US 3403223 A
Description (OCR text may contain errors)
KLEIS ET AL Sept. 24, 1968 I MICROPHONE comamnrons OF THE mun commsmc A PLURALITY OF DIRECTIONAL sounp UNITS Filed May 18, 1965 INVENTORS ING DIRK KLEIS 'WILHELMUS H. ID
nasur United States Patent 3,403,223 MICROPHONE COMBINATIONS OF THE KIND COMPRISING A PLURALITY OF DIRECTIONAL SOUND UNITS Derk= Kleis and Wilhelmus Hermanus Iding, Emmasingel, Eindhoven, Netherlands, assignors to North American Philips Company Inc., New York, N.Y., a corporation of Delaware Filed May 18, 1965, Ser. No. 456,768 Claims priority, application Netherlands, May 20, 1964, 6405564 7 Claims. (Cl. 179-1) ABSTRACT OF THE DISCLOSURE A microphone system in which the output of two groups of microphones are combined with opposite polarity in a frequency range below a predetermined frequency, and are combined with the same polarity in the frequency range above the predetermined frequency.
The invention relates to a microphone combination comprising a plurality of sound units arranged at a small distance from each other each of the sound units being connected through filter elements to a common output and formed by one or more microphones.
It is known to obtain a given directional characteristic in a microphone by causing the sound field to act upon both sides of the diaphragm, either directly with a time difference determined by the dimensions of the microphone and the direction of the incident sound (pressure gradient microphone) or by providing, in addition, on one side of the diaphragm an acoustical network (cardioid microphone). The microphones of the lastmentioned type may be considered to be a combination of an omnidirectional pressure-gradient microphone and a pressure gradient microphone with an eight-shaped directional characteristic. The directional characteristic thus obtained is, in principle, independent of the frequency as long as the dimensions of the microphone are small with respect to the wavelength.
It is furthermore known to obtain a sound receiver with a given directional characteristic by choosing a sound receiving surface which is larger than the wavelength. Examples thereof are a microphone with a parabolic mirror, a microphone with an acoustic lens, a microphone with a transit time tube and a combination of microphones arranged in line with each other (microphone column) or in one plane, said microphones cooperating in phase. The directional characteristic of these sound receivers depends upon the frequency and becomes sharper with an increasing frequency. The directional effect starts at the frequency, at which the wavelength is of the same order as the dimensions of the receiver, that is to say that such microphones, operating on the interference principle, are not very effective for the low tones as directional microphones, unless the dimensions are very large.
Apart from said interference microphones there are known difference microphones which are composed of two or more microphone systems, which may be connected with opposite polarities. Such a device is described in American patent specification 2,301,744.
In contrast to interference microphones, which have maximum sensitivity for that direction of the sound at which the individual microphone systems receive the sound in phase, the sensitivity of a difference microphone will just be zero under these conditions.
For all further directions the dilference microphone, also termed higher-order microphone, exhibits a frequency characteristic curve rising by 6 db/octave up to that frequency at which half the wavelength is equal to the distance between the microphones. With higher frequencies the frequency characteristic has a sequence of peaks and valleys.
The invention is characterized in that by means of the filter elements the microphone combination operates on the interference principle above a separation frequency 7'' between high and low tones, whereas below said frequency f it operates on the difference principle. The invention has for its object to combine the advantages of the two principles described above, so that with small dimensions of the microphone combination a very satisfactory directional characteristic of linear course from low to high notes can be obtained both for low frequencies and for high frequencies.
In the microphone combination described in American patent specification 2,301,744 separate microphones are arranged one behind the other. The frequency characteristic for high notes exhibits a wave-like variation.
By arranging the sound units side by side instead of arranging them one after the other, this wave-like variation does no longer appear, whilst this arrangement is particularly suitable for operation on the interference principle at high notes, which is not possible with an arrangement of the units one after the other.
A microphone combination according to the invention comprises two sound units M and M M being arranged centrally and M symmetrically around M It is characterized in that below the separation frequency f the output signals of M and M are joined with such amplitudes and opposite polarities so that in the direction of the height of the region of M the sensitivity of the microphone combination is zero, while the output signals of M and M have furthermore joined to them the output signal of M in a given ratio and below the frequency f the output signals of M and M are combined independently of frequency.
This means, in fact, that two directional characteristics are subtracted, for example an eight-shaped characteristic and an omnidirectional characteristic, so that one or more extinctions are produced laterally, which has the advantage that the microphone combination is insensitive for ambient sound in a wide range.
In a further embodiment of the invention the microphones of each sound unit are connected with equal polarities.
This permits of changing over the sound units so that they form together a single column.
In an advantageous embodiment of the invention the output signals of the sound units M and M are connected by a filter prior to the said addition, the frequency characteristic of said filter being inversely proportional to the square of the frequency lying between a given low frequency f and the separation frequency f The frequency characteristics of M and M combined in the preferential direction, are thus equalised to that of M alone.
By varying the amplitude of the added signal of the sound unit M the direction in which the sensitivity of the microphone combination is zero, can be varied.
When use is made of unilaterally sensitive microphones, which are directioned so that their preferential directions coincide, the directional characteristic of the combination is again multiplied by that of the individual microphone.
The invention will be described more fully with refer ence to the drawing.
FIGS. l-ll illustrate various arrangements of microphone units which may be used in the system of the invention; and
FIG. 12 is a circuit diagram showing the interconnection of microphone units to a common output circuit according to the invention.
Referring now to FIGS. 1 and 2, therein is illustrated a microphone arrangement in which cardioid microphones are aligned in side-by-side relationship. Each microphone is indicated by a circle. The plus or minus sign in each circle indicates the polarity of the microphone, i.e., the polarity with which it is connected in a circuit. An arrow pointing away from a circle indicates the direction of maximum sensitivity of the microphone. When a circle is shown without an arrow, it is considered that the direction of maximum sensitivity of the corresponding microphone is normal to the plane of the drawing.
FIGS. 1 and 2 thus show two views of a microphone arrangement having three cardioid microphones. As shown in FIG. 2, which may be considered to be a top view of the arrangement, the direction of maximum sensitivity of all microphones is the same (i.e., toward the left) and the microphones are aligned in one place. The view at FIG. 1, which may be considered to be a side view, shows that the microphones are also aligned in a second plane. The microphone in the center, with the positive polarity, is considered to be one microphone group or unit M and the two outside microphones with negative polarity are considered to be another group or unit M As shown in FIGS. 3 and 4, which also illustrate two views of a microphone arrangement, the arrangement of FIG. 1 may be modified to include additional microphones, so that the central group M includes two microphones and the outside group M includes four microphones on each side of the M group, for a total of eight microphones.
The arrangement of FIGS. 3 and 4 may be modified, as shown in FIGS. 5 and 6 (which also shows two views of a microphone arrangement) so that the microphones are aligned in the side view, but along a curved line as seen from the top view.
In a further microphone arrangement as shown in the side view of FIG. 7, a single microphone comprising group M may be surrounded by four microphones of the M group. This arrangement may be modified, as shown in FIG. 8, wherein the central group M comprises eight microphones equally spaced about a central microphone, for a total of 9, while the M group comprises three microphones extending radially outwardly from each of the eight equallyspaced microphones of the M group, for a total of 24 microphones.
FIGS. 9-11 show several arrangements of microphones for an omnidirectional system. In FIGS. 9 and 10 three nondirectional microphones are shown aligned in two directions, with the microphone in the center comprising the M group and the two outside microphones comprising the M group. In FIG. 11, five directional microphones are provided in a line, with the central microphone comprising the M group and the four outside microphones, having different directions of maximum sensitivity, comprising the M2 group.
In each of the arrangements of FIGS. 111, the microphones of each group are connected together.
Referring now to FIG. 12, therein is illustrated a circuit for combining the output signals of the microphone groups according to the invention. The output signals of the two groups of microphones M and M are combined so that for frequencies below a separation frequency of, for example, 1500 cycles per second, the signals are combined so that the system operates on the difference principle. For frequencies above the separation frequency, the signals are combined so that the system operates on the interference principle.
In the system of FIG. 12, the outputs of the two groups M and M of microphones are amplified in amplifiers 1 and 2 respectively, and applied to opposite ends of a potentiometer 6 by way of resistors 3 and 4 respectively, and the combined signal appearing at the arm of the potentiometer 6 is applied by way of RC filter 6, 7 to an amplifier 8. The output of the amplifier 8 is connected to a RC filter 9, 10. As indicated by the polarity signs on the microphone groups M and M and at the outputs of the amplifiers, the signal at the arm of the potentiometer 6, and hence at the output of filter 9, 10, is a difference signal from the microphones of the two groups.
The signal from amplifier 2 is also applied by way of an adjustable divider consisting of series variable resistor 12 and shunt resistor 13, to an inverting amplifier 14. The outputs of the RC filter 9, 10, and the output of the amplifier 14, and applied in combination to the input of a low-pass filter 23, 2-4, by way of separation resistors 11 and 26 respectively. The output of the low pass filter 23, 24 is applied by way of resistor 25 to the output terminals 30, 31.
The output of the inverter 14 is also applied to input of an amplifier 19 by way of an adjustable potentiometer circuit consisting of series adjustable resistor 15 and shunt resistor 16, and the output of amplifier 1 is also applied to the input of amplifier 19 by way of a series resistor 17 and shunt resistor 18. The output of amplifier 19, is applied by way of high pass filter 20, 21 to the output terminals 30, 31. The signal appearing at the output terminals due to this part of the circuit is thus the sum of the outputs of the microphones M and M in afrequency range above the cut-off frequency of high pass filter 20, 21.
What is claimed is:
1. A microphone combination of the type comprising a plurality of sound units of one or more microphones arranged at a small distance from each other and connected through filter eleemnts to a common output, comprising first means for combining the outputs of first and second of said sound units with opposite polarity, low-pass filter means for applying the output of said first means to said common output, second means for combining the outputs of said first and second sound units with the same polarity, and high-pass filter means for applying the output of said second means to said common output, whereby the microphone combination operates on the interference principle in excess of a separation frequency between high and low notes, whereas below said frequency f it operates on the difference principle.
2. A microphone combination as claimed in claim 1, characterised in that the sound units are arranged side by side in a uniformly curved line or in a plane.
3. A microphone combination as claimed'in claim 1, characterized in that the microphones of each sound unit are connected with the same polarities.
4. A microphone combination as claimed in claim 1, characterized in that the individual microphones are unilaterally sensitive microphones, which are positioned so that their preferential directions coincide.
5. A micro hone system comprising a plurality of closely spaced-apart microphones and means for connecting said microphones by way of first and second channels to a common output circuit, said first channel comprising first filter means for passing signals only less than a predetermined frequency and means for combining the outputs of said microphones with opposite polarity to provide a difference signal output, said second channel comprising second filter means for passing signals only above said predetermined frequency, and means for combining the signal outputs of said microphones with the same polarity to provide an interference signal output.
6. A microphone system comprising a plurality of closely spaced-apart microphones arranged to form first and second microphone groups, an output circuit, first channel means for combining the outputs of said first and second groups with opposite polarity and applying the combined signal output thereof to said output circuit, second channel means for combining the outputs of said first and second groups with the same polarity and for applying the combined signal output thereof to said output circuit, said first channel comprising filter means for passing signals only below a predetermined frequency and said second channel comprising means for passing signals only above said predetermined frequency, whereby said microphone system operates on a difference principle below said predetermined frequency and on an interference principle above said predetermined frequency.
7. The system of claim 6, in which microphones of said first and second groups are in the same plane, and said microphones of said second group surround those of said first group.
6 References Cited UNITED STATES PATENTS 2,301,744 10/1942 Olson 179-1 2,305,599 12/1942 Bauer 1791 3,204,031 8/1965 Gorike et a1 1791 KATHL'EEN H. CLAFFY, Primary Examiner.
R. P. TAYLOR, Assistant Examiner.