Directional microphones — myths and reality. Abalmazov E.I..

Апр 27, 2024
napravlennie mikrofoni mifi i realnost abalmazov ei

Directional microphones — myths and reality. Abalmazov E.I.

Directional microphones: myths and reality.

Abalmazov Eduard Ivanovich, Doctor of Technical Sciences, Professor

The article is reprinted from the journal «Security Systems» » No. 4 1996

There are a variety of rumors about the capabilities of directional microphones. Some sincerely believe in their long range, calling distances 100, 200 or more meters, others, on the contrary, believe that there is unjustified advertising bordering on misinformation. Let's try to find out the real state of affairs using simple mathematical calculations.

  Instead of introduction

Speaking about directional microphones, we primarily mean situations of acoustic monitoring of sound sources in the open air, when the effects of the so-called reverberation of acoustic panels can be neglected. For such situations, the decisive factor is the distance of the sound source from the directional microphone, which leads to a significant weakening of the level of the controlled sound field(In addition, at a large distance, sound attenuation becomes noticeable due to the destruction of the spatial coherence of the field, due to the presence of natural energy dissipators, for example medium- and large-scale atmospheric turbulence that creates wind disturbances).

So, at a distance of 100 m, the sound pressure is weakened by at least 40 dB (compared to a distance of 1 m), and then the volume of a normal conversation of 60 dB will be no more than 20 dB at the reception point. This pressure is significantly less than not only the level of real external acoustic interference, but also the threshold acoustic sensitivity of conventional microphones. So, unlike regular microphones, directional microphones must have:

— high threshold acoustic sensitivity as a guarantee that the attenuated audio signal will exceed the level of the receiver’s own (mainly thermal) noise. Even in the absence of external acoustic interference, this is a necessary condition for sound control at a considerable distance from the source;

— high directionality of action as a guarantee that the weakened audio signal will exceed the level of residual external interference. High directionality is understood as the ability to suppress external acoustic interference from directions that do not coincide with the direction of the sound source. Meeting these requirements in full in practice (for one microphone) is an extremely difficult task. It became more realistic to solve particular problems, for example, creating a low-directional microphone with high sensitivity or, conversely, creating a highly directional microphone with low sensitivity, which led to a variety of types of directional microphones.

2. Types of directional microphones.

There are at least four types of directional microphones:

— parabolic;
— flat acoustic phased arrays;
— tubular or traveling wave microphones;
— gradient.

A parabolic microphone is a parabolic-shaped sound reflector, at the focus of which is a regular (omnidirectional) microphone. The reflector is made of both optically opaque and transparent (for example, acrylic plastic) material. 


Fig. 1 Parabolic microphone.

The outer diameter of a parabolic mirror can be from 200 to 500 mm. The operating principle of this microphone is explained in Fig. 1. Sound waves from the axial direction, reflected from a parabolic mirror, are summed up in phase at focal point A. An amplification of the sound field occurs. The larger the diameter of the mirror, the greater the gain the device can provide. If the direction of sound arrival is not axial, then the addition of sound waves reflected from various parts of a parabolic mirror arriving at point A will give a smaller result, since not all terms will be in phase. The greater the angle of arrival of sound relative to the axis, the greater the attenuation. Thus, angular selectivity in reception is created. A parabolic microphone is a typical example of a highly sensitive but low directional microphone.

Flat phased arraysimplement the idea of ​​simultaneous reception of a sound field at discrete points of a certain plane perpendicular to the direction of the sound source (Fig. 2). At these points (A1, A2,A3…) either microphones are placed, the output signals of which are summed electrically, or, most often, the open ends of sound guides, for example, tubes of a sufficiently small diameter that provide in-phase addition sound signals from a source in some acoustic adder.

 

napravlennie mikrofoni mifi i realnost abalmazov ei 2
Fig. 2 Flat phased array.
 

A microphone is connected to the adder output. If the sound comes from the axial direction, then all signals propagating along the sound-water will be in phase, and addition in the acoustic adder will give the maximum result. If the direction to the sound source is not axial, but at a certain angle to the axis, then the signals from different points of the receiving plane will be different in phase and the result of their addition will be smaller. The greater the angle of arrival of sound, the greater its attenuation. Typically, the number of receiving points Аi in such arrays is several tens. Structurally flat phased arrays are built either into the front wall of the attaché case with subsequent camouflage, or into a vest vest, which is worn under a jacket or shirt. The necessary electronic components (amplifier, batteries, tape recorder) are located, respectively, either in a case or under clothing. Thus, flat phased arrays with camouflage are visually more secretive compared to a parabolic microphone.

Tubular microphones, or “travelling” wave microphones, unlike parabolic microphones and flat acoustic arrays

receive sound not on a plane, but along a certain line coinciding with the direction of the sound source. The principle of their operation is illustrated in Fig. 3. 

napravlennie mikrofoni mifi i realnost abalmazov ei 3
Fig. 3 Tubular microphone.

 

The basis of the microphone is a sound guide in the form of a rigid hollow tube with a diameter of 10-30 mm with special slotted holes placed in rows along the entire length of the sound guide, with a circular geometry for each of the rows. It is obvious that when receiving sound from the axial direction, there will be an addition in phase of the signals penetrating into the sound guide through all the slot holes, since the speeds of the axial propagation of sound outside and inside the tube are the same. When sound arrives at a certain angle to the axis of the microphone, this leads to a phase mismatch, since the speed of sound in the tube will be greater than the axial component of the speed of sound outside it, as a result of which the reception sensitivity decreases. Typically, the length of a tubular microphone is from 15-230 mm to 1 m. The longer its length, the stronger the suppression of interference from the side and rear directions.

High-order gradient microphones are practically not represented on the open supply market. The exception is the first order gradient microphone.

Unlike phased receiving acoustic arrays, which use the operation of adding acoustic signals, gradient microphones are based on the operation of subtraction in the direction of signal arrival. This puts them at a priori disadvantage in threshold sensitivity, since each subtraction attenuates the signal but statistically adds up the internal noise. At the same time, the subtraction operation itself makes it possible to design small-sized directional systems. The simplest gradient directional microphone is a microphone that implements a first-order gradient (Fig. 4). 

napravlennie mikrofoni mifi i realnost abalmazov ei 4
Fig. 4 The simplest gradient microphone.

It consists of two fairly miniature and closely spaced highly sensitive microphones M1 and M2, the output signals of which are electrically (or acoustically) subtracted from each other, realizing in finite differences the first derivative of the sound field along the microphone axis and forming a diagram of the form cos Q, where Q is the angle of arrival sound. This ensures a relative weakening of acoustic fields from lateral directions (O — 90°). High-order gradient microphones are systems that implement spatial derivatives of the 2nd, 3rd and higher orders.

3. How to compare and evaluate directional microphones? The main user characteristic of directional microphones is their range in specific conditions. For open space and external acoustic interference that is isotropic and independent in angular directions, the range R is related to:

a) with the spectral signal-to-interference ratio q at the output of a directional microphone,

b) with the spectral speech level Vr;

c) with the spectral level of external acoustic interference Vsh relationship of the form:

q=Bp-Bsh-20 lg R+G-Bp ( 1)

where

G is the so-called microphone directivity coefficient (dB),

Вп — threshold acoustic sensitivity of the microphone (dB).

 

The directional coefficient G included in formula (1) characterizes the degree of relative suppression of external acoustic interference: the greater it is, the stronger this suppression. Theoretically, it is related to the normalized microphone radiation pattern F (Q,j) a relationship of the form:

napravlennie mikrofoni mifi i realnost abalmazov ei 5

  where

Q is the angle of arrival of the sound wave relative to the axis of the microphone;

j — the angle of arrival of the sound wave in the polar coordinates of the plane

perpendicular to the axis. For example, for a tube microphone, when

napravlennie mikrofoni mifi i realnost abalmazov ei 6

where l is the wavelength of sound. and L is the length of the tube, we have (with L ? l . ):

 

G = 4 L/l. (4)

 

Similarly, an approximate formula is derived for the directivity coefficient of parabolic microphones and phased plane arrays:

G = 4p (S/l 2) (5)

 

where S— area of ​​the entrance aperture; l .- wavelength of sound. For nth order gradient microphones

with optimal signal processing

 

G=n (n+1) (6)

 

where n is the order of the gradient. For known values ​​of Gformula (1) is sufficient to obtain absolute estimates of the expected spectral signal-to-interference ratio if the conditions are known. But in many cases, knowledge of these conditions is inaccurate. Therefore, it is more justified to use not absolute, but relative estimates of range, as they do not require exact knowledge of the conditions, since comparison occurs when they are equal. Accepting this ideology, let us compare the capabilities of directional microphones with the capabilities of human hearing without special devices. Formally, a relation similar to (1) can be written for it. As a result of the comparison we get:  

R=R0 x 10 0.05 (G-G0) – 0.005 D Bp (7)

  Here R0— range of audibility of sound by the hearing organ;

R — range of a directional microphone with the same quality of control.

Go — directional coefficient of the human hearing organ (binoural listening mode).

D Bп — the difference in the threshold sensitivity of the directional microphone and the hearing organ. In Fig. 5 shows a graph of the relative range R/Ro of a directional microphone as a function of its directional coefficient G for the case when D Bп = O(the option is technically feasible). The coefficient Go of the directional action of the organ of hearing by humans is assumed to be 6 dB.

The graph shows that at G = 15 dB (this value of G approximately corresponds to the data for most fairly good microphones such as phased arrays and parabolic type)a directional microphone will allow you to realize a control range approximately 3 times greater than theRo distance, in which sound is perceived by a person without special devices. The comparison is carried out under the same conditions for the same sound source. In practice, this result means the following: if we are talking about acoustic monitoring of conversations in the city, on the street, when R0 = 2 — 4 m, then directional microphones will allow you to record conversations at distances of 6-12 m. In suburban conditions , with less noise when the value of Rocan reach 10 m or more, the control range using technical means can be more than 30 m.

napravlennie mikrofoni mifi i realnost abalmazov ei 7
Fig. 5. The range of action of a directional microphone R compared
with the range R» of sound audibility by an unequipped hearing organ.

These are the assessments of situations where directional microphones are used in open space conditions. But it is also possible to use directional microphones in enclosed spaces, for which it is necessary to take into account reverberation, that is, reflections of sound signals from the walls of rooms and interior items.

Formally, under these conditions, relation (7) remains valid if instead of G we use the reduced directional coefficient G0:

G0=(G+R)/(1+R) (8)

 where R is some parameter that takes into account the surface area of ​​the volume (so called acoustic ratio).

4. Thinking about the future

Speaking about the future of this special industry, we can highlight at least three areas for possible improvement of directional microphones. On the one hand, we should expect (by analogy with adaptive temporal filtering) the emergence of devices capable of adaptive spatiotemporal filtering of acoustic interference. The objective basis of such devices is achievements in the field of digital multichannel data processing. The second opportunity to improve directional microphones is associated with progress in the field of highly sensitive acoustic sensors, which fundamentally makes it possible to create microphones with a threshold sensitivity of minus 10 — minus 15 dB and a maximum control range in the absence of external noise. And finally, we cannot exclude the emergence of fundamentally new directional microphones that use nonlinear and parametric effects to implement large-sized organoleptic covert antennas and capable of providing c.n.d. 20-25 dB or more.   

 

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