Tubular Directional Microphone

Abstract

At least one directional tube communicates with a sound transmitter capsule and is provided with acoustic impedance elements spaced along said tube and having predetermined cut-off frequencies, which increase in one direction along said tube, and predetermined natural frequencies, which decrease as the distance of the impedance elements of the capsule increases, whereby the effective length of said tube decreases as the frequency of sound is increased. The spacing and natural frequencies of said impedance elements are selected so that the effective length of said tube is divided into two outer sections and an intermediate section. Said two outer sections have a relatively low sensitivity and are arranged to subject sound received from said impedance elements in said outer sections to mutually opposite phase displacements amounting to approximately .pi./2. Said intermediate section has a relatively high sensitivity and is arranged to subject sound received from said impedance elements in said intermediate section to zero phase displacement.

Description

This invention relates to a tubular directional microphone which comprises sound inlet openings, which are spaced apart along the tube and provided with acoustic impedance elements comprising preferably damped spring-mass plug systems, the directivity being due to the interference between the waves entering the tube through the sound inlet openings and the cut-off frequencies increasing in one direction.

When a tubular directional microphone is exposed to sound from a front source, the waves inside and outside the tube are substantially in phase and the sound energies entering through the several sound inlet openings are added. When the microphone is exposed to sound from a rear source, the waves inside and outside the tube are in phase opposition and an interference results which causes in part a complete extinction. The directional characteristic of a tubular directional microphone depends highly on the ratio L/.lambda., where L is the length of the tube and .lambda. the wavelength of the sound. If the ratio L/.lambda. is less than one-fourth, the directional characteristic of the microphone will approach an omnidirectional characteristic whereas there is a pronounced directivity for treble frequencies. It has been attempted in some cases to improve the directivity of directional microphones by the use of longer tubes. For instance, directional tubes having a length of several meters have been disclosed. Whereas such microphones have excellent directional characteristics, they can be used in practice only in exceptional cases. To avoid the strong dependence of the tubular directional microphones on frequency, it has been proposed to provide several discrete sound inlet openings along the shell of the tube and to provide acoustic low-pass filters preceding these openings and having cut-off frequencies which decrease along the tube so that the effective length of the directional tube for a given frequency range decreased as the frequency increases. According to another proposal the overall length of the microphone is decreased in that the directional tube is designed as an acoustic line which for frequencies having a wavelength in excess of the mechanical length of the tube have a transit time which increases as the frequency decreases. In known microphones of this type, the sound entered through a slot, which extended throughout the length of the tube and which may vary in width. The sound inlet slot is covered by a damping woven fabric, which acts as an acoustic resistance. In that microphone, the phase displacement is due to the cooperation of the acoustic resistance of the covering on the slot and the acoustic line. As a result, an increased phase displacement and a higher sensitivity are obtained only at the end portions of the tube.

These two known tubular directional microphones have the disadvantage that one of them has a relatively low directivity, whereas the other microphone has directional characteristics being still considerably dependent on frequency and has a large overall length. According to the invention, the disadvantages of the known microphones are avoided in that the impedance elements are so tuned and arranged along the tubular directional microphone that the effective length of the tube is divided into two sections having a low sensitivity and effecting mutually opposite phase displacements of about .pi./2 , and a section, which is disposed between said two sections and has a high sensitivity and effects no phase displacement, and the effective tube length is decreased for increasing frequencies in that the impedance elements are tuned to lower frequencies as their distance from the microphone capsule increases. Having a very short overall length, the tubular directional microphone according to the invention has an excellent directivity, which is highly independent of frequency in a very large frequency range. This result is due to the desirable distribution of the phase displacement and sensitivity over the effective length of the tube and the reduction of said effective length in dependence on frequency.

According to a desirable feature of the invention, the sound inlet passages, which lead into the directional tube and constitute mass plugs are covered on the outside, in known manner, by an acoustic resistance consisting e.g. of woven fabric or felt. The inlet passages serving as mass plugs have a length of a plurality of centimeters for bass frequencies. To enable satisfactory designs, the sound inlet passages consist preferably of non-straight acoustic lines.

It has already been proposed to use the interference action occurring in a tubular directional microphone in combination with a pressure gradient action in order to enable the use of smaller overall lengths and/or to improve the directivity of the microphone. In such microphones, the interference action is used only for treble frequencies whereas the pressure gradient action is used in the microphone for the bass frequencies. The tubes of such directional microphones have a small overall length of about 50 centimeters but have the disadvantage that interfering signals at bass frequencies are not sufficiently damped owing to the low directivity of directional pressure gradient microphones having cardioid, supercardioid and bidirectional patterns, although such interfering signals at bass frequencies occur rather often.

In accordance with another embodiment of the invention, at least one additional directional element is provided in known manner, which acts on the microphone capsule and is preferably similarly designed. The arrangement may be such that one directional tube acts on one side of a microphone diaphragm and a second directional tube acts on the other side of the microphone diaphragm.

Further features of the invention will become apparent from the subsequent description of some embodiments which are shown by way of example on the drawing, in which

FIG. 1 is a side elevation showing a tubular directional microphone according to the invention,

FIG. 2 is an enlarged longitudinal sectional view taken through the directional tube.

FIG. 3 is a sectional view taken on line III--III in FIG. 2.

FIGS. 4 and 5 are graphs illustrating the sensitivity distribution and the additional phase displacement along the tube length for a frequency of 200 cycles per second.

FIG. 6 illustrates diagrammatically the sensitivity distribution of a tubular directional pressure gradient microphone.

FIG. 7 is an exploded view showing a tubular directional pressure gradient microphone.

With reference to FIG. 2, a directional tube 1 has discrete sound inlet openings 2, which are spaced along the shell of the tube and provided with a covering 3 of felt or woven fabric. The sound inlet openings 2 are connected by passages 4 to the interior of the directional tube. Owing to its mass, the column of air in the passages 2 constitutes an acoustic inductance, which acts on an air volume having a certain compliance (acoustic capacitance) so that an oscillatable system (spring-mass plug) is provided. The oscillations of that system are damped by the friction of the air column at the passage surfaces and by the covering 3 of woven fabric or felt. The directional tube is connected at 5 to a microphone capsule, which is not shown. A chamber which precedes the diaphragm communicates directly with the interior of the directional tube. In a modification of this embodiment, the microphone capsule may be disposed directly in the directional tube. The natural frequencies of the several spring-mass plug systems are selected to decrease as the distance from the microphone capsule increases. For this reason, the spring-mass plug systems shown in the right-hand part of the illustration must have a relatively large length. To ensure that the tubular directional microphone has desirable dimensions, each air inlet passage 4 is angled to form a non-straight acoustic line. The frequency response of such spring mass plug system has two resonance frequencies, one of which is determined by the natural frequency of the air column which oscillates in the air passage whereas the second resonance frequency is determined by the air column in the air inlet passage (mass) and the air column between the inlet passage and the microphone diaphragm (spring).

The microphone according to the invention comprises three functionally significant sections. The effective acoustic length of the tube is divided into two sections, which have approximately the same length and impart phase displacements of about .pi./2 and -.pi./2 , respectively, to the sound (see FIG. 5). A relatively narrow section in which no phase displacement is effected and which has an increased sensitivity is disposed between said two sections (see FIG. 4). When the microphone is exposed to sound from a front source, only the intermediate section is effective because the sound energies entering through the two other sections substantially cancel each other to 0 by interference. As the microphone is exposed to sound arriving at increasing angles, the phase displacement of the sound energies entering through said two outer sections is further increased as a result of the transit time of the sound between the corresponding sound inlet openings and the canceling action is increased correspondingly. A further increase of the angle of sound incidence will result in a reduction of the amplitude to the extent which is required in view of the increasing phase displacement in order to avoid substantially negative resultant sound energies. As a result, the cancelation will be very substantial in large angular ranges. The resulting directional patterns may be defined by the function

and the mass plugs maintain the ratio L/.lambda. virtually constant. In the above function, c is a constant which can be selected by the covering which constitutes the acoustic resistance. For instance, c may be equal to 2 where L/.lambda.=1/4. The use of spring-mass plugs enables the design of a directional microphone which has a still higher directivity because the directivity which is due to the directional tube is increased by the directivity which is due to a pressure gradient. Such microphone is diagrammatically shown in FIG. 6 and has a diaphragm 6, to which two directional tube systems are coupled on opposite sides. In accordance with the embodiment shown in FIG. 2, each directional tube system has discrete sound inlet passages, which act as spring-mass plug systems. These systems should be tuned to provide the sensitivity distribution which is indicated in FIG. 6 whereas the phase-displacing action need not be utilized. The pressures stated are in accordance with the binomial coefficient

; this results in a directional characteristic defined by 1/2 n (1 + cos .alpha.).sup.n so that the directivity may be increased as desired by a decrease of n. To prevent a deaccentuation of the bass frequencies, the spacing of the inlet openings is desirably increased in proportion with the wavelength. This may be accomplished with the aid of the impedance elements. The increase in the directivity of that microphone is limited by the rapid decrease in sensitivity.

FIG. 7 is a diagrammatic perspective view showing another tubular directional pressure gradient microphone of this type. The two directional tubes are composed each of three plates 7, 8, 9. Each of the two outer ones of these plates (7 and 8) has a longitudinal passage 10, which communicates through openings 11, 12 with the chamber 14, which contains the microphone capsule 13. The plates 7 are provided with air inlet passages 15. In the assembled tube, the plate 9 closes the passage 10 and the air inlet passages 15 except for a small inlet opening. The openings of the air inlet passages are covered by a strip 16 of felt or woven fabric, which is inserted between the two plates 7 and 8 and ensures a damping action of the spring-mass plug systems.

Claims

What is claimed is:

1. A tubular directional microphone selective to a limited range of operating frequencies, comprising

an acoustic transducer capsule,

at least one tube having directional sound characteristics and selective to said range, said tube being coupled to said capsule,

an intermediate portion of said tube having a high sensitivity to said range and opposite end-portions to said tube having relatively low sensitivity to said range,

sound coupling means distributed along said tube and imparting the sensitivities to said portions of said tube, each of said sound coupling means extending between the inside and outside of said tube and having effective lengths, to sounds travelling through said sound coupling means, which lengths progressively change along the length of the tube, and

acoustic impedance elements in respective sound coupling means having respective cut-off frequencies which increase in one direction along the tube and respective natural response frequencies which decrease with increasing distance from the capsule, in which microphone the spacing of the second coupling means, their effective lengths and their acoustic impedance elements are so selected that sounds of the same frequency entering opposite end-portions of the tube via said means and emanating from a wave-front travelling parallel to the tube axis have a phase shift in the region of 180.degree. introduced between them so that such sounds have a self-cancelling effect on one another inside the tube.

2. A tubular directional microphone as set forth in claim 1, in which said impedance elements comprise damped spring-mass plug systems.

3. A tubular directional microphone as set forth in claim 2, in which

said spring-mass plug systems are formed by passages opening into the interior of said tube and

said passages are covered on the outside with acoustic resistance elements.

4. A tubular directional microphone as set forth in claim 3, in which said acoustic resistance elements consist of woven fabric.

5. A tubular directional microphone as set forth in claim 3, in which said acoustic resistance elements consist of felt.

6. A tubular directional microphone as set forth in claim 2, in which the spacing of said spring-mass plug systems along said tube increases in proportion with the lengths of sound waves at the natural frequencies of said spring-mass plug systems.

7. A tubular directional microphone as set forth in claim 1, in which at least part of said acoustic impedance elements consist of non-straight acoustic lines.

8. A tubular directional microphone as set forth in claim 7, in which said part of said acoustic impedance elements are spring-mass plug systems, at least part of which have natural frequencies in the bass range.

9. A tubular directional microphone as set forth in claim 7, in which at least part of said acoustic impedance elements consist of angled acoustic lines.

10. A tubular directional microphone as set forth in claim 1, wherein

at least two of said directional tubes are coupled to respectively opposite sides of the capsule, whereby sounds to which the two tubes are selective arrive in phase opposition at opposite sides of the capsule to produce an additive effect thereon.

11. A tubular directional microphone as set forth in claim 10, in which said directional tubes are similar to each other.

12. A tubular directional microphone as set forth in claim 10, in which

said capsule comprises a diaphragm,

said tubes are identical and extend side-by-side in the same direction with said sound coupling means extending along opposite sides of the tube combination, and

said capsule has a diaphragm on opposite sides of which sounds from the two tubes are respectively incident.

13. A tubular directional microphone as set forth in claim 12, in which

said impedance elements comprise spring-mass plug systems, which have natural frequencies selected so that the sensitivity distribution along the tube corresponds to binomial coefficients

, where n is the number of said acoustic impedance elements minus 1 and k is the number of each impedance element when counted from the diaphragm.

14. A tubular directional microphone as set forth in claim 13, in which the spacing of said acoustic impedance elements along said tube increases with their distance from said capsule.

15. A tubular directional microphone as set forth in claim 1, in which the spacing of said acoustic impedance elements along said tube increases with their distance from said capsule.