transmitted therethrough.

Another object of this invention is to provide a pipe microphone in which a mouth piece or cap for receiving sound waves from a sound source is connected to the upper end of a telescopic acoustic pipe and a resistance controlled type transducer or a microphone of this type is connected to the lower end of the telescopic acoustic pipe so that the Whole acoustic transmission system has an excellent acoustic matching to thereby improve its frequency response.

A further object of this invention is to provide a pipe microphone in which a telescopic acoustic pipe has minimum changes in transmission characteristics and losses even after adjustment of the length of the telescopic acoustic pipe.

Another and further object of this invention is to provide a pipe microphone in which an `acoustic pipe composed of a plurality of telescopic pipes can be easily and smoothly adjusted to any desired height thereof.

A still further object of this invention is to provide a pipe microphone in which a telescopic acoustic pipe is protected by an outer pipe to prevent external sound waves from being transmitted to the telescopic acoustic pipes and the telescopic acoustic pipes are reinforced by the outer pipe so as to mount the same rigidly onto a stand.

The other objects, features and advantages of this invention will be more apparent from the following description taken in connection with the accompanying drawing in which:

FIG. l is a partial sectional view of a pipe microphone, by way of example, according to this invention.

FIG. 2 is an enlarged sectional View of a mouth cap or mouth piece for receiving external sound Waves, connection mean-s between the mouth cap and the upper end of an acoustic pipe being also shown.

FIG. 3 is an enlarged sectional view of a resistance controlled type transducer or a moving coil type microphone preferably used in this invention, connection means between the microphone `and the lower end of an acoustic pipe being also shown.

FIG. 4 is an electrical equivalent circuit of a pipe microphone according to `this invention.

lFIG. 5 shows characteristic curves for illustrating the improved lresponse of a pipe microphone according to this invention, and

FIG. 6 shows another response curve of a pipe microphone according to the invention as compared with that of. an ordinary microphone.

`Referring to FIG. 1, an upper movable pipe 1 is airtightly fitted to the outside of a lower stationary pipe 2 to form a telescopic acoustic pipe as a whole. It is preferable to pour a small amount of lubrication oil such as grease between the contact surfaces of the two

pipes

1 and 2 to faciiitate the telescopic operation. The movable pipe 1 is preferably made of a metal pipe the inner diameter of which can be selected on the order of 6 to 8 mm., while the stationary pipe 2 is preferably made of a metal pipe the thickness of the wall of which is made very small such as 0.3 mm. in order to minimize the discontinuity of the sectional area at the junction end of the both pipes. That is, the thickness of the wall of the stationary pipe 2 is made as small as possible and the stationary pipe is coaxially and telescopically inserted into the movable pipe 1 so that sound waves passing from the movable pipe to the stationary pipe are prevented from reiiecting at the end of the stationary pipe 2. However, by making the thickness of the pipe 2 so small, it becomes structurally weak.

In order to reinforce the stationary pipe 2 it is sury rounded and supported by an outer pipe 3 which also serves as an acoustical shield to isolate the stationary pipe 2 from external sound waves. The stationary pipe 2 and the outer pipe 3 are coaxially connected by an upper annular member 4 and a lower annular member f4 inserted therebetween respectively at substantially the midpoint of the height of the stationary pipe and the lower end thereof. The upper annular member 4 serves also as a stopper for the movable pipe 1 the lower end of which abuts against the annular member and it is preferably made of soft rubber for preventing vibration which might be caused by the abutment.

The upper end of the outer tube 3 is externally threaded and screwed by a dish yshaped nut member 9 through the f center hole 2t) of the bottom of which the movable pipe 1 is passed.

Between the nut 9 and the upper end of the outer pipe 3 is inserted an annular member 4 made of rubber which is tightly pressed, by the screwed nut 9, onto the outer wall of the movable pipe 1 that the movable pipe is secured to the upper end of the outer tube 3 at its adjusted position.

Although the foregoing description has been taken, by way of example, in connection with the telescopic acoustic pipe which is composed of the movable pipe -1 and stationary pipe 2, it will be understood that a similar acoustic pipe can be made by using more than two movable pipes which are connected to one another and the lowest one is, in turn, connected telescopically, as has been described in the former example, to a stationary pipe which is mounted on a stand. It should be understood, however7 that the thicknesses of the walls of the pipes are made as small as possible and that the diameter of the lower pipe is smaller than that of the adjacent upper pipe which is telescoped to the lower pipe so as to minimize the sound reliection at the telescoped or assembled end of the pipes.

At the input end of the movable pipe 1 is mounted a mouth piece or mouth cap 7 which has a convexed contour for receiving sound waves. Inside the mouth cap 7 is inserted an acoustic resistance pad 11 which is sandwiched between two Wire nettings 12 secured to the open end of the pipe, as shown in FIG. 2.

This pad serves as an acoustic resistance at the sending end of the transmission pipe to suppress standing sound waves in the pipe.

It is preferable to use felt or silk layers having a thickness of 1 to 2 mm., as the acoustic resistance pad 11. Moreover, it is also preferable that a dust proof metal netting 13 be attached on the yfront face of the cap 7 to prevent dust from entering from the opening of the cap so as not to spoil and deform the pad.

The output end of the stationary pipe 2 is acoustically and closely coupled with a coupling pipe 6 of an electroacoustic transducer unit T contained in a housing 5. One part of the coupling pipe 6 may be formed by a connector 30 provided with a flange 31 which is mounted on and secured to the stand 3 by means of screws 40.

The upper part of the connector 3() is screwed into the lower end of a socket 32;. To the upper end of the socket 32 is screwed the lower end of the outer pipe 3, at the same time the lower end of the stationary pipe 2 is in registry with the center hole 33 of the socket 32 which also communicates with the pipe 6.

The lower end of the connector Sti is screwed to the upper end of the core member of the transducer unit and the other part of the coupling pipe 6 is formed by the pole piece 34. Thus an acoustic transmission pipe is constructed by including the cap 7, telescoped pipe 1, connection parts thereof, 4stationary pipe 2, coupling pipe 6 and the pole piece 34.

The sound transmission in the acoustic pipe is carried out through the cap 7 to which sound waves from a sound eld are received, the telescoped pipe 1, stationary pipe 2, coupling pipe 6 of the transducer unit T and nally the sound waves will arrive at a diaphragm 14.

It will be appreciated that a so-called resistance controlled type electro-acoustic transducer is well adapted as a transducer unit which is used for such a pipe microphone arrangement. In the transducer unit of this type, the diaphragm thereof is mainly controlled by a damping resistance which acts as a termination resistance of a pipe end when the diaphragm is acoustically coupled with the acoustic pipe. The damping resistance cooperates with a sending end resistance or pad 11 to substantially suppress standing waves in the pipe to a nullity. As the transducer unit of this type, a moving coil type, moving ribbon type or the like may be used.

FIG. 3 shows a cross sectional view of a moving coil type transducer unit by way of example. The sound waves pass through the coupling pipe 6 to the terminal opening of the pipe and vibrate the diaphragm 14 which is disposed in closely coupled relationship with the terminal opening. In order to vibrate the diaphragm with substantially the same intensity in the high frequency range,

. it is necessary that the gap g1 formed between the terminal opening of the coupling pipe 6 and the central spherical surface of the diaphragm is reduced to as low a value as possible. An acoustic resistance pad 16 is provided in the neighbourhood of the back side of the diaphragm. The pad 16 is preferably made of a felt sheet or the like and sandwiched by two metal plates 17 to form a spherical surface the peripheral edge of which is secured to the supported edge of the diaphragm. A moving coil 15 is attached to the peripheral edge of the diaphragm and suspended in the air gap of a strong magnetic field which is established by a permanent magnet 18. In the transducer unit thus manufactured, air molecules which are compressed or expanded by the sound pressure will vibrate the diaphragm through the coupling pipe 6, so that air molecules existing at the back side of the diaphragm are also compressed or expanded. The inner and the outer metal plate 17 are respectively provided with a plurality of holes h1 and h2 so that the unit will breathe in such a manner that air molecules at the back of the diaphragm escape through the holes h1 of the inner plate, the porous layer of the acoustic resistance pad 16 made of felt and the holes h2 of outer plate to the inner atmosphere of a sealed vessel 19 of the unit. The sealed vessel 19 serves to shield the unit acoustically so that extraneous sound waves are prevented from entering into the back side of the diaphragm 14 to vibrate the same and only the sound waves received from the mouth cap 7 are transmitted through the pipes to arrive at the front of the diaphragm 14. The diaphragm 14 is so damped by the pad 16 that the inherent vibration of the diaphragm is suppressed with the result that the diaphragm can operate in response to the sound pressure in the coupling pipe 6. The pad 16 `serves as a terminal matching resistance for the acoustic pipe so that there are a minimum number of standing waves in the acoustic pipe and the entire acoustic transmission system will respond equally for every frequency of the sound waves.

Thus a substantially constant velocity vibration of the moving coil 15 will be produced for a constant sound pressure in the operating frequency band so that a constant induced voltage can be obtained at the terminals of -the lead wires 1t).

FIG. 4 is an electro-acoustic equivalent circuit of a pipe microphone thus obtained, in which:

F=Vibrornotive force P A. given by a sound pressure P to the opening of the mouth having an effective area A.

rA=Sending end acoustic resistance which is given by the resistance pad 11 of the cap to the acoustic pipe.

m0=Equivalent mass of the acoustic pipe upon the dian phragm. m1,m2,m3 mn=Equivalent mass of the acoustic pipe at higher order resonance.

C1, C2, C3 Cn=Equivalent compliance of the acoustic pipe at higher order resonance.

r0,r1,r2 rn=Equivalent acoustic resistance of the pipe. U :Resistance controlled type electro-acoustic transducer unit.

r'B=Equiva1ent acoustic resistance or output resistance, for example given by the pad 16, at the receiving end of the acoustic pipe through the diaphragm.

The equivalent circuit shows that the acoustic pipe is terminated by the resistance I'A at the sending end and A the resistance rB at the receiving end.

FIG. 5 shows several curves for various conditions of Y the acoustic pipe.

The curve a shows a frequency response of the electromotive force at the output terminals of an electro-acoustic f transducer unit at a constant sound pressure when rA m0 of the pipe and the compliance of the diaphragm; a2 the fundamental standing wave determined by the length of the pipe. The fundamental frequency fome/1l, where C=velocity of sound, l=the length of the pipe. Curves a3, a4 -respectively show standing waves of higher order at the nodal points.

In the case oi the acoustic pipe in which the pad 11 stands for the adequate acoustic resistance rA and the pad 16 stands for the resistance in the microphone unit T to match both ends of the acoustic pipe, the standing waves of the acoustic pipe can substantially be eliminated, a1- though some degree of transmission loss occurs as shown by the curve b. But the response becomes gradually low at the higher frequency region above 5,000 c./s. as shown by the curve b. This is because of the fact that the mass of diaphragm 14 (including moving coil mass) is not neglected and acoustic coupling between the coupling pipe 6 and the diaphragm 14 is decreased. Moreover the transmission loss in the acoustic pipe is increased at this frequency region. In this case the response can be cornpensated by the diffraction effect of sound at the receiving opening of the mouth cap 7, which causes the sound pressure to rise at the front surface of the sending end of the pipe.

In fact, the diffraction of sound makes -it possible to flatten the response through the frequency region of 5010,000 c./s. to obtain improved transmission characteristics. The sound pressure rise caused by the diffraction effect depends upon the diameter D of the front surface of the mouth cap 7.

The curve C shows the response where the diameter D is mm. Such an improved response at comparatively higher frequencies of sound waves can be obtained by selecting the diameter D to the range of 18 mm. to 22 mm. In order to adjust the quality at higher frequency ranges of sound, however, the diameter D is selected as large as 60 mm. that the response h-as an ascending part at the neighbourhood of 6,000 c./s. as shown by the curve d but the curve suddenly descends at higher frequencies.

FIG. 6 shows another response curve C0 of a pipe microphone according to this invention as compared with `a response curve bo of an ordinary microphone. In this experiment, the compared microphones have the same conditions except that the upper movable pipe of the microphone according to this invention is telescopically tted to the outside of the stationary pipe which has a very thin wall as has been explained in connection with FIG. 1, while the ordinary microphone has a movable microphone which is telescopically inserted into a station- 'ary pipe which has a thicker wall. That is, the only one difference in conditions is the telescopic method of mounting of the movable and stationary pipes. From the comparison it is seen that the response curve C0 `of the microphone according to this invention is much superior to the curve bo of the ordinary microphone. Namely, the curve co is more flat and has higher sensitivity by substantially 6-7 db than the curve bo.

This is because mainly of the following facts:

The ordinary microphone has an appreciable air gap or chamber between the upper movable pipe and the lower stationary pipe which has a larger diameter 4and larger wall thickness in order to securely support the upper movable pipe, whereby a loose coupling occurs between the pipe and the transducer unit and standing waves are established in the pipes.

The microphone according to this invention, on the contrary, has no such air gap between the upper movable pipe and the lower stationary pipe, nor standing waves nor reflection of sound waves as has hereinbefore described.

The advantages of the microphone realized by the acoustic pipe according to this invention are as follows:

(1) It is possible to receive effectively sound waves by so adjusting the length of the telescopic pipes as to adequately :draw the cap 7 towards a sound source. Moreover, lan eiective acoustic resistance is inserted into the cap 7 so that noises caused by the air ilow of breath can be minimized when a user speak-s to the microphone.

(2) In the acoustic transmission path including the movable pipe, stationary pipe and coupling pipe, the discontinuity of the sectional area of the transmission path is also minimized from the sending to receiving end thereof. Besides, irregular sound reflection at the intermediate connection of the pipe is effectively suppressed by the fact that the sectional area of the acoustic transmission pipe is substantially uniform between sending side to receiving end according to this invention.

A flat response with respect to frequencies is also obtained by the adequate damping action of the pipe microphone.

(3) Changes of transmission characteristics and losses are small due to the fact that the discontinuity of the transmission path does not occur appreciably when the length of the acoustic pipe is adjusted.

(4) Lubrication oil is packed between outer movable pipe 1 and inner stationary pipe 2 so that the user does not touch the lubrication oil.

(5) The main acoustic transmission pipe is protected acoustically and mechanically by the outer pipe 3 so that a stand type microphone using such `an acoustic pipe has no response to noises induced from extraneous sounds and no tendency toward deformation which would occur by forces applied during use.

It will be understood that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.

What is claimed is:

1. A microphone assembly comprising a base, a thin walled stationary pipe extending from Said base, Said thin walled pipe having a thickness not substantially in excess of 0.3 millimeter, a thicker walled movable pipe having its inner periphery in sliding engagement with the outer periphery of the stationary pipe, a cap terminating the input end of said movable pipe, a iirst acoustical resistance within Said cap in communication with the interior of said pipes, a rigid pipe extending from said ibase and circumscribing said stationary pipe in spaced relation along its entire length, a microphone diaphragm in said base in communication with the interior of said stationary pipe, and a second acoustical impedance on the opposite side of said diaphragm from said pipe interior.

2. A microphone assembly comprising a base, a thin walled stationary pipe extending from said base, -said thin walled pipe having a thickness not substantially in excess of 0.3 millimeter, a thicker walled movable pipe having its inner periphery in sliding engagement with the outer periphery of said stationary pipe, a cap terminating the input end of said movable pipe, a first acoustical resistance within said cap in communication -with the interior of said pipes, a rigid pipe extending from said base and circumscri'bing said stationary pipe in spaced relation along its entire length, a resilient collar extending between said stationary pipe and said rigid pipe intermediate the ends of said Stationary pipe, said collar being positioned to serve as an abutment for limiting travel of said movable pipe, a microphone diaphragm in said base in communication with the interior of said stationary pipe, and a second acoustical impedance disposed within said base on the opposite side of said diaphragm from said pipe interior.

References Cited in the le of this patent UNITED STATES PATENTS 2,566,094 Olson et al. Aug. 28, 1951 2,717,932 Rackham et al. Sept. 13, 1955 2,852,088 Wood Sept. 16, 1958